Nader Ghafoori

ABRASION RESISTANCE OF FINE AGGREGATE REPLACED SILICA FUME CONCRETE

This investigation evaluated the resistance to abrasion of concrete proportioned to have four levels of fine aggregate replacement (5%, 10%, 15%, and 20%) with silica fume. Control mixtures containing no silica fume were also used for comparison purposes. Three cement factors, namely, 500 lb/cu yd (900 kg/cu m), 650 lb/cu yd (1,157 kg/cu m), and 800 lb/cu yd (1,424 kg/cu m), and two water-to-cementitious materials ratios (w/cm)--0.325 and 0.40)--were employed. The fresh and bulk characteristics, such as slump, air content, time of setting, bleeding, unit weight, and compressive strength, were examined to characterize the selected matrixes. The standard testing method--American Society for Testing and Materials (ASTM) C 779, Procedure C, Ball Bearing--was used to ascertain the resistance to wear. The influence of silica fume addition, cement factor, w/cm, and curing were studied. The relationship between depth of wear and compressive strength was also presented. Finally, the fresh properties, compressive strength (and strength development), and abrasion resistance of the fine aggregate-replaced silica fume concretes were compared with those of the reference mixtures. Laboratory test results concluded that the resistance to wear of concrete containing silica fume as a fine aggregate replacement was consistently better with increasing amounts of silica fume up to 10%. The abrasion resistance and compressive strength decreased with increases in w/cm and improved with increases in cement factor and curing age. Both compressive strength and resistance to wear of fine aggregate-replaced silica fume concretes were better than those exhibited by the equivalent control matrixes. A significant correlation was found between the depth of wear and compressive strength.

LABORATORY-MADE ROLLER COMPACTED CONCRETES CONTAINING DRY BOTTOM ASH: PART II - LONG TERM DURABILITY

Laboratory-made roller compacted concretes with various combinations of cement (Type I and Type V for sulfate-resistant concrete), lignite dry bottom ash, and crushed limestone coarse aggregate were tested to ascertain the suitability of this type of concrete for pavement applications. The fresh properties and strength and deformation of the hardened roller compacted concrete containing bottom ash have been discussed in the companion article (Part I). This paper describes the data pertaining to long-term durability of bottom ash roller compacted concretes. The analysis of the test results leads to the conclusions that durable concrete can be produced with the high-calcium dry bottom ash used in this investigation. Resistance to sulfate attack, rapid freezing and thawing, and wear improved with increases in cement and/or coarse aggregate contents. Length change caused by external sulfate attack varied from 0.0203% to 0.0388%, whereas no mass loss or reduction in strength were found in any of the test samples. Abrasion testing under wet conditions was consistently worse than under dry conditions. After 300 rapid freezing and thawing cycles, the mixture proportions of this investigation displayed a maximum mass loss of 2.3% and a minimum durability factor of 91.2%.

ABRASION RESISTANCE OF CONCRETE BLOCK PAVERS

Abrasion resistance of concrete pavement is a surface property that is mainly dependent on the quality of the surface layer characeteristics. A common assumption is that a concrete surface is abrasion-resistant when acceptable compressive strength is obtained. This indirect approach is for the most part effective and reasonable, but not always correct. This paper presents abrasion characteristics of various concrete paving blocks using American Society for Testing and Materials (ASTM) C 779, Procedure C, Ball Bearings. Effect of matrix proportions on abrasion resistance and the relationship between depth of wear and bulk properties are discussed. Influence of testing conditions and surface preparations on bulk and surface properties of concrete block pavers is thoroughly examined. It was found that the adopted testing procedure is simple and reliable. Abrasion resistance of concrete pavers was directly related to matrix proportions and their bulk properties. Testing under wet conditions was consistently worse than under air-dry conditions. The top surface of concrete pavers exhibited invariably superior abrasion resistance to the bottom and middle surfaces when tested in a dry condition. Both matrix proportion and testing condition had more impact on quality of the concrete surface than compressive or splitting tensile strength.

Compacted Non Cement Concrete Utilizing Fluidized Bed And Pulverized Coal Combustion By-Products

The engineering characteristics and long-term durability of laboratory-made compacted non-cement composites made with fluidized bed combustion (FBC) spent bed and pulverized coal combustion (PCC) fly ash, by-products of fluidized bed and pulverized coal combustion processes, respectively, natural siliceous fine aggregate, and crushed limestone coarse aggregate, were investigated. The concrete constituents were combined and fabricated, at their optimum moisture content, in accordance with the requirements of ASTM D 1557. Once cured, the engineering characteristics of hardened specimens were assessed at different curing ages, up to 18 months, under both sealed and saturated conditions. The PCC:FBC impact compacted non-cement concretes were studied for unconfined compressive and indirect tensile strengths; static modulus of elasticity; and resistance to internal sulfate attack, abrasion wear, and freezing and thawing. Laboratory test results conclude that higher strength and expansion properties, and an increased performance in long-term durability are attained with increases in PCC fly ash to FBC spent bed ratio of the matrix. The inclusion of natural fine aggregate improves paste quality and properties of PCC:FBC compacted concretes. Under saturated conditions, the engineering and long-term characteristics of the selected mixtures are inferior to those obtained under sealed conditions.

SCALING RESISTANCE OF CONCRETE PAVING BLOCK SURFACE EXPOSED TO DEICING CHEMICALS

This study reports on the relative performance of concrete block pavers subjected to repeated cycles of freezing and thawing with deicing chemicals. Six different mixture proportions, prepared at their optimum moisture content and fabricated at a commercial plant, were utilized to evaluate the surface scaling resistance of concrete pavers using the specifications of American Society for Testing and Materials (ASTM) C 672 deicer salt scaling test. Laboratory results indicate that ASTM C 672 can be successfully employed when a minimum cement content is used in the fabrication of block pavers. Test samples having a cement content below a specified threshold experienced infiltration of the salt solution and, consequently, failed by surface heaving with the first 25 freezing and thawing cycles. The remaining specimens endured 50 cycles of freezing and thawing with virtually no signs of surface scaling. When the experimental program extended to 200 cycles, some pavers displayed minimal scaling and remained crack-free throughout the testing.

Sulfate Resistance of Roller Compacted Concrete

This paper presents the results of a study on sulfate resistance of plain and fly ash roller compacted concretes (RCCs). A total of 12 plain, 24 cement-replaced, and 12 fine aggregate-substituted fly ash concretes were used in this investigation. Laboratory-made RCC specimens were prepared at their optimum moisture content and were fabricated in accordance with ASTM C 1170, Procedure A. The test samples were initially moist-cured for 28 days after casting, prior to immersion in a 5% sodium sulfate solution. Length change, mass loss, and compressive strength were monitored for a period of 180 days to evaluate the performance of specimens exposed to very severe sulfate attack. The influence of mixture variables (cement, coarse aggregate, and fly ash contents) on bulk characteristics and sulfate resistance were evaluated. The study shows that good sulfate-resistant RCCs can be attained with the use of Type V portland cement with or without low-calcium fly ash. The resistance to sulfate attack improves with increases in cement or coarse aggregate content, as concrete becomes more dense and impermeable. Length change of RCC samples increases with increasing immersion age and stabilizes within 3-4 months after the initial contact. No mass of concrete residues is found for any specimens tested in this study. However, after 6 months of immersion in a sodium sulfate solution, RCC mixtures with cement content of 12% or less (by mass of total dry solids) experienced slight reduction in strength. A 20-40% replacement of cement by low-calcium fly ash increases the sulfate resistance of RCC samples (excluding mixtures made with 9% cementitious binder and 20% fly ash), whereas 10% replacement has a contrary effect. Mixtures with 10-20% fine aggregate-replaced Class F fly ash exhibit lower sulfate expansion and higher compressive strength than those of plain and cement-substituted fly ash RCCs.

Influence of Hauling Time on Fresh Properties of Self-Consolidating Concrete

This study was intended to evaluate the influence of hauling time on fresh self-consolidating concretes (SCCs) made with slump flows of 508, 635, and 711 mm (20, 25, and 28 in.). Nine different hauling times—10, 20, 30, 40, 50, 60, 70, 80, and 90 minutes—were used. Fresh performance of the selected SCCs was affected by hauling time in the form of loss in flowability and gain in flow rate and dynamic stability. A remediation technique consisting of admixture overdosing was able to revert the adverse influence of hauling time on the fresh characteristics of the selected matrices, as it produced SCCs with similar flow characteristics, dynamic stability, and passing ability to those obtained at the reference hauling time.

Sulfate Resistance of Nanosilica and Microsilica Contained Mortars

Presented is a side-by-side comparison study intended to identify the effects of nanosilica (nS) on chemical sulfate attack resistance of portland cement (PC) mortars and its effectiveness in comparison to similar replacement levels of the more widely implemented microsilica (mS). Several mortar mixtures were prepared with a 4.1 and 7.2% tricalcium aluminate (C₃A) PC by progressive cement replacement with nS or mS. The mortars tested were measured for expansion, compressive strength, and mass loss. Results indicated that nS replacement benefited the studied mortars. However, in the dry powder form and method of mixing used in this study, poor dispersion and agglomeration of the nS was suspected to hinder mortar permeability in comparison to mS and low-C₃A cement mortars. Replacement with nS in aqueous dispersion, however, proved to be significantly more effective than equivalent replacement of dry powder nS and mS.

Effects of Hauling Time on Air-Entrained Self-Consolidating Concrete

Self-consolidating concrete (SCC) is a highly flowable concrete that is characteristically sensitive to mixing and hauling variables such as transportation time. In real-world applications, lengthy hauling times are often necessary for transportation to the job site and can result in deviations in the fresh properties of SCC. In this investigation, air-entrained SCCs were developed at a constant water-to-cementitious material ratio (w/cm) of 0.40 and air content of 6% for 3 distinct slump flows of 559, 635, and 711 mm (22, 25, and 28 in.). Test results revealed that the slump flow losses up to 39% were recorded after 90 minutes of hauling time. The air content increased with hauling time, ranging from 2.6-4.8%. The air void characteristics improved with hauling time, with an average increase in specific surface of 9.5 mm–1 (241.3 in.–1) and an average decrease in spacing factor of 64 μm (0.0025 in.).

Use of pulverised and fluidised combustion coal ash in secondary road construction

The research presented herein was undertaken to examine the viability of pulverised coal combustion (PCC) fly ash and fluidised bed combustion (FBC) spent bed, as a primary cementitious material and a secondary fine aggregate, respectively, for production of mixtures suitable for low-volume county and other secondary roads. To achieve the stated objective, the mechanical properties of a number of previously identified optimum PCC/FBC composites were studied in the laboratory and field. The characteristics such as unconfined uniaxial compressive strength, splitting tensile strength, modulus of elasticity and Poissons ratio, absorption, resistance to freezing and thawing, and length change were investigated. Results conclude that the engineering properties of the PCC/FBC composites exceed those of the conventional mixes used in secondary roads. The field results reaffirm that the engineering characteristics of the laboratory mixtures can be easily attained in field. The suggested thickness design requirements are conservative when the 90-day strength of the PCC/FBC composites is considered.