The ACI Tolerances committee spent a grueling three and a half hours in St. Louis on Nov. 4 working through 45 comments from the Technical Activities Committee (TAC) on revisions to ACI 117, Specification for Tolerances for Concrete Construction and Materials. The TAC review is one of the last steps in document development before it is released for publication.
Major topics of discussion included floor flatness, reinforcement location and cover, measuring plumbness on vertical walls, location of dowels and sawcut joints, and abovegrade floor slab thickness issues. The committee's responses now go back to TAC and the document continues to work its way through the process toward approval and publication.
One key point that resurfaced throughout the meeting was the document is written for the benefit of contractors. However, it cannot tell the specifier what to do. It is still up to the individual to incorporate tolerance provisions into the project specifications.
Tolerances also were the focus of a Tuesday afternoon session sponsored by the ACI Missouri Chapter. Ward Malisch, technical director of the American Society of Concrete Contractors (ASCC), opened with an historical overview. Tolerances were addressed first formally within ACI in a 1940 paper by John Nichols, Malisch said. In 1963, ACI 347-63, which dealt with formwork, became the first ACI document to include tolerances.
Research in the early 1970s documented that finished work often deviated much farther from design than commonly expected. The original ACI tolerances standard was put out for public discussion in 1980. After considering a large volume of responses, the first ACI 117 tolerances standard was published in 1981.
Refinements have been made since then, Malisch continued, including things like the addition of the F-number system for measuring floor flatness. However, he says, the ACI 117 standard still includes “many tolerances set as early as 1940, with no as-built data to validate the tolerance. And we've set new tolerances similarly.”
In summary, Malisch said that ACI tolerances not currently based on as-built data need to be; that loosening tolerances to be more realistic will not necessarily make structures unsafe; that measuring protocols are needed; and that tolerances must be written so they cannot be misunderstood. He also observed that progress is being made on these issues.
Heather Brown, associate professor at Middle Tennessee State University, followed with “Are Current Stair Tolerances Achievable?” She presented field data from a number of concrete stairways that her students recently measured. The first challenge, of course, was establishing a reasonable measurement method. The current construction tolerances in the National Fire Protection Association's widely adopted Life Safety Code allow no more than a 3/16-inch difference in riser height between any two adjacent treads and a 3/8-inch maximum difference in riser height for any of the stairs in a flight.
The students measured the nose-to-nose height 1 foot from the wall on each side of a stairway, sometimes adding an additional line of measurements where the stairs were more than 4 feet wide. Results showed three things. First, repeatability in measuring riser heights is not always within the 3/16-inch tolerance, making it difficult to get meaningful data. Second, a large portion of the riser heights measured fall outside the tolerance. Finally, the variation in riser height along an individual tread—from one side of the stairway to the other—can be surprisingly large.
Scott Tarr, of Concrete Engineering Specialists, used concrete ramps as the example by which to address the question “Are Current ADA Tolerances Achievable?” Tarr and his colleagues measured the longitudinal and cross slopes on a number of ramps in several different states and compared them to accessibility guidelines established by the Americans With Disabilities Act. The results show a need for the development of reasonable tolerances that can be used in this area as well, partly because the guidelines are fairly rigid.
To close the session, Bruce Suprenant, also with Concrete Engineering Specialists, presented an overview of the ASCC “Tolerance Manual for Concrete Buildings.” It brings together in one publication data from many different sources—90% or so from published sources, he notes. The new publication is expected to be available in early 2009.