Question: We contracted to pour a 22x38-foot slab in an existing basement. Because of headroom, it was decided the slab would be an 8-bag grout mix only 2 inches thick. We used #3 rebars 30 inches on center the long way and poured over an existing concrete floor. The existing floor was badly cracked, but reasonably true and showed no signs of ever having settled or shifted.Within two months the new slab developed many shrinkage cracks, but the puzzling thing was one crack that ran generally lengthwise approximately in the center. About 15 feet of this crack was displaced vertically nearly one-quarter of an inch. Why would one side of the new slab raise like this in the center? The existing slab was probably at least 25 years old. The new slab was poured in warm weather, about mid-May, and because there was a building over the basement, curing conditions were quite good.

Answer: The vertical displacement is probably due to curling of one side of the topping. Perhaps it bonded on one side of the crack but not on the other. If you tap the topping near the raised edge and hear a hollow sound, it's a good indication that the edges are curled. Curling is caused by uneven shrinkage of slab concrete. The top dries and shrinks more than the bottom of the slab or topping. This tends to lift the edges. If there isn't a good bond between the topping and the base concrete, the topping can raise up near a crack or at an edge. A sand-cement grout has a high shrinkage potential because of the high water and cement content.

The greater the shrinkage, the greater the possibility that curling will occur. Eugene Boeke of Beers Construction Company in Atlanta gives this advice for avoiding the curling problem with toppings:

  • Thoroughly clean the old floor. Waterblasting or sandblasting will work.
  • Wet the old floor for at least 12 hours by spraying it and covering it with wet burlap.
  • Just before applying the topping, scrub the floor with a slurry of neat cement, bonding agent, and water. Use a stiff bristle broom and lots of elbow grease.
  • Use the largest possible aggregate size for the topping mix, the lowest possible slump, and the lowest strength mix you can still work and finish. Boeke uses a 2500- or 3000-psi concrete. If the strength is less than that, his finishers have trouble closing the surface. Low strength, low slump, and large aggregate reduce the amount of water and cement in the mix. This reduces shrinkage.
  • Cut joints on a grid pattern after the topping is placed. Make them about 3/4 inch deep and 8 to 12 feet apart. These can later be closed by the finishers but will still give a plane of weakness and prevent major curling. Some people saw the joints after 12 hours.
  • Apply a good quality curing compound immediately after finishing is completed. Use twice the recommended application rate.

Boeke's crews have also had good results with a system they use for tying down the corners of a day's topping pour. They shoot three studs into the substrate using a triangular pattern as shown in the drawing (below). The studs are about 8 inches apart and 6 to 8 inches away from the edge at each corner. They are wrapped and laced with tie wire before the topping concrete placed. This helps hold down the corners. If reinforcement is needed only for crack control, Boeke generally uses polypropylene fibers in the topping rather than small diameter rebars. If the reinforcement is for structural purposes, an engineer should determine the bar size and spacing required.