THE KEY TO KEYWAYS

When considering keyways for longitudinal construction joints, the success of your job is a matter of three considerations: keyway necessity, keyway design and good construction.

Think of keyways as tongue and groove joints. The tongue and groove provide a mechanical interaction between two adjacent slabs. Keyways are often put in longitudinal construction joints to provide load transfer along the joint between two paving lanes. Load transfer across a longitudinal joint reduces edge stress in the loaded slab. The stress reduction helps to prevent cracking that might result under heavy loads. In some cases, longitudinal construction joint details also include tiebars with a keyway. The keyway provides interlock, while the bars hold the joint tight. (Tiebars are essential for highway pavements.) A keyway also helps keep the two slabs at the same level. This prevents any faulting which might develop along the construction joint due to differential subgrade settlement or swell.

Engineers at the Rigid Pavement Laboratories of the U.S. Army Corp of Engineers at Cincinnati Ohio, researched keyways back in the 1950s, their goal was to determine the optimum keyway design. They tested numerous designs and key dimensions oriented both vertically and horizontally. The researchers monitored the performance of the test models under load and concluded that a trapezoidal keyway (sized as shown) was the most efficient. The 1:4 slope of the top and bottom of the tongue was established to facilitate construction.

The researchers also concluded that the keyway tongue width is optimum at 20 percent the slab thickness. The tests showed that a thinner tongue will shear off under load, while the joint face will spall above or below the groove with a wider tongue. The location of the spalling or shearing of the female key depends on which side of the joint is loaded. If a load rests on the tongue side of the joint, failure will be at the bottom portion of the groove. Conversely, if the load rests on the groove side of the joint, the failure will occur toward the top of the slab.

Later research by the Portland Cement Association found that a half-round keyway performed essentially the same as a trapezoidal keyway. This shape is now used on many thinner pavements for municipal streets and parking lots.

Unfortunately, on many projects optimal keyway size and location is not carefully maintained. Often large keyways are formed, or the keys are located too close to the slab surface instead of at the midpoint of slab depth. These problems almost always result in 3-5 inch spalls which may run the full length of the longitudinal construction joint. Such spalls are difficult to repair and maintain. Oversize or misaligned keyways are a problem.

On thin slabs, less than 7 inches thick, an engineer may consider eliminating the keyway. A butt faced joint with tiebars will provide an adequate load transfer. These joints will not carry the risk of keyway failure. Small diameter (#4 or #5) tiebars at typical spacings are adequate for light traffic. Consider reducing the spacing to 12-24 inches for heavy traffic applications.

When designing and building keyways, attention to detail is necessary. Start by determining if the keyway is really necessary for your pavement application. If not, keep it simple and eliminate the keyway detail. If you must use a keyway, use the optimal designs discovered in the 1950s and shown on the accompanying diagrams - these designs were based on performance. Then, during construction try to keep the keyway located at mid-depth. Misorientation is equivalent to poor keyway design. These three simple considerations are the key to keyway performance.

American Concrete Pavement Association