lecture2-2015
DESCRIPTION
Cive 512: Rehabilitation of structuresConcrete mix designConcrete problemsTRANSCRIPT
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Spring 2015
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Part 1: Concrete mix design
Part 2: Concrete problems
Contents:
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Part 1: Concrete mix design
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1-Cement
Cement Type Use Type 10
Normal Portland
cement General purpose cement suitable for
all uses (where special properties are not requires)
Type 20
Moderate Portland cement
Used where precaution against moderate sulphate attack is important
Type 30
High early strength Portland cement
High strength at early period (usually a week or less)
Type 40
Low heat of hydration Portland
cement
When rate and amount of heat generated from hydration must be
minimized Type 50
Sulphate resistant Portland cement
For concrete exposed to severe sulphate action
According to CSA
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1-Cement ASTM C 150 provides 8 types:
-Normal
-Normal, air-entraining
-Moderate sulphate resistance
-Moderate sulphate resistance, air-entraining
-High early strength
-High early strength, air-entraining
-Low heat of hydration
-High sulphate resistance
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2-Aggregates: i- Generally, they must be:
-Clean, hard, durable -Free of absorbed chemicals -Free from coatings of clay -Free of fine materials in amounts that could affect the hydration and bond to the cement paste
ii-The nominal max size versus the maximum size (difference)
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2-Aggregates:
Nominal maximum size of an aggregate should not exceed: -1/5 the narrowest dimension between the sides of forms -3/4 of the minimum clear spacing between reinforcing bars and
forms -1/3 depth of the slab -For pumped concrete:
-1/3 smallest internal diameter of the hose or pipe -40 mm
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2-Aggregates: Property Effect
Potentially harmful material (as Organic impurities, fine
materials, soft particles, clay lumps)
pop outs, delayed setting , volume change of concrete, weak bond
between the cement paste and the aggregate
Resistance to freeze and thaw ( frost resistance of an
aggregate)
D-cracking of concrete
Wetting and drying properties Alternate wetting and drying cycles can cause severe strain in some aggregates leading to permanent increase in volume and concrete
breakdown Abrasion and skid resistance heavy duty floors and pavements
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3-Admixtures The main reasons for using admixtures:
1- To achieve certain properties in concrete
2- To maintain the quality of concrete during the stages of mixing,
transporting, placing and curing
3-To overcome certain emergencies No admixture of any type or amount can be considered a
substitute for good concreting practice
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3- Admixtures
Admixture Use Air entraining Improve durability in freeze and thaw
Water reducing To reduce water Typically 5-10%
Accelerating accelerate setting and early strength development
Retarding retard setting time Corrosion inhibitors Reduce steel corrosion in chloride
environment
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4-Supplementary cementing materials -They may be used in addition to or as partial replacement of the
portland cement . -They are used to improve a particular concrete property
Material Description
Fly ash Fine residue that results from the combustion of coal in electric generator
Slag made from iron blast furnace slag
Silica fume By product of the reduction of high purity quartz with coal in an electric arc furnace
Natural pozzolans
include calcined clay and calcined shale
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4-Supplementary cementing materials They are used to improve a particular concrete property
Property Effect Fly ash Silica fume Slag Natural pozzolans
water demand
dec. inc. dec. Little effect, may inc or dec
workability improve admixtures needed to maintain
workability
improve
improve
Bleeding and seggregation
dec dec
Bleeding inc.,
No effect on segregation
Little effect
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4-Supplementary cementing materials
Property Effect Fly ash Silica fume Slag Natural pozzolans
Heat of hydration
dec. Little effect ; may or may
not dec.
dec.
dec.
Permeability and absorbtion
dec.
dec. Very effective
dec.
dec.
Alkali aggregate reaction
dec.
dec.
dec.
dec.
Sulphate resistance
Inc. Inc.
Inc.
Inc.
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5-Concrete Mix design:
A proper proportioned concrete mix should posses the following: 1-Acceptable workability of fresh mix 2-Durability, strength, and uniform appearance of the hardened
concrete 3-Economy The design of the concrete mixture involves: 1- Establishing specific concrete characteristics 2-Selection of proportions of available materials to produce
concrete of required properties
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5- Concrete Mix design:
i-Strength: -The specified compressive strength (fc' ) at 28 days is the strength that is expected to be equal to or exceeded by the average of any set of 3 consecutives tests.No individual test (average of 2 cylinders) can be more than 3.5 MPa below the specified strength. -The average strength (fcr ' ) = the specified strength (fc' ) + allowance to account for variations in materials/mixing/placing..etc -The average strength (fcr ' ) is the strength required in the mix design
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5- Concrete Mix design
i-Strength: CSA A23.1:
fcr' =fc '+1.4S
fcr '=fc '+(2.4S-3.5) MPa
Where: S is the standard deviation (MPa)
If standard deviation is not available, Table 9-11 is used instead
to determine fcr
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5- Concrete Mix design:
ii-Classes of exposure: -Two classes for air content category 1: exposed to freeze and thaw, category 2: not exposed to freeze and thaw -Four classes of exposure for structures to Chlorides (CL) C1: Structurally RC exposed to CL (category 1 or 2) C2:Plain concrete exposed to CL (category 1) C3: Continuously submerged concrete exposed to CL (category 2) C4: P.C. exposed to CL (category 2) Three other classes for (air content and chlorides combination) F1 , F2 and N (page 155-156)
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5- Concrete Mix design:
ii-Classes of exposure:
-Three classes for Sulphate exposure:
S1, S-2 and S-3 as very severe, severe and moderate.
The degree of severity is determined based on the sulphate
concentration in soil or water.
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5- Concrete Mix design: It is the mass of water divided by mass of cementing materials selection of w/c ratio: 1-Must be the lowest value required to meet anticipated exposure conditions (Tables 9-1 and 9-2 co-relate the maximum w/c with class of exposure) 2-If durability and exposure are not the governing factors, it should be selected based on compressive strength ( from Table 9-3 and Figure 9-2)
iii-Water-Cementing Materials ratio (w/c):
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5- Concrete Mix design:
iii-Water-Cementing Materials ratio (w/c):
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5- Concrete Mix design:
iii-Water-Cementing Materials ratio (w/c):
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5- Concrete Mix design:
iii-Water-Cementing Materials ratio (w/c): a-amount of water in (kg/m3)
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5- Concrete Mix design:
iii-Water-Cementing Materials ratio (w/c):
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5- Concrete Mix design: b-Supplementary cementing materials: as Fly ash, natural pozzolans, Slag, silica fume
iii-Water-Cementing Materials ratio (w/c):
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5- Concrete Mix design: -The maximum size is determined as explained earlier. -From Table 9-4, the volume of dry coarse aggregate per unit
volume of concrete is obtained
iv- Aggregates:
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5-Concrete Mix design: -How can we get the volume of fine aggregates!!! - We need weights, so how can we translate volume to weight!!
iv- Aggregates:
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5-Concrete Mix design:
Additional information The code provides values for: - Recommended slump ( Table 9-6) -Minimum cement content:
To ensure satisfactory durability and guarantee suitable appearance of vertical surfaces
Concrete under water=390kg /m3 of cementing materials
Severe freeze and thaw and sulphate exposure=335kg /m3 of cementing materials
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Part 2: Concrete problems
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Part 2: Concrete problems
-See an EFFECT then determine the CAUSE
-The EFFECT of undesirable behaviour can be seen as: cracking, scaling, disintegration, spalling , erosion, seepage, distortion,
delamination, or popouts
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Part 2: Concrete problems
i- Cracks in concrete:
Types of cracks
After hardening
Physical Drying shrinkage
Crazing
Chemical Corrosion of RFT, AAR, Carbonation
Thermal Freeze/thaw, Temp.
variations, early thermal contraction
Structural Overload, Creep, Design loads
Before hardening
Plastic Shrinkage/settlement
Constructional movement formwork, subgrade 30
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Part 2: Concrete problems
i- Cracks in concrete: Cracks occur manifest themselves at different times and locations; For instance:
Plastic shrinkage: 30 minutes to 6 hours In roads , slabs, RC slabs due to rapid early drying ( random or diagonal). It could also occur over the RFT in the R.C. slabs if the RFT is near the surface.
Plastic settlement: 10 minutes to 3 hours In deep sections and top of columns ( Over RFT and arching).
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Part 2: Concrete problems
i- Cracks in concrete:
Corrosion: More than 2 years Beams and columns.
AAR: more than 5 years mainly damp locations (reactive aggregate + high alkali cement)
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Part 2: Concrete problems
i- Cracks in concrete:
Crazing: 1-7 days slabs
Early thermal cracking: 1 day-2/3 weeks thick wall
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Part 2: Concrete problems
i- Cracks in concrete:
Load induced crack:
Pure exure
Pure tension
Shear
Shear + moment
Bond
Compression load
Torsion
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Part 2: Concrete problems
ii-Other Symptoms of undesirable behaviour:
Scaling:
is local flaking or peeling away of the near surface portion of concrete.
Spalling:
deeper surface imperfection extending to the top layers of reinforcing steel
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Part 2: Concrete problems
ii-Other Symptoms of undesirable behaviour:
Delamination:
-is a separation along a plane parallel to a surface, in the case of a concrete slab, a horizontal splitting, cracking, or separation within a slab in a plane roughly parallel to, and generally near, the upper
-caused by the corrosion of reinforcing steel or freezing and thawing; similar to spalling, scaling, or peeling except that delamination affects large areas
- can often only be detected by non destructive tests,
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Part 2: Concrete problems
ii-Other Symptoms of undesirable behaviour:
Erosion
progressive disintegration caused by the actions of fluids or solids in motion
Seepage
Movement of water through pores and cracks
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Part 2: Concrete problems
ii-Other Symptoms of undesirable behaviour:
Popout
is the breaking away of small portions of a concrete surface;
small popouts leave holes up to 0.4 in. (10 mm) in diameter;
medium popouts leave holes 0.4 to 2 in. (10 to 50 mm) in diameter;
large popouts leave holes greater than 2 in. (50 mm) in diameter.
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Part 2: Concrete problems
ii-Cracks Widths Generally; Crack width is affected by: -tensile stress in reinforcement (most important variable) -thickness of cover -area of concrete around each bar
Fine crack
< 0.3 mm
No repair
Medium cracks
0.3-0.5 mm
loss monitor
Wide cracks
>0.5 mm
considerable loss, immediate repair
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Part 2: Concrete problems
Exposure Condition Crack width (mm)
Dry air or protective membrane 0.41
Humidity, moist air, soil 0.30
Deicing chemicals 0.18
Seawater and sea water spray wetting and drying
0.15
Water retaining structures 0.1
ii- Cracks Widths
According to CEB-FIP 1990 provisions
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How do we calculate the crack width for a given element?
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Part 2: Concrete problems
i-Crack control parameter
-Beams and One way slabs ( section 10.6 in CSA)
Bars in tension zone shall be spaced as follows: z = fs (dc A)1/3
less than 30 kN/mm for interior exposure less than 25 kN /mm for exterior exposure
What is Fs, dc, A!!!
Limiting (z) corresponds to crack widths of 0.4 mm and 0.33 mm,
respectively. ii-Skin Reinforcement
ii- Cracks Widths
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