blank space

South Dakota Department of Transportation
Project Synopsis

Title: Investigation of Low Compressive Strengths of Concrete in Paving, Precast and Structural Concrete
Project Researcher: Jon Kellar, SDSM&T, SDDOT
Project Manager: Jon Becker
Research Period: 2/15/1998 - 8/31/1999
Cost: $45,000.00

Problem Statement: Annually, reports of marginal or unacceptable compressive strengths of portland cement concrete from paving, precast and structural projects come in during the construction season. These reports normally come in from projects occurring at the end of the summer, and seem to represent isolated cases. In 1997, these reports occurred far more often than in the past. The 1997 failures occurred in mixes using various cements, aggregates and sands throughout the state. The failures were inconsistent, resulting in low compressive strengths on projects involving many different contractors, concrete mixes, and concrete plants. Most compressive strength problems have been in low production concrete placements - those involving barriers, culverts, or tight areas where the work is slow.

Although some of these low strengths may be due to poor cylinder preparation or handling, back up tests and cores have confirmed that many of the compressive strengths are below design strength and projects are being delayed while sufficient strengths are obtained. In most of these cases, strengths are below those obtained from laboratory mix designs and from earlier field results using the same cement factor and aggregates. Factors that may contribute to low compressive strengths may include but are not limited to high air content, flocculation of air bubbles at the aggregate-mortar interface, change in air entrainment admixture, change in cement chemistry, hot cement and aggregate, inadequate mixing, sand/rock ratio, temperature, and dirty aggregate.

Preliminary work has been done by SDDOT and SDSM&T to learn the extent of the problem. Scanning Electron Microscopy has revealed flocculation of air bubbles at the aggregate-mortar interface in two separate samples. Because of the work already underway, it is desirable for SDDOT and SDSM&T to continue the research in a cooperative effort. This may be the best way to get results prior to the 1998 construction season.

Findings: The researchers found the source for the low strengths to be poor aggregate-paste bonds associated with air void clusters and poorly formed cement paste in the interfacial region adjacent to the aggregate. An interaction between the “synthetic” air entraining admixtures, used as substitutes for vinsol resin, and low alkali cements was directly tied to the problem with high summertime temperatures also contributing to the problem. The "synthetics" seem to form thinner-walled air bubbles than vinsol resin, which can lead to significant reductions in strength. As a result of these findings, the Department now allows only vinsol resins for air entraining agents. Also, the maximum air content allowed was reduced from 8.0% to 7.5%. The technical panel recommended that current procedures for concrete plant inspection should be modified to provide a requirement for a minimum level of inspection.

Research Objectives:
1  Determine causes of low and variable strengths in concrete placed in South Dakota during the summer of 1997.
2  Determine solutions, and if applicable make recommendations for specification or policy changes.

Research Tasks:
1  Perform a literature search pertinent to low compressive strength problems in concrete.
2  Cooperatively develop a work plan to include a testing plan.
3  Meet with technical panel to discuss project, survey and scope of workplan.
4  Perform a survey of state DOT’s and other agencies nationwide (SDDOT). Contact South Dakota ready mix plants to obtain strength data and mix information.
5  Study the database of past SDDOT projects experiencing low strength failures. The analysis should identify any significant changes in the materials and construction practices used in 1997 construction projects with low compressive strengths when comp
6  Revise and submit the testing plan developed in the work plan that will analyze concrete constructed in 1997 with low compressive strengths and concrete placed in 1991-1995 using similar materials and construction practices. As a minimum the testing
7  Acquire samples of Vinsol Resin aire entrainment admixtures, concrete cores from projects constructed during 1991-1995, and 1997, as well as concrete mix design components representative of materials used in projects where low strengths were experien
8  Completed the laboratory testing as outlined in teh approved testing plan.
9  Meet with technical panel to discuss field testing and recommendations for the 1998 construction season.
10  Perform full-scale field testing as outlined in work plan and task 6.
11  Perform statistical analysis to determine relationship of possible factors to compressive strength (SDSM&T).
12  Prepare a final report and executive summary of the literature review, research methodology, findings, conclusions, and recommendations.
13  Acquire samples of Vinsol Resin air entrainment admixtures, concrete cores from projects constructed during 1991-1995, and 1997, as well as concrete mix design components representative of materials used in projects where low strengths were experienc

Documents Available:

blank space