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.

The maturity concept applying the Arrhenius equation is generally accepted as a proper way to model the temperature effects on concrete hardening. The Arrhenius equation gives the rate of hydration as a function of temperature depending on the activation energy for the cementitious binder materials. It is demonstrated how a simple three-parameter-model is sufficient to formulate the development of heat from the cement hydration process. Furthermore, the heat development is used to define the apparent degree of hydration. It is described how the heat of hydration may be determined experimentally at the con-crete plant or in nearby concrete laboratories. By means of a semi-adiabatic container the heat released from the hydration process is monitored. Finally, examples of the practical applications of the heat of hydration data for various concrete mixtures are addressed. It is demonstrated how the use of admixtures may influ-ence the heat of hydration and how difficult it is to model the complicated interactions between cement, mineral additions and admixtures without the use of experiments. The possibility of linking the heat of hydration data with the early-age mechanical properties is also illustrated.

Concrete paving mixtures are subjected to varying climatic conditions during the hydration process. The temperature of the concrete is a function of the heat generated by the cement paste and climatic conditions as well as curing procedures applied during construction. Temperature development in the concrete is closely related to the development of concrete properties and also affects the generation of internal stresses in the pavement that if not properly controlled may result in cracking and other distresses. With the FHWA HIPERPAV software, it is possible to assess the impact on the performance of the pavement that different concrete materials will have by evaluating their heat of hydration properties (heat fingerprint) and their interaction with the environment. Characterization of concrete mixtures in terms of their heat of hydration allows for a more rational selection of materials as a function of the climatic conditions to which they are exposed. Selected concrete mixtures with this approach can thus provide more confidence in that they will perform satisfactorily under the site-specific conditions to which they are subjected effectively reducing potential excessive stresses in the pavement. In this paper, the system approach to characterize concrete paving mixtures and its effect under various climatic conditions is presented.

Heat of hydration and its rate play key roles in concrete thermal cracking and high-early strength development for fast track construction. Heat properties are also unique indicators of both the quality and the performance of cement, cementitious materials, and chemicals in concrete. Heat Signature is an automated method of measuring concrete heat of hydration and its rate, and adiabatic temperature rise and its rate versus concrete maturity (equivalent curing age at 68 °F). The measurements are performed on full size concrete specimens via the Internet/Intranet, and together with mixture information are saved to a database. Heat signature data in combination with simulation enable projecting concrete field performance in terms of its temperature, maturity, and strength profiles as a function of job site weather conditions and placement and curing plans. The paper reviews the underlying heat signature theory, its history, and reviews and interprets typical data from the many measurements in the US. To better index a given mix design’s field performance, a consistent set of thermal cracking and high-early curing age indices are introduced.

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.