Innovative producers know a good concrete mix tests well in the lab and performs well in the field. Unfortunately, these dream mixtures are difficult to come by. Many producers have file cabinets full of mix designs containing unsuccessful labcretes, that on paper and in the lab tested well, only to fail to the rigors and variables of the field.
These days, innovative producers are working diligently to convert their labcrete designs for self-consolidating concrete (SCC) mix design into products suitable for field use. One of their challenges is the ability to prove the labcrete mix is field-worthy with user-friendly and timely test procedures. They realize that for contractors to accept this remarkable material, researchers must develop testing procedures that are repeatable and portable.
One of the key characteristics of a quality SCC is the fresh concrete's ability to maintain its designed viscosity. Technically, this quality is referred to as static segregation. If the mixture is too thin, the aggregates can't be supported by the mortar and tend to drop. In a form, aggregates become poorly distributed throughout the mix.
For several years, researchers have measured this quality using ASTM C 1610, “Standard Test Method for Static Segregation of Self-Consolidating Concrete Using Column Technique.” While a useful testing procedure for the lab, ASTM C 1610 is a labor-intensive procedure that is not suitable for rapid assessment needed on a jobsite or production floor.
As a result, many lab technicians use the Visual Stability Index (VSI) to judge an SCC's stability. This assessment is based on how an SCC mix looks after it's spread using ASTM C 1611, “Standard Test Method for Slump Flow of Self-Consolidating Concrete.” Technicians rate a mix's consistency by eye, and thus the index is qualitative and can be highly subjective.
At the 2007 Concrete Technology Forum, a research team from BASF Construction Chemical Admixture System's lab in Cleveland presented a paper outlining a new procedure that solves the problem of speed and accuracy. Their test method offers lab technicians a way to rapidly determine an SCC's stability
While currently under review by an ASTM subcommittee, the “Standard Test Method for Rapid Evaluation of Stability of Self-Consolidating Concrete” has generated excitement in the testing community. The BASF effort demonstrates that the procedure can allow lab technicians to determine an SCC's stability in the lab and the field.
A new mix of proven ways
In one respect, the newly proposed procedure is like a good mix design, combining well-proven testing methods into a new formulation. And based on the BASF research, the new procedure yields results that coordinate well with the column segregation lab test.
To start, technicians need their common pieces of field lab equipment—a flat, non-absorbent surface (used for slump flow test), a slump cone, and a strike-off bar.
The other main device is an alternative of a well-known testing procedure—a penetration apparatus. For this test, the penetration apparatus consists of a steel frame, a cylindrical slot, a reading scale, a release screw, and the penetration head.