Editor's note:This begins our regular every-other-month coverage of self consolidation concrete, one of the more innovative and important developments in the industry. The articles in this series will be adapted from presentations that were made at the November 2005 SCC symposium sponsored by the Center for Advanced Cement-Based Materials. We welcome your comments on this series.
Self-consolidating concrete (SCC) is one of the most innovative and flexible building materials introduced in recent years. Its introduction has provided concrete purchasers with a new means to maximize the productivity of their operations. Concrete construction contractors are now faced with the challenge of valuing this productivity benefit and deciding where to use SCC within their concrete practice.
This article provides an approach to quantifying the value to contractors of SCC. The value is primarily due to cost reductions in placement and finishing. Cost data from R.S. Means was used to develop a representation of the highest value SCC applications.
SCC has grown dramatically in worldwide usage and North American precast/prestressed concrete producers are converting to SCC in order to speed production, reduce labor costs, and generate precast members with high quality, bug hole free finishes. SCC accounts for over 40% of all non-DOT related precast concrete produced in North America today.
SCC is now under evaluation by cast-in-place concrete contractors. These contractors are interested in using materials that will provide overall cost savings, but they have little experience upon which to assess SCC's potential. Theoretically, SCC would enable faster placement of concrete into formwork with less labor and provide higher quality finishes with no need for “sack and rub” repairs.
Our study used average cost and productivity data for North America. This data was developed by R.S. Means under contract to Grace Construction Products to compare the costs of concrete placement and finishing within various cast-in-place applications. R.S. Means provided data on the overall cost of a concrete application in terms of cost per cubic yard of concrete placed. The cost components for any application are: formwork materials, formwork labor, reinforcement materials, reinforcement labor, concrete cost, placement cost, and finishing cost. Given that SCC impacts the costs of placement and finishing of concrete structures, this study identifies and targets those applications where the costs of placement and finishing are highest relative to the cost of concrete material. A group of contractors then estimated the degree of reduction possible within the target applications. This analysis formed the basis of an empirical model that estimates the potential savings for contractors who convert from conventional concrete to SCC.
Results and analysis
Figure 1 shows R.S. Means' detailed cost breakdown by building element. By averaging the data across building types for similar applications, the average costs per cubic yard of concrete placed for various concrete cast-in-place elements is determined. The contribution of placement and finishing cost to the overall cost of any element varies dramatically depending on the application considered.
Table 1 documents the ratio of placement cost and finishing cost to concrete cost for a various building elements or applications. This is based on an average cost for conventional concrete of $80 per cubic yard. The higher the number, the more expensive placement or finishing is in relation to the cost of concrete. In other words, the cost of concrete is less significant to the overall cost of constructing these elements—the other cost factors mentioned earlier are more important to the overall cost. The study targets applications with a greater than 0.35 placement cost ratio or a greater than 0.75 finishing cost ratio. These target segments are circled in Table 1.
Finishing costs seemed to increase as a “step” function, which is easily understandable, given that some elements do not require post-pour finishing. Strip footings, for example, are typically buried, so appearance is not considered important. With exposed walls, columns, and floors, however, appearance is critical and the impact of finishing cost becomes far more significant.
From this analysis, a list of applications was developed for further study. Contractors were interviewed with a focus on the potential benefits of SCC, the ability of SCC to meet the needs of the target applications, and the approximate magnitude of potential savings with each application. The initial conclusions were: