An accurate and practical method of determining the heat development of concrete mixtures under real mixing, cooling, hauling, placement, and curing conditions would greatly benefit contractors and engineers in helping predict in-place concrete member temperatures. Semi-adiabatic calorimetry was performed at several construction sites in temperature controlled rooms using concrete sampled from concrete placements. Semi-adiabatic calorimetry was also performed for comparison with concrete made under laboratory conditions from materials sampled at the respective batch plants. An energy balance-based finite difference method is presented for calculating the concrete non-linear heat generation using the measured heat of hydration determined from semi-adiabatic calorimetry. This method was used in a program which allows the direct input of values from semi-adiabatic calorimetry testing and estimates the development of in-place temperatures in mass concrete members of various geometries. Estimated concrete member temperatures are compared to the values measured on-site. Best practice suggestions are also given for performing semi-adiabatic calorimetry using concrete sampled on-site.

SP-241CD This CD-ROM contains nine papers presented at a technical session on Heat Development: Monitoring, Prediction, and Management, held in Atlanta, GA, in April 2007. Topics include innovative technology for concrete heat monitoring; use of the heat measurements to characterize concrete mixtures, evaluate the mixture performance, and detect potential incompatibilities of concrete materials; heat management in precast concrete; and modeling and prediction of in-place concrete temperature development, among others.

Abnormal early hydration resulting from "incompatibilities" of common concrete materials can result in erratic set and strength gain behavior and associated finishing, curing, and cracking issues. Contributing influences include high temperatures, cement sulfate levels, Class C fly ash content, chemical admixture use, and design approaches for retardation of hot-weather concrete. Simple, expedient test methods are needed to identify potentially incompatible materials and conditions and to verify appropriate modifications to concrete proportions. Thermal measurements of the early heat development of materials mixtures in the laboratory (semi-adiabatic calorimetry) have been shown very useful toward this end. Abnormal set and strength development of field concrete was reproduced in laboratory paste and mortar mixtures and studied using thermal measurements, verified by parallel mortar cube strengths. Sensitivities of various contributing influences were documented in extensive testing. Changing one or more of the key material or mixture characteristics was usually successful in restoring normal behavior. Recommendations are presented for avoiding related field issues and for the use of calorimetry testing programs for diagnosis of such problems.

The throughput of precast concrete plants can be improved by controlled heating of the cast products. Presently many systems do measure maturity or degree-hours providing information about the strength development, but not sufficient data for accurate decisions for the control of heating. A heat control system has been developed based on an on-line predictive calculation of the temperature behavior of concrete and a maturity-strength model. The temperature is measured continuously and every minute a complete prediction calculation up to the target maturity and strength is made. If the target strength cannot be reached without heating by the target time limit, the system opens the valve for heating the mould, until the temperature is high enough. The predictive algorithm also provides an accurate estimation of the time when the prestress release or demoulding strength is going to be reached. The parameters for the cement heat generation model are obtained by semiadiabatic measurements of the production concrete. The system has been in use since 1999 and applied in over ten precast factories in Europe in hollow-core and railroad sleepers production. The system has reduced significantly the heating costs; reduced rejections caused by too early demoulding and improved production planning in the factories.

Accurate characterization of the temperature rise in a concrete element requires an estimate of the adiabatic temperature rise of the concrete mixture. Semi-adiabatic calorimetry is commonly used to provide an estimate of the heat generation characteristics of a concrete mixture because of the relative simplicity of the test. This study examines the sources of variability in semi-adiabatic calorimetry, and an estimate of the confidence limits of the test is calculated. Then, twenty concrete mixtures are investigated using semi-adiabatic calorimetry. Activation energy values are calculated for each mixture using isothermal calorimetry. The adiabatic temperature rise is then calculated. The following mixture properties are investigated: cement type, cementitious content, water/cementitious material ratio, coarse aggregate type (siliceous river gravel and limestone), mixture placement temperature, and the effects of selected supplementary cementing materials. The following factors were the most important to reduce the adiabatic temperature rise: reduced cement content, use of a lower-heat cement, such as a Type V cement type, reduced aggregate specific heat, and substitution of cement with Class F fly ash.