Abdelhafid Khelidj

Effect of axial compressive damage on gas permeability of ordinary and high-performance concrete

Abstract Gas permeability is commonly used to evaluate durability characteristics of concrete. However, these values are often achieved using never stressed or damaged specimens. The objective of this study is to examine experimentally the effect of axial compressive loading on the permeability of three different types of concrete: ordinary concrete (OC), high-performance concrete (HPC), and high-performance steel fiber-reinforced concrete (HPFC). Monotonic and cyclic loads are applied on 220×110-mm diameter specimens. Stress levels vary between 60% and 90% of the ultimate strength. At the end of the loading phase, a disc is extracted from the middle part of the cylinders and is dried in a ventilated oven. Four different gas permeability tests are conducted during the drying procedure. The results show that, for each drying stage, the gas permeability of the discs increases with the load-induced strain. A correlation is worked out between the increase in permeability and the applied-strain/yield-strain ratio. Finally, a relationship between mechanical damage indicators and the increase in permeability is also discussed.

Hydration kinetics, change of relative humidity, and autogenous shrinkage of ultra-high-strength concrete

This research forms part of a vast experimental program designed to investigate the shrinkage and creep of compact reinforced composite (CRC), which is an ultra-high-strength concrete. In this paper, only results relating to autogenous shrinkage are presented. While shrinkage deformations were measured, cement hydration, pozzolanic reactions, and pore structure of concrete also were monitored to characterize the microstructural evolution of material. Very high autogenous shrinkage at an early age that stops 10 days after casting was observed. As far as we know, this sudden break in shrinkage kinetics has never been observed before in cement-based materials. The autogenous relative humidity measurements show high self-desiccation within the CRC. A relationship between autogenous shrinkage and self-desiccation of CRC is established and discussed.

A new approach to determine autogenous shrinkage of mortar at an early age considering temperature history

Shrinkage, settlement, and thermal deformations of concrete start to develop as soon as the casting is finished. The majority of standard tests currently used to measure free deformations are carried out on hardened concrete. Therefore, the prediction of thermal and other stresses at an early age cannot be determined. This article describes the design and the operation of an experimental device used for measuring the volume change of mortar in relation to the real temperature history. Measurements are carried out on a mortar having a water/cement (W/C) ratio of 0.35 and started approximately 20 min after the addition of water in the mixture up to 24 h of hydration. The measured deformation includes autogenous shrinkage and thermal expansion or contraction caused by the temperature changes induced by the heat of cement hydration. An experimental method is proposed to uncouple these deformations. The investigations show that the autogenous shrinkage amplitude is strongly affected by the temperature history of mortar. Therefore, for a good estimate of the shrinkage amplitudes of cement-based material, temperature must be taken into account in testing.

Experimental study of cement grout: Rheological behavior and sedimentation

Three basic elements (cement, water and admixture) usually make up injectable cement grouts used for prestressed cable coating, repair and consolidation of masonry, soil grouting, etc... The present study was divided into two parts. First, in order to characterize rheologically fresh cement paste with W/C ratios (water/cement ratio) varying between 0.35 and 1, an experimental study was carried out and has revealed that the cement past behaves like a shear-thinning material whatever W/C. second, to study the time evolution of their density, a gammadensitometer bench was used. Relying on the water content and the density measured, we demonstrate that the computation of the degree of hydration of cement is possible. The cement/geotechnics interdisciplinary approach proposed here has made it possible to obtain a large range of original results useful to improve our understanding of the sedimentation processes for cement pastes with different W/C ratios.

Extrudable reactive powder concretes: hydration, shrinkage and transfer properties

The behaviour of five reactive powder concretes (RPC), with four of them being extrudable, has been studied from the first minutes up to more than 90 days. The extrusion force, hydration rate, free autogenous shrinkage at early age, mechanical properties and durability parameters were measured. The performance analysis showed the benefits of a partial substitution of silica fume with crushed quartz on the RPC properties. The cost/performances analysis highlighted that the main advantage of using RPC lies in their potential of durability, which is 10 to 100 times higher than that of ordinary or high-performance concretes, according to the property considered. Le comportement de cinq bétons de poudres réactives (BPR), dont quatre présentent des propriétés d’extrudabilité, a été étudié depuis les premières minutes après leur fabrication jusqu’à plus de 90 jours de maturation. La force d’extrusion, la cinétique d’hydratation, le développement du retrait endogène libre au jeune âge, les propriétés mécaniques et les paramètres de durabilité ont été mesurés. L’analyse des performances a montré l’intérêt d’une substitution partielle de la fumée de silice par du quartz broyé sur les propriétés des BPR. L’analyse coût/performances a mis en évidence que le principal avantage de l’utilisation des BPR réside dans leur potentiel de durabilité, 10 à 100 plus élevé que celui de bétons ordinaires ou à hautes performances, selon la propriété considérée.

Coupling between Progressive Damage, Temperature and Permeability of Concrete: Experimental and Numerical Study

A synthesis of work carried out for several years within our laboratory is presented. The first part includes an experimental study. The tests are performed on hollow cylindrical concrete specimens, subjected to compressive loading. At stress levels lower than 80 % of the peak stress, the variation of permeability is small and it is slightly influenced by the stress, but as the load exceeds 80 % of the peak stress, micro-cracking increases rapidly, causing an increase of the permeability and a greater sensitivity to the applied load. In the post-peak phase the increase of permeability is much larger due to significant crack width growth. The effects of the applied load on permeability are greater with temperature. Finally, the experimental results seem to agree with the format of coupled evolution of the permeability due to damage and temperature assumed by Gawin et al. [10]. The second part of this paper includes a numerical study. The lattice mechanics model is extended to the hydraulic problem and, for this case, it appears that permeability is the size independent variable. Additionally, the evolution of permeability with damage and with stress ratio in the pre-peak phase is compared with experimental results on different types of concrete.

Properties evolution of phosphate film inhibitor as function of immersion time

Inhibitory properties of three chemical products based on phosphate (Na3PO4, K2HPO4 and Na2PO3F) have been studied in synthetic concrete mediums contaminated by chlorides and then in reinforced concrete. Electrochemical measurements revealed that immersion of 7 d in 1 M K2HPO4 offered a maximum efficiency “75%”, while for Na3PO4 and Na2PO3F, 1 d of immersion in molar solution provides inhibitory efficiencies “70 and 67%”, respectively. Scanning electron microscopy tests performed on samples rinsed following the optimal duration, show that the film deposited on the surface varies for each tested product; so that the evolution of film properties is connected with electrical parameters which constituted the equivalent electrical circuit.

Novel anticorrosive zinc phosphate coating for corrosion prevention of reinforced concrete

Deterioration of reinforcing concrete structures is a common problem among all the coast countries. Every year, several billion dollars are spent to repair and maintain reinforced concrete structures. Over time, the metal reinforcing bars used to improve the properties of concrete become susceptible to corrosion due to factors such as the presence of chloride and carbonation. Present work includes the use of novel anticorrosive coating. The use of zinc phosphate (ZP) coating has advantage of the low solubility of phosphates in medium- or high-pH solutions; also the resulting coating remains adhered to the metal surface even under extreme deformation. This study presents the efficiency of a new inhibitive pigment ZP containing zirconium compound obtained by chemical conversion (CC) method and cathodic protection to protect the steel rebars against localised corrosion in concrete. The corrosion behaviour of coated steel was assessed by open circuit potential, potentiodynamic polarisation and electrochemical impedance spectroscopy. Firstly, results demonstrated that the new coating show an adsorption on steel surface and provides an effective corrosion resistance compared to carbon steel rebar. Secondly, results showed that the increased weight of coating made by CC is consistent with the development of the polarisation resistance and corrosion potential of samples studied. Thirdly, a reduction in the corrosion rate is obtained once the coating covers the surface of the metal.

Déformations endogènes de pâtes de ciment au très jeune âge: Analyse critique et développement métrologique

ABSTRACT The objective of this paper is to analyse and to compare the results obtained with three different experimental methods used for the measurement of autogenous strain of hydrating cement pastes. Two (horizontal and vertical) linear measurement systems and a rotary volumetric device, especially designed for the study of very early-age strain, have been developed. The tests have been performed on Portland cement pastes with three different water-to-cement ratios (W/C) equal to 0.25, 0.30 and 0.40. The effects of bleeding on both the linear and the volumetric results and of the immersion liquid absorption on the volumetric results have been quantified. The critical analysis of these results yields the conclusion that, among the three test methods used and in the range of W/C ratios investigated, the dynamic volumetric method is the most adapted for the measurement of very early-age autogenous strain even if some improvements have to be made in the choice of the immersion liquid.

Perméabilité au gaz de bétons endommagés Interprétation de résultats expérimentaux

ABSTRACT Gas permeability is commonly used to evaluate durability characteristics of concrete. However, these values are often achieved using never stressed nor damaged specimens. The objective of this study is to examine experimentally the effect of axial compressive loading on the permeability of three different types of concrete: Ordinary Concrete, High Performance Concrete, and High Performance Steel Fiber Reinforced Concrete. Monotonic and cyclic loads are applied on 22 x 11-cm diameter specimens. Stress levels vary between 60% and 90% of the ultimate strength. After unloading, a disc is extracted from the middle part of the cylinders and is dried in a ventilated oven. The effects of both drying and maximum strain applied during loading are described and analyzed.