Modern concrete technology has uncoupled workability with water content through the use of surface active dispersants that interfere with the natural flocculation process of unhydrated cement in water. This has led to developments such as self-consolidating concrete (SCC) and high-strength concrete using silica fume. The use of these materials has not been as widespread in slabs on ground, in large part due to the concrete producer's poor understanding of possible side effects and reluctance to change, and to the installer's fear of unintended consequences.
The goal of this article is to discuss the benefits and risks of polycarboxylate-ether-based water reducers (PCEs) in slabs on ground and to review the precautions that should be taken to reduce the risks associated with their use.
Unlike the previous materials used as water reducers in concrete, PCEs are engineered chemicals that are made specifically for the concrete industry. They consist of two different parts: a main chain molecule that includes the surfactant portion, such as a conventional dispersant, which puts an electrical charge on the surface of the cement grains causing them to repel; and side chains, which impede the approach of water toward cement particle.
Chemists can design the main and side chains to change the properties of the PCE. The length of the main chain and the spacing of the charges along it impact the dose efficiency of the admixture. The length and spacing of the side chains affect the setting time of the concrete and the pot life of the admixture. The side chains also can be created long enough to have some viscosity-enhancing properties, thus improving the finishability and pumpability of the concrete, and allow the use of pozzolanic admixtures. The great advantage of these materials is that the dose efficiency and the ability to be accelerating, neutral setting, or retarding are uncoupled and under the manufacturer's control.
In order to discuss some of the reported problems, it is necessary to first discuss some aspects of how these admixtures work.
When you separate workability from the need to add water, the properties of concrete mixes offer new possibilities.
Credit: Joe Nasvik
How surfactants work
Air-entraining agents and water reducers are surfactants, meaning that they are active at the surface of things. The relevant surfaces in plastic concrete are the solid/water interface, such as at the aggregate surface or the cement particle surface, and the air/water interface, such as at the surface of a bubble. In the absence of surfactants, the surfaces of bubbles collide and two small bubbles become one large bubble. These larger bubbles quickly rise to the top of the concrete and dissipate. The addition of a surfactant, such as soap, creates bubbles that repel each other and won't coalesce—a phenomenon seen when washing your hands.
Air-entraining agents have a preference for the air/water interface. Water reducers prefer the solid/water interface. The surfactant nature of PCEs is such that it has no preference, and most, if not all, are blended during manufacturing with a defoamer.
Despite this, there are reported cases where the use of a PCE superplasticizer caused uncontrolled air content. Although it is difficult to hard trowel air-entrained concrete even when the finisher knows it is air entrained, it's virtually impossible if there is unintended air in the concrete. In part, the unintended air is due to the use of the admixture, the combination with the cementitious materials, and the timing of what was done.
Using polycarboxylate superplasticizers in concrete changes both the dynamics of placing and finishing concrete, as well as the ultimate functioning of floor slabs.
There are a number of flooring specialists who will not allow the use of PCEs in their concrete because of a few projects where there were problems with floor blistering. As with many things in the concrete industry, there is an attitude of one strike and you're out.
But experience can be misleading. The concrete slump of the concrete and the air-entraining agent dose required for a given air content are related. Anything that increases slump generally increases the air content. PCE is no exception. The addition of PCE at the jobsite can increase the air content significantly, particularly if there is not already PCE in the mix. This jobsite addition also can increase the setting rate of concretes that already have begun to set by distributing the hydrating materials. The purpose of PCEs is not to entrain air, but they are very efficient in supporting anything in the system that does generate air. In some low water-soluble alkali cements, this can include small changes in the alkali content, which can lead to a situation where trace elements in the cement can generate air.
These general rules of thumb can reduce problems with PCE dispersants.
- Determine the unit weight at the point of placement. No other test is cheaper or more reliable. This tool lets you catch changes in the mix, as well as detect the air without being confused by very fine bubbles not readily detected by the pressure meter.
- Trial batch the materials to be used on the job. If admixtures will be used for onsite adjustments, make sure that these adjustments are included in the trial batching. These are best performed at the anticipated temperature for the actual placement.
- Ensure that the concrete is adequately mixed after any PCE additions. This is critical not only at the plant but at the point of placement when adjustments are made onsite.
- If PCE will be added at the site, also add a small amount at the time of batching. This prevents rapid setting.
- Use a small amount of a low-range water-reducing admixture (ASTM C 494 Type A) to reduce the risk of air entrainment. The defoaming agents used vary by product and manufacturer, and a low-range water reducer will help if the defoamer is better than the superplasticizer.
- A dose of defoamer can be used to prevent accidental entrainment of air.
PCEs offer significant advantages over conventional materials for flatwork partly because of the viscosity modifying nature of their side chains. Some problems have resulted from a lack of care in the use of this material. This article has shed some light on these common problems and provided methods to address them when they occur.
Kevin MacDonald is vice president of engineering services for Cemstone, Mendota Heights, Minn.
Significant advances in dispersant chemistry have been made in the last decade. This includes the introduction and use of polycarboxylate dispersants across all segments of the concrete industry. Prior to that, most dispersant chemistries had limitations
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CC Field Test
Concrete Construction is conducting a field test to learn more about shrinkage and curling in warehouse floors. A 60,000-square-foot warehouse floor in Bartlett, Ill., was divided into 5000- to 10,000-square-foot sections in mid-February 2009, each receiving a different mix design. Naturally, curling and movement changes will vary in regard to the specific mix designs, as well as the ambient conditions, and each will be monitored for the next two years.