International Concrete Abstracts Portal

Green self-consolidating concrete (SCC) is the aim of the construction industry nowadays. The accumulation of steel slag wastes causes severe environmental problems. These wastes can be recycled and replace natural aggregates, resulting in sustainable green SCC. In this research, natural aggregates in SCC are replaced, wholly or partly, by steel slag coarse aggregates (SSA) that were produced by crushing by-product boulders obtained from the steel industry. The fresh properties, (workability, stability, and bleeding), can all be attained when the suitable amount of SSA is used. SSA concrete increased the air content. Higher values are reported under hot conditions. The study shows that the 28-day compressive strength of SCC increased by approximately 10% when natural aggregate is replaced by SSA. However, adverse effects are reported when the ratio of SSA is more than 50%. Under hot weather, the strength was less and the optimum replacement ratio is 25%. The tensile strength of SCC increased by approximately 20% when natural aggregate is replaced by SSA. Adverse effects are reported when the ratio of SSA is more than 75%. Under hot weather, the same is observed but the value of the 28-day strength was lower. Special strength development mathematical relations are obtained and discussed. The modulus of elasticity increased by the increase in slag. The optimum value was at 50% for both conditions. An adverse effect is observed when the ratio of slag exceeds 75%. The drying shrinkage of concrete was lower for concrete containing SSA.

The authors have used different types of acacia gum powders in self-consolidating paste systems. The response of self-consolidating paste (SCP) systems modified with a natural organic admixture—acacia nilotica gum (AN) powder—in varying dosages at a constant water-binder ratio (w/b) of 27% is reported to address missing links and provides in-depth information to readers. Some comparative information with a similar, previously published work using a typical AN gum powder content of 0.66% at its water demand of 32.5% in SCP systems is also given to explain the role of mixing water content on various properties of self-consolidating cementitious systems (SCCs). Five SCP formulations at varying dosages of AN gum powder were prepared, with or without cement replacement by secondary raw materials (SRMs) including fly ash and limestone powder. These formulations were tested in both fresh and hardened states for parameters such as high-range water-reducing admixture (HRWRA) demand, flow characteristics, compressive strengths, total linear early shrinkage, thermal conductivity, washout mass loss, density, and air content of the SCP systems. The role of mixing-water content was found to be very significant in terms of fresh and hardened properties of SCPs. The results revealed retarding behavior of SCP systems containing AN gum powder, slight offset able reduction in compressive strength at maximum recommended dose of AN gum powder, reduced thermal conductivity, reduced washout mass loss, and increased viscosity. Self-consolidating cementitious systems (SCCs) incorporating the recommended AN gum powder content can be used in situations such as mass concrete works, hot weather concrete, freezing-and-thawing environments, and to create energy-efficient buildings with reduced shrinkage.

Concrete paving blocks are used for several applications, such as the paving of squares, parking garages, and cycling lines. They are usually made of a dual concrete mixture and the surface layer is colored through the addition of pigments. In time, the surface layer of the blocks may fade as a consequence of weathering due to the climate and other factors; this will result in the loss of aesthetic requirements. In this paper, several factors that could affect the color and microstructure of concrete paving blocks are examined in specimens with different coloration. Color and microstructural variations were compared with new blocks. Color analysis carried out by means of spectrophotometry and macro- and microstructural analysis showed that the major factors that lead to color variation are wear and environmental exposure.

In hot weather, it is a common practice to cool concrete - particularly for mass concrete applications - to prevent thermal cracking and durability problems. An increasingly popular method of cooling is through direct injection of liquid nitrogen (LN2) into the drums of concrete trucks. During an investigation of the effects of LN2 on concrete properties, it was observed that the practice has unexpected impacts on concrete slump and setting time. While it is known that increasing temperature decreases slump for concrete with given mixture proportions, it is surprising that concrete cooled with LN2 has low slump similar to that of a hot mixture, a phenomenon that is not affected by the time at which the concrete is cooled. The initial setting time, however, is actually longer than predicted when there is a significant delay in LN2 cooling.

The rheological properties of fresh concrete incorporating various chemical admixtures were investigated as a function of the mixing time, temperature, and admixture dosage. Three chemical admixtures were used, namely, polycarboxylate-, melamine sulfonate-, and naphthalene sulfonate-based high-range water-reducing admixtures. Concrete mixtures were continuously agitated up to 110 minutes after mixing using a low shear rate concrete mixer under controlled temperature ranging from 22 to 45 °C (72 to 113 °F). The rheological testing was performed using a BML rheometer. The test results indicate that the Bingham constants of concrete are significantly affected by changes in temperature, mixing time, and admixture dosage. The yield stress was found to have an inverse logarithmic curvilinear correlation with the slump, whereas no correlation was found between plastic viscosity and slump. The relationship between the yield stress and plastic viscosity was investigated in an attempt to develop a rational means for the objective assessment of the rheology of concrete mixtures in hot weather. The findings of this study are of practical significance and provide realistic recommendations on the use of chemical admixtures in hot weather conditions. Such admixtures have been generally developed in areas with mild climates and, therefore, current technical information on their use is often unreliable when transposed to extreme hot weather conditions.