Ammar Yahia

Self-consolidating concrete

As self-consolidating concretes (SCCs) are easily pumpable and require little workmanship to be poured, their use is gaining acceptance in casting columns, walls and floors for high-rise buildings. SCCs are also currently used in pre-cast plants because they facilitate and accelerate the pouring of concrete, prolong the life of the moulds and decrease the noise level in the plant. This noiseless aspect is a great advantage when casting concrete in buildings during working hours or in cities where there are noise restrictions after 5 pm. The formulation of SCC is flexible enough to accommodate the addition of fine materials or viscosity-modifying admixtures (VMAs) that are used to adjust SCC viscosity. The overall cost balance of using SCCs must be considered in a global sense. Using SCCs requires tighter quality control, but knowledge and practical experience are now available to handle this properly.

Correlating Rheology of Self-Consolidating Concrete to Corresponding Concrete-Equivalent Mortar

The investigation reported in this paper seeks to develop a methodology to evaluate the rheological parameters and thixotropy of self-consolidating concrete (SCC) using those of concrete-equivalent mortar (CEM). The mixture proportioning of CEMs are derived from their corresponding concrete mixtures by eliminating the coarse aggregate fraction and replacing it by a certain mass of sand of the same surface area. SCC mixtures with water-cementitious material ratios (w/cm) of 0.35, 0.38, and 0.42 and coarse aggregate-total aggregate volume ratios (CA/A) of 0.44 to 0.53 were investigated. The mixtures with a w/cm of 0.38 and 0.42 incorporated low and moderate dosages, respectively, of a viscosity-modifying admixture to enhance stability. For each SCC, the dosage of high-range water-reducer (HRWR) was varied to cover a wide range of slump flow consistencies ranging between 570 and 730 mm (22.4 and 28.7 in.). All SCC mixtures were proportioned with 450 kg/m 3 (758.5 lb/yd 3 ) of ternary silica fume-ground granulated blast-furnace slag cement. A simple method is also proposed to determine the HRWR demand of SCC from that of the corresponding CEM. Test results showed that a good correlation can be established between the yield stress, plastic viscosity, and thixotropy of SCC and their corresponding CEM mixtures. Both thixotropy and plastic viscosity of SCC mixtures and their corresponding CEM mixtures are shown to vary primarily with variations in the w/cm and relative volume of coarse aggregate.

New Test Method to Evaluate Dynamic Stability of Self-consolidating Concrete

Stability is one of the crucial properties for self-consolidating concrete (SCC). Dynamic stability, which describes the stability during flow of concrete, remains, to date, one of the least investigated aspects of SCC workability. A novel dynamic stability test, tilting box, is proposed in this paper. The test involves the tilting of a rectangular box numerous times to simulate concrete flow and evaluating the homogeneity of the concrete with increasing flow distance. Results show that the test has an adequate repeatability and sensitivity to variations in dynamic stability stemming from changes in mixture design and flow conditions. Dynamic segregation is shown to increase as the flow distance increases or the flow velocity decreases. Workability characteristics are shown to have significant effect on dynamic segregation.

Evaluation of cement grouts for embedding anchors under water

Approximately 110 pull-out tests were conducted on grouted anchors cast in the laboratory to investigate the effects of the rheological and mechanical properties of cement grouts, the initial free drop distance of grout, as well as the age of testing on the behavior of anchorages cast in dry and submerged conditions. The anchors consisted of 19-mm, conventional steel bars with an embedment length fixed at five times the bar diameter. Two different casting procedures corresponding to initial free drop of the grout in water of 20 and 200 mm were investigated. The cement grouts used in this investigation incorporated either a Type 10 or a blended silica fume cement and a fixed water-binder ratio of 0.40. The grouts incorporated various concentrations of a high-range water reducer, a rheology-modifying admixture, and silica fume. The rheology-modifying admixture and high-range water reducer were jointly incorporated to enhance both fluidity and washout resistance.The test results indicate that properly designed grouts can be easily cast into place, and yet be cohesive enough to resist the washout of cementitious materials. The spread in load-carrying capacity between anchor bars cast in dry versus submerged conditions can decrease when cement grouts having a greater washout resistance level are used which can be secured by incorporating a rheology-modifying admixture and silica fume. The bond strength is shown to increase when incorporating silica fume regardless of the casting condition.RésuméDes essais d’arrachement ont été réalisés en laboratoire afin d’étudier l’effet des propriétés rhéologiques et mécaniques des coulis de scellement, de la longueur de scellement, de la procédure de mise en place ainsi que de la période de mûrissement sur le comportement des ancrages installés sous l’eau et en dehors de l’eau. Des barres d’armature de 19 mm de diamètre sont utilisées comme tirants d’ancrage. La longueur de scellement est fixée à une longueur égale à cinq fois le diamètre du tirant. Les coulis de scellement ont été mis en place en utilisant deux hauteurs de chute du coulis dans l’eau de 20 et de 200 mm.Les coulis de scellement ont été formulés en utilisant un ciment Type 10 et un ciment contenant de la fumée de silice et un rapport eau/liant de 0,40. Les coulis utilisés contiennent différents ajouts. En particulier, le superplastifiant et l’agent colloïdal sont utilisés pour améliorer la fluidité et la résistance au lessivage des coulis.Cette étude a montré en particulier que l’utilisation des coulis de ciment ayant une bonne résistance au lessivage permet d’obtenir une bonne résistance à l’arrachement des ancrages mis en place sous l’eau. Une bonne résistance au lessivage est assurée par l’utilisation de l’agent colloïdal et de la fumée de silice.

New insight into the equivalent concrete mortar approach for self-consolidating concrete

The concrete equivalent mortar (CEM) approach consists in replacing the coarse aggregates (CA) of concrete by a mass of sand to provide an equivalent specific surface area at mortar scale. Such approach can be used to reduce time and cost needed to design self-consolidating concrete (SCC). The approach is extended with less success to evaluate rheological properties of SCC using those of the corresponding CEM. The limitation of the approach is mainly due to the fact that the surface area of aggregates is the only used criterion. The objective of this investigation is to evaluate the applicability of the original CEM approach and establish correlations between rheological properties of CEM and SCC. Various SCC mixtures were proportioned with different CAs and binder contents varying from 27 to 36% and 300 to 500 kg/m3, respectively. The obtained test results show that the excess paste theory is a key factor in studying relationship between CEM and SCC. Good correlations between flow properties of CEM and SCC are obtained when the excess paste thickness and CA content are taken into account.

Efficiency of waste tire rubber aggregate on the rheological properties and compressive strength of cementitious materials

A serial of experiments were carried out to investigate the efficiency of waste tire rubber on consistency and rheological behavior of cementitious materials at fresh state and on the mechanical strength at hardened state was evaluated. Results indicate that the addition of rubber aggregates significantly influences yield stress of the sample in the fresh state and the compressive strength at 28 days, depending on the grain size distribution of the rubber aggregate and its volume percentage in the mixture. The yield stress of the sample in fresh state is more influenced by an increasing volume of rubber compared to adding sand with equal grain size distribution. The percentage of reduction of compressive strength of cementitious materials caused by increasing the volume of rubber aggregates by 1% is about 4.5%.

Unified Formulation for a Triaxial Elastoplastic Constitutive Law for Concrete

A constitutive model to describe the triaxial load-response spectrum of plain concrete in both tension and shear was developed. The inelastic phenomena are described using the plastic flow with direction determined by the gradient of the plastic potential. A new plastic potential is introduced and experimentally fitted to ensure better estimate of the load direction. This approach allows to control the inelastic dilatancy in terms of the inelastic deformation of the material. By overlaying the plastic potential on modified Etse and Willam’s yield surface (both defined on the Haigh–Westergaard coordinates), the results showed that the two curves do not undergo similar stress states for a given strength level. It is, therefore, necessary that each surface goes through the current stress state to ensure adequate evaluation of normal vectors. A closed-form solution to accurately predict the triaxial stress state in concrete has been proposed. The predictive capabilities of the proposed model are evaluated by comparing predicted and measured stresses. The proposed model is shown to be accurate in predicting stress state of concrete.

Assessing the individual and combined effects of uncertainty and variability sources in comparative LCA of pavements

PurposeSeveral efforts have attempted to incorporate the sources of uncertainty and variability into the life cycle assessment (LCA) of pavements. However, no method has been proposed that can simultaneously consider data quality, methodological choices, and variability in inputs and outputs without the need for complementary software. This study aims to develop and implement a flexible method that can be used in the LCA software to assess the effects of these sources on the conclusions.MethodsA Monte Carlo analysis was conducted and applied in a comparative LCA of pavements to assess the preferred scenario. The uncertainty of the results was first estimated by considering the data quality using the ecoinvent database. Moreover, the variabilities of the materials, construction methods, and repair stages of the pavement life cycle were included in the analysis by assigning continuous uniform probability distributions to each variable. Ultimately, the probability of methodological choices was modeled using uniform distributions and assigning a portion of the area of the distribution to each scenario. The individual and combined effects of these uncertainty and variability sources were assessed in a comparative LCA of asphalt and concrete pavements in a cold region such as Quebec (Canada).Results and discussionThe results of the Monte Carlo analysis show that the allocation choices can change the environmentally preferred scenario in four midpoint categories. These categories are significantly dominated by the crude oil supply chain. The variability in construction materials and methods can change the preferred scenario in the damage categories, namely, human health and global warming. Additionally, parameter uncertainty has a significant effect on the conclusion of the preferred scenario in ecosystem quality. The worst qualitative scores are given to the geographical uncertainty of the elementary flow that primarily contributes to this category (i.e., zinc). The simultaneous effect of the uncertainty and variability sources prevents the decision-maker from reaching a less uncertain conclusion about ecosystem quality, human health, and global warming effects.ConclusionsThis study demonstrates that it is feasible to assess the cumulative effects of common uncertainty and variability sources using commercial LCA software, including Monte Carlo simulation. Based on the variability and uncertainty of the results, the identification of a certain conclusion is case specific at both the midpoint and endpoint levels. Increasing the quality of the inventory is one solution to decreasing the uncertainties related to human health, ecosystem quality, and global warming regarding pavement LCA. This improvement can be achieved by avoiding the adaptation of a similar process to match the considered process and using practical construction efficiencies and realistic construction materials. The effectiveness of these tasks must be assessed in future studies. It should be noted that these conclusions were determined regardless of the uncertainty in the characterization factors of the impact assessment method.

Modelling the properties of pervious concrete using a full-factorial design

Although the greatest concern regarding pervious concrete (PC) is permeability, the strength performance should not be neglected. The trade-off between permeability and mechanical properties is however a difficult task and often requires several trials to establish optimum mix design. The objective of this study is to model the effect of main mixture parameters on permeability, effective porosity, unit weight, and compressive as well as tensile strength of PC using a full-factorial design. The modelled factors are paste volume-to-inter-particles void ratio (PV/IPV), water-to-cement ratio (W/C), and dosage of water reducing agent (WRA). The modelled domain is limited by values of PV/IPV ranging between 33% and 47%, by volume, W/C between 0.27 and 0.33, and WRA dosages between 30 and 190 mL/m3 of concrete. The modelled responses include hardened unit weight (HUW), effective porosity, permeability, compressive, and tensile strength characteristics. Test results showed that the PV/IPV parameter shows the most significant influence on permeability and strength development.

Effectiveness of rotational shear in dispersing concentrated cement suspensions

Buildup of cement suspensions has a great impact on their structural behavior. Cement suspensions differ considerably in their buildup kinetics because of the various mixture and testing parameters. Accurate assessment of buildup of cement suspensions relies on their dispersion state before measurements. The effectiveness of various rotational shear rates to disperse concentrated cement suspension proportioned with a water-to-cement ratio (w/c) of 0.35 is evaluated. The storage modulus and the shear stress were measured before and after applying disruptive shear rate using small-amplitude oscillatory shear. Test results indicated that the highest dispersed state can be attained using a rotational shear rate corresponding to the transition shear rate from shear thinning to shear thickening. Beyond this value, shear-thickening effects dominate and flocculation can take place, resulting in less dispersed state.