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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.
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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
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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.
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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
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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.
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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.
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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).
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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 -
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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