concrete technology

Concrete Technology

BP 2158-1 GB

For Pumps

2nd edition, March 1996

Published by:Putzmeister AGMax-Eyth-Strasse 10D-72631 Aichtal

Author:PMW Central Service

Assembly, layout, production:Dipl.-Ing. Andreas Hartmann

Specialist support:Dr.-Ing. Dieter Bergemann

Nominal fee: DM 10,–

© 2001Copyright, also just excerptsonly when permission is granted by the publisher

Index

Page

Preliminary comments ........................................................................ 4

1 Concrete components – base materials and their influences ................................................ 6

1.1 Cement................................................................................................... 61.2 Addition of water .................................................................................... 71.3 Concrete aggregates ............................................................................. 81.4 Concrete additives ................................................................................. 111.5 Concrete composition – mix calculation................................................ 13

2 Properties of freshly-mixed concrete (general) ................................ 182.1 Bulk density............................................................................................ 182.2 Workability .............................................................................................. 19

3 Properties of hardened concrete and their alternating relation to concrete technology ..................... 23

4 Properties and conditions of freshly-mixed concrete when pumping ...................................................................... 25

4.1 Pumpability and willingness to pump .................................................... 254.2 Origination and properties of the “boundary zone layer“...................... 264.3 The behaviour of freshly-mixed concrete in the concrete pump........... 304.4 The behaviour of freshly-mixed concrete in the delivery line................ 33

5 Short guide to avoid faults and eliminate them ................................ 36

6 Specifications and regulations of the“Technical Regulations“ ...... 37

7 Further literature .................................................................................. 38

Concrete Technology For Pumps

3BP 2158--1 GB

4

Preliminary comments

“Concrete is an artificial stone which is made out of a mix of cement, concrete aggregate and water –and if necessary also concrete additives and comes into being by the hardening of the cement paste(cement-water mix)“.1 Very different concrete properties are attained depending upon the choice ofits composition. Before hardening the so-called freshly-mixed concrete is more or less “fluid“ and canbe made into almost any shape chosen and when it has hardened as an artificial stone it retains thisshape.

Different distinctions and categories are the result of the many possible compositions and applicationsof the construction material concrete:

⇒ one subdivides as follows depending upon the reinforcement

◆ steel-reinforced– conventionally reinforced concrete– pre-stressed concrete

◆ non-reinforced concrete

⇒ one subdivides according to the dry bulk density:

◆ light-weight concrete not heavier than 2.0 t/m3

◆ normal concrete heavier than 2.0 t/m3, but not heavier than 2.8 t/m3

◆ heavy concrete heavier than 2.8 t/m3

⇒ One subdivides according to the specifications for manufacture and monitoring

◆ B I concrete– concrete of strength class B 5– concrete of strength class B 10– concrete of strength class B 15– concrete of strength class B 25

◆ B II concrete – concrete of strength class B 35– concrete of strength class B 45– concrete with special properties

✳ watertight concrete✳ concrete with high resistance to freezing✳ concrete with high resistance to frost and de-icing salt✳ concrete with high resistance to chemical corrosion✳ concrete with high resistance to wear✳ concrete for high working temperatures up to 250° C

⇒ One subdivides according to the state of hardening

◆ Freshly-mixed concrete as long as it can be still be placed– According to its consistency, one subdivides the freshly-mixed concrete into

✳ stiff-plastic, soft and free-flowing.

– According to the type of conveying and placing ✳ no-fines concrete, pumping concrete, underwater concrete and shotcrete.


BP 2158-1 GB 1DIN 1045 “Concrete and reinforced concrete“

– According to the type of compaction one recognises:✳ tamped concrete, rod concrete, vibrated concrete, jolted concrete and spun con-

crete.

◆ Green concrete Concrete after initial setting and during hardening;can no longer be placed, but does not have anystrength yet.

◆ Hardened concrete after hardening

– According to the type of the surface condition, hardened concrete is subdivided into:✳ Exposed concrete, concrete with exposed aggregates by washing, etc.

⇒ Depending upon the place of preparation one distinguishes between:

◆ Construction site concrete the mobile mixing plant is on the construction site

◆ Ready-mixed concrete a stationary concrete works makes the concreteand the freshly-mixed concrete is delivered byspecial vehicles to the construction site ready forplacing.

⇒ Depending upon the place of placement, one distinguishes between:

◆ Cast in-situ concrete placement of freshly-mixed concrete and harde-ning at the final site

◆ Concrete products components manufactured in a precast factory oron the construction site which are not placed untilthey are hardened.

Concrete technology comprises all tasks which especially serve the purpose of guaranteeing theconstruction material properties of concrete aimed at with the base materials available. After deter-mining the mix contents this mainly concerned all freshly-mixed concrete processes starting withmixing via transport, placing and compaction to any after-treatment of the green concrete that maybe necessary. Here it is the duty of concrete technology to also purposely influence the properties ofthe freshly-mixed concrete for the placing stages planned in a serviceable way, but if possible with-out any negative impact on the later properties of the hardened concrete.

The pumping nowadays of freshly-mixed concrete is a link of the process chain that one can scarcelyimagine not having. It was not very long ago that it was also the task of concrete technology to spe-cially design the concrete composition as pumping concrete. Now for the latest stage of concretepump technology, pumping concrete is no longer a special concrete. It is a concrete normed by theconcrete technology in a composition as is required for reinforced components of reinforced concrete.

However, every good pump operator should have a basic knowledge of concrete technology. On theone hand, he ought to know which pump-technical consequences are a result of the different pro-perties of the material, and on the other hand, he must recognise what the construction material con-sequences would be if the freshly-mixed concrete was handled incorrectly. This present print“Concrete Technology For Pumping“ serves this purpose. Further information can be found in the“Technical Regulations“ (List – see section 6) as well as in more specialised literature (list: refer to section 7).


5 BP 2158-1 GB

6

1. Concrete components – base materials and their influences

1.1 Cement

Cement is usually a grey powder which is fabricated by burning and grinding certain limey and clayeyrock. It rigidly combines (cements together) the individual aggregate (s), i.e. sand or gravel particles,to form artificial stone.

The different types of cement have different concrete grades, arranged according to the strengthclasses, for example, in Germany from Z 25 to Z 55. Depending upon the chemical composition andfineness of grinding, the different types of cement develop their strengths at different speeds.Portland and Iron Portland cements usually belong to the cement with a higher early strength. Blastfurnace cement can considerably improve the chemical strength. The standard norms for cementare:

◆ in Germany DIN 1164

◆ in Austria the ÖNORM B 3310

◆ various country-specific norms

The numerical data of the strength classes usually refer to the minimum strength to be attained byassay grains after 28 days at a certain w/c value, measured in the respective country-specific unit(e.g. in Germany: N/mm2, in Austria: kp/cm2). The development of the strength is by no way completeafter 28 days; this value does, however, usually form the basis for the strength calculation and per-mission is granted to use the building. The cement does not reach its final strength until quite a fewyears later. This is then, however, almost the same for all cements with the same w/c factor. Also theprotection against corrosion of the steel reinforcements is the same for all cements.

The setting of the cement (hydration) is a very complicated process where water is combined chemi-cally and physically. When mixing cement and water a cement paste results and here the cementimmediately starts to form new microscopically small crystal connections with the water. These finecrystals mat together closer and closer, and this first results in the setting and then the hardening ofthe cement paste to hardened cement paste. This has the following special properties:

◆ Both in the air and under water, it remains solid and volumetrically stable

◆ The steel parts in the concrete (e.g. reinforcement) are protected against rust

◆ When temperatures increase, it expands to the same extent as steel

If one of these properties was to be lacking, there would not be any reinforced concrete. The cementmay only begin to set at the earliest one hour after mixing and up to this point in time, the freshly-mixed concrete can be placed.

For complete hydration, approximately 40% of the cement mass must be water; only approximately25% of this is, however, combined chemically whereas the rest stays as steam in the gel pores; i.e.physically combined. For a water cement value below 0.40, the cement grain cannot hydrate completely even with constant water immersion; whereas for a w/c value of over 0.40, even after complete hydration, finest capillary pores which at first are still filled with water, Fig. 1 illustrates theseconditions. The diameter of these capillary pores is approximately 1000 times larger than that of thegel pores.


BP 2158-1 GB

To fabricate a placeable concrete usually more than just 40% of the cement mass is required as water.The amount of water required is stipulated in the mixture breakdown.

CautionEvery unauthorised addition of water on the construction site drastically impairs the quality!

This impairs the strength (15 to 30%) and correspondingly also the imperviousness of the concrete.

1.2 Adding water

There are also norms, resp. specifications for the metered water to restrict the content of harmfulmaterials which develop corrosion or disturb hardening. In principle note the following: drinking wateris always suitable as added water.

CautionThe mix of water and cement is greatly alkaline and has a causticeffect on skin and mucous membrane. Always wear gloves andgood shoes. If direct contact is made inadvertently flush immedia-tely with sufficient clean water.

Fig. 1: Diagram showing the reaction of cement and water (hydration)


7 BP 2158-1 GB

Not enough water => unused cement is left over!

Complete hydration with 40% water!

Too much water =>capillary pores

Water Cement grain Pore

Hydration

w/c = 0.20

w/c = 0.40

w/c = 0.60

Capillary pores

8

1.3 Concrete aggregates

Concrete aggregates are usually natural rock from gravel pits, rivers (gravel and sand) or quarries(chippings) and they give the concrete certain properties. The requirements regarding quality that areto be monitored are stipulated in the respective standards:

◆ in Germany the DIN 4226◆ in Austria the ÖNORM B 3304◆ diverse country-specific norms.

Along with the designation of the aggregate and the usual grain groups, these standards comprisethe requirements regarding:

◆ shape of the grain◆ resistance to frost and thawing agent◆ content of material of organic origin◆ content of sulphates◆ pressure resistance◆ content of components that can be clarified by filter presses◆ content of swellable components◆ content of water-soluble chloride

Concrete aggregates are sub-divided according to grain size into different grain sizes. Here the smal-lest and largest grain are quoted, e.g. 0/2; 0/4; 2/8; 8/16; 16/32. The aggregate of a type of concertusually consists of a mix of fine, average and coarse grain. This composition may be present in naturein a mine. Usually, however, the grain mix that arises naturally or the mix that results when rocks arebroken is classified immediately with regard to size, i.e. it is separated by large screen plants accor-ding to grain size and it is then delivered to concrete mixing plants and stored in separate boxes.


BP 2158-1 GB

Fig. 2: Sieve analysis and grading curve

Screen hole size (mm)

Scr

een

hole

(w

eig

ht %

)

Screen analysis and grading curve

25.9 Gew.-%

Scr

een

hole

siz

e (m

m)

23.1 Gew. -%

14.7 Gew.-%

5.6 Gew.-%5.3 Gew.-%8.2 Gew.-%

11.2 Gew.-%

5.2 Gew.-%

0.5 Gew.-%

When preparing the concrete in the mixer, the portions of the different grain sizes are mixed in thecomposition rquired. The composition of a grain mix is measured by screen analysis and representedgraphically as a grading curve. For this, a sample previously measured is separated in a laboratoryby a set of vibrating screens which are stacked on top of each other, and which have the prescribedmesh or square hole screens. Fig. 2 illustrates this process. The top screen has the largest meshwidth and the lowest the smallest one. Right at the bottom the floor is closed to retain of the finestcomponents. The sample to be examined is evenly distributed onto this vibrating set of screens. Herethe individual grains drop downwards from the floor of one screen to the floor of the nest screen untilthe mesh or hole width is too small for the respective grain size.

The shaking of the screens really leads to each grain often having the opportunity to pass through amesh or a hole in different positions. The amount of grain remaining on each screen floor is then weighed and its proportion to the total weight of the sample is calculated. The grading curve as thegraphical representation of the grain composition, is attained by placing the percentual shares of thesample that have passed through the respective screen floor above the screen hole.

These individual values are attained as a sum of all the shares of residue of the respective screenfloor and the ones below it with a smaller mesh width. One therefore sometimes also calls the curvethat arises the “throughs” grading curve. The horizontal axis (screen hole size) in the grading curvediagram is divided for better vividness in logarithmic scale. As the shape of the grain is very irregu-lar, one would attain varying grading curves for different screen opening shapes (e.g. round hole orsquare mesh). This is why the quadratic shape of the test screen openings is prescribed along withthe mesh width. If one uses screens with a different shape (e.g. round or long hole) when classifyingin production, a screen analysis conforming to standard requirements is required if the actual grainsize is to be quoted or the so-called oversize and undersize particles checked.


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Grain Vol Surface No. of grain(cm3) (cm2)

16 / 32 186 0.052 378.0 / 16 164 0.0921 2574.0 / 8.0 104 0.1169 13052.0 / 4.0 39.8 0.0895 39981.0 / 2.0 37.6 0.1691 302160.5 / 1.0 58.2 0.5234 3742360.25 / 0.5 79.5 1.4712 44548690.125 / 0.25 36.9 1.329 152442100 / 0.125 3.55 0.2424 9989144Sum 709.55 4.0856 30098272

Fig. 3: Geometrical relations in the grain mix

10

The grain composition favourable for normal concrete is a compromise between complete filling ofthe cavities between the larger aggregate grains with fine material and a restriction of the share offines due to the higher cement consumption. The smaller a grain is, the greater the surface is per unitof mass (m2/kg) which must be covered with cement paste. Fig. 3 shows, taking the grain composi-tion represented in Fig. 2 as an example, the relations with regard to volume, specific surface andnumber of grain of the different fractions.

To be able to assess the amount of water needed for a certain grain mix when preparing concrete,one can use the so-called grain figure k which is calculated from the grading curve: the sum of theresidue quoted in % on the screens 0.25; 0.5; 1; 2; 4; 8; 16; 31.5 and 63 mm divided by 100. The valuesto be used here for the ‘residue’ are not, however, not the share residue on the individual screen floorsfor the screen analysis but are the residue that would arise if one were to begin the screen analysiswith the smallest mesh width. In this way, the coarse grain sizes are ‘weighed’ several times and theirshare forces up high the grain figure, i.e. the more coarse grain and less fine grain an aggregate mixhas, the greater the grain figure. Thus a pure grain group 32/63 would have the maximum possiblegrain figure kmax = 9 and a pure fine sand grain 0/0.25 the minimum possible grain figure kmin = 1.

The grain figure, however, only considers the influence of the grain size but the total surface and thereby the amount of cement paste needed or the amount of water needed by an aggregate mixalso depends upon the shape of the grain. Fig. 4 illustrates this with the example of a cube repre-senting a “compact” grain and a plate with the same volume representing a ‘platy’ grain which has asurface 2/3 times greater than that of the ‘compact’ grain. For grain liable to chip, this difference iseven greater, whereas the surface of a ‘round grain’ (ball) with the same volume is 1/5 smaller thanthat for the cube. In addition, the shape of the grain also directly influences the workability of the con-crete. Concrete with round, compact and smooth grain “flows” better and can also be compactedbetter than concrete with large platy or crackable aggregate with a rough surface.

Usually the largest grain of the aggregate for concrete is restricted to 32 mm diameter. For especial-ly bulky components, this value can be increased to 63 mm (can, then, however, no longer be con-veyed by standard pumps through standard pipes. For piece parts with fine members and finely rein-forced, the greatest grain is restricted to 16 mm or even 8 mm diameter.


BP 2158-1 GB

Fig. 4: Influence of the shape of the grain on the surface with the same volume

V = volume O = surface area

The grading curve to be carried out is chosen according to the intended application on the basis ofthe experience gained over a number of years by the Technical Regulations and recorded as speci-fications and recommendations. The standardised grading curves ranges serve as an orientationhere, e.g. in Germany DIN 1045 according to Fig. 5 for aggregate mixes 0/8, 0/16, 0/32.

1.4 Concrete additives

With concrete admixtures the properties of the freshly-mixed or hard concrete are improved or newproperties are attained. One distinguishes between concrete additives and reagents. Concrete addi-tives in Germany must comply with a standard or a test mark of the German Institute for StructuralEngineering. These are usually powdery admixtures which are added to the concrete in amounts ofsome % of the cement content. They mainly work physically and usually serve as an aid for betterworkability, less water repellent (bleeding), higher structural imperviousness or as coloration.

The most important additives are:

◆ Trass (DIN 51043) ◆ Silica dust (test mark)◆ Limestone dust or siliceous dust (DIN 4226) ◆ Silica suspension (test mark)◆ Coal fly ash (test mark)

Pigments in accordance with DIN 53237 are used to dye concrete


11 BP 2158-1 GB

Fig. 5: Screen curve ranges according to DIN 1045 for aggregate mixes 0/8, 0/16 and 0/32

Aggregate mix 0/8 Aggregate mix 0/16

Aggregate mix 0/32

Scr

een

thro

ughp

ut (

wei

ght

%)

Screen hole width Screen hole width

Scr

een

thro

ughp

ut (

wei

ght

%)

Scr

een

thro

ughp

ut (

wei

ght

%)

Screen hole width

12

Concrete reagents are usually liquid and are only added in very small amounts whilst mixing the con-crete. They act chemically-physically and are classified into so-called efficiency groups dependingupon their effect in freshly-mixed or hardened concrete:

◆ Concrete liquefier (BV)These reagents detension the water and one improves the workability at the same timeas reducing or keeping the prescribed water cement value.

◆ Solvent (FM)These reagents are further advanced concrete liquefiers. They have a very strong lique-fying effect, but not for very long. This is why they are not added until just at the point ofplacement before the mix is placed. After the solvent has been added the slump measure(refer to section 2) increases from, for example, 41 cm to max. permissible 60 cm. Afterabout half an hour this value returns to its initial value again. Within this space of time theconcrete can spread out without bleeding.

◆ Air-entrainment agent (LP)Concrete with high resistance to frost and thawing salt must have a minimum content ofmicro air voids (Ø 0.01 .. 1 mm) which can be obtained by adding air entrainment agent.Ice has a larger volume than water. If the expansion of the frozen water is prevented inthe concrete then the concrete may burst. The additional air voids offer the necessaryspace for this extension.

◆ Water repellent (DM)These ought to improve the water imperviousness of the concrete. Its efficiency is, how-ever, limited and is no replacement for a correct concrete composition.

◆ Setting retarder (VZ)These postpone the thickening and the point of initial setting of the concrete. This maybe necessary due to a number of reasons, e.g. hot weather or large and jointless com-ponents. Over-metering can, however, have the opposite effect and turn the VZ into ansetting accelerator agent!

◆ Setting accelerator (BE)These chemically accelerate the setting of, for example, shotcrete or sealing mortar upto just a few seconds after spraying or placing. An alternative without the disadvantageof considerable reduction of the 28 day- and final strength is physically-active micro silicadust.

In addition there a number of other reagents which are, however, not used everywhere. Test specifi-cations, standards and approval recommendations are available for the different types of reagents.

Caution!Adding concrete additives at the point of placement is forbidden!

This usually leads to the concrete being damaged. An exception here are solvents. These may beadded into the mixer vessel on the job-site in accordance with the concrete producer’s instructions.If these instructions are not abided all claims for warranty are void.


BP 2158-1 GB

1.5 Concrete composition – mix calculation

The quality of the concrete can be attained with the respective composition of the green concrete.DIN 1045 contains binding data for the composition of concrete both with a certain strength and alsowith special properties. The requirements are different for the concrete groups B I and B II.

Conditions are normal for personnel and machines in the company for the production and placing ofconcrete B I (strength class B 5 up to and including B 25). For the composition of concrete B I thereare two possibilities:

◆ without previous qualification test as so-called “mix-formula concrete“1 or◆ with qualification test

Even when using mix-formula concrete, the person responsible for the production of the concrete isnot exempt from carrying out the prescribed quality tests. When using ready-mixed concrete, it usual-ly suffices to test the data on the delivery note or in the register for the type of concrete. When usingB II concrete (strength classes B 35 to B 55 and concrete with special properties) the company isallowed a great choice in concrete composition. The further use of the concrete that is now possibledoes, however, expect more from the personnel and machines, and requires monitoring of the value(own and outside monitoring). All the regulations for this are included in DIN 2045 and DIN 1084.Then the company must have a constant concrete testing point E run by a specialist experienced inconcrete technology and the manufacture of concrete. Concrete B II requires its own testing at alltimes.

The task concerning the mix calculation is to determine the amounts of cement, water and aggregateneeded for 1 m3 compacted concrete for a strength class required or “special property” of the con-crete. For this one uses a form or suitable calculation programme with respective results as shown inFig. 6 for a B 25. Here one proceeds according to the following design operation.

◆ water cement value ω

The quantity ratio of water added and cement (water cement value W/C or ω is one of the most impor-tant characteristic index for the quality of the concrete.

◆ the strength of the concrete decreases◆ the water permeability and the sensibility to weather increases ◆ concrete dries out quicker and shrinks more, consequently high shrinkage stress and

cracks◆ concrete tends to “bleed“ and segregate and there is more dust formation on surface

Fig. 7 shows the dependence of the cube compressive strength of the concrete on the cementstrength class and the water cement value. The result for this example is, however, the max.W/C valueof 0.6 due to the special requirements.

◆ Water content W


13 BP 2158-1 GB1mix tables, see DIN 1045

14


BP 2158-1 GB

Fig. 6: Example for mix calculation

The amount of water needed for 1 m3 concrete is a result of the dependence on the grain design(grain fig. of the grading curve), in the example k = 4.62) and the consistency (in the example: KR)from the following table from DIN 1045 – W = 180 dm3 (litre) = 180 kg.


15 BP 2158-1 GB

Fig. 7: Dependence of the cube compressive strength on the cement strength class and w/c value

Standards for the water required in dm3 per m3 freshly-mixed concrete:

Grading curve Grain figure Consistency

k KS KP KR

A 32 5.48 130 150 170

A 16 4.60 140 160 180

B 32 4.20 150 170 190

B 16 3.66 160 180 200

C 32 3.30 170 190 210

C 16 2.75 190 210 230

Co

ncr

ete

com

pre

ssiv

e st

ren

gth

(N

/mm

2 )

Water cement value

◆ Cement content Z

The cement content Z is calculated from the water content W and the W/C value ω

Z = W / ω = 180 / 0.6 = 300 kg

This value coincides with the given minimum cement content and therefore does not need to be cor-rected.

◆ Aggregate content G

The aggregate content G is calculated with the aid of the so-called material volume calculation, i.e.the volume of the cement, water and air voids (1.2% of 1000 dm3 = 12 dm3) are deducted from thevolume 1 m3 = 1000 dm3 of the compacted freshly-mixed concrete to be calculated. The rest valuecorresponds to the volume to be filled with concrete aggregate. Here the cement content Z, that wascalculated in kg, must first be divided by the value known of its bulk density rz = 3.05 kg/dm3.

300G = 1000 – ––––3.05

– 180 – 12 = 710 dm3

◆ Mo grain content

Mo is the share of solid matter which has a grain size smaller than 0.125 mm i.e. the mo content iscomposed of cement, the share of grain 0/0.125 contained is the concrete aggregate and any con-crete additive added. In the following example the mo content is:

300 kg cement + 0.5% · 710 dm3 · 2.8 kg/dm3 = 310 kg

Mo improves the workability of the freshly-mixed concrete and leads to a tight texture of the hardenedconcrete. A sufficient share of mo is therefore important for pumped concrete, exposed concrete,concrete for thin-walled, tightly reinforced components and for water-impermeable concrete.

A too high share of mo can, however, be of disadvantage. The content of mo as well as the contentof mo and finest sand is therefore restricted in DIN 1045 to 0.25 mm for external components as wellas for concrete with a high resistance to frost and thawing salt, and to wear depending upon thecement content.

The values may be increased by max. 50 kg/m3 when:– the cement content exceeds 350 kg/m3 and– a puzzolane concrete aggregate is used– the largest grain is 8 mm

Highest permissible content for concrete with largest grain of 16,32 and 63 mm(intermediate values are to be lineally interpolated)

Content of cement Mo = cement + aggregate Mo + finest sand + additive 0/0.125 + additives 0.125/0.25

[kg/m3] [kg/m3] [kg/m3]

up to 300 350 450

up to 350 400 500

16


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17

◆ Mortar content

Mortar is defined as the shares of cement, water, air voids, aggregate 0/2. Its content is given in dm3

for 1 m3 compressed freshly-mixed concrete, In the following example, this is

98 dm3 cement + 180 dm3 water + 12 dm3 air voids + 30.4% · 710 dm3 aggregate 0/2 = 506 dm3

The consistency of the freshly-mixed concrete primarily depends upon the amount of mortarand not, as is often assumed upon the water/cement content.

Also the pumpability2 is influenced by the mortar content. The following are valid as standard values3

for pumpable concrete:

◆ Mix formula calculation

For the final calculation of the mix formula for 1 m3 compacted freshly-mixed concrete, note that the% shares of the individual grain groups from the grading curve have both very different bulk densi-ties and also the water contained in aggregate is usually different to each other. Due to this, the drymass, the water contained in the aggregate and the total mass to be weighed when mixing, are to becalculated for every grain group. The amount of water actually added when mixing is a result of thereduced water content of the water contained in the aggregate of all the grain groups.

The above-mentioned form for the mix calculation additionally contains a mix formula table, for resp.one mix fill which depends upon the mixer size available and its filling rate for the respective concreteconsistency. Modern, computer-controlled mixer plants usually calculate this independently.

By choosing and determining a concrete mix formula especially when using components that areavailable locally, the properties of the concrete are determined which are needed to fulfil the job sitetask. The unerring guarantee for these properties depends lastly upon how good the knowledge ofall participants is on concrete technology. Here it is not just necessary to be acquainted with the mostimportant properties of the concrete but one must also know what positively or negatively influencesthem and to what extent.


BP 2158-1 GB

Largest grain (mm) Mortar content (dm3 / m3)

32 500-550

16 550-600

2 Refer to section 4.13 Refer to PM concrete mix formula

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2. Properties of freshly-mixed concrete (general)

The most important properties of freshly-mixed concrete are:

◆ bulk density (incl. degree of compaction and pores content) and ◆ workability (incl. consistency, deformation behaviour, homogeneity etc.)

2.1 Bulk density

When one talks about the bulk density of freshly-mixed concrete one means the mass in t per m3

of fresh concrete compacted correctly, including the remaining air voids.

After careful compacting, the air content remaining in the concrete for normal concrete with 32 mmlargest grain is still 1…2 vol.-%, i.e. 10…20 litres per m3. For finely-grained concrete this value canbe up to 60 litres per m3. These residual air voids vary greatly in shape and size. Another type of airvoid is produced on purpose by deliberately adding additive (LP) that forms air voids. This increasesthe resistance to frost and thawing salt. These voids are distributed very finely and are also verysmall, if possible below 0.3 mm diameter. A too great an air content, no matter what type, would howe-ver impair the strength of the concrete.

Concrete that is insufficiently compacted can therefore not be compared to a concrete that has beentreated with LP. The freshly-mixed concrete placed in the component contains more or less voidsdepending upon the consistency and the aggregate mix. These voids that are first filled with air mustbe removed as far as possible by compaction. With the aid of an exterior vibrator on the formwork ora vibrating cylinder that is immersed into the freshly-mixed concrete, the freshly-mixed concrete ismade to vibrate so that it seemingly becomes fluid within the zone of action of the vibrator and the airfrom the air voids rises to the surface as a result of natural ascending force. So as not to make thispath up to the surface insurmountably, respectively. to unnecessarily increase the duration for com-paction and therefore the danger of segregation connected with this, the concrete layer to be com-pacted by vibrating should not be higher than approx. 0.5 m.

The compaction of freshly-mixed concrete comprises, however, more than just this. The concretecomponents on the construction component surface formed by a form-work as well as the surface ofthe reinforcement rods or mats that are found in the construction components have to be rearrangedin such a way that also these surfaces are completely covered with cement paste. Unsatisfactorycompaction is very often the cause for later damage to the structure or for complaints already whenthe acceptance test on the structure is carried out. The degree of compaction of compacted concretethat has just been placed can, however, not be measured. Just in a few exceptional cases can thedegree of compaction be checked later by core lifting from the hardened body of concrete. With theso-called air-entrainment meter, a standardised testing unit, one can only determine the air void con-tent of a sample of freshly-mixed concrete removed if possible at the point of placement; but the com-paction of the concrete must not implicitly coincide with the actual conditions.


BP 2158-1 GB

2.2 Workability

The consistency is a measure for the stiffness and thereby the workability of the concrete. With anotherwise constant concrete quality, it does not depend upon the w/c value but upon the amount ofcement paste. The consistency is measured, resp. tested by different standardised methods of test.

The most common methods in Germany are the slump test and in certain cases the compactiontest according to Walz DIN 1048. Part I differentiates four consistency ranges:

The European Concrete Standards ENV 206 differentiate between the following, similar consistencyclasses:

Note here that the F and C classes are not the same.

The compaction test (refer to Fig. 8) is suitable for determining the consistency of stiff, plastic andsoft concrete, but not for free-flowing concrete. This method may be more suitable than the slump testfor the consistency ranges KP and KR when using chip concrete, which is concrete with a high mocontent or lightweight and heavy-weight concrete.

Used here are:a 40 cm high plate container, closed at the bottom with quadratic cross-section (20 cm x 20 cm):alternatively a 20 cm cube form can also be used with a 20 cm high add-on frame.

The container, wiped out wetly or slightly oiled, is loosely filled with concrete according to Fig. 8 andany concrete jutting out is simply struck off without any effect on compaction. Then the concrete inthe container is compacted – if possible by vibrating – until it no longer drops into itself. The propor-tional number of the original filling height (400 mm ) to the filling height after compaction (h) is themeasure for the consistency, i.e. the so-called compaction measure v:

v = 400=

400h 400 – s

Class Slump measure according to ISO 9812(mm)

F1 ≥ 340

F2 350 - 410

F3 420 - 480

F4 490 - 600

19


BP 2158-1 GB4 only as flow concrete by adding FM

Consistency Compaction Slump Slump Property Type of range measure measure (inch) when compaction

a (cm) placedKS: stiff (K1) ≥ 1.2 – – still loose vigorous vibrating,

stamping for thin packed layer

KP: plastic (K2) ≥ 1.19...1.08 35 - 41 1.2 - 2,5 clod-like to Vibrate, rod just adhesive and stamp

KR; soft (K3) 1.07...1.02 42 - 48 2.6 - 4.2 slightly Rod, flowing lightly vibrate

KP4; free-flowing – 49 - 60 4.3 - 6.3 flowing only deaerate (K4) by rodding,

lightly vibrate

Class Slump measure according to ISO 4111(mm)

C0 ≥ 1,46

C1 1.45 -1.26

C2 1.25 - 1.11

C3 1.10 - 1.04

20

The slump test is carried out in Germany and Austria under the same test conditions to determinethe consistency for plastic, soft and flowing concrete in accordance with Fig. 9:

The following are used here: – a 70 cm x 70 cm large work bench that is to be set up horizontallyand which must not yield. Its surface consists of a 2 mm thick, flatmetal plate on a wooden frame. On the one side are hinges whichconnect the table to the lower frame; on the other side are a handleand try square for restricting lift to 40 mm (check occasionally!)

– a 20 cm high conical shape with 20 cm diameter at the bottom and13 diameter at the top

– a wooden rod

The metal plate and the inner surface are to be wiped wet just before testing is carried out. The conemould is placed on the middle (!) of the table and kept in position with your feet. Now the concrete isfilled in two layers and each layer is slightly compacted with the wooden rod. After filling the mould,the concrete over the top of the edge is struck off and the table plate is cleaned around the mould.About half a minute after striking off the top edge, the mould is slowly pulled up perpendicularly. Thenthe table plate is lifted up by the handle and allowed to drop in 15 seconds fifteen times – but withoutbanging it hard. The concrete spreads out. Then the diameters of the spread-out concrete parallel tothe edges of the table are measured. The mean value of both diameters in cm is the slump measure a.When the concrete does not stay together but falls apart, the slump test is not suitable for determiningthe consistency. If necessary, the compaction test is to be applied, but such concrete as stiff as thiscan usually not be pumped.

In most of the other European countries as well as in the US (where the slump test, is, however notcustomary) the test parameters deviate more or less from this. The test readings can therefore not becompared.


BP 2158-1 GB

Fig. 8: Compaction test according to WALZ

4. Calculate compaction measure v = 40/h

1. Fill loosely with concrete

2. Completely compact filling

3. Measure s

In the US the consistency of the freshly-mixed concrete is usually quoted with the so-called slumpTest (slump) according to Chapman/Abrams (ASTM) according to Fig. 10. This test is also very wide-spread and well-known in many countries – apart from in Germany. This test is not standardised inGermany but is permissible for monitoring according to DIN 1048, Part 1, table 1.

Used here are: – a 30 cm high conical mould with 20 cm diameter at the bottom and 10 cmdiameter at the top

– a steel rod Ø 16 mm, rounded at the end.


21 BP 2158-1 GB

Fig. 9: Slump test; on the left: different deformation of the concrete depending upon the consistency rangeafter lifting up the conical mould. On the right: concrete cake on completion of test

KS(K1)

KP(K2)

KR(K3)

22

The mould, on a level and firm base (e.g. the work bench for the slump test), is filled with the fresh-ly-mixed concrete to be tested in three layers each with about the same volume. Each layer is com-pacted by rodding with the rounded-off end of the rod. Specified here are 25 penetrations per layer.At the end of this filling process the concrete is struck off at the top edge of the mould; then the mouldis carefully pulled up perpendicularly and placed next to the concrete cone. The slump s is the dif-ference in height of the concrete cone to the height of the mould (30 cm) – measured in cm.

The consistency of the freshly-mixed con-crete continuously changes from the time itleaves the mixer to the end of workability –approximately as shown in Fig. 11. This process, generally known as “stiffening” iscompletely normal and meets the require-ments for the later development of strengthof the concrete and is not to be mistaken forthe effect of solvents (FM) that are also limi-ted to a certain time.

The temperature of the freshly-mixed concrete is of importance when concretingduring extremely cold and extremely warmoutside temperatures. This should normallybe between +5° C (+41° F) and +30° C (+86°F) when placing. When the temperature ofthe air is below -3° C (+27° F), the minimumtemperature of the freshly-mixed concretemust be +10° C (+50° F) and this for at leastthe next 3 days.


BP 2158-1 GB

Fig. 10: Slump Test

Fig. 11: Time-dependence of the consistency

1. Fill with concrete, compact and strike off

2. Slowly pull up mouldperpendicularly

3. Measure slump s

KF

(fre

e-flo

win

g)

KR

(so

ft)K

P (

pla

stic

-like

)

Time after manufacture (mm)

slum

p (

cm)

Increased temperatures of freshly-mixed concrete (considerably above 20° C (+68° F)) in generalusually accelerate the stiffening. High summer temperatures or artificially increased temperatures offreshly-mixed concrete (warm concrete for winter construction) considerably shorten the length of timebetween the mixing and the initial setting.

If a longer period of time has to be bridged between the fabrication and placing of concrete, then thestiffening of the concrete must be taken into account accordingly. This means, for example, thatready-mixed concrete must be made soft enough whilst being prepared in the works – and that boththe travelling time and the temperature are taken into consideration – so that it has the consistencywanted when it arrives on the construction site.

Caution!If water were to be added unauthorised on the construction site forrenewed “softening“ of the concrete this would drastically damagethe quality!

The different consistency parameters only, however, reflect a part of the qualities of the freshly-mixedconcrete with regard to workability. Important here are also the water-retaining capacity, the pump-ability and the pump-willingness (refer to section 4.1) the deformation and alternating behaviourduring compaction (refer to section 2.1) etc.

3. Properties of hardened concrete and their alternating relationto concrete technology

The most important properties of hardened concrete are:

◆ compressive strength◆ protection against corrosion◆ water imperviousness◆ resistance to chemical corrosion◆ resistance to frost, resistance to frost and thawing salt◆ resistance to wear

The compressive strength is the most important concrete property. This is determined by the com-pression test carried out on specimens specially made for this (cube, cylinder) or, if necessary on testcores from the structure. The standardised test of the standards (in Germany according to DIN 1045)is carried out after 28 days usually on test cubes with arris lengths of 20 cm. The compressivestrength is calculated from the max. charge in the test press (before it breaks) in Newton’s divided bythe surface in mm2 of the specimen charged here in mm2. Depending on the compressive strength,the concrete is assigned to one of the strength classes already mentioned. A certain cube compressivestrength may also be necessary for a period of time earlier than after 28 days, e.g. when strippingwalls or floors. It can, however, also be arranged for a later date, e.g. when using slowly-hardeningcement.

In addition to these possibilities of wanting to adapt, resp. use the development of the concretestrength for concrete technology, there are also unintentional, usually negative reactions of concretetechnological mistakes to the really attainable concrete compressive strength. The main sources ofmistakes here are:

– unauthorised addition of water on the construction site– placing of freshly-mixed concrete after the initial setting– insufficient compaction especially as a result of fill lifts being too great and– improper post treatment, e.g. insufficient protection against premature drying out.


23 BP 2158-1 GB

24

A permanent protection against corrosion of the reinforcement can only be attained by the concretesurrounding it but only when the hardened cement paste is sufficiently leakproof and the concretecover thick enough. Unfortunately, already when determining the size of the largest grain, mistakesare made in assessing the actual amount of space available for the concrete to ‘slip through’ whenplacing between the reinforcement rods.

Likewise the ‘mixing work’ necessary for complete covering of the reinforcement is also not to beunderestimated whilst compacting. One can also envisage the reinforcement as an additional “steel”grain fraction which is not ‘mixed’ along until concreting (without agitator). For example, 80 kg rein-forced steel per m3 concrete are the equivalent of 10 volume %, and a steel Ø of 16 mm has the samesurface to cover as a round grain with 24 mm Ø. What makes it more complicated here is that thereinforcement is necessarily concentrated in the areas near the surface where the concrete also stillhas to be ‘arranged’ so that the surface is enclosed by the concentration of the fine particles.

The necessary concrete cover which is a requirement for sufficient protection against corrosion, mustbe guaranteed for by sufficient bar spacers. The forces which the falling or flowing freshly-mixed con-crete exert on the reinforcement are often very great and the subsequent displacement of a correct-ly plaited reinforcement is covered by concrete. The damage does not come to light until quite a longtime afterwards when the reinforcement rusts and the concrete chips.

The impermeability to water of the concrete does not just serve to guarantee the corrosion protectionfor the reinforcement but also prevents the penetration of water that stands under pressure, e.g. fordams or building foundations below the groundwater table. Testing of the water impermeability is carried out according to DIN 1048 by the reaction of a water pressure of 0.5 N/mm2 (5 bar) for 3 dayson several specimens taken. Then the average penetration depth of the water must not be more than50 mm. Special care must be here for intensive compaction avoidance of working joints between theindividual concreting sections. It must be made sure that the concreting layers are “sewn up” to eachother well – “fresh concrete on fresh concrete”. Concrete layers of not more than 30 .... 50 cm guarantee that, for example, the vibrating cylinder with a normal penetration depth also reaches intothe previous layer before this has reached their initial setting.

The resistance to chemical attacks is mainly realised by the normal, resp. increased water imper-meability. For strong and very strong chemical attacks, the water penetration depth measured onsamples as mentioned above, must not be more than 30 mm. When the attacking water is highly char-ged with sulphate (more than 0.6 g per litre) cement with a high resistance to sulphate (HS-cement)is to be used. When sea-water attacks the concrete, no special choice of cement is necessary –despite the high content of sulphate here – experience over a number of years has shown this.Concrete that is, however, exposed to “very strong” chemical attacks over a large period of time mustbe protected both reliably and long-term by a protective cover before they are attacked by such.

Concrete resistant to frost must be concrete that is impermeable to water with sufficient strength andwith additives that are resistant to frost. The resistance to frost and thawing salt is improved by air-entraining reagent (LP): Please note that every volume % air voids results in a loss of strength ofapprox. 5%.

A high resistance to wear is required by concrete with a surface that is exposed to a great mechani-cal load, e.g. lots of traffic, slipping bulk material, the movement of heavy objects or water with astrong current and water that carry solids. Here usually special measures are required for the con-crete composition.


BP 2158-1 GB

4. Properties and conditions of freshly-mixed concrete whenpumping

4.1 Pumpability and willingness to pump

Pumping concrete is not a special concrete. It fulfils all requirements made on a concrete that is usedfor reinforced concrete for reinforcement work. But not every one of these types of concrete also fulfilthe requirements on pumping concrete even though the parameter ranges accepted by modern con-crete pumps have been considerably extended. The question regarding the pumpability of freshly-mixed concrete is to be asked and answered in two steps:

1. Is the concrete at all pumpable under the given conditions?2. If yes, how can the concrete be pumped, i.e. at what cost?

A freshly-mixed concrete is pumpable when it is structurally leakproof during the whole course ofpumping and stays as such. Structurally leakproof means that all firm components are completelyenclosed by liquid (water) and can move against each other. Not only the “support” on the pipe wallbut also the pressure exchange inside the concrete may only flow via the liquid. On the one hand therefore in every cross-section along the conveying path, the aggregate-cement mix must be at leastsaturated with water; even better if it is somewhat over-saturated. On the other hand the flow resistancefor the water inside the aggregate-cement mix must be greater than the wall friction resistance andfinally the water excess at the beginning of the conveying path must be greater than the water dis-placement through the concrete during one stroke.

The concrete composition in the finest grain range is thus very important. The cement and the otherfinest grain shares therefore do not just provide the “lubrication” on the pipe wall and thereby a reduction of the wall friction resistance but also provide an almost complete “blockage” of the grainstructure. As you know the inner flow resistance of a grain mix (e.g. a gravel filter layer) is directly(proportionally) dependent upon the specific surface of the mix. Hence Fig. 3 results in a specific sur-face of the aggregates of approx. 4,100 m2/m3 freshly-mixed concrete for the repeatedly mentionedconcrete mix; this corresponds to the specific surface of approx. 20 kg cement. The actual cementcontent of 300 kg/m3 in the chosen example therefore results in a “blockage” of the aggregate mix toabout 16 times the original inner flow resistance. In other words, the flow resistance for the water inthe freshly-mixed concrete is about 95% the result of the cement share.

Freshly-mixed concrete is pumpable because it behaves like a blocked gravel filter.

The pumpability, resp. structure imperviousness of a freshly-mixed concrete is not just a question ofits composition but also of the pipeline diameter and the ”boundary zone layer” connected with this– more details about the special features of this are given in a further publication.

Experience shows that the following are needed for pumpability:

◆ a grain composition according to grading curve DIN 1045 in the upper range betweenthe limit lines A and B (refer to Fig. 5)

◆ a cement content of at least 240 kg/m3 for concrete with a largest grain size of 32 mm◆ a mo content of at least 400 kg/m3 for concrete with a largest grain size of 32 mm◆ a pipe line diameter of at least three times that of the largest grain size diameter.


25 BP 2158-1 GB

26

The willingness to pump when pumpable does not just imply the specific conveying resistancedepending upon the consistency and flow velocity but also the inner mobility of the freshly-mixed con-crete when sucked in as well as when passing through pipe elbows and other changes in cross-section. Whereas the first part of the willingness to pump can be expressed in the so-called “concretepressure performance diagram” (refer to section 4.4.), it is not (yet) possible to express the innermobility in figures.

The number of different ways to describe the consistency and the wide range when comparing theirmeasured values that cannot be precisely described physically, show just how complicated this matteris. We will nevertheless, however, try to give you an idea of this property.

4.2. Origination and properties of the “boundary zone layer“

When concrete is conveyed through pipes, the necessity of a “lubricating film” made of cement pasteright before the pipe wall is emphasised at all times. A concentration of fine grain can be clearly seenon the outside of the “concrete sausage” when the concrete emerges out of the pipeline. The causesand effects of this “boundary zone layer” are as yet only a little familiar.

As has already been mentioned, pumpable freshly-mixed concrete is structurally leakproof in everypart of the delivery line, i.e. the aggregate mix “swims” free and easily in the “concrete paste”. Thegrain interspaces are saturated with water and cement. The air voids that are also present and thathave a liquefying effect are pressed together by the delivery pressure that is needed for pumping tojust a fraction of their natural size and thereby loose their liquefying effect when pumping.


BP 2158-1 GB

Fig. 12: Filling the space of a pipe section (1 litre) with ball grains of the mix set as an example a) grain size 16/32 b) grain sizes 8/16 and 16/32.

As an example the air void content falls from 10% in a loose bulk concrete to a residual share of just0.12% with a conveying pressure of 85 bar. The aggregate grains of the concrete participate in fillingthe space according to their share of volume (refer to mix formula calculation, Fig. 6). To illustrate thisbetter, one can, for example, examine a section of line Ø 100 mm, 126 mm long with a volume of 1litre and all aggregate grains of the grain 8/6 and 16/32 as balls of different sizes. Fig. 12 shows alikely, random arrangement of these balls in just such a pipe section.

As you know the largest “bulkiest” grain have a diameter of up to a third of that of the pipe that encloses them. Each grain can, however, just approach the pipe wall with its surface. If one looks atthe layer parallel to the pipe wall, e.g. at a distance of 1 mm, one only “meets” the outside layers ofthe coarse grains, whereas all grains with a diameter of less than 1 mm, contribute towards filling thevolume with their whole volume, and they can compensate for the “lack” of coarse grain. In otherwords, to fill the pipe cross-section completely with the concrete components, the large grains mustbe pressed inwards and a suitable share of the smaller grain and water must be pressed outwards –at least in the boundary zone. This process is comparable to flattening the concrete surface with atrowel.


27 BP 2158-1 GB

Fig. 13: Boundary layer segregation for a delivery pipe diameter of 100 mm for the concrete set as an example (composition dependent upon the relative distance from the pipe middle axis)

Aggregate content (dm3/m3)Average aggregate content according to mix formula 710 dm3/m3

Mortar content (dm3/m

3)

Average mortar content according to mix formula 494 dm3/m

3

28

The “boundary zones segregation” is always carried out when a space is filled with concrete, there-fore already when filling the delivery cylinders as well as for final placement, e.g. into a wall formwork.A requirement here, however, is the inner mobility of the freshly-mixed concrete, as already mentioned.If this is sufficiently available, then the rearrangement is carried out inside the total cross-section, andcan be calculated accordingly.

As a result of the calculation, one obtains a change of the mix composition that is constantly dependentupon the radius. In the boundary layer, there is a constantly increasing fineness of the mix from thethickness of the coarsest grain at the edge of the layer to the pure cement mortar on the pipe insidewall. This accordingly results in an enrichment of coarse grain in the core zone. A requirement, however,for the pumpability of the concrete, is that the structural impermeability of the cone zone remainsdespite boundary zone segregation. This explains why concrete is only pumpable up to a certainminimum pipe diameter.

Fig. 13 shows a calculation, e.g. for the mix already given as an example (Fig. 6) for a pipe diameterof 100 mm. This results in a relative increase of aggregate to about 120% for the cone zone and adecrease of the mo content to about 50% of the respective mean value. Fig. 14 shows the alterationsto the grading curve at different distances to the pipe wall as a result of this.


BP 2158-1 GB

Fig. 14: Changes to grading curve in the core and boundary zone

Output grading curve according to mixGrading curve in the core cross-sectionGrading curve at a distance of 2.00 mm to the edgeGrading curve at a distance of 0.25 mm to the edgeScreen mesh width (mm)Grading curve A 32 according to DIN 1045Grading curve B 32 according to DIN 1045

29

This radius-dependent concrete composition in the pipe cross-section shows that the freshly-mixedconcrete properties are also dependent upon cross-section and radius, and that they alter accordingto changes during pumping. On the way through the delivery line the freshly-mixed concrete is subjectto different stress and changes in shape which it opposes with a certain resistance. When conveyingin the straight cylindrical pipe an exclusive shear stress τs increases linearly with the radius accordingto Fig. 15a.

The concrete opposes this stress with a shear stress τω which is dependent upon the speed but isin no way – as is usually assumed in literature on the subject – constant over the course of the cross-section. Rather, the tenacity of the concrete coincides with the ‘denticulation’ of the cement paste withaggregate grain that greatly decreases towards the wall (refer to Fig. 13). The share of aggregate inthe cone zone is a multiple of the share of cement paste, whereas towards the edge the share ofaggregate practically drops to zero. If one compares the average grain size of cement (approx. 0.01mm) to the largest aggregate size (e.g. 32 mm) the result is the course shown in Fig. 15 b for the con-crete tenacity: the tenacity on the wall is approximately equivalent to that of the cement paste as isknow from rheological readings; increasing towards the cone zone to a multi-thousand fold.

The existence of a so-called limit shear stress τo below which the concrete is not sheared and is there-fore conveyed as a firm plug is often advocated in theoretical statements. This has, however, neitherbeen proven in practice nor in tests in the laboratory. The considerably greater ‘tenacity’ of the conezone compared to the boundary zone (refer to Fig. 15 b) and shear stress increasing with the radius(refer to Fig. 15 a) result in a shear speed χ that greatly increases toward the edge, according to Fig.15 c, and a speed profile for the flow of concrete in the pipe (Fig. 15 d) that is very similar to the so-called plug conveyance. The tests carried out on normal concrete in the laboratory by RÖSSIG5

already more than 20 years ago, show that a deformation due to shear amounts of 0.3 .. 0.5 insidethe cone zone after a conveying distance of 10 m. This is the equivalent of about 100 -200 times moredeformation caused by shear in the total boundary zone than in the cone zone. It thus follows that thepipe conveying of freshly-mixed concrete has no additional mixing effect. Merely after leaving thedelivery line does a certain remix ensue when placing and packing and in this case, as already mentioned, renewed zone segregation occurs, e.g. on the surfaces of the formwork as well as in thereinforcement.


BP 2158-1 GB

Fig. 15: Current influences on the pipe conveying of freshly-mixed concrete a) Shear stress b) Shear resistance c) Shear speed d) Speed profile

5 Rössig, M: Conveying freshly-mixed concrete,research reportNRW no. 2456, Westdeutscher Verlag 1974

Shear stress Concrete tenacity Shear speed Flow rate

Boundary zoneCore zone

Pipe longitudinal axis

30

4.3 The behaviour of freshly-mixed concrete in the concrete pump

The concrete technological task of the pump is to press the freshly-mixed concrete as a closed andcontinuous conveying current into the delivery line, and then through this to the point of placementand to carry this out as far as possible without any impairment to its given composition and pro-perties. The behaviour of the freshly-mixed concrete in the concrete pump includes on the one handits passive behaviour as a result of the active reaction of the concrete pump to it, and on the otherhand its own reactive effect on the concrete pump and the behaviour of this. The freshly-mixed con-crete and the concrete pump run through different ‘operating phases’ here.

One must distinguish between on the one hand the operating state of the pump (starting up pum-ping, normal conveying operation, emptying and cleaning the line, malfunctions) and, on the otherhand, the operating state of the concrete (transfer and sojourn time in the hopper, suction, filling ofthe conveying space, passing through the valve system and the tapering after this). The type of con-crete pump used (piston pump or squeezed tube pump) and the type of valves used for a pistonpump (e.g. trunk of S-pipe valve) have a considerable influence on the behaviour of the freshly-mixedconcrete inside the concrete pump. We will not go into further details here about the characteristicfeatures and characteristics of the two principle types of pump as well as the different valve systemsof piston pumps (refer to Fig. 16). With the following print “Concrete Technology for Pumps” we simplywant to make what happens inside the concrete pump comprehensible from a concrete technologicalpoint of view.

Concrete can only be pushed through the delivery line when this has previously been sucked out ofan open vessel (hopper) by increasing the volume of the conveying space of the pump, and the con-crete fills the conveying space completely as far as possible. By decreasing the volume of the con-veying space, the concrete is pushed out into the delivery line whilst displacing the whole concretecolumn in the delivery line. When observed more clearly, the sucking-in is also pushing; the volumeincreasing of the conveying space (i.e. movement of the delivery piston in the delivery cylinder awayfrom the suction opening) brings about a low pressure compared to the atmosphere, which pushesthe concrete out of the hopper into the conveying space with max. 1 bar, but only when there is nota continuous “air bridge” between the conveying space and the atmosphere.

The low pressure level for suction and filling requires a low as possible resistance to flow and deformation of the concrete. Here the agitator of the hopper and its geometrical shape contributeconsiderably towards this. The agitator does not just serve to keep the concrete free-flowing duringthe breaks in conveying but also moves and pushes the concrete in such as way during suction thatthe concrete can flow “from this movement” and flow without congestion into the suction opening thatis as large as possible. The filling rate of the conveying space is an essential criterion for the efficiencyof a pump.

An increase of the speed of the delivery pistons, resp. the rotor does not lead to an improvement ofan insufficient filling rate as a result of poorly-flowing concrete as the atmospheric difference in pressure of 1 bar can not be increased. On the contrary, the filling rate and therefore the efficiency ofthe concrete pump deteriorate rather. It is well known that stiff concrete and mixes out of crushedaggregate cannot be sucked in as well as thin-plastic and round, granular ones. For optimum suctionconditions the suction openings and the conveying area diameter are kept the same and also largeas far as possible. Here are the essential differences between the piston and the hose squeeze pipes:piston pumps suck in the concrete through large cross-sections and reduce the cross-section whenpressing out the material; large conveying outputs can be realised here. Hose squeeze pumps arelimited with regard to their delivery pressure to approx. 30 bar, and they therefore suck in the con-crete preferably with the same cross-section as that through which the material is conveyed throughthe line afterwards. Its delivery performance is above all restricted by the suction performance.


BP 2158-1 GB


31 BP 2158-1 GB

Fig. 16: Concrete pump constructions: a) Piston pump with trunk valve b) Piston pump with S-pipe valve c) squeezed tube pump

32

For piston pumps the suction behaviour of the freshly-mixed concrete is not just determined by thesize of the suction opening and the efficiency of the agitator hopper but also by the ‘hindrance’ of thesuction due to the valve system used.

The filling of the conveying space also comprises the “boundary zone segregation” described in section 4.2 for the complete space-filling and the emergence of the more free-flowing boundary layerconnected to this. There is only little time available for this as when the delivery pistons reverse directionthe conveying space must be tightly filled immediately and the concrete must be pumpable.

When the concrete is pressed out of the delivery cylinders of a piston pump into the delivery line, theconcrete current experiences a reduction in cross-section to the diameter of the delivery line (100 mm,resp. 125 mm) whilst passing through the valve (trunk or S-pipe) and also after this. For the concretethis does not just mean a considerable deformation but also a great increase in speed as well as acorresponding increase of the boundary layer per volume unit of the concrete. To reduce the con-veying resistance connected here, the cross-section reduction is carried out continuously as far aspossible over a sufficiently long section. This reduction of the cross-section inside or immediatelyafter the pump also provides a pumpability test for the concrete. If a “difficult” concrete passes this“obstacle” without any problems then it really is pumpable and the danger of a blockage over thecourse of the delivery line due to the result of a wrong concrete composition is very improbable.

An essential condition to maintain the pumpability of the concrete inside the pump is the reliableimperviousness of the valve system during the pressing phase. A valve system that is not water-tightmeans a loss of water or cement paste in the boundary zone and thus the danger of the concrete notbeing water-tight any longer and its wall friction is no longer pressure-independent which inevitablyleads to blockages. The same is similar for hose squeeze pump. Here there is the danger that in-sufficient sealing of the squeeze gap leads to the water or cement paste flowing away and the con-crete loosing its pumpability just in front of the squeezing roller.

Under high pressure (above 80 bar) an effect arises in the concrete at points of leakage which in job-site jargon is called “encrustation”. Finest mortar stores along the gaps and a part of the mix water ispushed through this. Under the influence of pressure and time the encrustation increases in theshape of a ring from the outside to the inside. Narrowing of the cross-section by more than 50% is notrare. The result of this is the tendency to form blockages. As this encrustation hardens during opera-tion it is not possible to remove it by the usual methods when cleaning the concrete pump afterwards.If the concrete encrustation is not noticed by the operator frequent blocking is caused the next timethe pump is used after the preliminary slurry has been pumped.

It is very important for piston concrete pumps that the delivery space is completely emptied as far aspossible for every pump stroke as a so-called dead volume remains in the delivery space at least upto the next time the pump is cleaned, especially remaining on the delivery piston, hardening or settingthere and this can lead to the destruction of seals, the delivery piston, resp. the delivery cylinder insidewall. This danger does not exist for hose squeeze pumps as the concrete only passes through thedelivery space (the pump hose) in one direction and is therefore always flushed through with freshconcrete. The special operating states of the concrete pump described above (starting up pumping,emptying, etc.) have a considerable smaller influence on the behaviour of the concrete in the deliveryline than on the behaviour inside the delivery line. This is why these problems don’t arise until in thefollowing section. Besides the reaction of the concrete behaviour to the concrete pump already mentioned, there is, along with the stress as a result of concrete conveying pressure, especially thewear effect of the concrete on all parts that come into contact with the concrete.


BP 2158-1 GB

The wear effect of the concrete inside the concrete pump as well as also later in the delivery line isabove all dependent upon the consistency and speed just like the delivery resistance, but it is notindependent of the pressure. The enormous abrasiveness of the concrete, especially everywherewhere the concrete does not flow “cylindrically” but where the contact surface moves relativelytowards the concrete, i.e. in the hopper, in the agitator, in the valve system, in the reductions and inthe elbows (outside) is due to the fact that the coarse aggregates are embedded in “deeper” layers.They therefore have a greater relative speed with which they “occasionally” scratch the contact sur-face through the flexible boundary layer zone. Their irregular shape and the tight toothing of the grainmix also prevents a rolling off of the contact surface which would reduce wear, but also lead to a twistingeffect on neighbouring grains which are thereby additionally turned towards the contact surface.

4.4 The behaviour of freshly-mixed concrete in the delivery line

When flowing through a straight, cylindrical pipe vertically upwards this process calms down after ashort time by making use of the available ‘toothing play’ between the grains, provided that the pipesections do not have any identations and do not leak. The latter leads in extreme cases to the loss ofpumpability and therefore to blockages or to “just “ the formation of a firm concrete crown constrictedby the cross-section with increased resistance to conveying. For great vertical conveying heights andwhen pumping through high quality delivery pipes, the wall contact of coarse aggregate is practicallycompletely “quietened down” and there is therefore both lesser conveying resistance and considerablyless wear. This process known as the ‘flotation effect’ was first observed in 1976 during the high risepumping world record at the time of 310 m at the Post Office Tower in Frankfurt /Main (Germany) witha Putzmeister Elephant pump. With a horizontal delivery line the ‘flotation effect’ can only occur in areduced form as just a slight settling of the coarse aggregates leads to “occasional” wall contact andall the consequences of this already mentioned, however, mainly on the lower pipe inside wall.

Flowing through pipe elbows means an additional bending and shearing stress for the freshly-mixedconcrete. As a pipe elbow in the ‘outside curve’ has a greater surface than a straight pipe, the boundaryzone with more fine grain becomes thinner; whereas in the ‘inner curve’ it becomes thicker. The verythick cone zone displaces the softer and weak outside boundary zone and is diverted wear-intensivelywhen hitting the pipe wall due to shearing and bending. This may well lead to some local zone nolonger being leakproof and therefore to an even greater conveying resistance and wear. Moreover,the flow of concrete needs a consolidation and quietening phase after a pipe elbow.

The through-put of freshly-mixed concrete through a delivery line is a result of the performance of theconcrete pump (engine performance [kW], eff. output [m3/h], eff. delivery pressure [bar]), geometryof the delivery line (diameter [mm], length of line [m], delivery height [m]) and consistency of the freshly-mixed concrete (tenacity factor). The mutual dependence of these diameters is illustrated byFig. 17 with the concrete pressure performance monogram which is independent of the concretepump used.

The example shown here starts from an effective delivery performance of Q = 40 m3/h. For the assumeddelivery pipe diameter of D = 125 mm, one can read an average flow speed of approx. 1 m/s in thefirst quadrant. The dependence of the delivery pressure to the delivery pipe diameter is even greaterthan the dependence of the flow speed: a reduction of the pipe diameter from 125 mm to 100 mm isfor example the equivalent of increasing the speed of the concrete in the pipe to just 1.5 m/s where-as the necessary delivery pressure is almost doubled. The consistency dependency represented isin keeping with experience gained over a number of years and meets the requirements for a roughestimate. If more exact values are necessary for a certain application case, then pump trials must becarried out with the planned concrete mix formula. For the example in Fig. 17 for a plastic concretewith a slump of a = 40 cm, flow resistance is 0.2 bar per metre run. The assumed length of deliveryline of L = 300 m results in a delivery pressure of p = 63 bar, which is to be enlarged by the share of0.25 bar per meter of height resulting from the high-rise conveying, in the example 20 bar for 80 mdelivery height. If one can use the “Rotation effect” for high-rise conveying, then this value 0.25 bar/m


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Fig. 17: Concrete pressure performance nomogram

is to be reduced by 60 % of the flow resistance mentioned already (in the example: approx. 0.13 from0.21 bar/m). The resistance to flow in the pipe elbows as well as an the leaking pipe couplings withencrustation is usually converted to an equivalent pipe length:

The pump operator must direct his full attention to the behaviour of freshly-mixed concrete in the delivery line when starting to pump. The problem here is the necessary wetting of the inside wall ofthe pipe with cement paste until starting the stationary pump operation. The amount needed here permeter run is exactly the same amount which would remain in a 1 m section if one first fills it comple-tely with freshly-mixed concrete and then lets this flow out again (10 m of a 125 mm delivery line havean inner surface of approx. 4 m2 to be covered). When starting to pump, this amount of cement pasteis “removed” from the concrete that first flows through the delivery line. For this reason, one ought tofirst pump a starting-up mix enriched with cement excess or even a cement slurry mixed extra in thepump hopper before pumping the concrete (refer to operating instructions).

A more favourably-priced and effective solution to provide a start-up mix is to use a PM slurry for staringup pumping which is available as powder and only water needs to be added. The substance that isready after just a few minutes is emptied in via the cleaning opening. When starting to pump this substance is pushed in front of the concrete and thereby covers the pipe inside wall.

The method greatly used in the field of covering the delivery pipes with water before pumping is onlyto be used if all else is lacking, and can only be used for short delivery pipe lengths. If nothing is carried out one can already reckon with a blockage when starting to pump as a dry concrete plugthat cannot be pumped arises after just a relatively short conveying period, and it stops the flow ofconcrete at one of the first elbows.

An important requirement for a trouble-free flow of concrete is also the correct emptying and cleaningof the delivery line during a longer break in conveying so that no old, hardened concrete or cementpaste residue remains in the line which would also lead to blockages when starting to pump again.


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Elbow radius Equiv. pipe length

Large pipe elbow 90o 1000 mm 3 m

Pipe elbow 90o 281 mm 1 m

Leaking coupling – 1 m

Identifiable irregularities Possible causes Reccommended measures

For the delivery of concrete and charging of the concrete pump

Gravel and broken stone noises inthe truckmixer drum

Swelling up and down noises of theconcrete in the truckmixer drum

When leaving the truckmixer drumthe concrete breaks sharp-edged

Change of consistency during con-crete delivery

Frequent blocking of the agitatorshaft

Delivery pressure considerablyabove that of the value expected

Quick rise in pressure above normalvalue

Slow rise in pressure above normalvalue

Poor filling rate of the delivery cylin-ders

Blockages in the delivery cylinder ofthe pump

Blockages in the delivery line

5. Short guide to avoid faults and eliminate them

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Check delivery note

Check delivery note or determineslump measure

Check delivery note; add sufficientcement paste when starting uppumpingInterrupt transfer and intensivelyremix the concrete (several minutes)

Check delivery note

Cover the delivery line

Pump in reverse for several strokesMix well hopper fill, add water andcement if necessary

Pump in reverse for several strokes,continue to pump slowly. If neces-sary, locate blockages with hammerhandle test, and dismount the linebeginning at the endCheck delivery note, determineslump measure if necessaryHopper filling level above the agitator shaft

Refer to aboveRefer to above

Refer to aboveCheck setting of valve and switch-over

Remove the obstacle

Check pipe coupling, repair cracks

alternative laying

Eliminate bend

Refer to aboveRefer to above

Share of fines is too small

Concrete consistency is tooliquid

Concrete consistency is toostiff

Segregation

Share of fines is too small

The residual effect of BV, FMor VZ has been exceeded orshortened (summer heat, hotdelivery line)

Blockages in or just after theconcrete pump

Blockages more at the end ofthe delivery line

– consistency is too stiff

– filling level of hopper is toolow

– shares of fine is too small– concrete consistency is too

stiff– segregation– valve system is not leakpro-

of or does not switch over

– old concrete residue or foreign particles in the delivery line

– leaky pipe joints or weldingseam cracks

– unfavourable laying of delivery line

– bent end placing hose orbent delivery lines

– share of fines is too small– consistency is too stiff

When pumping

6. Specifications and regulations of the “Technical Regulations“

Designation Issued Title

DIN 1045 07.88 Concrete and reinforced concrete: dimensions and design

DIN 1048 06.91 Test method for concrete – Section I: Freshly-mixed concrete

DIN 1084 12.78 Monitoring (quality monitoring) concrete and reinforced concrete – Part I: Concrete B II on construction sites, supply

DIN 1164 03.90 Portland, Iron Portland, furnace and trass cement – Part I:Terms, requirements, supply

DIN 18551 03.92 Shotcrete; manufacture and monitoring of quality

DIN 18560 05.92 Floorscreeds in civil engineering

DIN 18999 NE/05.91 Concrete technology; additives for concrete

DIN 4187 Testing screens with square holes

DIN 4188 Testing screens (mesh screens)

DIN 4219 12.79 Lightweight concrete and reinforced lightweight concrete withclosed structure – Part I: Requirements made on the concrete,manufacture and monitoring; Part II: Dimensioning and design

DIN 4226 07.83 Aggregate for concrete

DIN 4227 07.88 Pre-stressed concrete – Part I: components made of normalconcrete with restricted or full pre-stress

DIN 4235 12.78 Compacting concrete by vibrating

DIN V ENV 206 10.90 Concrete, properties, manufacture, placing and proof of quality

ISO 2736 08.86 Part I: concrete tests; manufacture of test bodies, testingfreshly-mixed concrete

ISO 4109 06.91 Freshly-mixed concrete; determining the consistency; slump test

ISO 4110 06.91 Freshly-mixed concrete; determining the consistency; Vebe test (slump time test)

ISO 4111 12.79 Freshly-mixed concrete; determining the consistency

ISO 4848 03.80 Freshly-mixed concrete; determining the void content of thefreshly-mixed concrete;

ISO 6276 01.82 Compacted freshly-mixed concrete: determining the strength

ISO 9812 not yet avail. Freshly-mixed concrete, determining consistency, slump test01.86 Guide lines for concrete with solvent and for flow concrete;03.83 Preliminary guidelines for concrete with extended handling

time (retarding concrete) DAfStb11.91 Guidelines for using DIN V ENV 206/10.90 – Concrete,

properties, manufacture, handling and quality01.82 Code of practice – added water for concrete, Deutscher

Betonverein e.V.

DAfStb Booklet 422 Testing concrete. Recommendations and notes as comple-ment to DIN 1048


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7. Further literature1 DIN 1045 “Concrete and reinforced concrete“

2 Mix formula tables, refer to DIN 1045 or e.g. in: R. Weber, R. Tegelaar, Good Concrete, Beton-Verlag GmbH, Düsseldorf, 1993.

3 Putzmeister concrete mix formula 73: Concrete mixes according to the new technical constructionregulations whilst observing the new standards.

5 Rössig, M.: Conveying freshly-mixed concrete, especially lightweight concrete, through pipelines,research reports from Nordrhein-Westfalen, No. 2456, Westdeutscher Verlag 1974.


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concrete technology

Documents

concrete of concrete

x green concrete concrete

exposed concrete

concrete aggregates

concrete pump

concrete additives

vibrated concrete

jolted concrete