International Concrete Abstracts Portal

Two different types of latex modifier were used to determine how curing conditions and latex-modifier content influenced compressive strength and durability. Freeze-thaw resistance in the presence of a 2.5 percent NaCl solution was evaluated by measuring both relative dynamic modulus and mass of scaled material. While all specimens were 14 days old at the start of testing, compressive strength increased as the period of initial wet-curing increased, durability factor values (ASTM C 666) were insensitive to the period of initial wet-curing, and scaling resistance was improved by increasing the wet-curing time. The air-void system, described by the spacing factor, was found to have a greater influence on durability and scaling than either latex-modifier content or duration of wet cure. A control mix made using a high-range water-reducing admixture (HRWRA) was used as a basis of comparison.

In October, 1994, CANMET in association with the American Concrete Institute sponsored a fourth conference on the superplasticizers and chemical admixtures in Montreal. The objective of this conference was to bring to the attention of the concrete community the new developments in chemical admixtures since the last conference in 1989. A total of 25 papers were accepted for publication in this special proceedings from the conference. If you are involved with superplasticizers and chemical admixtures, this special publication is a must. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP148

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.