International Concrete Abstracts Portal

The paper describes the physical characteristics of pumped high-strength, high-performance concrete for a large (26,480m3) 50m high concrete slipform placement where the air content in the formwork was intentionally varied from 2-3% in the lower portion of the placement to 4-6% in the upper portions of the placement. Included are evaluations of the slump, air content and density of the unhardened concrete at the batch plant, in distribution hoppers after pumping, at the formwork before vibration, and in the formwork after vibration and re-vibration. A special tub test was developed to approximate the unhardened characteristics of the pumped concrete in the formwork without having to remove concrete from the formwork to do the testing. The concrete was a modified normal density concrete where a portion (45% by volume) of the normal weight coarse aggregate was replaced with structural lightweight aggregate to reduce the concrete density. High slumps (210-230mm) were used to facilitate pumping, and to accommodate extremely congested reinforcing bar situations. Concrete cylinder strengths of 74 to 78 MPA were obtained at 28-days age for concretes having air contents from 4 to 6%. Hardened concrete air-void parameters indicated fewer but larger air voids than what might normally be expected for durable concrete yet the freezing and thawing behavior in water (ASTM C666, Procedure A) for 500 cycles showed no change in the quality of the concrete.

Curing of concrete is essential for reliable performance of concrete structures. The recommendations concerning curing of high-strength concrete are contradictory. The traditional ways of curing fail in the case of high strength concrete. A higher porosity in the vicinity of edges, microcracks due to self desiccation and shrinkage, and reduced compressive strength affect the durability of high performance concrete. Therefore, another approach is followed which consists of a new idea for supplying curing water in the interior of the concrete, by using lightweight expanded clay aggregates. When a shortage of water in the hydrating cement paste occurs, the water from the lightweight aggregates is transported by capillary suction or by capillary condensation into the smaller pores of the cement paste, thereby permitting continuous hydration. About 25% by volume of the aggregates are lightweight. The improved durability due to the higher degree of hydration, an improved density of the hydrated cement paste, less drying shrinkage and higher com-pressive strength have been shown by experiments. Test results from differen-tial thermal analyses, X-ray diffraction, mercury porosimetry, water absorp-tion, drying shrinkage and compressive strength are presented and compared with normal weight concrete with 100% natural aggregates.

This paper outlines the historical developments in the use of fly ash in Australia, Malaysia and Thailand. The slow but marked change in the philosophical approach to the use of fly ash concretes is discussed. With more focused research into the durability performance of fly ash concretes for specific environments in Malaysia and Australia, fly ash concretes in both countries are gaining acceptance into this new area of applications. This is also reflected in some of the most advanced specifications. It has been found that for concretes of equal 28-day strength, the use of fly ash in concretes resulted in a better resistance to chloride ingress. This was also shown in the lower coulomb values in the ASTM C 1202. Higher strength grade portland and fly ash concretes gave lower coulomb values. Other related work had shown that the use of fly ash could result in a lower corrosion rate of steel reinforcement in concretes. Hence a significantly longer service life could be expected from fly ash concretes compared with portland cement concretes of equal strength. In high-performance concretes, it has been found that the use of triple blends of either a slag or a silica fume added to the fly ash/Portland chloride resistance of the concretes. cement system could further enhance the

The corrosion resistance of reinforced concrete, containing fly ash and calcium nitrite-based corrosion inhibitor, under sea water attack was studied in this research. Aggressive and normal construction conditions were simulated. Test results indicate that the use of calcium nitrite-based corrosion inhibitor is an effective way to protect reinforcing steel from corrosion in aggressive environments. No serious detrimental effect was found for the corrosion inhibitor on concrete properties. Replacement of 20% cement by fly ash gives no apparent improvement in preventing the corrosion of steel in sea water exposure.

This research was carried out to evaluate the effects of source and amount of fly ash on s,trength and durabil ity properties of concrete. Mechanical properties considered were compressive strength, tensile strength, tlexural strength, and modulus of elasticity. The durability-related properties considered were: shrinkage, abrasion resistance, air and water permeability , chloride permeability and salt scaling resistance of concrete. A reference concrete was proportioned to attain the 28-day compressive strength of 41 MPa. Three sources of Class C fly ash were used in this work. Fly ash from each source was used at three levels of cement replacements (40, SO, and 60% ) in producing, concrete mixtures. The water-to-cementitious materials ratio was maintained at 0.30 + 0.02 for all mixtures. In general strength and durability-related properties of concrete were considerably affected by both the tly ash source and amount of fly ash. Also, the strength properties and durabi lity for the 40% fly ash mixture were either comparable or superior to the no-tly ash concrete. The salt scaling resistance of fly ash concrete was either comparable to or better than the no-fly ash concrete, except for one source of fly ash at 60% cement replacement level. All the mixtures, with and without tly ash, tested in this investigation conformed to the strength and durability requirements for excellent quality structural grade concretes.