360R-06 Design of Slabs-on-Ground
DESIGN OF SLABS-ON-GROUND 360R-27
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- 5.3—Sawcut contraction joints
| DESIGN OF SLABS-ON-GROUND 360R-27
slab is less than 3%. This percentage is relatively minor when compared with the potential impact of variations in the restraint stresses due to the different coefficients of subgrade friction ( Fig. 13.3 ) and curling stresses. Round, square, and rectangular smooth dowels for slab- on-ground installation should meet the requirements of ASTM A 36 or A 615. The diameter or cross-sectional area, length, shape, treatment for corrosion resistance, and specific location of dowels as well as the method of support should be specified by the designer. Refer to Tables 5.1 and 5.2 and Fig. 5.9 through 5.13 . For long post-tensioned floor strips and floors using shrinkage-compensating concrete with long joint spacing, care should be taken to accommodate significant slab movements. In most instances, post-tensioned slab joints are associated with a jacking gap. The filling of jacking gaps should be delayed as long as possible to accommodate shrinkage and creep. In traffic areas, armor plating of the joint edges is recommended (Fig 5.14). A doweled joint detail at a jacking gap in a post-tensioned slab (PTI 1996, 2000) is shown in Fig 5.15. 5.3—Sawcut contraction joints The following three families of tools can be used for sawcutting joints: conventional wet-cut (water-injection) saws; conventional dry-cut saws; and early-entry dry-cut saws. Timing of the sawing operations will vary with manu- facturer and equipment. The goal of sawcutting is to create a weakened plane as soon as the joint can be cut, preferably without creating spalling at the joint so the floor slab will crack at the sawcut instead of randomly, thus creating the desired visual effect. Conventional wet-cut saws are gasoline-powered and, with the proper blades, are capable of cutting joints up to 12 in. (300 mm) depth or more. Both types of dry-cut tools can use either electrical or gasoline power. They provide the benefit of being generally lighter than wet-cut equipment. Most early- entry dry-cut saws cut to a maximum depth of 1-1/4 in. (32 mm). Early-entry dry-cut saws, however, which can cut to a maximum depth of 4 in. (100 mm), are now available. The timing of the early-entry process allows joints to be in place before development of significant tensile stresses in the concrete; this increases the probability of cracks forming at the joint when sufficient stresses are developed in the concrete. Care should be taken to make sure that the early-entry saw does not ride up over hard or large coarse aggregate. The highest coarse aggregate should be notched by the saw to ensure the proper function of the sawcut contraction joint. Early-entry dry-cut saws use a skid plate that helps prevent spalling. Timely changing of skid plates in accordance with manufacturer’s recommendations is necessary to effectively control spalling. Typically, joints produced using conven- tional processes are made within 4 to 12 hours after the slab has been finished in an area—4 hours in hot weather to 12 hours in cold weather. For early-entry dry-cut saws, the waiting period will typically vary from 1 hour in hot weather to 4 hours in cold weather after completing the finishing of the slab in that joint location. Longer waiting periods may be necessary for all types of sawing for floors reinforced with steel fiber or where embedded mineral-aggregate hardeners with long- slivered particles are used. In all instances, sawing should be completed before slab concrete cooling occurs subsequent to the peak heat of hydration. The depth of sawcut using a wet conventional saw should be at least 1/4 of the slab depth or a minimum of 1 in. (25 mm), whichever is greater. The depth of sawcut using an early- entry dry-cut saw should be 1 in. (25 mm) minimum for slab depths up to 9 in. (230 mm). This recommendation assumes that the early-entry dry-cut saw is used within the time constraints noted previously. Some slab designers are requiring that the slab be cut the following day to 1/4 of the slab depth to deepen the 1 in. (25 mm) nominal early-entry sawcut and ensure that the joint is activated. Restricted joint activation using a nominal 1 in. (25 mm) sawcut is a particular tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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