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CIP 42- Thermal Cracking of Concrete

CIP 42- Thermal Cracking of ConcreteWHAT is Thermal Cracking ?WHY Does Thermal Cracking Occur? Thermal Cracking occurs due to excessive temperature dif-ferences within a Concrete structure or its temperature difference causes the cooler portion to con-tract more than the warmer portion, which restrains the con-traction. Thermal cracks appear when the restraint results intensile stresses that exceed the in-place Concrete tensilestrength. Cracking due to temperature can occur in concretemembers that are not considered mass of cementitious materials generates heat for sev-eral days after placement in all Concrete members. This heatdissipates quickly in thin sections and causes no thicker sections, the internal temperature rises and dropsslowly, while the surface cools rapidly to ambient tempera-ture.

2009 References 1. ACI 207.2R, Report on Thermal and Volume Change Effects on Cracking of Mass Concrete, American Concrete Institute, www.concrete.org

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Transcription of CIP 42- Thermal Cracking of Concrete

1 CIP 42- Thermal Cracking of ConcreteWHAT is Thermal Cracking ?WHY Does Thermal Cracking Occur? Thermal Cracking occurs due to excessive temperature dif-ferences within a Concrete structure or its temperature difference causes the cooler portion to con-tract more than the warmer portion, which restrains the con-traction. Thermal cracks appear when the restraint results intensile stresses that exceed the in-place Concrete tensilestrength. Cracking due to temperature can occur in concretemembers that are not considered mass of cementitious materials generates heat for sev-eral days after placement in all Concrete members. This heatdissipates quickly in thin sections and causes no thicker sections, the internal temperature rises and dropsslowly, while the surface cools rapidly to ambient tempera-ture.

2 Surface contraction due to cooling is restrained by thehotter interior Concrete that doesn t contract as rapidly asthe surface. This restraint creates tensile stresses that cancrack the surface Concrete as a result of this uncontrolledtemperature difference across the cross section. In mostcases Thermal Cracking occurs at early ages. In rarer instancesthermal Cracking can occur when Concrete surfaces are ex-posed to extreme temperature members will expand and contract when exposedto hot and cold ambient temperatures, respectively. Crack-ing will occur if this bulk volume change resulting from tem-perature variations is restrained. This is sometimes calledtemperature Cracking and is a later age and longer term concreteThe main factor that defines a mass Concrete member is itsminimum dimension.

3 ACI 301 suggests that a Concrete mem-ber with a minimum dimension of 4 feet ( m) should beconsidered as mass Concrete . Some specifications use a vol-ume-to-surface ratio. Other factors where precautions formass Concrete should be taken even for thinner sections arewith higher heat generating Concrete mixtures - highercementitious materials content or faster hydrating main concern with mass Concrete is a high Thermal sur-face gradient and resulting restraint as discussed conditions can result during the initial stages due toheat of hydration and during the later stages due to ambientHOWTo Recognize Thermal Cracking ?temperature changes. Another factor is a temperature differ-ential between a mass Concrete member and adjoining ele-ments.

4 As the mass member cools from its peak temperature,the contraction is restrained by the element it is attached to,resulting in Cracking . Examples are thick walls or dams re-strained by the StructuresTemperature Cracking can occur in structures that are notmass structures. The upper surface of pavements and slabsare exposed to wide ranges of temperature while the bottomsurface is relatively protected. A significant temperature dif-ferential between the surface and the protected surface canresult in Cracking . Concrete has a Thermal coefficient of ex-pansion in the range of 3 to 8 millionths/ F ( to mil-lionths/ C). A Concrete pavement cast at 95 F (35 C) duringthe summer in Arizona may reach a maximum temperature of160 F (70 C) and a minimum temperature in winter of 20 F (-7 C), resulting in an annual temperature cycle of 140 F (75 C).

5 Expansion joints and spacing between joints have to be de-signed to withstand such temperature induced expansionand contraction to prevent cracks caused by excessive temperature differen-tials in mass Concrete appear as random pattern Cracking onthe surface of the member. Checkerboard or patchwork crack-ing due to Thermal effects will usually appear within a fewdays after stripping the formwork. Temperature-related cracksin pavements and slabs look very similar to drying shrink- Thermal cracks in a thick slabCourtesy CTLG roup2009 References1. ACI , Report on Thermal and Volume Change Effects onCracking of Mass Concrete , American Concrete Institute, . Mass Concrete for Buildings and Bridges, John Gajda, EB547, Port-land Cement Association.

6 ACI , Causes, Evaluation, and Repair of Cracks in ConcreteStructures, American Concrete Institute4 . Contractor s Guide to Mass Concrete , Bruce A. Suprenant and WardR. Malisch, Concrete International, ACI, Jan 2008, pp.. Controlling Temperatures in Mass Concrete , John Gajda and MarthaVanGeem, Concrete International, Jan 2002, pp. cracks. They usually occur perpendicular to the longestaxis of the Concrete . They may become apparent any timeafter the Concrete is placed, but usually occur within the firstyear or summer-winter Minimize Thermal Cracking ?The key to reducing Thermal or temperature-related crackingis to recognize when it might occur and to take steps tominimize it.

7 A Thermal control plan that is tailored to thespecific requirements of the project specification is recom-mended. See Ref. 2 for specifications for mass Concrete include a maximumtemperature and a maximum temperature differential. Themaximum temperature addresses the time it takes for the con-crete member to reach a stable temperature and will governthe period needed for protective measures. Excessively highinternal Concrete temperatures also have durability implica-tions. A temperature differential limit attempts to minimizeexcessive Cracking due to differential volume change. A limitof 35 F (20 C) is often used. However, Concrete can crack atlower or higher temperature differentials. Temperature differ-ential is measured using electronic sensors embedded in theinterior and surface of the peak temperature of a Concrete mixture can be estimatedassuming perfectly insulated conditions.

8 See Ref. 1 and modeling can also be used to predict temperatureand potential for Cracking based on Thermal controls models are HIPERPAV ( ) for pave-ments and ConcreteWorks ( )for pavements and other mass Concrete members. Consult-ants can also assist with these large part of the responsibility to minimize Thermal crack-ing lies with the designer and contractor. Steps include es-tablishing the Concrete mixture, specification limits for tem-perature of Concrete as delivered and in the structure, insu-lating the structure and termination of protective measures,and in critical conditions, post-cooling of structural steps to minimize Thermal Cracking are: Concrete mixture - Reduce heat of hydration by optimizingthe cementitious materials using supplementarycementitious materials like fly ash or slag; or using a port-land cement that generates a lower heat of hydration.

9 Avoidspecifying an excessively low w/cm. Retarding chemicaladmixtures may delay but not reduce peak Concrete tem-peratures. A cooler initial Concrete temperature will reducethe peak temperature in the structure but needs to be bal-anced with practical feasibility and project costs. Mass Concrete - Ensure that Thermal control measures areagreed upon in a pre-construction meeting. Some thingsto consider include placement method and details, estab-lishing temperature requirements for Concrete as deliv-ered and temperature monitoring of in-place Concrete ,curing methods and duration that do not increase tem-perature differentials, use of insulation - including whenand how the insulation will be removed, and use of cool-ing pipes if necessary.

10 Placing Concrete in lifts along withtiming of successive lifts can minimize the overall peaktemperature and time of Thermal control but this needs tobe balanced against construction joint preparation andthe design requirements. Water curing will cool concretesurfaces and water retention curing methods may be moreappropriate. Wood forms provide insulation while metalforms do not. Covering forms with insulating blanketsmay be necessary. The removal of insulation or formworkshould be scheduled based on monitored in-place tem-perature and Thermal shock to the surface should beavoided. Reinforcing steel protruding from a massive beamcan act as a heat sink to draw heat out of the interior of thebeam.


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