Showing papers on "Silica fume published in 1996"

High-Performance Concrete Incorporating Rice Husk Ash as a Supplementary Cementing Material

Abstract: This paper presents results on the physical and chemical properties of rice husk ash (RHA), and deals with the properties of fresh and hardened concrete incorporating the same ash. The properties of fresh concrete investigated included workability, bleeding, setting time, and autogenous temperature rise, and those of the hardened concrete included compressive, splitting tensile, and flexural strengths, modulus of elasticity, drying shrinkage, resistance to chloride ion penetration, resistance to freezing and thawing cycling, and salt-scaling resistance. In addition to the effects of the percentage of RHA and the water-cementitious materials ratio on the properties investigated, the properties of the RHA concrete were also compared with those of the control Portland cement concrete and silica fume concrete.

Autogenous deformation and change of the relative humidity in silica fume-modified cement paste

Abstract: Sealed curing at a constant temperature is not enough to keep hardening cement paste from deforming and the relative humidity within its pores to decrease This autogenous deformation and autogenous relative humidity change may be so significant that the cement paste cracks if the deformation is restrained This article focuses on the influence of silica fume addition on autogenous deformation and autogenous relative humidity change Continuous measurement of autogenous deformation and autogenous relative humidity change for more than 1 and 1 1/2 years, respectively, was performed Findings reveal that silica fume addition markedly increases autogenous shrinkage as well as the autogenous relative humidity change

Mechanical properties of reactive powder concretes

Abstract: Reactive Powder Concretes (RPC) are a set of ultrahigh-strength concretes reinforced with steel fibers. Their compressive strength is between 200 and 800MPa, and their flexural strength can reach 140MPa.RPC200 has been studied with respect to compressive strength and two-point loading strength to define its mechanical behavior.RPC800, which has been mostly studied from the point of view of compressive strength, displays hardening elastic non-linear behavior at low stress. This behavior is similar to that of some natural rocks. The critical stress intensity factorKIc, and the average fracture energy,\(\bar G_F \), ofRPC200 andRPC800 have been studied experimentally by applying the theory of linear fracture mechanics (compliance method). The fracture energy, which is a measurement of ductility, can reach 40,000 J/m2 forRPC200, as compared to 100 to 150 J/m2 for ordinary concretes. Fracture energy depends on the volume of fibers added to the concrete. The optimum content is between 2 and 3% by volume.

Shrinkage cracking of high-strength concrete

Abstract: Pozzolanic materials and high-range water-reducing admixtures have improved the strength and durability of concrete construction. Since compressive strength is the most commonly used parameter to describe the quality of concrete, high-strength or high-performance concrete, has been used in many structures that are sensitive to shrinkage cracking. No systematic study has been conducted to evaluate the restrained shrinkage cracking behavior of high-strength concrete. This paper presents research on the restrained shrinkage cracking on several strength levels of concrete. Higher strength levels were derived by partial substitution of cement with silica fume and by reducing the water content. Ring-type specimens were used for restrained shrinkage cracking tests. Free shrinkage, creep, weight loss, compressive, and splitting tensile strength were also examined. Findings reveal that the free shrinkage for different concretes does not depend on the unit water content or weight loss and that high-strength silica fume concrete shows higher shrinkage and lower creep. Cracking for high-strength silica fume concrete develops much faster and is significantly wider than that of normal-strength concrete.

Cementitious gypsum-containing compositions and materials made therefrom

Abstract: A cementitious composition useful for water-resistant construction materials, including floor underlayments, backing boards, self-leveling floor materials, road patching materials, fiberboard, fire-proofing sprays, and fire-stopping materials includes about 20 wt. % to about 75 wt. % calcium sulfate beta-hemihydrate, about 10 wt. % to about 50 wt. % Portland cement, about 4 wt. % to about 20 wt. % silica fume and about 1 wt. % to about 50 wt. % pozzolanic aggregate. The Portland cement component may also be a blend of Portland cement with fly ash and/or ground blast slag.

Mechanical properties of high-performance concrete

Abstract: To understand and document the mechanical properties of high-performance concrete, a test program was initiated. High-performance concretes with 56-day compressive strengths of 65 to 120 MPa were examined. Findings and discussions are presented regarding compressive strength gain with time, effect of type of cement, effect of drying, specimen size effect, static modulus of elasticity, Poisson's ratio, tensile splitting strength, and modulus of rupture. A modification of the American Concrete Institute (ACI) 318-M equation for the modulus of elasticity of normal-weight high-performance concrete is proposed. The validity of equations recommended by ACI 363R for estimating tensile splitting strength and modulus of rupture can be extended for high-performance concretes with or without supplementary cementitious materials and with 28-day compressive strengths up to 120 MPa.

Low-drying-shrinkage concrete containing carbon fibers

Abstract: This paper provides a low-drying-shrinkage concrete (with fine and coarse aggregates) that is practically attractive when strength (compressive and flexural), flexural toughness, freeze-thaw durability, chemical attack resistance and cost are all considered. The use of short carbon fibers (just 0.19 vol%), together with silica fume, in concrete results in an 84% decrease of the drying shrinkage strain at 14 days of moist room curing; the shrinkage strain is just 1.9 × 10 5 at 14 days. The fibers dominate the silica fume in lowering the drying shrinkage and increasing the flexural toughness. However, both fibers and silica fume contribute to lowering the drying shrinkage and increasing the flexural strength, toughness and freeze-thaw durability. On the other hand, the fibers decrease while the silica fume increases the compressive strength and chemical attack resistance, so the combined use of fibers and silica fume is recommended. The adverse fiber effects are associated with an air content increase.

Strength development and drying shrinkage of high-strength concretes

Abstract: Ten high to very high strength concretes with total cementitious contents of 550 and 600 kg/m3 were made. The concretes consisted of plain cement, 5 and 10% condensed silica fume (CSF) and 10% silica fume with 20% fly ash or 20% ground granulated blast furnace slag. The specimens were either continuously fog cured or 3 or 7 days initially cured and then exposed to 40% R.H. drying regime. The results suggest that all laboratory made specimens irrespective of their composition (with or without cement replacement materials) give lower strength when inadequately cured and exposed to drying regimes for a long period of time. However, concretes made with 5 and 10% CSF resulted in the least loss in strength and drying shrinkage and had a superior quality cover concrete as indicated by the 24 h water sorption test.

Properties of fiber reinforced structural lightweight concrete

Abstract: The authors of this paper present the results of an experimental study of the behvaior of fiber reinforced structural lightweight concrete. Properties investigated include workability and behavior under compression, splitting tension, and flexure. The independent variables were composition of fine aggregate, fiber content, fiber length, and presence of silica fume. Fine aggregate consisted of all lightweight aggregate or combinations of lightweight aggregate and nautral sand. Two fiber lengths were investigated with various fiber contents. The lightweight aggregate used was made of expanded shale. High-range water-reducing admixture was used to improve workability. An air-entraining admixture was used to reduce the unit weight and improve workability. Results reveal that a compressive strength of approximately 42 MPa can be obtained for concretes with an equilibrium density of 1650 kg/m3. Fibers increase flexural and splitting tensile strengths significantly. The modulus of elasticity is enhanced by about 30 percent. Fiber reinforced concrete displays excellent ductility.

A preliminary investigation of strength development in Jamaican red mud composites

Abstract: The 28-day strength of 50 mm cubes of composites formed by the addition of various amounts of hydrated lime, condensed silica fume and limestone to Jamaican red mud is investigated. The aim is to produce a red mud composite suitable for use as a construction material, without employing Portland cement as binder. The identities of compounds formed in the composites are deduced from XRD scans, in combination with DTG and electron micrography where appropriate. The strongest composite found in this preliminary study has compressive strength in the range 15–18 MPa at 28 days, with the strength increasing slowly with age to a maximum so far, in the range 18–22 MPa at 122 days. The strength development is observed to be associated with the formation of stratlingite, and possibly also with the formation of complex carbonates such as hydrogrossular. This composite compares favorably, in terms of compressive strength and durability, with the one other composite reported in the literature, which is formed similarly from red mud with additives, not including Portland cement.

Durability aspects of precast prestressed concrete. part 2: chloride permeability study

Abstract: A laboratory study was undertaken to investigate the good past performance of low water-cement ratio, heat-cured precast, prestressed concrete in highway bridges, parking garages, and other applications. The study included salt water ponding testing, AASHTO T 277 or ASTM C 1202 coulomb tests, compressive strength tests, and absorption and volume of permeable voids tests. Heat-cured, water-cured, and moist-cured concretes with water-cementitious ratio values of 0.46, 0.37 and 0.32 with and without silica fume were tested. Using the measured chloride contents, chloride diffusion coefficients were calculated and estimates of the time-to-corrosion were developed. The water-cement ratio was found to be the most important influence on the performance of the concrete, with low w/c, heat-cured conventional concretes having comparable performance to realistic silica fume concretes having 0.37 to 0.46 water-cementitious ratios. It was also found that the use of heat curing could reduce the permeability of AASHTO-grade, 0.46 w/c concrete by 40 to 50 percent. The addition of silica fume to concrete caused an increase in the absorption and volume of permeable voids in concrete, while heat curing was seen to decrease the absorption and volume of permeable voids in concrete.

Tensile and compressive strength of silica fume-cement pastes and mortars

Abstract: The effect of silica fume on the compressive and uniaxial direct tensile strength of portland cement paste and mortar is reported. Sixteen and 25% of the cement used in the paste and in the mortar, measured by mass, was replaced by silica fume. Four different W/C ratio mixtures were tested: 0.22, 0.25, 0.28, and 0.31. The super-plasticizer content was adjusted for each mixture to provide a sufficient amount for efficient dispersion of the cement and silica fume particles, but also to make sure that there would be no excess of this additive that might lead to effects such as bleeding. Results show that the partial replacement of cement by silica fume increases the compressive strength of mortar but has no effect on the compressive strength of paste. Results also show that the partial replacement of portland cement by silica fume decreases the tensile strength of both paste and mortar. The reduction in the strength of paste was greater than the reduction in the strength of mortar. Results were verified by statistical analysis using hypothesis testing at a 95% confidence level. The uniaxial tensile strength results were obtained using the cementitious composites axial tensile technique (CCATT). The relation between the tensile strength and compressive strength of materials with and without silica fume was obtained. The ratios of tensile to compressive strength of paste and of mortar were decreased with increasing silica fume content.