Abdelhak Kaci

The Compressive Strength of High-Performance Concrete and Ultrahigh-Performance

The compressive strength of silica fume concretes was investigated at low water-cementitious materials ratios with a naphthalene sulphonate superplasticizer. The results show that partial cement replacement up to 20% produce, higher compressive strengths than control concretes, nevertheless the strength gain is less than 15%. In this paper we propose a model to evaluate the compressive strength of silica fume concrete at any time. The model is related to the water-cementitious materials and silica-cement ratios. Taking into account the authors and other researchers’ experimental data, the accuracy of the proposed model is better than 5%.

Influence of recycled sand and gravel on the rheological and mechanical characteristic of concrete

Abstract The objective of this paper is to investigate the influence of type and dosage of recycled sand (RS) and gravel on the fresh and mechanical properties of concrete. Experimental program was conducted on concretes made with different ratio of substitutions (15, 30, 70, and 100%) of natural sand and gravel with RS and gravel. At the fresh state, slump, air content, and density were measured at the exit of the mixer, and then at 30, 60, and 90 min after mixing. Tests were also performed for compressive strength at the age of 1, 7, and 28 days, whereas elastic modulus measurements were done at 28 days. The results indicated that maintaining the workability of recycled aggregate concrete depends on sand or gravel substitution and their rates. Up to 30 min, slump values were decreased, but after that, no substantial change in slump values was observed. Air content increased and density decreased, with increasing recycled aggregate content (sand or gravel). Mechanical properties, such as compressive strength and modulus of elasticity, were lower than those of reference concrete.

Estimation of the Pumping Pressure from Concrete Composition Based on the Identified Tribological Parameters

A new method is proposed to estimate pumping pressure based on concrete composition without experimental measurements. Previous studies show that the pumping pressure depends on the interface friction between concrete and the wall of the pumping pipes. This friction is determined by the thickness and the rheology of the boundary layer formed at the interface. The latter is mainly formed by water, cement, and fine sand particles which come from concrete. Hence, interface parameters, which are the viscous constant and the interface yield stress, are directly related to concrete composition. In this work, at the first time the interface yield stress model is suggested and validated thanks to an experimental database also carried out in this study with a precision of around 13%. Then, the pressure estimation method is proposed using the two models to calculate the interface parameters. The validation of the method is carried out basing on the comparison with real measurements on the building site. This method enables the calculation of the pumping pressure with a precision of around 15%.

Application of Empirical Models to Optimizing Concrete Pumpabiltity

Pumbability of freshly-mixed concrete is a great challenge in the construction field; this attracts a growing number of researchers willing to study. This study pays attention on the use of empirical models that are established from the experimental data to evaluate the concrete pumpability. The purpose of the study is to reduce the amount of cost and time involved in the project construction. The models permit to directly estimate interface parameters (i.e., the viscous constant (η) and the interface yield stress (τ0t)) from the concrete formulation. These parameters directly affect the interface friction (τ) between the concrete and the wall of the pumping pipe, and thus the concrete-mix mobility (i.e., the pumping flow rate) can be improved by reducing the interface friction. The calculated interface parameters are then validated with measurement parameters using a tribometer. In this study, a parametric approach is also employed by varying formulation parameters such as water/cement ratio, cement paste volume, and aggregates/sand ratio of the concrete-mix. The obtained results demonstrates that the empirical model is reliable with a high precision. They permit to validate a theoretical approach to estimate and to optimize the pumping parameters and the concrete pumpability.