Concrete Foundation Walls - Structural Deterioration And Damage
Problem: Shrinkage cracks.
Cause: Normal drying of concrete.
Shrinkage occurs when particles of cement and aggregate draw together to fill voids that appear when excess water evaporates from the concrete.
Solutions: Minimize the amount of shrinkage.
- Shrinkage can be minimized by;
- not adding water on the construction site. Adding air entrainment or a super-plasticizer to the mix will improve workability and reduce the potential for shrinkage cracks,
- using the maximum allowable aggregate size,
- ensuring proper curing of concrete (moist-cure concrete surfaces for three days, and leave formwork on walls for a minimum of 24 hours at temperatures close to 56°F (13°C),
- adding fly ash to the concrete mix to slow the setting time.
Use control joints to minimize random cracking and leakage. Control joints help you to predetermine where shrinkage cracking will occur: they intentionally weaken the slab or wall. the use of control joints can reduce foundation callbacks by 90%
- To be effective, control joints must reduce the thickness of the wall by 25%. Attach beveled wood strips to formwork or saw-cut immediately after forms are removed as shown in Figure 9.
Figure 9 - Vertical Control-Joint Detail
- A common building code requires that control joints shall be provided in foundation walls more than 82 feet (25 m) long, and at intervals of not more than 49 feet (15 m) as shown in Figure 10.
Figure 10 - Foundation Control Joint
- On the exterior, joints must be sealed with gun-grade, oil-based caulking applied over a bond breaker as shown in Figures 11a, 11b and 11c. Sealant should be protected from the backfill soil with asphalt paper, heavy polyethylene, or rigid insulation installed to grade.
Figure 11a - Crack closed - Sealant in foundation control joint
Figure 11b - No bond breaker - crack opened
Figure 11c - Crack opened with bond breaker
- Tool in control joints in floor slabs when you are finishing. Make saw cuts 6 to 18 hours after placement, and space that at 15 feet to 20 feet (4.5 to 6 m) for basements and 12 feet to 15 feet (3.5 to 4.5 m) for garages. The depth of the control joints should be one quarter of the slab thickness.