Medhat H. Shehata

DEVELOPMENT OF STATISTICAL MODELS FOR MIXTURE DESIGN OF HIGH-VOLUME FLY ASH SELF-CONSOLIDATING CONCRETE

Self-consolidating concrete (SCC) in the fresh state is known for its excellent deformability, high resistance to segregation, and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Such concrete can be obtained by incorporating either mineral admixtures such as fly ash (FA) or viscosity-modifying admixtures (VMA). The use of VMA has proven very effective in stabilizing rheology of SCC, and recent research has focused on development of new, cheaper VMAs compared with currently available, costly commercial ones. Research to produce an economical SCC with desired properties has been conducted in recent years into the use of FA. In this paper, 21 statistically balanced concrete mixtures were investigated to minimize the use of high-range water-reducing admixtures (HRWRA) and to optimize the use of fly ash in SCC. The minimum use of HRWRA and optimum use of FA were desired in this study. Four independent variables were used for design of SCC mixtures. The fresh concrete properties were determined from slump flow, V-funnel flow, filling capacity, bleeding, air content, and segregation tests. The mechanical properties and durability characteristics of SCC such as compressive strength, freezing/thawing resistance, rapid chloride permeability, surface scaling resistance, and drying shrinkage were determined to evaluate the performance of SCC. Four statistical models to predict the slump flow, 1- and 28-day compressive strength, and the rapid chloride permeability of SCC were developed and their performances were validated.

Performance Limits for Evaluating Supplementary Cementing Materials Using Accelerated Mortar Bar Test

The accelerated mortar bar test (AMBT) was originally developed for the purpose of identifying alkali-silica reactive aggregates, but has been widely used to evaluate the preventive action of supplementary cementing materials (SCM). Indeed, a modified version of the AMBT for testing the effectiveness of pozzolans and slag for controlling expansion due to alkali-silica reaction (ASR) was recently developed and published as an ASTM standard test method (ASTM C 1567). In this paper, results from accelerated mortar bar tests on reactive aggregate-SCM combinations are compared with the performance of the same combination of materials in concrete structures, field-exposed concrete blocks, and laboratory expansion tests on concrete prisms (ASTM C 1293). It is concluded that the use of a 14-day expansion limit of 0.10% in the AMBT produces an outcome that agrees well with the performance of concrete in the laboratory or under field conditions. Combinations of reactive aggregates and SCM that pass this limit when tested in mortar have a very low risk of resulting in damage when used in concrete. Furthermore, the minimum level of SCM required to control expansion with a given reactive aggregate can be determined using the 14-day expansion limit and the result is in good agreement with the amount of SCM required to prevent cracking in concrete. Extending the duration of the test (for example to 28 days) is overly conservative and results in estimates of much higher levels of SCM (by 1.5 times on average) to control expansion than that actually required in concrete.

Applicability of the Accelerated Mortar Bar Test for Alkali-Silica Reactivity of Recycled Concrete Aggregates

Using recycled concrete aggregate (RCA) as a replacement for natural aggregate in new concrete is a promising way to increase the overall sustainability of new concrete. This has been hindered, however, by a general perception that RCA is a sub-standard material because of the lack of technical guidance, specifically related to long-term durability, on incorporating RCA into new concrete. The goal of this research was to determine whether current testing methods (namely, ASTM C1260) for assessing natural aggregate susceptibility to alkali-silica reactivity could be used to assess the potential reactivity of concrete incorporating RCA. Seven different RCA sources were investigated. It was determined that ASTM C1260 was effective in detecting reactivity, but expansion varied based on RCA processing. Depending on the aggregate type and the extent of processing, up to a 100 % increase in expansion was observed. Replicate testing was performed at four university laboratories to evaluate the repeatability and consistency of results. The authors recommend modifications to the mixing and aggregate preparation procedures when testing the reactivity of RCA using ASTM C1260.

Evaluation Protocol for Concrete Aggregates Containing Iron Sulfide Minerals

Several cases of concrete deterioration involving sulfide-bearing aggregates have been reported over the years. However, no specific guidelines are currently available to enable making a precise decision on the deleterious potential of aggregates containing iron sulfide minerals. The aim of this paper is to provide an innovative assessment protocol to evaluate the potential deleterious effects of iron-sulfide-bearing aggregates prior to their use in concrete. The findings of this paper are based on tests developed within the past few years as part of a major research project. The protocol is divided into three major phases: 1) total sulfur content measurement; 2) oxygen consumption evaluation; and 3) an accelerated mortar bar expansion test. Tentative limits are proposed for each phase of the protocol, which still need to be validated through the testing of a wider range of aggregates.

Optimizing a Test Method to Evaluate Resistance of Pervious Concrete to Cycles of Freezing and Thawing in the Presence of Different Deicing Salts

The lack of a standard test method for evaluating the resistance of pervious concrete to cycles of freezing and thawing in the presence of deicing salts is the motive behind this study. Different sample size and geometry, cycle duration, and level of submersion in brine solutions were investigated to achieve an optimized test method. The optimized test method was able to produce different levels of damage when different types of deicing salts were used. The optimized duration of one cycle was found to be 24 h with twelve hours of freezing at −18 °C and twelve hours of thawing at +21 °C, with the bottom 10 mm of the sample submerged in the brine solution. Cylinder samples with a diameter of 100 mm and height of 150 mm were used and found to produce similar results to 150 mm-cubes. Based on the obtained results a mass loss of 3%–5% is proposed as a failure criterion of cylindrical samples. For the materials and within the cycles of freezing/thawing investigated here, the deicers that caused the most damage were NaCl, CaCl2 and urea, followed by MgCl2, potassium acetate, sodium acetate and calcium-magnesium acetate. More testing is needed to validate the effects of different deicers under long term exposures and different temperature ranges.