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Imperfect Concrete & Cracks

A spot light on "Why Concrete Cracks"

Poorly compacted or prepared sub-grade: Excavations beneath the slabthat are not to be filled with concrete (usually plumbing or other mechanical trenches) should be brought back to grade in compacted lifts. This means a 24" (60 cm) trench would be back filled 6" (15 cm) at a time, each "lift" being mechanically vibrated so it is left compact. If these excavations are not compacted when backfilled, this loose soil will settle over time leaving the concrete over that area with no earth under it. This becomes a prime place for concrete to settle and crack. Excess water in the mix: Concrete requires very little water to hydrate properly.Typically concrete flat work is poured with toomuch water. For every pound (or kilogram or any unit of weight of cement, about 0.25 pounds (or 0.25 kg or corresponding unit) of water is needed to fully complete the hydration reactions. This requires a water-cement ratio of 1:4often given as a proportion: 0.25. However, a mix with a w/cratio of 0.25 may not mix thoroughly, and may not flow wellenough to be placed, so more water is used than is technically necessary to react with the cement. More typical water-cement ratios of 0.4 to 0.6 are used.

Rapid Drying of the concrete: Water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopicpores that will reduce the final strength of the concrete.Concrete slabs can shrink as much as 1/2 inch per 100 feet (1.5cm per 30 meters). This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces. Rapid drying of the slab significantly increases the possibility of cracking.The chemical reaction, which causesconcrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after the concrete ispoured. Improper strength concrete poured on the job: Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.                                                                                                                  

Example: 4000 psi concrete with air - Cold weather mix 3000 psi concrete no air - warm weather mix Lack of control joints: Control joints are planned cracks – Control joints allow for movement and proper cracking. Temperature change and poor sub base preparation contribute to slab movement. When the concrete cracks, we want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly. Control joints can be cut at the time of placement or the next day with a diamond saw blade. (When installing decorative concrete work saw cuts are preferred over hand tooled joints).