technical report writting curing of concrete.pdf

8/10/2019 Technical Report Writting Curing of Concrete.pdf

1/9

Technical Report on possible Problems and Remedy in Curing of Concrete

Department of Civil EngineeringUniversity of Engineering and Technology, Lahore

November 17, 2009

This technical report is an informative study of the Problems and their Remedy in curingof concrete. The report describes the different problems and their recommended remedies

as well as a complete cycle of operation of the process of curing. Also included are

sections concerning economic feasibility and advantages of the better solution of the

problems of curing.


2/9

Civil Engineering Department - UET Lahore Construction Management & Planning

Technical Report on possible Problems and Remedy in Curing of Concrete 2

TABLE OF CONTENTSTITTLE . . . . . . . . . . . . . . . . . . . . . . . . . . . i

TABLE OF CONTENTS . . . . . . . . . . . . . . . . . . . . . . ii

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . 3

INTRODUCTION . . . . . . . . . . . . . . . . . . . . 3

OBJECTIVE . . . . . . . . . . . . . . . . . . . . 3

RELATED THEORY. . . . . . . . . . . . . . . . . . . . 4

Curing and its effects on concreter . . . . . . . . 4

Problems in curing of cold concrete . . . . . . . 4

Problems in curing of hot concrete . . . . . . . . 4

Curing timings problems . . . . . . . . . . . . . . 4

PROBLEMS IN THE METHODOLOGY OF CURING . . . . . . . . 5

Protection from environmental effects . . . . . . 5

Problems with curing blanket . . . . . . . . . . . 5

Cure and Seal Problems . . . . . . . . . . . . . . 5

METHODS TESTS FOR EVALUATION OF CURING . . . . . . . 6

Effectiveness of curing of compounds . . . . . . . 6

Infrared moisture test . . . . . . . . . . . . . . 6

Plastic shrinkage cracking test . . . . . . . . . . 6

Concrete curing strength test cylinders . . . . . 6

RESULTS . . . . . . . . . . . . . . . . . . . . . . . 7

DISCUSSION OF RESULTS . . . . . . . . . . . . . . . . 8

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . 8

RECOMMENDATION . . . . . . . . . . . . . . . . . . . . 8

LIST OF REFERENCES . . . . . . . . . . . . . . . . . . . 9

LIST OF TABLES

Strength increases with time in curing 7

LIST OF FIGURES

Figure Page

1. Reference of Plastic Shrinkage test w.r.t field . . . . 5

2. Curing of cold concrete is in site . . . . . . . . . . 4

3. Curing blanket in a top floor of a building . . . . . . 6


3/9



ABSTRACT

Curing concrete is critical to a high-quality finished surface.

Keep evaporation retardant on hand for initial curing.

For final curing, water curing is the best method if feasible.

For colored concrete, most contractors use cure & seal, which is effective for both

curing and sealing.

Use a cure & seal material that complies with the requirements of ASTM C1315,

Type I, Class A.

For sealing, use solvent-based high-solids-content acrylics.

INTRODUCTION

This technical report contains the detail about the curing of beams and

slabs and the problems we faced during the curing process of the concrete relating to thestrength, feasibility and efficiency of concrete and the remedies methods recommended

in the field by engineers to enhance the strength in the process of curing. The problems

were mainly related to the environmental conditions like hot weather and reactivity ofcementitious materials.

The problems related to curing portland cement concrete pavement

were due to changes in concrete technology that developed since the curing guidancewas written or from some details of paving construction practice that differed from the

types of concrete construction around which curing guidance was developed.

OBJECTIVE

The scope of this technical report is limited to strength of bema and slabs. It does not

include curing of bridge decks or other structures associated with pavements.

The objective of this technical report is to develop practical, quantitative guidelines on

curing of portland cement concrete beams and slabs that will give the structural engineer

tools with which to anticipate the critical details of curing practice and to be able to plan

for contingency conditions. Specific information includes recommendations on:

Selection of curing procedures.

Selection of curing materials.

Accounting for climatic conditions.

Duration of curing.

Sequence of critical events.

Verification of curing.

Accounting for concrete materials and concrete mixture proportions.


4/9



RELATED THEORY

CURING AND ITS EFFECTS ON CONCRETER:

Most of the people when dealing with moisture content, they think only of maintaining

moisture on the surface of the concrete. But curing is more than that; it is giving the

concrete what it needs to gain strength properly. There should be a large amount of waterin the curing area, if there isn't enough water, the crystals can't grow and the concrete

doesn't develop the strength it should. If there is enough water, the crystals grow out like

tiny rock-hard fingers wrapping around the sand and gravel in the mix and intertwiningwith one another.

Almost sounds like a horror movieour concrete baby has turned into a

monster!

N.L Manker (Phd. University of Buffalo, Texas)

PROBLEMS IN CURING OF COLD CONCRETE:

The other important aspect of curing is temperaturethe

concrete can't be too cold or too hot. As fresh concrete getscooler, the hydration reaction slows down. The temperature

of the concrete is what's important here, not necessarily the

air temperature. Below about 50 F, hydration slows down a

lot; below about 40 F, it virtually stops.

PROBLEMS IN CURING OF HOT CONCRETE:

Hot concrete has the opposite problem: the reaction goes too fast, and since the reaction

is exothermic (produces heat), it can quickly cause temperature differentials within the

concrete that can lead to cracking. And cement that reacts too quickly doesn't have timefor the crystals to grow properly so it doesn't develop as much strength as it should.

CURING TIMMINGS PROBLEMS:

So the objective is to keep our young and impressionable concrete damp

and at the right temperature (ideally between 50 and 85 F). The most frequentlyoverlooked curing aspect is keeping exposed concrete surfaces moist while they are

hydrating.

The problem is that if the exposed surfaces dry out then the concrete can't

hydrate and our young concrete ends up with very sensitive skineasily scratched and

sometimes actually dusty. Curing must be started as soon as possible after finishing and


5/9



stamping.

Keeping the concrete surface wet for 7 days is still the best way to cure concrete.

PROBLEMS IN THE METHODOLOGY OF CURING:

There are three ways to cure concrete

1) Adding water to the surface to replace the water that is evaporating2) Sealing the concrete to prevent the water from evaporating in the first place.

3) Combination of the above two methods

Note that adding water to the surface is NOT adding water that will be worked into the

concrete mix--that would increase the water-cement ratio of the surface concrete and

weaken it, ruining all our curing efforts.

Protection from environmental effects:

The best approach for decorative concrete is to try to alter conditions so you don't need to

do initial curing: block the wind, keep the sun off the concrete, and get cooler concrete. If

that's not possible, fogging just enough to keep the surface damp is possible, but thesimplest approach is to use evaporation retardant. This chemical can be sprayed on to

form a thin membrane on the surface that prevents the water from evaporating. It

completely dissipates during finishing operations. Keep some of this around for dry

windy conditions.

Problems with curing blanket:

Wet curing really is the best method and thereare some good curing blankets out there now

that work well on colored concrete surfaces.

The problem has always been that it was

impossible to get the blankets down smoothly

so that curing was even. Small spots where the

blanket is touching or where the surface dries

out can lead to a mottled appearance. With the

new single-use blankets that combine an

absorbent material with plastic sheeting.

Cure & Seal Problems:

One difficulty with stamped concrete can be when a colored release powder is used

during stamping. The cure & seal can't be sprayed on until the release agent has been

washed offwhich could be several days later. In dry, windy conditions, that's probably

too late to do much good.


6/9



If it's an interior application it will never go away by itself--it would have to be removed

after 28 days, a messy job. That's not the right product for that application.

METHODS

TESTS FOR EVALUATION OF CURRING:

Effectiveness of curing compounds Test

The test method is very similar to ASTM C 156(ASTM C 156-44T) existed at that time,

but it is not clear from this report whether the Bureau of Reclamation test method is the

same or some slight variant). Moisture loss of a standard mortar specimen was measured

after 7 days at 38 C, 21 percent RH (saturated CaCl2). Application rate was 3.7 m2/L. A

maximum moisture loss of 40 g (equivalent to 0.87 kg/m2) of water was determined to

represent material that would result in water retention equivalent to 14 days moist curing

(the standard Bureau of Reclamation moist-curing requirement). The method was

reported to have "reasonably close duplication."

WaterLoss (CuringCompound1) = 0.25-0.34* time+ 0.18* appl.rate R2= 0.85

Infrared Moisture Test

The infrared method that could be used for concrete curing is infrared thermography.

This method uses an infrared sensitive video camera image of a concrete surface.

Moisture conditions affect heat flow through the surface, resulting in variations in surface

temperature that could be sensed by the infrared camera. This method would appear to

have application in analyzing the quality of cover of curing compounds.

Plastic Shrinkage Cracking Test

Wang, Shah, and Phuaksuk (2001) investigated

effects of fly ash and fibers on plastic shrinkage

cracking. Fibers generally reduced crack areas on

laboratory specimens as did a Class F fly ash when

used as 30 percent replacement for cement. A Class

C fly ash increased the amount of plastic shrinkage

cracking.


7/9



Concrete Curing Strength-test Cylinders

The curing method varies with the purpose for testing the cylinders. Test results may be

used as the basis for acceptance of the concrete. Acceptance-test results represent thepotential strength of the concrete thats discharged from the truck. The test isnt intended

for determining the in-place strength of the concrete because it makes no allowance forthe effects of placing, compaction, or curing. When cylinder tests are used to indicatewhether or not the producer delivered concrete of the specified strength, standard curing

is mandatory. The details for standard curing are given in ASTM C 31, Practice for

Making and Curing Test Specimens in the Field.

RESULTS

Standard initial curing for test cylinders requires them to be stored immediatelyafter molding, for a period up to 48 h, at a temperature range from 60 and 80 F

(16 and 27 C) and in an environment preventing moisture loss. The cylindersmust also be shielded from direct sunlight. After this period, the cylinders are

cured with free water maintained on their surfaces at all times, and at a

temperature of 73 3 F (23 2 C).


8/9



From the test we determined the extent of effectiveness of the compounds and theconstituents in the concrete should be maintained in accurate proportions.

The infrared test is very necessary for the better and efficient curing.

DISCUSSION OF RESULTS

The results obtained from the above shrinkage test were reliable up to extent but the

recommendation cannot be done until the results are analyzed and checked a few moretimes to obtained following main points for the conclusion of the data. Hydration and

hardening of concrete during the first three days is critical. Abnormally fast drying and

shrinkage due to factors such as evaporation from wind during placement may lead to

increased tensile stresses at a time when it has not yet gained significant strength,resulting in greater shrinkage cracking.

CONCLUSION

Curing concrete is critical to a high-quality finished surface. Keep evaporation retardanton hand for initial curing. For final curing, water curing is the best method if feasible. For

colored concrete, most contractors use cure & seal, which is effective for both curing and

sealing. Use a cure & seal material that complies with the requirements of ASTM C1315,Type I, Class A. For sealing, use solvent-based high-solids-content acrylics.

RECOMMENDATION

During this period concrete needs to be in conditions with a controlled temperature and

humid atmosphere. In practice, this is achieved by spraying or pending the concretesurface with water, thereby protecting concrete mass from ill effects of ambient

conditions. The pictures to the right show two of many ways to achieve this, ponding

submerging setting concrete in water, and wrapping in plastic to contain the water in themix.

Properly curing concrete leads to increased strength and lower permeability, and avoids

cracking where the surface dries out prematurely. Care must also be taken to avoidfreezing, or overheating due to theexothermic setting of cement (theHoover Dam used

pipes carrying coolant during setting to avoid damaging overheating). Improper curing

can cause scaling, reduced strength, poor abrasion resistance and cracking.

The internal forces in common shapes of structure, such asarches,vaults,columns and

walls are predominantly compressive forces, with floors and pavements subjected totensile forces. Compressive strength is widely used for specification requirement and
http://en.wikipedia.org/wiki/Exothermichttp://en.wikipedia.org/wiki/Hoover_Damhttp://en.wikipedia.org/wiki/Archhttp://en.wikipedia.org/wiki/Vault_%28architecture%29http://en.wikipedia.org/wiki/Vault_%28architecture%29http://en.wikipedia.org/wiki/Archhttp://en.wikipedia.org/wiki/Hoover_Damhttp://en.wikipedia.org/wiki/Exothermic


9/9



quality control of concrete. The engineer knows his target tensile (flexural) requirements

and will express these in terms of compressive strength.

LIST OF REFERENCES

www.cement.org/basics/concretebasics_curing.asp

www.tpub.com/content/construction/14043/.../14043_201.htm

en.wikipedia.org/wiki/Concrete

en.wikipedia.org/wiki/Concrete_curing

www.nrmca.org/aboutconcrete/cips/11p.pdf

www.cabmphandbooks.com/Documents/Construction/NS-12.pdf

www.kraftenergy.com

www.cement.org/basics/concretebasics_faqs.asp

www.freepatentsonline.com/y2009/0241463.html www.concrete.net.au/publications/pdf/Curing06.pdf

www.tkproduct.com/Curing%20Concrete.PDF

www.constructiondir.com/concrete_curing-service.html

www.intrans.iastate.edu/pubs/t2summaries/curing.pdf

books.google.com.pk/books?isbn=0870311719

Microsoft ENCARTA Encyclopedia

www.encyclopedia.com/doc/1G1-95844832.html
http://www.cement.org/basics/concretebasics_curing.asphttp://www.tpub.com/content/construction/14043/.../14043_201.htmhttp://www.nrmca.org/aboutconcrete/cips/11p.pdfhttp://www.cabmphandbooks.com/Documents/Construction/NS-12.pdfhttp://www.kraftenergy.com/http://www.cement.org/basics/concretebasics_faqs.asphttp://www.freepatentsonline.com/y2009/0241463.htmlhttp://www.concrete.net.au/publications/pdf/Curing06.pdfhttp://www.tkproduct.com/Curing%20Concrete.PDFhttp://www.constructiondir.com/concrete_curing-service.htmlhttp://www.intrans.iastate.edu/pubs/t2summaries/curing.pdfhttp://www.encyclopedia.com/doc/1G1-95844832.htmlhttp://www.encyclopedia.com/doc/1G1-95844832.htmlhttp://www.encyclopedia.com/doc/1G1-95844832.htmlhttp://www.intrans.iastate.edu/pubs/t2summaries/curing.pdfhttp://www.constructiondir.com/concrete_curing-service.htmlhttp://www.tkproduct.com/Curing%20Concrete.PDFhttp://www.concrete.net.au/publications/pdf/Curing06.pdfhttp://www.freepatentsonline.com/y2009/0241463.htmlhttp://www.cement.org/basics/concretebasics_faqs.asphttp://www.kraftenergy.com/http://www.cabmphandbooks.com/Documents/Construction/NS-12.pdfhttp://www.nrmca.org/aboutconcrete/cips/11p.pdfhttp://www.tpub.com/content/construction/14043/.../14043_201.htmhttp://www.cement.org/basics/concretebasics_curing.asp