Self-consolidating concrete (SCC) is one of the newer “state-of-the-art” concrete products in the marketplace. It has enjoyed wide acceptance in the precast concrete industry and is beginning to be used by cast-in-place contractors, especially wall contractors. Most decorative contractors have had little experience with SCC, but it can greatly improve the look of decorative work such as precast countertops, walls using formliners and architectural surfaces, and concrete sculpture using molds. SCC is ideal for filling artistic molds. It will fill small spaces and negative draft areas with few bug holes. And because SCC produces high-early strength, molds can be stripped more quickly.
SCC is very flowable. When you place it in one location it flows and moves under its own weight without any segregation of aggregates. Depending on your requirements, you can make it flow as far as 100 lineal feet from its point of placement in a form. SCC's advantages include higher strength than customary concrete mixes, external vibration isn't required for consolidation, very few bug holes will appear on finished surfaces, less labor is required for placement, and it is easier to place into forms with congested steel reinforcement.
SCC mixes can be designed by adapting more standard gap-graded concrete mixes or by developing “well-graded” mixes with several sizes of aggregates to reduce the voids between aggregates. Well-graded mixes make it possible to use less cement which results in less shrinkage. Sometimes SCC mixes are “thickened” by adding a viscosity modifying admixture (VMA) to minimize aggregate segregation; polycarboxylate superplasticizers are always used to provide the flowability.
SCC for walls and formliner work
A typical problem for wall construction using decorative formliners is that bug holes can compromise the look. Internal and external mechanical vibrators can reduce bug holes but placing SCC can practically eliminate them. George Ryan, the engineer of project implementation for the Illinois Department of Transportation (IDOT) District 4, says they decided to use SCC for the entire retaining wall fascia for the rebuilding of the I-74 freeway through Peoria, Ill. “To date we have placed over 40,000 cubic yards of SCC with spreads running between 26 and 28 inches and the results are amazing. The work is free of bug holes and in locations with congested steel reinforcement the placements were easy and fast,” he says. IDOT contracted with the University of Illinois during the second stage of construction to measure form pressures, believing that the liquidity of SCC would cause higher lateral pressures against forms. They discovered, however, that a structure develops within the concrete matrix shortly after it is placed in the forms and before it actually sets. This gelling effect reduces form pressure.
Concrete counter tops
One of the problems with precasting concrete countertops is that the finished surface is usually at the bottom of the mold where bug holes can form and they aren't discovered until it's too late. SCC can solve that problem but there are a few things to keep in mind.
- Mix and place all the concrete at one time, if possible. Multiple mixes can result in observable pour seams and discolorations when the mixes aren't identical, especially when integral colors are being used.
- When you fill a bucket with SCC, tap the sides to release entrapped air from the mixing process.
- Start pouring into the form at one location, working from one end of the form to the other.
- To minimize entrapped air, place the concrete on top of itself so that the concrete moves outward from its own weight. A good practice is to move your hands (in rubber gloves) through the concrete along all the form surfaces to push concrete into corners and ensure that small air bubbles aren't trapped against the form.
Learn more about SCC
Although SCC is still new its use is growing worldwide. Some contractors (including decorative) are experimenting with SCC to pour (in this case the word “pour” is more descriptive than the word “place”) flatwork in difficult areas and to reduce labor costs. A great source of information on SCC is the proceedings from the Second North American Conference on the Design and Use of Self-Consolidating Concrete and the Fourth International RILEM Symposium on Self-Compacting Concrete sponsored by the Center for Advanced Cement-Based Materials. It is available at http://acbm.northwestern.edu/index.html