International Concrete Abstracts Portal

Water penetration and premature deterioration are widespread problems in reinforced concrete structures, particularly in the tropics. Many different chemical and mineral admixtures have been proposed as possible solutions. This paper examines high quality concretes modified with fly ash, silica fume, polypropylene fibre or certain chemical admixtures so as to reduce the permeability and hence make concrete more durable and watertight. Experiments were conducted at the National University of Singapore using different combinations of these admixtures supplied by four local manufacturers. A total of 39 mixtures were tested for pressure penetration and water absorption as well as compressive strength. The results show significant variation in the penetrability characteristics of the different concretes tested. Most products had some beneficial effect on pressure penetration, but little influence on absorption. One proprietary ingredient tested was found to greatly reduce both pressure penetration and absorption producing a concrete that was virtually completely watertight.

Granulated ground blast furnace slag is widely used as a partial substitute for normal portland cement in concrete. Among others, one of the perceived benefits is that slag cement improves the durability of reinforced concrete structures. Numerous investigations have reported enhanced resistance to steel reinforcement corrosion. There are two aspects to enhancing resistance to reinforcement corrosion. The first is the ability of the concrete to resist the penetration of chlorides. The second is the degree of corrosion once the chlorides have reached the reinforcing steel. This presentation considers the performance of specimens once chlorides have reached the depth of steel reinforcement. The Ministry of Works & Agriculture has monitored the performance of 96, 100 by 100 by 300 mm reinforced concrete specimens for ten years; exposed to the Bahrain environment. The environment is classified as an ultra hot climate with extreme temperature, extreme solar radiation and extreme chloride salt concentrations in the air and seawater. The specimens were cast with three types of cementitious material: 1) normal portland cement, 2) site-blended slag cement, consisting of 30 percent portland cement and 70 percent slag and 3) preblended slag cement, typically consisting of 30 percent portland cement and 70 percent slag. The exposure program includes two concrete mixtures (220 and 330 kg/m3 cementitious material), two curing regimes (good and poor) and two depths of cover (10 mm and 20 mm). The concrete was contaminated with 0.4 percent chloride and 1.5 percent sulfate by weight of cementitious material. The exposure specimens made with the slag cements had substantially greater carbonation depths and steel corrosion losses than the specimens made with normal portland cement. This is the result regardless of curing regime,cover to reinforcement and content of cementitious material.

In most concrete markets these days, there are several varieties of pozzolans and ground slag available for use in regular and high-performance concretes. Each one has its strong points when blended with portland cement in concrete and, properly used, will provide concrete of enhanced durability. Recently, concrete containing more than one such material has become common, even to the point of being available as ternarv or quaternarv blend. This paper reviews the data available on durability of concrete produced from‘ multiple blends and discusses some of the potential benefits to specifiers and users.

Choosing the optimum cementing material system is a key question when designing a concrete structure to last for a long time in a chloride environment. There is a lack of reliable comparative information on the short and long term performance of cementing material blends, incorporating materials such as silica fume, fly ash and slag. In this contribution, various cementing material blends are examined for their potential ability to resist chloride ion penetration. A range of mixtures was tested to assist designers in material selection for concrete in a chloride environment. Silica fume, blast furnace slag, Class C and F fly ash were examined at 0.30 and 0.40 water to cementing materials ratios (w/cm) for mortars and concrete. Ternary blends of normal portland cement/slag/silica fume and normal portland cement/slag/fly ash were also tested. The diffusion coefficients were determined by chloride migration using a D.C. potential gradient, and by chloride pending/profile grinding. Silica fume was found to be essential to obtain low diffusion coefficients particularly at an early age. Some additional reduction in diffusion coefficient is obtained with ternary blends of silica fume plus slag or silica fume plus Class F fly ash.

The resistance to chloride penetration is one of the prime parameters in specifying concrete in marine applications and in a quality assurance scheme during construction. The aim of this research was to compare alternative accelerated laboratory test procedures for the assessment of chloride penetration into concrete containing supplementary cementitious materials. Three binder systems, a normal portland cement (PC), one with 30% fly ash and a third with 50% slag, were investigated under three curing conditions, 7 days water curing, air curing, and 12 hours 65 C water curing. Chlorides penetration was measured by static ponding (5% NaCl solution) and cyclic ponding (2%, 5% and 15% NaCl solutions with 12 hour wetting and drying cycles). The effect of the age at start of testing and the test duration were also investigated. The water absorption was tested by measuring weight gain, and the results were compared with that of the chloride penetration tests. It was found that the cyclic ponding test with 15% NaCl solution resulted in accelerated chloride penetration and a clearer chloride penetration front compared to the use of 2% and 5% NaCl solutions. Good correlation was found between the results from cyclic chloride ponding and static ponding. While the chloride penetration depth was found not vary significantly with test age from 7 to 56 days for all the three mixes, the increase of chloride penetration with the test duration from 3 to 14 days was much pronounced for the PC mix than the fly ash and slag mixes. The chloride penetration was highest in the air cured specimens and the lowest in the water cured specimens for all the three mixes, and the slag concrete had the lowest chloride penetration within the three mixes under each of the three curing conditions. It was also found that water absorption correlated poorly with chloride penetration.