ICE - Institution of Civil Engineers - Virtual Library

Engineers often have to deal with materials of ill-defined behaviour such as cement-based materials in order to perform special design tasks. There is usually great difficulty in predicting the engineering properties of such materials due to various factors, including their non-homogeneous nature, their composite behaviour with dissimilar ingredients and sometimes the dual and/or contradictory effects of some components on the overall performance. Until recently, the methods used to predict the engineering properties of cement-based materials have been based mainly on statistical and mathematical models, which in turn are derived from human observation, empirical relationships and assumptions with limited ability to account for the effects of and interaction between all variables involved. An alternative approach, termed artificial neural networks (ANNs), has recently emerged in different engineering fields as a popular tool to predict the behaviour of materials. Due to the relatively recent adoption of ANNs for modelling the behaviour of cement-based materials, a good understanding of its fundamental basis and a critical assessment of its performance are essential. This paper examines the most widely used ANNs in materials modelling (the feed-forward, back-propagation (FFBP) neural networks). Guidelines for building, training, and validating such networks are provided. A critical assessment is presented of the effects of various parameters on the training and performance of FFBP networks and their use as an alternative approach to traditional modelling methods is evaluated through a case study. Recommendations are made to optimise the performance of ANNs.

Synchrotron radiation is used for X-ray computed tomography to characterise the hydration process of Portland cement. Quantitative determination of the three-dimensional absorption coefficient distribution due to the use of monochromatic radiation and the achieved high spatial resolution allows the study of the spatial microstructure of cement volume during hydration. The present study has demonstrated the potential of the method for real-time imaging of the evolution of cement structure after solidification. The method yields three-dimensional (3-D) images of the sample volume in a non-destructive manner. Thus, by reiterated imaging of the same sample region at different times, processes in the sample volume can be tracked. During cement hydration, effects such as the formation and further evolution of microcracks and pores – mainly related to autogeneous shrinkage – could be observed for the first time in the cement volume with up to 1 μm resolution. Image analysis of the acquired 3-D data sets allows the determination of the temporal evolution of microporosity and of the fraction of contiguous pore volume for different water : cement ratios of the initial cement paste.

A quantitative understanding of the efficiency of fly ash as a mineral admixture in cement-based materials is essential for its effective utilisation. The present paper is directed towards a specific understanding of the efficiency of fly ash in cementitious materials systems by considering the independency of fly ash and its dependency on the characteristics of the cementitious materials system. A new method of quantitatively evaluating the strength effect of fly ash is proposed, in which two parameters, the strength-effect index and the strengthening factor, are employed to study strength development in fly ash mortars and concretes and to further analyse the influences of water to binder ratio (w/b) and the replacement amount of fly ash on strength. Results indicate that the strength effect of fly ash in mortar systems is different from that in concrete; the strength effect of fly ash varies with both amount of fly ash and w/b. Furthermore, in a concrete system, two different optimal w/b ratios are used to maximise the strength-effect index and the strengthening factor of fly ash, respectively. An optimum amount of fly ash exists for an optimal unit strength-effect index in concrete. It is shown that the method presented in this paper is reasonable and effective in assessing the efficiency of fly ash. This information will strengthen the effective utilisation of fly ash in cementitious materials and design of fly ash concrete.

The advisability of controlling the temperature rise and fall in concrete at early age is well recognised, and the choice of an appropriate, low-heat cement with suitable heat of hydration characteristics can assist in this control. This is particularly pertinent with respect to water-retaining and massive concrete structures where the need to prevent early-age thermal cracking is paramount. Portland cement/ground granulated blast furnace slag (PC/ggbs) or PC/fly ash cements are often used in these structures due to their low heat hydration properties. This paper presents the results of isothermal conduction calorimetry tests carried out on PC/ggbs and PC/fly ash cements and describes a model that uses these results to simulate the heat evolution processes in hydration concrete sections at early ages. The tests covered a range up to 90% ggbs and up to 65% fly ash content by mass of cement, at temperatures from 5° to 60°C. For PC/ggbs cements, the total heat of hydration can be considered as a composition of three components, that is heats from an initial Portland cement reaction, a latent ggbs hydraulic reaction and co-reactivity effects of PC and ggbs; whereas for PC/fly ash cements, the initial PC reaction dominated with a small co-reactivity effect.