Working with Self-Consolidating Concrete

Question:
I have been hearing a lot about the advantages of using self-consolidating concrete. What is it, and what are the applications for decorative concrete?

Answer:
Self-consolidating concrete (SCC) is a highly flowable concrete that can spread through and around dense reinforcing under its own weight to adequately fill voids without segregation or excessive bleeding, and without the need for significant vibration. In the decorative concrete arena, it is used to produce castings with a high surface quality, often with few or no pinholes. Less labor, quicker casting times, better surface finish, and increased concrete densities are common reasons for choosing SCC.

Because SCC flows so readily, the flowability is measured in terms of spread instead of slump. The slump flow test (ASTM C 1611) is similar to a standard slump test (ASTM C 143), but instead of measuring a vertical height change, a horizontal spread measurement is made. Typical spread values range from about 20 to 30 inches (see photo).



Self-consolidating concrete is highly flowable and can spread
as much as 30 inches. Note how there is almost no
segregation, bleeding, or variation.

SCC is achieved by designing a mix that has a low yield stress and an increased plastic viscosity (see figure). In other words, the mix should require minimal force to initiate flow yet have adequate cohesion to resist aggregate segregation and excess bleeding. Lowering the yield stress without increasing the viscosity causes segregation and bleeding, both of which result in poor-quality concrete. The yield stress is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity-modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.
















The preferred admixture for reducing yield stress in self-consolidating concrete is a polycarboxylate-based admixture, due to its superior water-reduction capabilities and high-early-strength gains at low dosing rates. This new generation of synthetic admixture has been specially designed to increase the dispersion of the cement particles, which aids in plasticity and strength and can help with pigment dispersion.

However, conventional concrete cannot be transformed into SCC merely by adding the right admixtures. Aggregate shape, sizing, gradation, and cement and water contents all have a powerful influence on the self-consolidating properties. The admixtures and all of the ingredients must be carefully selected and proportioned to preserve the SCC properties. A well-graded aggregate distribution minimizes cement paste content as well as admixture dosage.

When working with SCC, keep the following factors in mind:

• -Self-consolidating concrete has thixotropic characteristics, meaning that in the absence of energy (vibration) the concrete will stiffen on its own. This phenomenon may lead to problems with pour lines between consecutive lifts. These pour lines can be eliminated by simply vibrating for a short period of time, causing the two pours to flow together.
  


• -Moist curing is beneficial to SCC because it often has a very low water-cement ratio. Providing a continuous water source to the concrete as it cures will help ensure that the capillary pores are filled and the hydration reaction continues to take place. Increasing coarse aggregate contents will also reduce plastic shrinkage, but again this may affect the fresh properties of the SCC mix.
  


• -With concrete countertops,shrinkage-induced curling is always a concern. Drying shrinkage for SCC is very close to that of conventional concrete. However, self-desiccation could cause plastic shrinkage during the first 24 hours. This occurs in very low water-cement-ratio mixes (below 0.40) and is the result of hydrating cement particles consuming the available free water during the hydration process. As the water is consumed, the capillary pores within the concrete partially empty, causing the internal relative humidity to drop considerably. This could lead to bulk shrinkage, resulting in internal microcracking and affecting the overall strength and durability of the product.