Many American Concrete Institute (ACI) documents, especially guides and reports, contain statements unaccompanied by any cited data that supports them. Some of these are remnants of language used in earlier versions of the documents, which may or may not have cited data or explained the rationale for the statement. In the consensus process for developing such documents, anecdotal evidence is almost certainly responsible for some of the unsupported statements. But we know that any beneficial use of anecdotal evidence in creating guides for construction can be limited by the number of uncontrolled — and unreported — variables present on every jobsite.
Construction tolerances based on what?
Section 9.4 of ACI 522R-10, “Report on Pervious Concrete,” includes the following statement:
Tolerance for thickness reported as the average of three cores of each lot should not be more than 14 inch (6 mm) less than the specified thickness, with no single core exceeding 12 inch (13 mm) less than the specified thickness, nor should the average compacted thickness be more than 112 inches (40 mm) more than the specified thickness.
There is no citation or reference to data that shows this to be a reasonable tolerance. But there is a wealth of data showing that this is an unreasonable tolerance for concrete floor slabs, which are usually placed on a smoother base course than the open-graded base courses often used under pervious pavements.1
So how was the tolerance set: Arbitrarily? Using anecdotal evidence from a committee member or committee members? Remember the often used statement that two anecdotes don’t comprise a data set.
What causes layer lines?
Section 3.8 of ACI 309.2R-98, “Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces,” defines layer lines and causes as follows:
Layer lines are dark horizontal lines on formed surfaces that indicate the boundary between concrete placements. Layer lines are caused by premature stiffening or insufficient consolidation of the previous layer of concrete due to lack of penetration of the vibrator into that layer, or the use of a mortar bonding layer between placements.
Again, no citation shows that the list of causes is complete or indicates the conditions that lead to conclusions about causes. As stated by Steven Novella in his article in Science-Based Medicine:
The primary weakness of anecdotes as evidence is that they are not controlled. This opens them up to many hidden variables that could potentially affect the results. We therefore cannot make any reliable assumptions about which variable (for example a specific treatment) was responsible for any apparent improvement.2
Are there hidden variables that affect the formation of layer lines? We think so.
Effects of form release agents on rebar
Section 5.3 of ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” states the following:
At the time of concrete placement, reinforcing steel and embedded items should be clean and free from mud, oil, and other materials that can adversely affect the steel’s bonding capacity.
This statement is based on a nearly identical requirement that has appeared for many years in ACI 318, “Building Code Requirements for Structural Concrete.” The Code uses the phrase “…that decrease bond,” but in the ACI 318 Commentary, no publications with data are cited to show that oil, in particular, decreases bond. Research published in 1968 showed that a normal amount of rust on reinforcing steel increased bond. This research is cited in the ACI 318 Commentary. Research on the effect of form release agents on bond showed that there was no adverse effect.3 However, contractors are often required to remove form release agents from rebar, usually increasing labor costs and slowing progress.
Moving concrete laterally
Section 5.4.1 of ACI 304R-00 states that concrete should be deposited at or near its final position because it tends to segregate when it has to be flowed laterally into place. Section 126.96.36.199 in the 2007 ACI Manual of Concrete Inspection includes an even stronger statement: