Showing papers on "Silica fume published in 1998"

High performance concrete—An overview

Abstract: It is suggested that high performance concrete is not fundamentally different from the concrete used in the past, although it usually contains fly ash, ground granulated blastfurnace slag and silica fume, as well as superplasticizer. The cost aspects of the use of silica fume are considered. The content of cementitious material is high and the water/cement ratio is low; the maximum size of aggregate is small. Although ordinary Portland cement is used, it must be compatible with a given superplasticizer; the causes of incompatibility are discussed. The distinct shrinkage behaviour of high performance concrete is considered and the reasons for an absolute necessity of wet curing are given. Some uses of high performance concrete are mentioned. A ‘prediction’ of the future of high performance concrete and of concrete in general is offered.

Effect of supplementary cementing materials on the specific conductivity of pore solution and its implications on the rapid chloride permeability test (AASHTO T277 and ASTM C1202) results

Abstract: The American Association of States Highway and Transportation Officials (AASHTO) Test Method T277--Rapid Determination of the Chloride Permeability of Concrete and the American Society of Testing and Materials (ASTM) C1202--Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration have specified a rapid test method to rank the chloride penetration resistance of various concretes by applying a potential of 60 V DC to a concrete specimen and measuring the charge passed through the specimen during 6 hours of testing. The method is essentially a measurement of electrical conductivity of concrete, which depends on both the pore structure and the chemistry of the pore solution. Analyses based on published results have indicated that the replacement of portland cement with supplementary cementing materials, such as silica fume, can reduce the electrical conductivity of concrete more than 90% because of the change in pore solution composition in the concrete. Chemical composition of pore solution has little to do with the transport of chloride ions in the concrete; thus, it is not correct to use passed charge to rank the chloride penetration resistance of concrete made with supplementary cementing materials.

Use of Viscosity-Modifying Admixture to Reduce Top- Bar Effect of Anchored Bars Cast with Fluid Concrete

Abstract: With the increasing use of highly fluid concrete to facilitate the casting and consolidation of congested reinforced concrete sections, it is essential to ensure adequate stability of fresh concrete to reduce micro-structural defects at the interface between the hardened cement paste and embedded reinforcement Accumulation of bleed water under the reinforcement and minute separation of fresh paste from the reinforcement caused by segregation and settlement can significantly reduce bond with anchored or overlapped reinforcement The reduction in bond with horizontally embedded bars located in the upper sections of structural elements as opposed to those located near the bottom is known as the top-bar effect (top-bar factor) This paper aims at investigating the effectiveness of incorporating viscosity-modifying admixture to enhance the stability of high-slump concrete and reduce the top-bar factor of anchored bars A total of 25 specimens measuring 500 mm, 700 mm, and 1,100 mm in height were cast with various mixtures with slump values of 220 mm and 190 mm and self-consolidating mixtures with slump flow values of 600-690 mm The specimens were prepared to evaluate the effect of viscosity-modifying admixture content, specimen height, and mode of consolidation on external bleeding, surface settlement, segregation, and relative bond strength to horizontally embedded bars The findings indicate that regardless of the slump, specimen height, and mode of consolidation, the reduction in surface settlement (that is related to bleeding and segregation) resulting from incorporating a viscosity-modifying admixture (welan gum) can significantly reduce the top-bar factor Highly stable, self-consolidating concrete with low settlement had low top-bar factors comparable to specimens cast with 190- and 200-mm slump concrete The use of 007% welan gum in concrete made without any silica fume developed greater stability and lower top-bar factor than similar concrete containing 0035% of welan gum and 8% silica fume replacement

Cementitious gypsum-containing binders and compositions and materials made therefrom

Abstract: Cementitious binders include calcium sulfate beta-hemihydrate, a cement component comprising Portland cement, and either silica fume or rice-husk ash. The silica fume or rice-husk ash component is at least about 92 wt.% amorphous silica and has an alumina content of about 0.6 wt.% or less.

Bond strength of reinforcement in high-performance concrete: the role of silica fume, casting position, and superplasticizer dosage

Abstract: Little research is reported in the literature on the effect of pozzolans such as silica fume on structural behavior of reinforced concrete, namely on bond and anchorage characteristics of reinforcing bars in concrete. The objectives of the study were to investigate the effect of silica fume on bond and anchorage characteristics of reinforcing bars in high performance concrete, to study the validity of the upper limit of 70 MPa (10,000 psi) imposed by the American Concrete Institute (ACI) Building Code 318-95 on the concrete compressive strength for determination of development length, and to evaluate the reliability of the empirical equation of Orangun, Jirsa, and Breen in estimating the bond strength of deformed bars embedded in high strength concrete. Sixteen beam specimens were tested. Each beam was designed to include two bars in tension, spliced at the center of the span. The splice length was selected so that bars would fail in bond, splitting the concrete cover in the splice region, before reaching the yield point. The beams were loaded in positive bending with the splice in a constant moment region. The variables used were the percentage replacement by weight of cement by silica fume, casting position, and the superplasticizer dosage. Test results indicated that replacement of 5-20% of the cement by an equal weight of silica fume resulted in an average 10% reduction in bond strength regardless of casting position or the superplasticizer dosage used.

Alkali-aggregate expansion inhibiting admixtures

Abstract: Many concrete structures have exhibited deterioration due to the alkali-aggregate expansion reactions. A review has been prepared to describe the nature of the alkali-aggregate reactions, test methods that are adopted to evaluate the aggregates for their potential to cause expansion and the methods suggested to counter this reaction. Some methods seem to be effective but further work has to be carried out before establishing the efficacy of others. Results obtained using additives such as silica fume and other siliceous materials such as fly ash, slag, and rice husk ash have been discussed. Some chemicals based on lithium compounds have shown promise as, also, have other non-lithium bearing compounds.

Effects of Water-Cement Ratio, Curing Age, Silica Fume, Polymer Admixtures, Steel Surface Treatments and Corrosion on the Bond between Concrete and Steel Reinforcing Bar

Abstract: The bond between concrete and steel reinforcing bars was evaluated by electromechanical pullout testing that involved measuring the shear bond strength and contact electrical resistivity of each sample. The bond strength was increased by steel reinforcing bar surface treatment (acetone, water, ozone, or sandblasting, with ozone treatment being the most effective and acetone treatment being the least effective), adding silica fume and polymer to the concrete, an increase in the water-cement ratio of concrete (particularly from 0.45 to 0.50), and a decrease in the curing age (particularly from 14 days to 7 days). The origins of these effects are reinforcing bar cleansing for acetone treatment (accompanied by a decrease in contact resistivity), reinforcing bar surface oxide film formation for water and ozone treatments (accompanied by increases in contact resistivity), reinforcing bar surface roughening for sandblasting, polymer interface layer formation for the addition of polymer (accompanied by an increase in contact resistivity for the addition of latex, but not for the addition of methyl-cellulose), a decreased interfacial void content (accompanied by a decrease in contact resistivity) for an increase in the water-cement ratio (because of an increase in fluidity) and for a decrease in the curing age (because of a decrease in shrinkage), and an increase in the matrix modulus for the addition of silica fume. Corrosion initially caused the bond strength to increase while the contact resistivity increased, but further corrosion caused the bond strength to decrease while the contact resistivity continued to increase.

Chloride initiated reinforcement corrosion in marine concrete

Abstract: Factors affecting the total chloride threshold for initiation of pitting corrosion in reinforced concrete were investigated by means of field and laboratory exposure tests. Factors affecting the chloride penetration into concrete were also investigated by means of field tests. The thesis consists of a summary (86 p.) and 11 papers attached. Measured total chloride thresholds for uncracked concrete based on sulfate resisting portland cement with w/c ratio 0.3-0.5 and exposed in the splash zone were in the range of 1.1-1.5 % total chloride by weight of binder. The effect of replacing the portland cement with 5 % silica fume on the chloride threshold was insignificant. Defects in the concrete micro structure and the steel surface at the steel-concrete interface were found to have a major negative effect on the chloride threshold. A linear relationship between free and total chlorides was found in concrete submerged in sea water. Concentration profiles measured for the free chloride and hydroxide ions indicated that these ions penetrate concrete in opposite directions but at approximately the same rate. It was suggested that hydroxide counter diffusion increases the chloride binding close to the exposed uncarbonated surface, which leads to an almost linear chloride binding. The effective chloride diffusivity calculated by fitting experimentally obtained total chloride concentrations to a solution to Fick´s second law of diffusion, decreases over time in field exposed concrete. The results after 5 years of field exposure indicated that the effective chloride diffusivity can be reduced by a factor of 8-10 by using 5 % silica fume in the binder and by reducing the w/c ratio from 0.40 to 0.30, as compared to a typical Swedish bridge concrete with w/c 0.40 and no pozzolan in the binder. The corresponding reduction of the required minimum cover would be 50 % for an initiation time of 100 years.

Improving the vibration damping capacity of cement

Abstract: Cement pastes containing latex (20–30% by weight of cement), methylcellulose (0.4–0.8% by weight of cement) and silica fume (15% by weight of cement, either as received or acid treated) were compared in terms of the dynamic flexural mechanical properties, as expressed by the loss tangent (damping capacity), storage modulus and loss modulus at 0.2–2 Hz (loading frequency) and 30–150 °C. Treated silica fume and latex are by far the most effective admixtures for enhancing the loss tangent (up to 0.18, an increase of up to 390%). Silica fume (whether as received or treated) is the most effective admixture for enhancing the storage modulus (up to 15 GPa). Latex tends to give a high loss modulus (up to 0.18 GPa) at 2 Hz; silica fume tends to give a high loss modulus (up to 2.2 GPa, an increase of up to 2200%) at 0.2 Hz.

Influence of High-Reactivity Metakaolin and Silica Fume on the Flexural Toughness of High-Performance Steel Fiber Reinforced Concrete

Abstract: For steel fiber-reinforced concrete with practical fiber volume fractions, the major post-peak energy dissipation mechanism is the pull-out of fibers across a crack. With undeformed, smooth fibers, post-peak energy dissipation of "toughness" is mainly a function of fiber-matrix adhesional bond, whereas for the highly stressed deformed fibers, properties of the bulk matrix also become important. High-performance matrices tend to be brittle, and addition of pozzolanic admixtures, particularly silica fume, further increases the brittleness. An increased matrix brittleness can cause crushing and splitting of the matrix and in turn, curtail the ability of fibers to transfer stresses during pull-out, thus reducing the overall toughness. This paper examines the influence of two pozzolanic materials--high-reactivity metakaolin (HRM) and silica fume--on the toughness characteristics of high-performance fiber-reinforced concrete. It is concluded that HRM is particularly effective in improving the post-peak energy absorption capacity of concrete with fibers, and unlike silica fume, no particular post-peak brittleness is seen to occur.