Q: Why are grouted shear keys used at control joints in concrete masonry units?
Is there any advantage to using a grouted shear key instead of a rubber or plastic preformed gasket?
A: The answer is related to costs, availability of materials, and required fire ratings of the wall system. Preformed control joint gaskets are set into concrete masonry sash units to transfer lateral out-of-plane loads, primarily wind, across a joint, as shown in Fig. 1.
Whenever plastic joint gaskets are used, it is important to review the allowable capacity of the shear key printed by the manufacturer to verify that the product has sufficient capacity for the calculated loads. Many plastic preformed control joint gaskets also meet the requirements of a two-hour fire resistance rating. (See Standard Method for Determining Fire Resistance of Concrete and Masonry Construction, ACI 216.1-97/TMS 0216-97) Check with the manufacturer for more information related to fire resistance ratings using plastic control joint gaskets.
In some cases, the cells of the concrete masonry on either side of the control joint are grouted and reinforced. With this approach, the two cross webs on either side of the control joint need to be filled to prevent the grout from flowing into the joint.
Grouted shear keys are formed using typical stretcher concrete masonry units. Therefore, there is no need to purchase any special sash units. The grouted key is constructed by using a release paper (such as building paper or polyethylene) on one side of the key so that when the grout is poured into the void space, it bonds only to the wall on one side of the control joint, as shown in Fig. 2.
The building paper should be extended into the head joint and be held in place by the mortar or backer rod put in the joint. By extending the release paper into the head joint, the mortar bond is interrupted through the depth of the wall at the joint. The mortar or backer rod prevents the grout from flowing out of the head joint when the grout key is poured. The building paper must be cut or folded back prior to installing the sealant joint to prevent any interference with the bond of the sealant to the concrete masonry.
As is the case with other control joints, the cells of the concrete masonry on either side of the control joint are sometimes grouted and reinforced. When these cells are not reinforced or grouted, the cross webs adjacent to the control joint must still be filled to prevent grout from flowing into the cell adjacent to the control joint. Grout shear keys meet a four-hour fire resistance rating. (See Standard Method for Determining Fire Resistance of Concrete and Masonry Construction, ACI 216.1-97/TMS 0216-97)
In summary, grouted shear keys should be used instead of plastic shear keys when sash block units are not readily available or were not purchased, when a fire rating greater than two hours is required by the job, or, in some cases, to reduce material costs.
Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.