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.

A variety of new literature and data on the properties of cement pastes and concentrated slurries of various types of mineral particles are examined to elucidate the origin of the fluidification of cement pastes by superplasticizers. The influence of sodium poly-¯-naphthalene sulfonate superplasticizers (NaPNS) of different molecular weights on the rheological properties of pastes and on the early heat of hydration of cement, together with results from other physicochemical measurements (adsorption, zeta potential), suggests that the unique fluidification effect of these admixtures depends on at least four distinct phenomena. With reference to fluidification of slurries of "inert" minerals, the superplasticizer effect in concrete can be understood in terms of nonspecific (physical) and specific (chemical) effects. The "physical" effects comprise: adsorption of the superplasticizer molecules by van der Waals and electrostatic forces (direct or assisted by cations); surface charging that induces long-range interparticle repulsive forces; steric hindrance between adsorbed polymer molecules on neighboring particles, leading to added short-range repulsive forces. The "chemical" effect involves a reaction of the PNS superplasticizer molecules with the most reactive sites of cement particles (particularly C 3A), substantially reducing the initial surface hydration rate. This description is largely based on data relevant to PNS-type superplasticizers, but with proper allowance for specific chemical effects, it should also be valid for other types of superplasticizer.

Influence of an acrylic polymer on the rheology of mortars was investigated using a mixer in which the torque on the impeller shaft was continuously measured. The polymer was added to specific mortars either alone or in combination with aqueous solutions of sulfonated naphthalene formaldehyde condensate or sulfonated melamine formaldehyde condensate. Two cements were ground from two different clinkers to specific surfaces of 270 and 400 m 2/kg, respectively. The flow properties of these fresh mortars closely approximate the Bingham model, whatever the time after initial mixing may be. When used alone, the polymer decreases the plastic viscosity of the mortar. When used in combination with sulfonated melamine or naphthalene formaldehyde condensates, it decreases the yield value.