Technical Report Writting Curing of Concrete.pdf

Technical Report on possible Problems and Remedy in Curing of Concrete

Department of Civil Engineering University of Engineering and Technology, Lahore

 November 17, 2009

This technical report is an informative study of the Problems and their Remedy in curing of concrete. The report describes the different problems and their recommended remedies as well as a complete cycle of operation of the process of curing. cu ring. Also included are sections concerning economic feasibility and advantages o f the better solution of the  problems of curing.

Civil Engineering Department - UET Lahore

Construction Management & Planning

TABLE OF CONTENTS TITTLE

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

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 ABSTRACT

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INTRODUCTION

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OBJECTIVE

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RELATED THEORY. . . . . . . . . . . . . . . . . . . . Curing and its effects on concreter . . . . . . . . Problems in curing of cold concrete . . . . . . . Problems in curing of hot concrete . . . . . . . . Curing timings problems . . . . . . . . . . . . . .

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PROBLEMS IN THE METHODOLOGY OF CURING . . . . . . . . Protection from environmental effects . . . . . . Problems with curing blanket . . . . . . . . . . . Cure and Seal Problems . . . . . . . . . . . . . .

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 METHODS – TESTS FOR EVALUATION OF CURING . . . . . . . Effectiveness of curing of compounds . . . . . . . Infrared moisture test . . . . . . . . . . . . . . Plastic shrinkage cracking test . . . . . . . . . . Concrete curing strength test – cylinders . . . . .

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RESULTS

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DISCUSSION OF RESULTS CONCLUSION

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RECOMMENDATION

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

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LIST OF TABLES Strength increases with time in curing

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LIST OF FIGURES Figure

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

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2. Curing of cold concrete is in site .

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

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Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

Construction Management & Planning

ABSTRACT    

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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 the strength, 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 of cementitious materials. The problems related to curing portland cement concrete pavement were due to changes in concrete technology that developed since the curing guidance was 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 dev elop 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.

Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

Construction Management & Planning

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 water in 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 intertwining with one another. ―Almost sounds like a horror movie— our 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 temperature — the concrete can't be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. T he temperature of the concrete is what's important here, not ne cessarily 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 time for 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 frequently overlooked 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 skin — easily scratched and sometimes actually dusty. Curing must be started as soon as possible after finishing and Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

Construction Management & Planning

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 evaporating 2) 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 the simplest approach is to use evaporation retardant. This chemical c an 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 there are 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 wh en a colored release powder is used during stamping. The cure & seal can't be sprayed on until the release agent has been washed off   — which could be several days later. In dry, windy conditions, that's probably too late to do much good. Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

Construction Management & Planning

If it's an interior application it will never go away b y 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 2 after 7 days at 38 °C, 21 percent RH (saturated CaCl2). Application rate was 3.7 m /L. A 2 maximum moisture loss of 40 g (equivalent to 0.87 kg/m ) 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." 2

WaterLoss (CuringCompound  1) = 0.25-0.34* time + 0.18* appl.rate  R  = 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 wh en used as 30 percent replacement for cement. A Class C fly ash increased the amount of plastic shrinkage cracking.

Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

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Construction Management & Planning

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 the  potential strength of the concrete that’s discharged from the truck. The test isn’t intended for determining the in-place strength of the concrete because it makes no allowance for the effects of placing, compaction, or curing. When cylinder tests are used to indicate whether 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 immediately after 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 cylinders must 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).

Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

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

Construction Management & Planning

From the test we determined the extent of effectiveness of the compounds and the constituents in the concrete should be maintained in accurate proportions. The infrared test is very necessary for the better an d 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 more times to obtained following main points for the con clusion 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 retardant on hand for initial curing. For final curing, water curing is the best method if feasible. For colored concrete, most contractors use cure & sea l, 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 b y spraying or pending the concrete surface with water, thereby protecting concrete mass from ill effects of ambient conditions. The pictures to the right show two o f many ways to achieve this, ponding –  submerging setting concrete in water, and wrapping in plastic to contain the water in the mix. Properly curing concrete leads to increased strength and lower permeability, and avoids cracking where the surface dries out prematurely. C are must also be taken to avoid freezing, or overheating due to the exothermic setting of cement (the Hoover 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 as arches, vaults, columns and walls are predominantly compressive forces, with floors and pavements subjected to tensile forces. Compressive strength is widely used for specification requirement and Technical Report on possible Problems and Remedy in Curing of Concrete

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Civil Engineering Department - UET Lahore

Construction Management & Planning

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

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