International Concrete Abstracts Portal

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.

This paper describes the use of a recently developed, inexpensive portable semi-adiabatic calorimeter for monitoring cement hydration in concrete and mortar. The calorimeter measures the temperature as a function of time at the bottom of eight individual 3x6 cylinders with concrete or mortar. The measured temperature profile is used to evaluate the overall hydration performance of cementitious mixtures, with special emphasis on the timing and the size of the main hydration exotherms that strongly affect setting and early strength development of cementitious mixtures. Furthermore, a method has been developed for a more precise calculation of "thermal set", with good correlation to manual set times according to ASTM C403. The field calorimeter is useful to screen the effect of type and dosage of admixtures and supplementary cementitious materials on "thermal" setting times in concrete and mortar.

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.

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.

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.