Showing papers on "Silica fume published in 1989"

Pozzolanic and Cementitious by-Products in Concrete--Another Look

Abstract: Updates of a 1983 critical review on pozzolanic and cementitious by-products for use in concrete are presented in this paper. The by-products included in this report are fly ash, granulated blast-furnace slag, and condensed silica fume. Recently available worldwide statistics on production and utilization rates of these mineral admixtures are given. New information is presented on their physical and chemical characteristics, structure, and reactivity of the glassy phase, mechanisms by which concrete properties are enhanced, and engineering properties of concrete containing siliceous by-products. A special emphasis is given to durability aspects of concretes incorporating fly ash, blast-furnace slag, or condensed silica fume. Finally, the status of standard specifications and test methods is reviewed, and the contribution of siliceous by-products to make concrete an environment-friendly material of construction is emphasized.

Bond effects in high-strength silica-fume concretes

Abstract: The strengthening effects in high-strength silica-fume concretes were evaluated in terms of water-reducing effect associated with the reduction in water requirement in the silica fume system and in terms of an inherent effect that reflects the increase in strength of the silica-fume concrete over a similar water/cement ratio concrete without silica fume. The inherent effect was found to be as important as the water-reducing effect, and it is suggested that its origin is in the improved aggregate-matrix bond. This enhanced bonding is associated with the formation of a dense microstructure in the transition zone of the silica-fume concrete.

Curing Effects, Strength and Physical Properties of High Strength Silica Fume Concretes

Abstract: The physical properties of high strength silica fume concretes and their sensitivity to curing procedures were evaluated and compared with reference Portland cement concretes, having either the same concrete content as the silica fume concrete or the same water to cementitious materials ratio. The marked increase in the strength of the silica fume concrete over the two reference concretes, which was observed even at one day, was not accompanied by liberation of excessive heat. The shrinkage of the silica fume concretes was much lower due to the smaller weight loss on drying. The effects of poor curing procedures on the strength, and the skin properties, were found to be equally detrimental in the reference and in the silica fume concretes.

Air-void stability part i: influence of silica fume and other parameters

Abstract: Thirty concrete mixes were prepared to analyze the influence of silica fume and other parameters on the stability of air-void system during mixing and agitation. For each mix, four sets of samples were cast over a period of 90 min after initial water-cement contact. The modified point count method (ASTM C 457) was used to determine the characteristics of the air-void system on each set of samples. Results indicate that silica fume and type of mixing have little influence on the production and stability of the air-void system but that superplasticizers can cause a significant increase of the spacing factor value. This inflkuence of superplasticizers was found to vary quite significantly with the characteristics of the cement and also with the type of superplasticizer and air-entraining agent. Results show that it is difficult to estimate the value of the spacing factor from the air content of fresh concrete.

Fly Ash and Silica Fume Chemistry and Hydration

Abstract: Hydration of cements containing the supplementary cementing materials fly ash (FA) and silica fume (SF) is discussed and compared with the hydration of ordinary portland cement (OPC) in this paper. Early stage heats of hydration, changes in the chemistry of the solution (both at early stages, and later pore solution compositions), microstructural development, and pore structure are also compared in the paper. The hydration rates normally follow the order: SF > OPC > FA. The complex hydration processes may be controlled so that the use of these cements enables development of materials having superior strength and durability.

Marine Curing of Steel Fiber Composites

Abstract: With the attractive possibility of offshore concrete casting in mind, the effects of marine curing on the pull-out behavior of steel fibers were investigated. Single fiber pull-out specimens were chosen to clearly examine the behavior of individual fibers. Three curing temperatures of 2°, 22°, and 38°C were chosen. Deformed fibers with hooks on both ends were chosen. The effects of silica fume addition were also investigated. Pull-out resistances were continuously monitored by conducting tests starting at the age of 1 day up to about 3 months. In some cases, the tests were extended to an age of 1 year. Some fibers were retrieved and examined in a scanning electron microscope. Curing at a low temperature of 2°C was not found to adversely affect the pull-out resistance even after one year of continuous marine exposure. High temperatures, however, were found to promote an early corrosion leading to substantial reductions in pull-out resistances. The presence of silica fume was not found to promote strength retrogression in any particular way. The deformed locations of the fibers, perhaps because of the residual stresses, were found to be particularly susceptible to anodic pitting.

The hydration of C3S and ordinary Portland cement with relatively large additions of microsilica

Abstract: The microstructure of pastes of ] part of C3S blended with 1·3 parts of precipitated silica, and 1 part of ordinary Portland cement (OPC) blended with 1·4 parts of silica fume, hydrated for two months at 25°C and three months at 40°C respectively, have been studied by analytical transmission electron microscopy and solid state NMR. The hardened pastes were found to consist, solely in the case of C3S/silica and principally in the case of OPC/silica fume, of silica particles embedded in CSH gel with a Ca/Si ratio significantly less than unity. The gel contained silicate units in chains of approximately 8·5 units in average length. Similar pastes prepared with alkali addition differed little in microstructure, but the OPC/silica fume paste with alkali addition contained isolated regions of low calcium and high alkali content with a distinctive morphology.

Resistance of Microsilica Concrete to Steel Corrosion Erosion and Chemical Attack

Abstract: This paper discusses how the use of silica fume (microsilica) to improve the compressive strength at a given cement level or as a cement replacement is on the rise. Additional benefits of adding silica fume to improve the corrosion resistance of embedded steel and improve concrete durability in erosive or severe chemical exposure were investigated. Concretes with embedded steel were produced with silica fume levels varying from 0 to 15 percent by mass of cement. Additional variables were water-cement ratio and calcium nitrite content. All concretes were air-entrained and had high-range water-reducers. Plastic properties of the concretes are reported as well as compressive strength, freeze-thaw, and resistivity and rapid chloride data. Corrosion rates and chloride contents are reported and show substantial improvements with silica fume and/or calcium nitrite. An accelerated hydraulic erosion test was conducted, in which ball bearings impact the concrete surface, simulating abrasive action of waterborn particles. Mass loss was measured for concretes with 0 to 15 percent silica fume by mass of cement. Silica fume significantly improved erosion resistance. Chemical testing was performed in 5 percent acetic acid, 1 percent sulfuric acid, 5 percent formic acid, and mixed sulfates. A cyclic method involving drying, weighing, and wire brushing was used. Results show that silica fume concretes had superior chemical resistance that improved as silica fume levels increase.

Fire Resistance of High Strength/Dense Concrete With Particular Reference to the Use of Condensed Silica Fume--A Review

Abstract: This paper will discuss that the use of silica fume is an accepted technology in the Norwegian concrete environment and how high-strength concrete incorporating silica fume is being used increasingly. However, in recent years, conflicting reports regarding the fire resistance of high-strength concrete have been published. This review indicates that eventual fire-safety problems seem to be linked less to the materials used than to physical properties of hardened concrete or conditions under which high strength concrete is used.

Influence of Curing on the Salt Scaling Resistance of Concrete With and Without Silica Fume

Abstract: This paper will discuss ASTM C 672 scaling tests that were carried out on normal concretes and concretes containing 5 percent silica fume, with air-void spacing factors in the 100 to 200 µm range. Five curing methods were compared: a 24 hr heat cycle with a maximum temperature of 70° C, 2 and 14 days moist curing, and two different curing compounds. Results indicate that the use of silica fume does not improve the scaling resistance of concrete. Concretes cured with one particular curing compound were found to have a scaling resistance similar to that of those cured in water for 14 days, weight losses after 50 cycles being lower than 1 kg/m(2) for all mixes. Concretes cured with the other curing compound had a lower and more variable scaling resistance. As expected, specimens cured for only two days in water also had a lower scaling resistance. All mixes cured using the heat cycle exhibited a poor performance, although, in this case only, silica fume reduced very significantly, but not sufficiently, the damage due to the freeze-thaw cycles in the presence of deicing salts.

Chemical Resistance of Concrete Containing Condensed Silica Fume

Abstract: This paper studied chemical resistance of concrete containing condensed silica fume (CSF), 5 percent H2SO4, 5 percent HCl, and 10 percent Ha2SO4. The deterioration of concrete was investigated by measuring the changes in weight and modulus of elasticity. The penetration depth of chloride ion by 3 percent NaCl solution spray and the carbonation depth by 5 percent CO2 gas were also measured. Chloride was measured by colorimetry. Mortar and concrete containing CSF show higher compressive strengths at 28 and 91 days than the control mix. The penetration depth of chloride into CSF concrete was reduced and at 10 percent CSF, the penetration depth was reduced by 65 percent. At 30 percent CSF, carbonation depth is increased with respect to the control concrete.

Rapid Chloride Ion Permeability Test: Data on Concretes Incorporating Supplementary Cementing Materials

Abstract: This paper gives the results of an investigation on the chloride ion permeability of concretes incorporating supplementary cementing materials, and it uses the Rapid Determination of Chloride Permeability Test (AASHTO T277-83). A total of 18 concrete mixtures were made and these included mixtures incorporating silica fume (8 percent replacement or addition to the cement by mass) or ground granulated blast-furnace slags (50 percent replacement by mass), or fly ash (25 percent replacement by mass). The water cement (w/c) of the mixtures investigated ranged from 0.21 to 0.71. From each mixture, a number of 152 x 305 mm cylinders for compressive strength testing and 102 x 203 mm cylinders for determining the chloride permeability were made. Porosity measurements were also performed on some of the concrete specimens. The test results showed that the use of supplementary cementing materials significantly reduced the chloride ion permeability of concrete. Silica fume and blast furnace slags investigated seem to be particularly efficient for producing concrete almost impermeable to chloride ions.

Optimum use of pozzolanic materials in steel fiber reinforced concrete

Abstract: The effects on fresh and hardened material properties for fly ash caused by substituting cement with fly ash and silica fume in steel fiber reinforced concrete were studied experimentally. The percentage substitution of cement ranged from 0 to 40% and from 0 to 20% for silica fume. The workability of fresh fibrous mixtures was characterized by measuring the inverted slump cone time. The hardened material was tested at 28 days under compression and flexural loads. The development of compressive strength with time was also assessed in steel fiber reinforced concrete incorporating fly ash. The generated test data were used to decide the optimum ranges of cement substitution with fly ash or silica fume in steel fiber reinforced concrete for achieving desirable fresh mix and hardened material characteristics.

Silica fume in pcc: the effects of form on engineering performance

Abstract: Silica fume is commercially available in 3 forms: water-slurry, dry compacted powder, and dry densified powder. Results to clarify uncertainties on the effects of the of form of silica fume on 28- and 56-day compressive strengths, resistance to freezing and thawing cycles, and resistance to chloride permeability of high-quality air-entrained concrete are presented and discussed. The effects of the form of silica fume on the fresh properties (slump and percent air content) of concrete are also presented and discussed. Based on the results obtained, the authors do not advocate the use of any specific form of silica fume as there were significant differences observed in the engineering properties of concretes containing either of the 3 forms of silica fume.

Alkali-aggregate reaction in concrete containing fly ash final report

Abstract: The main objective of this study was to determine the influences of the constituents of portland cement, aggregates, and mineral admixtures on alkali-silica reaction. The testing program included 3 natural aggregates, 3 Type I and 1 Type IP portland cements, 4 Type A and 5 Type B fly ashes, tap and distilled (low pH) water, and 1 source of silica fume. ASTM C 227 was the primary guideline for the evaluation and comparison of these materials, most of which are available for use in highway projects in the state of Texas. The results of this testing program demonstrated that: (1) Each of the natural aggregates studied can be used safely in concrete without mineral admixtures, if the cement is chosen carefully with respect to alkali content; (2) Each of the cements tested can be used in concrete with any of the aggregates tested, if a suitable mineral admixture is used as a partial replacement for cement in the proper proportion; (3) The silia fume and seven of the nine fly ashes tested reduced expansions considerably when used in proper proportions as partial replacements for cement; and (4) Fineness of Type B fly ash does not affect significantly its effectiveness in reducing alkali-aggregate related mortar bar expansions.

Relative Durability Properties and Strengths of Mortars Containing Finely Ground Silica and Silica Fume

Abstract: This paper will discuss the effect of finely ground silica with a high specific surface of about 12 m²/g and 20 m²/g in mortar and was compared with that containing silica fume. The dosage requirement of a high-range water-reducing agent to maintain a constant flow of fresh mortar was determined for 1:3 mortar containing 5 to 25 percent finely ground silica or silica fume as cement replacement. The compressive and flexural strengths of the mortar were investigated at different ages. Also, the resistance of mortar specimens to chemical attack and chloride penetration was determined. It was concluded that the use of finely ground silica was as effective as silica fume for improving quality of mortar.

Performance of Gravelstone Concrete Incorporating Silica Fume at Elevated Temperatures

Abstract: This paper will report on the performance of gravel concrete incorporating silica fume after 72 hr exposure at 150, 300, and 450o C. A total of eight concrete mixtures, each 0.09 m3 in volume, were made at water-cement ratios of 0.23, 0.35, 0.50, and 0.71. The mixtures at each w/c consisted of one control mixture and one incorporating 8 percent silica fume by weight of cement. The cylinders and prism specimens were cast for testing in compression and flexure before and after exposure to the elevated temperatures. Before exposure to the elevated temperatures, the test specimens were moist cured for 7 days and air dried for 21 days at ambient temperature and 50 percent relative humidity. After exposure, the test specimens were cooled to room temperature and tested in compression and flexure. The weight loss and pulse velocity determinations were made before and after heat exposure. The test results show that after 72 hr exposure at 150, 300, and 450o C, the performance of the control concrete in the compression testing mode is marginally superior to that of the silica fume concrete. The reverse is true when the two types of concretes are tested in the flexural mode.

Use of Silica Fume and Superplasticizers in Cement Grouts for Injection of Fine Cracks

Abstract: This paper will discuss how hydraulic grouts have been used for a long time for injecting sufficiently large cracks or ducts in prestressed concrete structures. However, they cannot penetrate into narrow spaces, such as millimetric cracks, because of clogging. For some grading curves of cement, it is possible to obtain grouts injectable into narrow cracks. A thorough study of optimum gradings and penetrability characteristics of grouts obtained through the combination of cement and ultrafines (such as silica fume) has enabled the formation of hydraulic grouts for injection. Possessing properties similar to the original materials and being relatively economical to use, such grouts would be particularly useful for repairing masonry structures and concrete. Materials so developed can be used under certain conditions for prestressing steel ducts.

A new mineral admixture for high-strength concrete

Abstract: A new silica fume admixture in liquid form has been developed that is easier to handle than conventional powder forms. With this product, consistently high strength can be obtained regardless of portland cement source. The strength increase does not appear to be related to reduced porosity. The permeability of concrete is reduced when this new admixture is used. The use of high-strength aggregate did not increase mortar strength. It was found that in concrete made with the new admixture, a water layer does not form around the aggregates, and the amounts of calcium hydroxide and ettringite are reduced, leading to a stronger interfacial paste. Silica fume does not densify concrete in the usual sense, but enhances the paste-aggregate bond, which is usualy the weakest part of concrete.

Bond Mechanisms in Fiber Reinforced Cement-Based Composites

Abstract: : This report presents a comprehensive investigation of the mechanisms of bond in steel fiber reinforced cement based composites. Following a state-of- the-art review on bond in reinforced and prestressed concrete as well as fiber reinforced concrete, the results of an experimental and an analytical program are described. The experimental program focuses primarily on the behavior of fibers under pull-out conditions. Pull-out load versus end slip behavior and bond shear stress versus slip relationship are studied extensively. Keywords: Fiber concrete; Cement composites; SIFCON; Bond; Interfaces; Mathematical modeling; Constitutive modeling; Pull-out test; Friction; Latex; Fly ash; Microsilica; Shear.

Effect of Condensed Silica Fume on the Mechanism of Chloride Diffusion Into Hardened Cement Paste

Abstract: This paper reports on a study of the chloride distribution profile in hardened cement paste cylinders of 5 cm diameter. The specimens were made from ordinary portland cement and blended cement with 10 percent fly ash. The condensed silica fume was used as cement replacement, with replacement levels of 5, 10, and 15 percent by weight of cement. Other experimental variables were water-to-(cement + silica fume) ratio of 0.5, 0.7, and 0.9. The specimens were immersed in stagnant seawater at 20 C. After 6 months of exposure, the specimens were cut, ground, dried, and the chloride ion content determined by a potentiometric titration procedure. By applying Fick's second law, the effective diffusion coefficient and the effective supply concentration of chloride were calculated by using an approximation method. Results show the effective diffusion coefficient is reduced highly when condensed silica fume is used as cement replacement.

Comparison of Creep and Shrinkage of High-Strength Silica Fume Concretes With Fly Ash Concretes of Similar Strengths

Abstract: This paper describes how the specific creep and shrinkage of five high-strength concrete mixtures were monitored for 400 days at the University of Michigan, Ann Arbor. One 52.6 MPa (7630 psi) silica fume (SF) concrete was compared with a fly ash concrete of similar compressive strength, and SR concrete and fly ash concrete having compressive strengths of approximately 69 MPa (10,000 psi) were compared. A 106.6 MPa (15,450 psi) SF concrete was also studied in the paper. The creep of SF concretes was not significantly different from that of the fly ash concretes. Furthermore, the relationship between creep and compressive strength was consistent with that reported in the open literature for high-strength portland cement concretes. Several other concrete properties were studied, including slump retention, time of setting, compressive strength development for one year, split-tensile strength, modulus of rupture, and for the nominal 69 Mpa concretes only, rapid freezing and thawing durability and the hardened concrete air-void system.