Concrete that flows into rebar-congested forms under its own weight, with no vibration and no segregation, is the dream of every contractor. Today, most ready-mix producers can provide that dream material—self-consolidating concrete (SCC). So why then are SCC mixes still the exception and not the rule?
A basic understanding of how to produce self-consolidating concrete has existed in North America for at least 10 years. Combine the proper amounts of coarse and fine aggregate with cementitious materials, fines (or “powder”), water, and admixtures, and the concrete will flow. Such a mix is easy to place, even with congested reinforcement, is easy to pump, and produces very high quality concrete surfaces.
Most importantly for the contractor, SCC needs no consolidation, eliminating vibrators.
“I could never keep a good vibrator operator,” says one contractor, “because the job usually was assigned to the lowest ranking laborer and as soon as I got him trained he would find a new job.”
With all those things going for it, SCC would seem an obvious choice for any concrete going into formwork. So why is it still relatively rare in site-cast construction? One reason is that simple standardized test methods did not exist for several years. That mostly has been resolved with the slump flow test and the J-ring test. But the two greatest barriers to acceptance are cost and the difficulty producers have had with consistency and robustness.
Today new methods are available that overcome these obstacles, including a new way of proportioning concrete and reducing cement content by a company called iCrete, Beverly Hills, Calif. Properly proportioned SCC mixes can achieve all the attributes of SCC at less cost and with more robustness—you get the same concrete every time regardless of slight variations in the ingredients.
Testing and tolerances
There are three primary characteristics of a good self-consolidating concrete:
- high flowability, which allows the SCC to fill forms without consolidation
- passing ability, which allows the concrete to flow through closely spaced reinforcement
- resistance to segregation, often referred to as stability, which implies no separation of the paste and aggregate
Flowability is measured using the slump flow test, now standardized as ASTM C 1611, “Slump Flow of Self-Consolidating Concrete.” This is basically a standard slump test, although the slump cone often is inverted to make it easier to fill. Rather than measure the vertical slump, because the concrete spreads out to a thin layer, the testing technician measures the diameter of the resulting concrete patty. Typical SCC mixes have slump flows between 19 and 30 inches.
Establishing tolerances on slump flow is critical to preventing rejection of batches. For example, the new Revel resort in Atlantic City, N.J., includes two 1900-room hotel towers and 650,000 square feet of casino and dining/retail/entertainment space. SCC proportioned with the iCrete system is being used for all formed concrete, such as columns and walls. The tolerance on the SCC mix is ±2 inches on a specified spread of 25 inches. This tolerance is the same regardless of the specified strength, which ranges from 6000 to 12,000 psi. Other project specifications have different tolerances—the tolerance on a recent Australian project was 700 mm ± 20 mm (27.5 inches ± 0.8 inches). Such a tolerance is unreasonable.
Based on testing done for ASTM using multiple laboratories and multiple technicians, spreads can vary up to 3 inches between testing technicians, even if they use the same test apparatus. According to Colin Lobo of the National Ready Mixed Concrete Association, ASTM subcommittee C 09.40 has balloted a tolerance to include in C 94, “Specification for Ready Mixed Concrete,” that would establish a tolerance of ±2.5 inches on slump flows greater than 22 inches (±1.5 inches on slump flows less than 22 inches). This seems a reasonable tolerance on such an imprecise test.