Since its introduction in Japan in the late 1980s, self-consolidating concrete (SCC) has demonstrated substantial commercial benefits to the construction industry in terms of ease of placement, need for minimal or no vibration, improved form finish, and the ability to use highly congested reinforcement as well as formwork with restricted access.
Successful construction with SCC can only be achieved when the SCC mixture exhibits proper rheological parameters such as yield stress (which can correlate with slump-flow), viscosity (rate of flow once yield stress has been exceeded), and thixotropy (change in viscosity as a function of agitation) during mixing, at the time of discharge, and after placement. These parameters can be used to understand the key SCC performance attributes of workability, segregation resistance, and passing ability, and can be generally correlated with three tests described in the ASTM C1611 standard slump flow, T20 (or T50), and VSI, which are commonly used for routine quality control operations. Alternately, the more fundamental rheological properties such as yield stress and viscosity can be measured using a concrete rheometer.
Admixtures for SCC
Advances in the development of new admixtures such as polycarboxylate-based superplasticizers, viscosity modifying agents, and powerful slump retaining products have been a key enabling factor for the production, placement, and service life of high-quality SCC. Polycarboxylate (PC)-based superplasticizers have become the most common form of superplasticizer technology for SCC mixtures because of a number reported benefits such as powerful cement dispersing ability to achieve high slump-flows, extended workability retention, and increased thixotropy of the in-place concrete to reduce surface settlement upon placement (static stability).
These performance attributes are made possible because the polymeric structure of polycarboxylate-based superplasticizers can be configured for a wide range of concrete applications, including SCC mixtures. There are circumstances when use of a polycarboxylate-based admixture may not be feasible due to high clay content contamination with certain aggregate sources or where very low dosages of air-entraining admixtures are required to meet air content specifications. In such cases, a naphthalene-sulfonate-based product may be necessary.
Viscosity modifying admixtures (VMAs), comprised of biopolymers, cellulose ethers, high molecular weight glycols, and various synthetic anionic polymers, impact the flow property of concrete, whereby they can increase the viscosity of the cement paste, enhancing its capability to suspend heavier materials in the concrete matrix. Figure 1 illustrates the fundamental role that superplasticizers and VMAs have in changing the slump-flow and viscosity of concrete mixtures. Figure 2 illustrates the impact of VMAs in controlling concrete viscosity as a function of added water content to an SCC mixture. The balance between the slump-flow and plastic viscosity (T20 value) is key to obtaining the optimum SCC rheology and stability.
Euclid Chemical’s William Phelan discusses the Self-Consolidating Concrete program at World of Concrete. Planning and communication are critical when dealing with any high-performance concrete, Phelan says.