Architects seem to be designing thinner walls with more reinforcing. We are having some trouble measuring the lap splice length of the rebars. Specifications require the lap splice to be 50 bar diameters or 12 inches, whichever is greater. When we puddle the bars into the grout we try to make sure the lap splices are the correct length. But we have no way of verifying this for the architect. Is there some nondestructive method for determining the lap splice length of vertical bars in grouted cores?
The lap splice length of the rebars is important to the structural performance of a building. American Concrete Institute 531, Building Code Requirements for Concrete Masonry Structures, states that the minimum length of lap for bars embedded in grout, in tension or compression, shall be 0.002 db Fs, in inches, but not less than 12 inches. Fs is the full allowable stress in the reinforcement and db is the bar diameter. For Grade 40, Fs = 20,000 psi, and for Grade 60, Fs = 24,000 psi.The lap splice length for Grade 40 bars, then, equals 0.002 x 20,000 x db or 40 bar diameters. For Grade 60 bars, the lap splice length equals 0.002 x 24,000 x db or 48 bar diameters. Regardless of the reinforcement grade, the splices must be a minimum of 12 inches.Most specifications, including ACI 531.1, require that "reinforcement shall be supported and secured against displacement before grouting." While it is easier to grout the masonry units without the rebar in the wall, this practice violates most specifications. If you install the rebar after grouting, the architect may require you to demolish the wall and rebuild it, or he may ask you to verify that the lap splice lengths are adequate. Installing the bars after grouting also reduces the bond between the bar and the grout.Verifying the lap splice length in an existing structure is difficult. Magnetic methods, such as a cover meter, have been used to successfully locate reinforcing bars in masonry structures. But they can't determine lap splice length. We are aware of two studies that were inconclusive.X-ray methods have been used to determine the position and condition of reinforcement, voids in concrete, and grouting of post-tensioned construction. Perhaps they also can be used to solve your problem. If readers know of other methods of determining lap splice length of rebars after the walls have been grouted, please tell us about them.
The best solution to your problem may be to avoid it. Follow the specifications and install the vertical rebar before grouting. We can suggest three techniques to make this relatively easy. Two methods involve the installation and securing of the reinforcement. The third requires the purchase of special blocks. Check the building codes that govern in your area to make sure these methods are allowed.The first method can be used in high-lift grouting operations. The procedure: Install the rebar after laying a one-story height of wall. Use the cleanout holes to securely tie the rebar splice, then pump the cores full with grout.The second method can be used in low-lift grouting operations where cleanouts are not used. The procedure: Lay up the first 4-foot height of wall. In the process, install bar chairs (various types are available) in cores where vertical rebar must be installed. Then insert 6-foot-long vertical rebars in the cores, properly positioning them in the rebar chairs. Next fill the cores with grout and repeat the process--lay another 4-foot height of wall.This second procedure does not completely eliminate threading blocks over bars but it does minimize the problem. Note that the 6-foot bars provide a 2-foot lap splice. If a longer lap splice is required, longer bars must be used, making it more difficult to place block.The third method can be used in low-lift or high-lift grouting operations. The procedure: Install and tie the vertical rebar first, then lay up the block. Use special A-shaped and H-shaped blocks (see page 71). Made with one or two open ends, these blocks can be placed around the vertical steel instead of threading block over the top of the rebar.In California, the State Division of Architects requires that the rebars be installed before the blocks are placed. This means special shaped blocks are the only acceptable, practical method. One note of caution: because of their shape, these special blocks are easily damaged during shipping.In high-lift grouting, holes knocked into the masonry to clean out the cores before grouting also can be used to tie the rebar splice.