Strength and Wear Resistance of Sand-Replaced Silica Fume Concrete

This investigation evaluates: 1) the influence of silica fume content on the strength and resistance to wear of 3-, 7-, 28-, and 91-day moist-cured concretes made with 0, 5, 10, 15, and 20% silica fume replacing a portion of the fine-aggregate; and 2) the influence of various combinations of curing schemes on the strength and resistance to abrasion of the selected fine aggregate-replaced silica fume concretes. A uniform cement factor of 385 kg/m^sup 3^ (650 lb/yd^sup 3^) and a constant water-cementitious material ratio (w/cm) of 0.325 are used in all trial mixtures. The fresh and bulk characteristics, such as slump, air content, time of setting, bleeding, unit weight, and compressive strength are obtained to characterize the selected matrixes. ASTM C 779, Procedure C, Ball Bearings, is used to evaluate the resistance to wear. The compressive strength and abrasion resistance of the fine aggregate-replaced silica fume concretes cured under a continuous moist-curing condition and various combinations of wet-dry curing cycles are compared. The relationships among depth of wear, compressive strength, percentage of silica fume content, and curing age are also studied.

Laboratory test results conclude that both the compressive strength and resistance to wear peaked at 10% silica fume content. Silica fume incorporation in concrete by way of fine aggregate replacement did not alter the samples' compressive strength when subjected to the various combinations of wet-dry curing cycles. When compared with continuous moist curing, the selected cycled wet-dry curing conditions caused a modest reduction in resistance to abrasion that varied with silica fume contents, curing cycles, and curing schedules. There was statistically a significant correlation between the dependent variable (depth of wear) and the independent variables (compressive strength, percentage of silica fume content, and curing age) for the samples aged under continuous moist-curing conditions.

Keywords: aggregate; compressive strength; durability; silica fume.

INTRODUCTION

Concrete engineers design mixture proportions based on the quality required for construction purposes. The dosage and the quality of the matrix constituents (cement or cement plus pozzolan, fine and coarse aggregates, water, and admixtures) specify the quality of concrete. In predicting concrete performance, concrete engineers must assume high quality workmanship and proper curing. When properly cured, concrete becomes stronger, more durable, and more resistant to abrasion, sulfate attack, and freezing and thawing.1 Proper curing requires sufficient moisture content and temperature in concrete during its early stages (relative humidity within capillary pore of approximately 80%) so that desired properties may develop.2 Seven days of curing under normal temperature conditions (11 to 23 °C [52 to 73 °F]) is sufficient for normal portland cement (ASTM Type I or II) to develop the required 28-day compressive strength. The rate of strength improvement decreases slowly thereafter for an indefinite period so long as unhydrated cement particles and a suitable curing environment are available. Several techniques of curing exist. The most commonly used are: water ponding or immersion; fog spraying or sprinkling; burlap, cotton mats, or rugs earth curing; and sand, sawdust, straw, or hay curing.2 All of these are moist or moist-related curing techniques. Sealed curing methods are used to cover concrete surface with water proofing papers, plastic sheets, or curing compound. Accelerating curing is also used by supplying heat and additional moisture to concrete (steam curing).3

The review of literature shows that much consideration has been given to the adverse effect of improper curing on concrete performance. In practice, curing condition varies widely depending on the type of construction. Large horizontal surfaces such as roads, bridge decks, footpaths, and parking floors are usually subjected to natural phenomenon such as snow, rain, wind, solar radiation, and hot weather, which can be simulated by various wet-dry cycling curing schemes. Although it is easy to resume moist curing in the laboratory, the moist recuring is relatively difficult in field for large horizontal concrete surfaces. When moist curing is interrupted, the exposed skin can experience an internal relative humidity drop resulting from the partial evaporation of mixing water. This loss of water leads to more shrinkage and cracking, resulting in a decrease in concrete performance. Nanni4 (in 1988) and Khan and Ayers5 (in 1992) reported that the concrete strength development could reactivate if the moist curing resumed.

Silica fume is defined as "very fine non-crystalline silica produced in electric arc furnaces as a by-product of the production of elemental silicon or alloys containing silicon."6 It is usually categorized as a supplementary cementitious material. In recent years, silica fume has gained acceptance for use as a mineral admixture in concrete because of its filler and pozzolanic effects.6