Mohamed Lachemi

Self-compacting concrete incorporating high volumes of class F fly ash: Preliminary results

ABSTRACT In recent years, self-compacting concrete (SCC) has gained wide use for placementin congested reinforced concrete structures with difficult casting conditions. Forsuch applications, the fresh concrete must possess high fluidity and goodcohesiveness. The use of fine materials such as fly ash can ensure the requiredconcrete properties. The initial results of an experimental program aimed atproducing and evaluating SCC made with high-volumes of fly ash are presented anddiscussed. Nine SCC mixtures and one control concrete were investigated in thisstudy. The content of the cementitious materials was maintained constant (400kg/m 3 ), while the water/ cementitious material ratios ranged from 0.35 to 0.45. Theself-compacting mixtures had a cement re placement of 40, 50, and 60% by Class Ffly ash. Tests were carried out on all mixtures to obtain the properties of freshconcrete in terms of viscosity and stability . The mechanical properties of hardenedconcretes such as compressive strength a nd drying shrinkage were also determined.The self-compacting concretes develope d a 28-day compressive strengths rangingfrom 26 to 48 MPa. The results show th at an economical self-compacting concretecould be successfully developed by incor porating high-volumes of Class F fly ash.

PERFORMANCE OF NEW VISCOSITY MODIFYING ADMIXTURES IN ENHANCING THE RHEOLOGICAL PROPERTIES OF CEMENT PASTE

Abstract The use of viscosity modifying admixtures (VMA) has proved to be very effective in stabilizing the rheological properties and consistency of self-compacting concrete (SCC). SCC is known for its excellent deformability, high resistance to segregation and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Most of the commercial VMAs currently available in the market are costly and increase the price of such a concrete. Identification or production of new low-cost VMA is then essential. This paper presents the performance of four new polysaccharide-based VMAs in enhancing the rheological and consistency properties of cement paste. The study of the rheological properties and consistency of cement paste to screen the dosage and type of new VMA to be used in SCC is a promising approach. Investigation was carried out on cement pastes with combinations of various dosages of new VMAs and of a superplasticizer (SP) to study the influence on rheology, consistency and washout mass loss. A commercial VMA designated in this paper as “COM” was tested for comparison. The study on new VMAs is encouraging and confirms that pastes with satisfactory rheological and consistency properties comparable with or even better than commercial VMA can be developed. The combined use of proper dosages of VMA and SP is shown to clearly contribute to securing high-performance cement pastes that is highly fluid yet cohesive enough to reduce water dilution and enhance water retention. Attempt has also been made to correlate rheological properties (yield stress) to consistency (slump) of pastes.

Mechanical Properties and Durability of Two Industrial Reactive Powder Concretes

Two reactive powder concretes (RPC) were produced on an industrial scale at the Universite de Sherbrooke and in a nearby precast plant. A 2.6 cubic meter mix was prepared in the central mixer of the precast plant. The ready mix RPC was sampled before and after the addition of steel fibers while the one produced at the precast plant was sampled only at the end of the mixing process. These RPCs were tested for compressive strength, modulus of elasticity, freezing and thawing cycling resistance, scaling resistance to deicing salts, and resistance to chloride ion penetration. The results show that a 200 MPa compressive strength could be achieved in both cases: after curing in hot water at 90 degrees C or in the low pressure steam chambers at the precast plant. Confinement of the RPC in a steel tube greatly increases its compressive strength and its ductility.

DEVELOPMENT OF COST-EFFECTIVE SELF-CONSOLIDATING CONCRETE INCORPORATING FLY ASH, SLAG CEMENT, OR VISCOSITY-MODIFYING ADMIXTURES

The concept of self-consolidating concrete (SCC) as a building material has gained worldwide acceptance in recent years. The development of a cost-effective SCC with desirable fresh and hardened properties is important for such concrete to be used in structural construction in the future. SCC can be manufactured by incorporating either supplementary cementitious materials such as fly ash and slag cement, or viscosity-modifying agents (VMAs). Commercially available VMAs are expensive, making it important to explore new cost-effective VMAs to manufacture more economical SCC. This paper compares the performance of SCCs incorporating fly ash, slag cement, and VMAs based on fresh and mechanical properties. The feasibility of manufacturing such an SCC is assessed based on cost analysis. Results indicate that an economical SCC with the desired properties could be successfully developed by the incorporation of fly ash, slag cement, or VMA.

Influence of Aggregate Type and Size on Ductility and Mechanical Properties of Engineered Cementitious Composites

The influence of aggregate type and size on engineered cementitious composite (ECC) mechanical and ductility properties was investigated and the results presented in this paper. A micromechanically-based high-performance fiber-reinforced cementitious composite, ECC enjoys improved durability and high ductility due to tight crack width. Microsilica sand (200 μm [0.008 in.] maximum aggregate size) is typically used to produce standard ECC mixtures. There was investigation of ECC mixtures containing either gravel sand or crushed dolomitic limestone sand with maximum sizes of 2.38 or 1.19 mm (0.094 or 0.047 in.) in this study. Three different ECC mixtures with 1.2, 2.2, and 4.2 fly ash/portland cement (FA/C) ratios were cast for each aggregate type and maximum aggregate size. There was experimental determination of crack development, drying shrinkage behavior, and the effects of FA/C, aggregate type, and maximum aggregate size on compressive, flexure, and unixial tensile properties. Experimental results show that strain-hardening behavior with strain capacities, provided that the matrix employs a high FA content, can be compared with the standard microsilica sand ECC mixtures, in ECC mixtures produced with gravel sand and crushed dolomitic limestone sand with higher maximum aggregate sizes. Tensile strengths of these mixtures can be 3.57 to 5.13 MPAa (0.52 to 0.74 ksi), and tensile ductility can maintain, at 28 days of age, 1.96 to 3.23%. Material behavior can be further improved by using crushed dolomitic limestone sand and gravel sand, since they can be drying-shrinkage arrestors in the paste.

DEVELOPMENT OF STATISTICAL MODELS FOR MIXTURE DESIGN OF HIGH-VOLUME FLY ASH SELF-CONSOLIDATING CONCRETE

Self-consolidating concrete (SCC) in the fresh state is known for its excellent deformability, high resistance to segregation, and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Such concrete can be obtained by incorporating either mineral admixtures such as fly ash (FA) or viscosity-modifying admixtures (VMA). The use of VMA has proven very effective in stabilizing rheology of SCC, and recent research has focused on development of new, cheaper VMAs compared with currently available, costly commercial ones. Research to produce an economical SCC with desired properties has been conducted in recent years into the use of FA. In this paper, 21 statistically balanced concrete mixtures were investigated to minimize the use of high-range water-reducing admixtures (HRWRA) and to optimize the use of fly ash in SCC. The minimum use of HRWRA and optimum use of FA were desired in this study. Four independent variables were used for design of SCC mixtures. The fresh concrete properties were determined from slump flow, V-funnel flow, filling capacity, bleeding, air content, and segregation tests. The mechanical properties and durability characteristics of SCC such as compressive strength, freezing/thawing resistance, rapid chloride permeability, surface scaling resistance, and drying shrinkage were determined to evaluate the performance of SCC. Four statistical models to predict the slump flow, 1- and 28-day compressive strength, and the rapid chloride permeability of SCC were developed and their performances were validated.

Mixture Design, Strength, Durability, and Fire Resistance of Lightweight Pumice Concrete

The authors discuss lightweight volcanic pumice concrete (VPC) development in which 21 concrete mixtures are illustrated through use of pumice as aggregate. The authors describe VPC mixture fresh, mechanical, and durability properties, including water permeability, drying shrinkage, modulus of elasticity, density, tensile strength, compressive strength, air content, and slump. The authors also describe the fire resistance of VPC subjected, for different durations (with a 2 hour maximum), to elevated temperatures (up to 800° C [1472° F]). The basis of performance judgment is VPC physical changes and residual strength compared with normal density concrete (NC). The investigation suggests for structural applications, VPC production having both durability characteristics and satisfactory strength. With the increase of fire and temperature duration, both NC and VPC strength is found to decrease. Compared with NC after elevated temperature exposure for different durations, VPC shows better strength retaining capacity and residual strength.

DETERMINATION OF ELASTIC PROPERTIES OF HIGH - PERFORMANCE CONCRETE AT EARLY AGES

Knowledge of the elastic properties of concrete at early ages is very useful for designers, especially from a serviceability point of view. This paper discusses the determination at early ages of the elastic properties of three different concretes with water-cementitious materials ratios of 0.45, 0.35, and 0.30. Static and dynamic tests on the modulus of elasticity were carried out to monitor the evolution of this important parameter from 8 h of age and thereafter. A correlation factor between the static and dynamic moduli of concretes with a water/binder ratio between 0.30 and 0.45 is proposed. The relationship predicting the static modulus of elasticity of concrete was found to be proportional to the square root of the concrete compressive strength. The Poissons ratio was found to decrease sharply in very little time to reach a 0.14 value at a very young age, and then increased to approximately 0.24 after 7 days.

Assessing Mechanical Properties and Microstructure of Fire-Damaged Engineered Cementitious Composites

In recent years, a number of investigations of engineered cementitious composites (ECC) have been carried out, and the mechanical behavior and durability characteristics of this type of composite are now increasingly better understood. The fire-resistant behavior of this specialized concrete, however, has not yet been studied as extensively. This investigation develops important data on the mechanical properties and microstructure of ECC exposed to temperatures up to 800°C (1472°F). In this study, the mechanical properties (residual compressive strength, stress-strain curve, and stiffness) and mass loss were determined after air cooling, subsequent to temperature exposure. Changes in the microstructure, porosity, and pore size distribution of the fire-deteriorated ECC specimens were identified using scanning electron microscopy and mercury intrusion porosimetry techniques. Test results revealed no significant changes in the mechanical properties for tested specimens exposed to temperatures up to 400°C (752°F) for 1 hour. Microstructural analysis showed the creation of supplementary pores and channels in the matrix due to polyvinyl alcohol (PVA) fibers melting in the 200-400°C (392-752°F) temperature range. After a 1-hour exposure to temperatures of 600 and 800°C (1112 and 1472°F), the mechanical performance of fire-deteriorated ECC mixture is similar to or better than that of conventional concrete incorporating polypropylene or steel fibers, despite a significant reduction in compressive strength and stiffness. Moreover, no explosive spalling occurred in any specimens during the fire test. The promising performance of ECC under fire exposure may be due to the presence of PVA fibers and high-volume fly ash. The beneficial influence of fly ash can be ascribed to the pozzolanic reaction consuming calcium hydroxide in the hydrates. PVA fiber is also beneficial in that it prevents explosive spalling. This introduces additional channels for vaporized moisture in ECC to escape without creating high internal pressure in the material.

Effect of Fly Ash and PVA Fiber on Microstructural Damage and Residual Properties of Engineered Cementitious Composites Exposed to High Temperatures

This paper discusses the influence of high volumes of fly ash and micro polyvinyl alcohol (PVA) fibers on the fire resistance and microstructure of engineered cementitious composites (ECC). Composites containing two different contents of fly ash as a replacement for cement (55 and 70% by weight of total cementitious materials) are examined. To determine the effects of microfibers and ultrahigh ductility of ECC, ECC matrix mixtures of similar composition except PVA fiber are also produced and tested for the fire resistance. The mixtures are exposed to temperatures up to 800°C for one hour. Fire resistances of the mixtures are then quantified in terms of the residual mechanical properties (strength, stress-strain curve, deflection, and stiffness) and mass loss. The role of PVA fibers and fly ash is discussed through the analysis of microstructure and fiber-matrix interactions as a function of heat treatment. The microstructural characterization is examined before and after exposure to fire deterioration by ...