Title: Concrete Q & A: Roughening Construction Joints

Author(s): Alan Mattock, Lawrence Kahn, and Gary Klein

Publication: Concrete International

Volume: 32

Issue: 10

Appears on pages(s): 80

Keywords:

DOI:

Date: 10/1/2010

Abstract:
At a construction joint with reinforcing perpendicular to the plane of the joint, the nominal shear capacity along the joint Vn is defined by Section 11.6.4 of ACI 318- 81 Vn = Avf fyμ where Avf is the area of the bars crossing the joint, fy is the specified yield strength for the bars, and μ is the coefficient of friction at the interface created by the joint. Designers assume a μ value based on the surface quality of the hardened concrete at the joint. To use a μ value of 1.0λ (where λ is 1.0 for normalweight concrete), Section 11.6.4.3 of ACI 318-08 requires that the surface of the hardened concrete must be intentionally roughened to a full amplitude of approximately ¼ in. (6 mm) (Section 11.6.9). But roughening the entire area of a joint surface can be a problem. For example, if two layers of vertical bars project out of a horizontal construction joint in a wall, the zone between the bars and forms is too narrow to finish. It’s therefore feasible to roughen only the concrete between the two layers of vertical bars (for example, by scratch finishing the plastic concrete). While it may also be possible to roughen the concrete after it has hardened (using high-pressure water blasters, sand blasters, or pneumatic tools), it’s still difficult to roughen the narrow space between the reinforcing and the forms without spalling the edges of the concrete or perhaps damaging the forms. Is it acceptable to roughen only the area between bar curtains? How much surface area needs to be roughened to comply with Section 11.6.4.3 requirements? Also, we aren’t sure how to verify the 1/4 in. (6 mm) amplitude, so how was it checked in the experiments used to develop the requirements?