International Concrete Abstracts Portal

High-performance concretes (HPC) typically have low w/c to achieve the desired levels of strength and durability. As a result, HPC have a tendency to be stiff and lose their workability rather quickly. Often, high-range water-reducing admixtures (HRWRA) are used to improve the workability of HPC. Care must be exercised when using any admixture, or combination of admixtures, to insure that there are no detrimental side effects that might shorten the life of the concrete. Research has shown that, although retempering concrete with an HRWRA will generally improve workability and maintain the strength of low-w/c concretes, it may also reduce freeze-thaw resistance. Therefore, an experimental study was

Presents a detailed investigation of the role and effectiveness of ground granulated blast furnace slag and a high-range water reducer (HRWR) on the quality of concrete in terms of bleeding, setting times, heat evolution, strength development, and pore structure. The tests were carried out in two parts. In the first, a slag of normal fineness was used, and both the replacement level and water-binder ratio were varied. It was found that both the slag and HRWR acted as set retarders in terms of setting times and heat evolution. The water-binder ratio was the predominant factor affecting the rate of bleeding. The presence of slag, on the other hand, caused low-early strength and slow strength development, but had significant beneficial influence on the total pore volume and pore size distribution. In the second part, fineness of slag was varied from 453 to 1160 m 2/kg and the replacement level was kept constant at 50 percent. It was then possible to obtain compressive strength in excess of 30 MPa at 3 days and 100 MPa at 28 days, with substantial reductions in total porosity and water permeability. The bleeding rate was also reduced and the setting times also improved. The overall conclusion of this study is that a judicious combination of HRWR and slag fineness can lead to a very effective synergic interaction to produce concretes of high strength, high modulus, and low porosity.

It is well known that to successfully pass ASTM C 666 (Procedure A) for rapid freeze-thaw resistance, normal strength concrete must contain an adequate amount of entrained air composed of minute air bubbles with the right spacing factor. As concrete slump is increasingly restored at the jobsite using superplasticizer instead of retempering with water, it is essential that slump increase does not alter the total air content and air-void system if the concrete is to be frost-resistant. Since mixed results have been reported when superplasticizer is added to air-entrained concrete at the jobsite, a research program was undertaken to study the compatibility between three air-entraining agents, four water reducers, and one polynaphthalene sulfonate superplasticizer currently used in Eastern Canada. Experimental results conducted on 12 different combinations of admixtures with a Type 10 (ASTM Type I) portland cement show that the addition of superplasticizer nearly always increased the air content without changing the bubble spacing. The only case in which the air bubble spacing was significantly altered was when the air content of the concrete was lower than 4.5 percent 70 min after batching. In this case, the total air content decreased after the introduction of the superplasticizer, while the spacing factor increased significantly. A second Type 10 cement was used to duplicate these results. No significant difference was found between the results of the two sets of experiments.

The key to casting high-strength concrete with compressive strength of more than 100 MPa into complicated reinforced structures is to give the concrete high fluidity as well as to improve strength. The authors developed an acrylic copolymer-based new superplasticizer that can improve fluidity of concrete with water-binder ratio of around 0.20. Paper presents results of a series of studies conducted to determine the properties of fresh and hardened high-strength concrete using the newly developed superplasticizer. The effect of the new superplasticizer was examined with varying water-binder ratio, type of cement, and temperature compared with conventional superplasticizers. The new superplasticizer needed a much lower dosage than conventional superplasticizers to attain a certain slump (250 mm) for a water-binder ratio of around 0.20, and it significantly reduced concrete viscosity. Sufficient workability was kept for 2 hr without much delay in setting time, while conventional superplasticizers showed large slump loss and excessive delay in setting time. Results of strength development, drying shrinkage, and freeze-thaw resistance did not show any harmful effect. Field studies were conducted on application of the high-strength concrete to a prestressed concrete bridge with design strength of 100 MPa using the new superplasticizer. Workability and strength development of concrete were tested and resulted in sufficient quality.

High-strength concrete with air-entraining high-range water-reducing admixture, 0.35 water-cement ratio, 148 kg/m 3 unit water content, and about 60 MPa compressive strength, was produced, and the admixture content and sand percentage were varied. The consistency property was measured by the slump test, and the compaction property and segregation resistance were determined by the flow time through an inverted slump cone. An appropriate mixture that balanced these properties relative to the admixture content and sand percentage was determined from the test. The appropriate mixture can be consolidated by using a shorter vibrating time and lower frequency than in the case of ordinary concrete.