FIELD EVALUATION OF MASS CONCRETE SPECIFICATIONS TO PREVENT EARLY-AGE CRACKING

Abstract

Mass concrete is defined in the American Concrete Institute (ACI) as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from the hydration of the cement and attendant volume change to minimize cracking”. There are two important limits in specification for mass concrete. These two limits are the maximum temperature and the maximum temperature differential, and are set to be 180 °F and 35 ° F, respectively in the mass concrete specification of Florida Department of Transportation (FDOT) . Many methods and software are used to ensure these limits are not exceeded. The Schmidt method is one of these methods used by contractors for FDOT. Concrete Works software is also used by the contractors. There is a need to evaluate how well the present FDOT specification can prevent cracking in mass concrete and how well Schmidt method and Concrete Works can predict maximum temperature and maximum temperature differential in mass concrete. Case studies on mass concrete projects were conducted using available mass concrete reports from FDOT. The methodology of analysis included (1) comparison of the predicted concrete temperature from the Schmidt Method as performed by the contractor with the actual measured concrete temperature, (2) conductance of an adjusted Schmidt Method analysis using adjusted parameters which more accurately reflect the actual conditions, and (3) conductance of analysis using Concrete Work software. The FDOT mass concrete specification which limits the maximum temperature differential of 35 °F and the maximum temperature of 180 °F were found to be quite effective in predicting early-age cracking of mass concrete. The Schmidt method was found to be not very effective in predicting early-age cracking of mass concrete. When the adjusted Schmidt method was performed using the actual concrete placement temperature and the actual ambient temperature, there was a slight improvement in cracking prediction. The errors in the temperature prediction from the Schmidt method analysis could be attributed to the inaccurate values of the heat of hydration used. The Concrete Works was shown to give more conservative predictions of early-age cracking of mass concrete. The more conservative predictions from Concrete Works could be attributed to the fact that Concrete Works used more accurate values of heat of hydration and the actual shape and dimensions of the structure in the analysis. Based on the results of this study, it is recommended that (1) the FDOT mass concrete specification which limits the maximum temperature differential of 35 °F and the maximum temperature of 180 °F be continued to be used, (2) the use of heater be continued to be used during early-age curing to reduce the maximum temperature differential and chances for cracking, and (3) accurate adiabatic temperature rise data due to heat of hydration of the cementitious materials used in the specific mass concrete projects be determined through adiabatic testing and used in the Schmidt method analysis, and (4) the use of Concrete Work or other similar finite element software for thermal analysis of mass concrete is recommended for use for checking the results of Schmidt method analysis.