Modelling the temperature, maturity and moisture content in a drying concrete block

In this paper we continue work from a previous Study Group in developing a model for the maturation of concrete. The model requires equations describing the temperature, moisture content and maturity (or amount of cement that has reacted with the water). Non-dimensionalisation is used to simplify the model and provide simple analytical solutions which are valid for early time maturation. A numerical scheme is also developed and simulations carried out for maturation over one day and then two months. For the longer simulation we also investigate the effect of building the block in a single pour or two stages

Early-age Thermal Characteristics of Clinker Cements in Relation to Microstructure and Composition: Implications for Temperature Development in Large Concrete Elements

The thermal response of concrete due to hydration of cement is a predominant factor in the potential for early-age cracking of large concrete elements. An anal¬ysis of this cracking potential requires an ability to quantify both the amount of heat that is evolved by the cement as well as the rate at which this heat is evolved [1]. Both these parameters are strongly influenced by the chemical and mineralogical composition of the cement, insofar as it affects the kinetics of the hydration reac¬tions of cement. Furthermore, clinker morphology has been shown [2] to influence the compressive strength and, by inference, the hydration development of cement. Clearly, an ability to estimate the thermal response of cement in concrete, based on a knowledge of clinker characteristics would be of assistance to mass concreteThis paper presents an assessment of the heat response of nominally similar cement clinkers from a range of cement production facilities in South Africa. Clinker samples were collected at nine cement plants and cements were prepared by grinding each clinker with a uniform quality of gypsum. XRF and optical microscope techniques were then used to characterise each clinker and cement in terms of chemical composition and cement compound morphology

Early-age heat evolution of clinker cements in relation to microstructure and composition: Implications for temperature development in large concrete elements.

This paper presents an assessment of the range and extent of variation of heat evolution of nominally similar cement clinkers from a range of cement production facilities in South Africa. Clinker samples were collected at nine cement plants and cements were prepared by grinding each clinker with a uniform quality of gypsum. X-ray fluorescence and optical microscope techniques were then used to characterise each clinker and cement in terms of chemical composition and cement compound morphology. Concretes were then prepared with the laboratory-manufactured cements and these were tested in an adiabatic calorimeter in order to determine the rate of heat evolution from each of the clinker samples. The results of these tests were related to the chemical and morphological characteristics of the corresponding cement clinkers. The results indicate a clear differentiation of clinker cements into low, medium and high heat cements. The relationships between this classification of the heat performance of the cements and the chemistry and morphology of the clinker is not clear at this stage. However, using a finite difference heat model, the paper presents an indication of the implications of the measured heat characteristics of the cement for early-age temperature distributions in large concrete elements.MvdH2016http://www.journals.elsevier.com/cement-and-concrete-composite

The effects of supplementary cementing materials in modifying the heat of hydration of concrete.

This paper is intended to provide guidance on the form and extent to which supplementary cementing materials, in combination with Portland cement, modifies the rate of heat evolution during the early stages of hydration in concrete. In this investigation, concretes were prepared with fly ash, condensed silica fume and ground granulated blastfurnace slag, blended with Portland cement in proportions ranging from 5% to 80%. These concretes were subjected to heat of hydration tests under adiabatic conditions and the results were used to assess and quantify the effects of the supplementary cementing materials in altering the heat rate profiles of concrete. The paper also proposes a simplified mathematical form of the heat rate curve for blended cement binders in concrete to allow a design stage assessment of the likely early-age time–temperature profiles in large concrete structures. Such an assessment would be essential in the case of concrete structures where the potential for thermally induced cracking is of concern.Financial support from South African cement industry, Cement and Concrete Institute, Eskom and the National Research Foundation (South Africa).MvdH2016http://link.springer.com/journal/volumesAndIssues/1152

A Maturity Approach to the Rate of Heat Evolution in Concrete

Early-age cracking as a result of temperature induced stresses can be a serious problem in mass concrete structures or in concrete structural elements in which a high cement content concrete is used. These stresses are induced by temperature differences in the concrete as a result of the heat liberated by hydrating cement. A strategy that is aimed at controlling or limiting such cracking must include a reliable determination of the space-time distribution of temperature throughout the concrete element under consideration.This paper discusses the use of the concept of maturity as a means of combining the effects of time and temperature in describing the rate of heat evolution from hydrating cement in concrete. The proposed maturity approach allows the rate of heat evolution determined from an adiabatic test to be expressed in a form which is independent of the starting temperature of the test. This relationship can then be directly used in a time-temperature prediction model which requires a solution of the Fourier equation for heat flow

Modelling surface heat exchanges from a concrete block to the environment

The presented problem was to determine an appropriate heat transfer boundary condition at the surface of a concrete slab exposed to the environment. The condition obtained involves solar radiation and convective heat transfer, other terms were shown to be small compared to these. It is shown that this boundary condition leads to a temperature variation that has qualitative agreement with experiments carried out by the Cement and Concrete Institute

Modelling the cooling of concreate by piped water

Large concrete structures are usually made sequentially in a series of blocks. After each block is poured it must be left to cool and shrink for a period depending on its size, but typically for around 1 week, before the next block is poured. The reason for the delay is that the mixture of cement and water, which constitute the binding agent of the concrete, results in a series of hydration reactions that generate heat.Piped water is used to remove hydration heat from concrete blocks during construction. In this paper we develop an approximate model for this process. The problem reduces to solving a one-dimensional heat equation in the concrete, coupled with a first order differential equation for the water temperature. Numerical results are presented and the effect of varying model parameters shown. An analytical solution is also provided for a steady-state constant heat generation model. This helps highlight the dependence on certain parameters and can therefore provide an aid in the design of cooling systems

Maturity effects in concrete dams

Model equations for determining the coupled heat, moisture and maturity changes within a concrete block are introduced and briefly examined. Preliminary results are obtained for the heat exchange between concrete slabs in contact driven by maturity differences

Modelling the cooling of concrete by piped water

Large concrete structures are usually made sequentially in a series of blocks. After each block is poured it must be left to cool and shrink for a period depending on its size, but typically for around 1 week, before the next block is poured. The reason for the delay is that the mixture of cement and water, which constitute the binding agent of the concrete, results in a series of hydration reactions that generate heat.The chemical reaction can lead to temperature rises in excess of 50 K and it can take a number of years before the concrete cools to the ambient temperature. Prior to construction of the Hoover dam engineers at the U.S. Bureau of Reclamation estimated that if the dam were built in a single continuous pour the concrete would require 125 years to cool to the ambient temperature and that the resulting stresses would have caused the dam to crack and fail (U.S. Bureau of Reclamation 2005)

Piped water cooling of concrete dams

Piped water is used to remove hydration heat from concrete dams during construction. By examining simple models we obtain an estimate for the temperature rise along the pipe network and within the concrete. To leading order, for practically useful networks, the temperature distribution is quasi-steady, so that exact analytic solutions are obtained. The temperature in the water increases linearly with distance along the pipe and varies logarithmically with radial distance from the pipe in the concrete. Using these results we obtained estimates for the optimal spacing of pipes and pipe length. Some preliminary work on optimal network design has been done. This is work in progress