International Concrete Abstracts Portal

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.

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.

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.

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.

There is a growing interest in monitoring the temperature of cement paste, mortar and concrete, particularly at early ages. However, there also seems to be confusion about what is being achieved by this activity, and what to do with the information once it is recorded. This paper outlines the tools and techniques in use, and discusses their applications, benefits and limitations. The discussion will cover concepts such as heat of hydration, maturity, isothermal calorimetry and semi-adiabatic temperature monitoring for assessing setting times, and potential incompatibility between the reactive ingredients (cements, supplementary cementitious materials, and chemical admixtures) in a mixture.