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Cover of Welded Wire Fabric in Slabs and Pavements By Luke M. Snell, Published in the July 1997 issue of Concrete Construction Many design professionals choose not to use welded wire fabric (WWF) in slabs or pavements because they believe that the amount of cover can't be controlled. In some cases, their concerns are justified. I have investigated projects where workers used the "hook-and-pull" method to control WWF cover. This method is usually doomed form the start, since workers attempt to use hooks to pull the fabric up into position while they are standing on it. In these projects, the WWF usually ends up in the bottom inch of the slab. Design specifications, however, require WWF to be in the upper portion of the slab. WWF cover - or vertical spacing - can be controlled when supports or a two-course placing method is used. (The horizontal spacing of the steel is predetermined, since the wires are held in position by the fabric's welds.) Unlike the hook-and-pull method, using supports — such as chairs or precast concrete cubes — holds the steel firmly in place during construction. If a two- course placing method is used, the bottom layer of concrete supports the WWF. Even when these support methods are used, some design professionals still have two major concerns about the use of WWF:
If the answer to these questions is yes, the WWF will be at the specified location and the slabs or pavements will perform as designed. To address these concerns, research was conducted in 1996 to determine if WWF was at its correct location in a completed slab or pavement in which supports or a two-course concrete placement was specified. Three projects in the United States were investigated, two of the slabs within buildings and one a pavement (see Case Histories, below). The projects had been in use from two to 30 years, and the owners were satisfied with their performance. Tolerance Limits for WWF Cover ACI 117-90 (Ref. 1) does not provide tolerance limits for WWF. The standard's preface to the specification checklist (P5) states, "Tolerance for fabrication, placement, and lap splices for welded wire fabric must be specified by the specifier." Thus, specifications that give a single value for cover do not provide enough information about acceptable tolerances. I contacted several designers who specialize in slabs and asked them what tolerance for cover they use or would consider acceptable. Following is a summary of their comments:
The tolerance limits of the upper one-third to one-half of the slab thickness were consistently cited. Therefore, these limits were used to evaluate each of the slabs in the research. In pavements, WWF must meet state department of transportation (DOT) specifications. Because the pavement that was investigated is in Illinois, the Illinois DOT tolerance limits for cover of WWF in pavements were used. The Illinois DOT specifies "a placement tolerance for individual bars or individual wire of ±25 mm (±1 inch) horizontally and vertically" (Ref. 2). Determining Specification Compliance Fortunately, statistics provide a way of determining is a slab or pavement is acceptable. In this research, there were only two possible conditions: The WWF either was or was not within the tolerance limits. A slab or pavement was considered to be acceptable if at least 95% of the WWF was withing the tolerance limits. In acceptable slabs, the chance of WWF not meeting the 95% requirement was considered to be less than 5% (Ref. 3). For this investigation, the slab or pavement was evaluated and the number of locations with WWF not within the tolerance limits was compared to the critical values shown in the table above. Slabs with noncompliance measurements that equaled or exceeded these critical values were considered unacceptable. Equipment used. The equipment used to measure the cover in these investigations was a portable battery-powered cover meter. This device induces a magnetic field into the concrete through a hand-held probe. Steel disturbs the induced field, resulting in a variation of the field proportional to the depth of steel. Previous research has shown this equipment to be very accurate in determining the amount of cover. Over 95% of the variations between concrete cover and instrument reading can be explained with a cover accuracy of approximately 1/8 inch (Ref. 4). Analysis of Test Results In all three cases, it also appears that the WWF was purposely placed lower than specified. In Case History I, the contractor placed the WWF with about 2 3/8 inches of cover, yet the specifications required the WWF to be at 2 inches. No tolerance was specified. The extremely low standard deviation of 0.11 inch indicates a great attention to detail to hold the WWF at this location. In Case History II, the contractor placed the WWF with about 2 5/8 inches of cover. Again, the specifications required the WWF to be at 2 inches, with no specified tolerance. The standard deviation of 0.25 inch indicates that adequate control was used to hold the WWF at this location. In Case History III, the specifications required the WWF to be at 2 ½ inches with a ±1-inch tolerance. The contractor, however, placed the pavement fabric with about 3 inches of cover. Again, the extremely low standard deviation (0.07 inch) shows a great attention to detail to hold WWF at this location. Note that for Cases I and II, the specified location of the WWF is at 2 inches, which is at one- third of the slab thickness - the upper tolerance limit. If each contractor had tried to place the WWF at exactly that location, about 50% of the WWF would be above 2 inches an 50% would be below that value. Therefore, about 50% of the WWF would be unacceptable, making the placement of the WWF unacceptable. The decision of each contractor to place the WWF with greater cover than specified appears to be warranted. Based on this research, the concerns some design professionals have about the use of WWF do not appear to be justified. The cover of WWF can be controlled within specification limits if adequate supports are provided. PROJECT CASE HISTORIES Case History I: Thirty locations in the slab were randomly selected to determine the cover of the WWF. Test results show that the average cover is 2.36 inches with a standard deviation of 0.11 inch. At all measured locations, the WWF is between one-third to one-half the slab depth; no measurements are out of tolerance. These tests indicate that 95% or more of the WWF cover measurements are within compliance and that the cover for the WWF is acceptable. Case History II: Fifty-two locations were randomly selected to determine WWF cover. The test results show that the average cover is 2.64 inches with a standard deviation of 0.25 inch. Four values were outside the tolerance limits (not in the upper one-third to one-half of the slab). Since the critical value is 6 (see the table), the tests indicate that 95% or more of the WWF measurements are in compliance and the cover for the WWF is acceptable. Case History III: Forty locations were randomly selected to determine cover of the pavement fabric. Test results show an average cover of 3 inches with a standard deviation of 0.07 inch. At all measured locations, the pavement fabric was within the stated tolerance of 1 ½ to 3 ½ inches. Thus, 95% or more of the WWF cover measurements are withing compliance, and the cover for the fabric is acceptable. REFERENCES 1. ACI Committee 117, Standard Specifications for Tolerances for Concrete Construction and Materials (ACI 117-90), American Concrete Institute, Farmington Hills, Mich., 1990. 2. Standard Specifications for Road and Bridge Construction, Illinois Department of Transportation, Springfield, Ill., 1994. 3. L.M. Snell and R.B. Rutledge, "A Proposed Method for Determining Compliance With Floor Thickness Specifications," Concrete International, April 1986. 4. L.M. Snell, N. Wallace, and R.B. Rutledge, "Locating Reinforcement in Concrete," Concrete International, April 1986. 5. L.M. Snell, "Locating Steel Withing Concrete and Masonry," The Construction Specifier, Dec. 1987. URL: http://www.siue.edu/~lsnell/wwr.htm
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