Chloride Permeability and Corrosion Risk of High-Volume Fly Ash Concrete with Mid-Range Water Reducer

This study investigated the effect of local high-volume fly ash (HVFA) incorporating a mid-range water reducer on compressive strength of concrete and on chloride penetration and steel corrosion under tropical climate conditions. The results of electrically accelerated corrosion tests were compared with normal tests. Half-cell potential measurements and visual examination indicated the effectiveness of the corrosion reduction, independent of compressive strengths. A mid-range water reducer improved the 28-day strength by 50 to 75%, compared with 8 to 36% of concrete with 0 to 35% replacement. It also yielded lower chloride permeability and corrosion risk. HVFA concrete with a replacement of 50 to 65% was ranked at greater than 90% probability of no steel corrosion, and the corrosion rank improved from a severemoderate level to slight or no corrosion at all. Fly ash content, water reducer, and water-binder ratio (w/b), respectively, were three parameters that influenced corrosion risk, while w/b was the most important factor for normal concrete.

Keywords: concrete; corrosion; fly ash.

INTRODUCTION

The use of local lignite fly ash as pozzolan for partial cement replacement in concrete in Thailand has increased significantly during the past decade. The usual amount of fly ash used is in the range of 15 to 20% by weight of binder. Due to the savings, as well as the reported uniformity and good quality of Type C fly ash from a major source, however, there is a trend of increasing the percentage to the 30 to 50% range. This material, which is classified as highvolume fly ash (HVFA) concrete behaves differently from normal fly ash concrete. Malhotra and Ramezanianpour1 compiled reported behaviors on the physical and mechanical properties of HVFA, their developments, and the mechanisms involved. While both strength and durability are of concern in reinforced concrete structures, there is little available data relating to these issues for local HVFA in a tropical climate. The hot and humid weather affects these issues in two ways: 1) While it yields a faster rate of strength gain of fly ash concrete, it also results in the rapid deterioration of concrete structures; and 2) Corrosion and concrete deterioration are commonly reported, especially in the coastal area in the south and the east of the country.

The dense microstructure and the smaller size of capillary pores resulting in improved long-term strength and chloride diffusion retardation, probably from pozzolanic reaction and filler effect, have been reported.2,3 There are few reports on chloride diffusion retardation of concrete with a replacement percentage higher than 50%.4 The study of high-strength concrete in a hot, humid environment indicates significant reductions in permeability of concrete incorporating 20 to 40% local moderate calcium oxide fly ash to the very low level, compared with the high and low levels of conventional and silica fume concrete.5 The reduction of corrosion risk and its severity have also been reported for fly ash concrete with no water reducer, using 15 to 50% replacement.6 In practice, however, water reducer is frequently used in fly ash concrete even though the available information is rare. The beneficial effect of water reducer on the better dispersion of fly ash particles than of cement has also been recognized.

The positive effect of water reducer and dense microstructure from the addition of fly ash are generally expected, but the slow pozzolanic reaction from high volume fly ash and the possible depassivation of reinforcing steel from the lower alkalinity are of general concern.

This study investigated the effect of high-volume fly ash together with a mid-range water reducer on the compressive strength of concrete and its effect on chloride penetration resistance and corrosion risk of steel reinforcement. To reduce setting problems, the mid-range water reducer is normally incorporated with fly ash concrete in Thailand. The study is essential for understanding an appropriate use of high-volume fly ash concrete, as well as for the prediction of corrosion risk of reinforced-concrete structures in moderately hot, humid conditions.

RESEARCH SIGNIFICANCE

The results of this study provide information on chloride permeability and corrosion risk of concrete incorporating high-volume fly ash exposed to tropical climates. This is important in the prediction of structural performance in moderately hot, humid conditions. The results are expected to be used as a guide for the design and construction of structures to extend service life.

MATERIALS AND SPECIMEN PREPARATION

In this study, properties of HVFA concrete were compared with those of conventional concrete. Local Type C fly ash from a major source in Mae-Moh, located north of Thailand, is partially replaced cement at three levels, that is, 35, 50, and 65%. The chemical compositions and physical properties of fly ash used are shown in Table 1. A ligno-sulphonatetype mid-range water reducer was added at 0.5% by weight of cement during mixing. Note that the results of this study may not be valid for non-lignin-based water reducers. Type 1 ordinary portland cement was used. The water used was tap water. The aggregates used were crushed limestone with a maximum size of 13 mm and coarse river sand. The mixture proportions are shown in Table 2. The slumps of mixtures with water reducer were kept at the same levels as those in the mixtures without. In this study, three programs were conducted to investigate chloride permeability and corrosion risk of HVFA concrete with a mid-range water reducer. A rapid chloride permeability test was conducted to study the ion penetration using 102 × 203 mm cylindrical specimens that were moist-cured for 28 days and continuously air-cured until the age of testing. Before the test at 90 days, specimens were cut into 51 mm slices at midheight for testing. The results of the rapid chloride permeability test were compared with those of 90-day ponding tests in a 3% NaCl solution.