INTRODUCTION Plastic shrinkage cracking occurs when the concrete is in the plastic state, which is within the first 24 hours. Plastic shrinkage cracks are usually spaced 1 to 3 feet apart, are generally parallel to one another.

PLASIC SHRINKAGE CRACKING : THE AMERICAN CONCRETE INSTITUTE [ACI] defines plastic shrinkage cracking as the “ cracking that occurs on the surface of fresh concrete soon after it is placed and still it is in plastic’’. FIG: Plastic shrinkage cracking

HOW IT OCCURS PLASTIC SHRINKAGE CRACKS are caused by the rapid loss of water from the surface of concrete before it has set. The critical condition exist when the rate of evaporation of surface moisture exceeds the rate at which bleed water can replace it.

CAUSE FOR CRACKING

CONDITIONS THAT CAUSES HIGH EVAPORATION Wind velocity in excess of 5mph Low relative humidity High concrete temperature

FACTORS CONTRIBUTING PLASTIC SHRINKAGE CRACKING 1.Concrete strength 2.Depth of section 3.Fines content 4.Admixtures 5.Fibers 6.Surface sprays

PREDICTION OF EVAPORATION RATE : 1. By the usage of Nomograph 2. By using Formula

NOMOGRAPH : Nomo graph is a graph that relates air temperature, relative humidity and wind velocity to predict the evaporation rates for a concrete surface with bleed water.

NOMOGRAPH FOR DETERMINING THE RATE OF EVAPORATION FROM CONCRETE SURFACES

BY THE USAGE OF FORMULA METRIC UNITS: E = 5 ( [ Tc + 18]2.5 - r [ Ta+ 18 ]2.5 )(V +4)X10-6 here,E=Evaporation Rate, kg/m2/hr Tc =Concrete(watersurface)Temperature,C Ta = Air Temperature, C r = Relative Humidity (percent)/100 V = Wind Velocity, kph

EVAPORATION RATE CAUSING PLASTIC SHRINKAGE CRACKING When the evaporation rates exceed 1.0 kg/m2/hr plastic shrinkage cracks are expected. Precautionary measures are almost manitory. Some of these are erecting a wind screen, cooling the concrete, using a fogging system, and placing concrete at night

Conti…… When the evaporation rate is between 0.5 and 1.0 kg/m2/hr plastic shrinkage cracking may occur.. When the evaporation rates are 0.5 kg/m2/hr, plastic shrinkage cracks are not expected.

WHY NOMOGRAPH AND FORMULA NOT WIDELY USED 1. Contractors, Ready mix concrete companies and designers are unaware of nomograph. 2.The nomograph and equations are not easy to use 3.Unaware of how to get weather information.

COMPUTER PROGRAM Computer program makes the Nomograph and the equations easy to use. The user simply enters the air temperature, concrete temperature, relative humidity, and wind velocity.

COMPUTER PROGRAM Cont… The program will determine the evaporation rate and warn the user if precautionary methods are needed. To further emphasize the fact that some conditions may cause or might cause plastic shrinkage cracking, the answers are color coded.

COMPUTRE PROGRAM cont… The color codes are similar to a traffic light; The output is in red when plastic shrinkage cracks are expected.

COMPUTER PROGRAM Conti… The output is in yellow when the plastic shrinkage cracks can occur. The output is green is used when the plastic shrinkage cracking is not expected.

IMPACT OF PLASTIC SHRINKAGE CRACKING Plastic shrinkage cracking rarely impairs the strength of a concrete element. It will have a dramatic impact on the appearance of the concrete. Where it penetrates full depth it may lead to water penetration

WATER PENETRATION THROUGH CRACKS Figure showing water penetrating through the cracks formed due to plastic shrinkage.

REPAIR OF PLASTIC SHRINKAGE CRACKS In pre-hardened concrete: Surface vibrators can be used to close the cracks over their full depth and the surface finishing can then be completed as specified.

REPAIR contin…. Figure shows surface vibration to close the plastic shrinkage cracks

REPAIR contin… Figure showing finishing of concrete surface.

REPAIR contin.. In hardened concrete: If the cracks are not wide at the surface then it may be best to tolerate them otherwise, the cracks should be filled with a suitable proprietary filler.

HOW TO MINIMISE PLASTIC SHRINKAGE CRACKING : Erecting temporary wind breakers to reduce the wind velocity Usage of fog sprays to reduce the rate of evaporation Avoiding the usage of vapor retarders Covering the concrete with moisture retaining coverings such as polythene sheeting, building paper etc.

Contin….. Curing the concrete as soon as possible Usage of synthetic fibers Accelerating the setting time of concrete

Contin.. Figure shows the use of fog sprays

Contin.. Figure shows moisture retaining covering on concrete surfaces

CONCLUSION This paper alerts people about the impact of plastic shrinkage cracks on concrete. Makes the people realise that nomograrh and equations still exist. Computer program makes nomograph and equations easy to use.

REFERENCE 1. American Concrete Institute (ACI) Committee 308, “Guide to Curing Concrete”, ACI 308R-01, American Concrete Institute, Concrete Q & A, “Estimating Evaporation Rates to Prevent Plastic Shrinkage Cracking”, Concrete International, March Uno, P ‘Plastic Shrinkage Cracking and Evaporation Formulas’ ACI Materials Journal Vol 95, No 4, July– August, 1998, pp 365–375.