Q. We are considering whether to bid to provide concrete storage tanks on a high-profile residential development project. The developer has opted to install a large septic system, rather than paying to hook up to the local waste water treatment system.
His decision has brought his development plan under the scrutiny of the local county sanitation department and opponents of the project. One of the requirements for the development's onsite treatment system is that pipe and tank connections must be free of leaks.
What standards can we provide the developer to help him assure the county engineer a quality connection will be the result?
A. ASTM Committee C-27 on Precast Concrete Structures recently approved a new standard covering this concern. C-1644-06, “Standard Specification for Resilient Connectors Between Reinforced Concrete On-Site Wastewater Tanks and Pipes,” provides guidance to the minimum performance and material requirements for connectors. This information is applicable to the connectors used on structures referenced in Specifications C 913 (Precast Concrete Water and Wastewater Structures) and C 1227 (Precast Concrete Septic Tanks).
The specification only covers the design, material, and performance of the connector. Field experience has demonstrated that a system's overall success also depends on properly selecting bedding and backfill, in addition to the care of the contractor's installation.
Nonetheless, the standard does provide the specifier assurance that resilient connectors meeting the testing requirements will provide a positive seal between the connector and the tank, and between the connector and the pipe in most applications.
Copies of the new standard specification are available for purchase at the ASTM Web site at www.astm.org.
Determining Mix Design Adjustments
Q. We've just installed a new batching system at our ready-mix plant. After start-up we continued production using our old mix designs. After a few months, we noticed our 28-day cylinders are consistently breaking at higher compressive strengths. It seems that the new equipment has not helped us run smoother, but it has enhanced our quality.
We'd like to demonstrate to upper management that these higher breaks are part of a pattern. We'd like to suggest to our managers that we can economize our mix designs, without the risk of ruining any of the trust we've built with our customers.
What is our best approach to this problem?
A. From your comments, it appears that your new batching operation has eliminated a principal source of concrete strength variation—poor mixing. The best way to analyze your situation is to adopt a statistical evaluation of your testing program.
Your best guide is the American Concrete Institute (ACI) document, “Evaluation of Strength Test Results of Concrete,” developed by ACI Committee 214. This provides an introduction to evaluating concrete strength tests and includes statistical procedures users will find useful in interpreting test result variations.
This might be a good time to reacquaint your quality control team with the document. Committee 214 just completed a thorough review, updating several sections. When it was originally introduced in the late 1970s, most statistical calculations were conducted by hand. The revision now takes into account the effect of new computer programs that can quickly analyze data.
It's important to remember that your test results represent the potential strength of the concrete. To develop a statistically accurate trend analysis, it's important to adopt an unbiased testing program. This includes establishing a random testing procedure. For example, you shouldn't only sample each day's first load of a particular mix design.
At the ACI Convention in November, Committee 214 presented a technical session on the Evaluation of Concrete Test Results, which provided information on how producers could use the statistical analysis proposed in the ACI 214 information to determine if a mixing operation is in control.
Committee member John Luciano, manager of product support at BASF Admixtures, demonstrated a software program that automated and simplified the statistical calculations found in ACI 214. The program allows users to enter raw test data and then calculate important information such as the graph of Probability of Low Strength.
Floor Coating Problems
Q. We are about to submit a bid for concrete that will be the slab for a new high school gym floor. We've heard that there have been problems on these types of flooring projects because of concrete dryness and flooring adhesive issues. Where can we research the most current technical advice on these projects?
A. Costs associated with construction delays, pending legal action, and loss of productivity resulting from flooring failures on slab-on-grade concrete are the leading claim categories in the concrete industry. Several years ago, the American Society of Concrete Contractors (ASCC) invested in a research project to help support their members in promoting the best practices in this area. Under the leadership of Bruce Supprenaut, ASCC developed “Moisture-Sensitive Floor Coverings on Concrete Slabs Position Paper #10,” describing how contractors view the problem.
Fortunately, there's an even better resource. Last month, ASCC released the Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. The book covers concrete moisture basics and testing, and vapor retarders/barriers, along with design and construction recommendations.
ACI 302 Committee and ACI's Technical Activities Committee reviewed the 42-page guide. You can buy the book at www.ascconline.org.