The construction of reinforced concrete structures in hot weather dictates fast placement of fresh concrete to prevent hot weather concreting difficulties such as rapid loss of workability, pumping problems, acceleration of cement hydration, fast evaporation of mixing water, and forming of cold joints. In this process, the selection of adequate chemical admixtures is of paramount importance. In the current study, the viscoelastic properties of portland cement pastes with a water–cement ratio (w∕c) of 0.35 and 0.50 were investigated at different temperatures in the range of 20–45°C through oscillatory rheological tests conducted using an advanced shear-stress/shear-strain controlled rheometer. The influence of water reducing and retarding admixtures, melamine-based, polycarboxylate-based, and a new generation of polycarboxylate-based high-range water reducing admixtures on the oscillatory rheological properties of cement paste at various temperatures was also examined. The results indicate that the viscoel...

Placement of cement-based materials in hot weather conditions encounters many problems due to the rapid evaporation of mixing water and the accelerated hydration of cement. Such materials are also adversely affected at elevated temperatures by an increased water demand, shorter setting time, and more rapid slump loss. These problems can be mitigated using adequate chemical admixtures. However, technical data on such admixtures are commonly developed at normal temperatures, and there is a real need to investigate their performance at hot temperatures. In the current research, the rheological properties of portland cement pastes with a water-cement ratio (w/c) of 0.35 and 0.50 are investigated at different temperatures in the range of 20 to 45°C using an advanced shear-stress/shear-strain controlled rheometer. The influence of water-reducing and retarding admixture, polycarboxylate, naphthalene sulfonate, and melamine-based high-range water-reducing admixtures (HRWR) in addition to mid-range water-reducing admixtures (MRWR) on the rheological properties of cement paste at various temperatures was also examined. The paper aims at formulating recommendations for the effective use of chemical admixtures at high temperature, which should enhance the rheological properties of cement-based materials in hot weather conditions.

The time-dependent rheological behavior at high temperature of cement paste incorporating various chemical admixtures is critical for hot-weather concreting applications. This paper investigates the combined effects of temperature and time on the performance of polycarboxylate-, melamine sulfonate- and naphthalene sulfonate-based high-range water-reducing admixtures used to enhance the flow of cement paste. Rheological parameters, including yield stress, plastic viscosity, and thixotropy of cement pastes with a water/binder ratio of 0.38, were measured as a function of the superplasticizer dosage and temperature (22–45°C) from 20 to 110 min from the time of mixing with 30 min between successive measurements. The rheological tests were conducted using an advanced shear-stress/shear-strain controlled rheometer. This paper aims at developing a better understanding of the coupled time and temperature effects on the performance of various superplasticizers, and formulating more realistic recommendations for us...

The evaluation of thermal properties is commonly conducted to characterize non-structural materials, such as lightweight concrete, that are used for thermal insulation. Such materials are designed for thermal resistivity applications. Due to the increased demand to adopt sustainable practices in the construction industry, municipalities in the United Arab Emirates (UAE) emphasize the use of sustainable materials in construction, such as green concrete. The cement in green concrete is partially replaced with supplementary cementitious materials (SCMs); these materials are by-product waste from other industries. The SCMs can contribute to sustainability by reducing the concrete carbon footprint. They can also help in extending concrete durability and service life. However, there is still a lack in the literature regarding the effects of these materials on the thermal properties of concrete. This paper investigates the thermal properties of sustainable concrete mixes incorporating various types of SCMs. The SCMs that are considered in this investigation are fly ash, ground granulated blast-furnace slag (GGBS), and microsilica. Another way to improve the sustainability of the concrete is to partially replace the natural aggregates with recycled aggregates. Thus, a group of the concrete mixes in this investigation were prepared by replacing 40% of natural aggregates with recycled aggregates to investigate the effects of recycled aggregate on the thermal properties of concrete. Further, the thermal properties of three lightweight concrete mixtures commonly used in construction were evaluated. All concrete mixtures were examined for thermal conductivity and resistivity in accordance with ASTM D5334. The results of this investigation showed that SCMs and recycled aggregates have a significant impact on the thermal properties of concrete. The high replacement of ground granulated blast-furnace slag (GGBS) resulted in a remarkable increase in thermal conductivity. This investigation provides significant conclusions and recommendations that are of practical importance to the construction industry in the UAE to promote sustainability. This research aims at formulating recommendations for the effective use of SCMs in the construction industry in the UAE based on their effects on the thermal properties of concrete.

Complex permittivity measurements combined with artificial neural networks (ANNs) are investigated as a method for assessing and identifying heavy metal contamination in soil. The measurements are carried out with a custom-built device on 164 compacted samples of a natural clayey soil, artificially contaminated with various simple salts including heavy metals (Cu, Zn, and Pb). The soil samples are prepared by mixing solutions of the various salts with the soil at various concentrations and water contents. A database has been set up consisting of complex per mittivity measurements made between the frequencies of 200 and 500 MHz and measured physical and chemical properties of the soil samples. Using this database as input, two ANN models are designed, the first to detect the presence or absence of heavy metals in the soil samples and the second to determine whether the heavy metal, if present in a given sample, is Cu, Zn, or Pb. Both ANN models perform reasonably well. Overall, the first model is able to detect the presence of heavy metals in 92.7% of cases, and the second is successful in distinguishing the particular type of heavy metal in 76.4% of all the samples containing heavy metals. These encouraging results underscore the potential of complex permittivity and ANNs as promising tools for nondestructive subsurface contamination assessment.Key words: heavy metals, subsurface contamination, complex permittivity, artificial neural networks, contaminant detection.