Cracks in the Slab-on-grade for Warehouse building
Cracks in the Slab-on-grade for Warehouse building
(OP)
A slab was poured in July 2012 on granular A materials. The granular 'A' materials was compacted to a 98%. The saw joints were placed at a distance of 10' 6". The average measured thickness of the slab was 6". Approximately after 8 months of the pour the cracks appeared on the concrete slab. The thickness of the cracks was measured between 0.5 mm to 1.5mm. The depth of the crack was around 2/3 depth of the slab. The designed strength of the concrete was 3600 PSI @ 28 day. The strength of the concrete core samples tested was 6100 PSI. The cracks appears to be a drying shrinkage cracks. During placement of the concrete the water was added in some of the concrete trucks prior to placement.
What factors causes concrete to crack? Is the higher strength of the concrete is one of them?
Please provide some comments on various causes for cracking.
What factors causes concrete to crack? Is the higher strength of the concrete is one of them?
Please provide some comments on various causes for cracking.





RE: Cracks in the Slab-on-grade for Warehouse building
When concrete strength is increased, the cement content has to increase. With higher cement content comes higher shrinkage, even considering a constant water-cement ratio. If water gets added to concrete in the field, the water-cement ratio typically goes up and drying shrinkage will go up as well.
If concrete is pumped, it usually is necessary to reduce the nominal coarse aggregate size for pumping convenience. When the coarse aggregate size is reduced, the cement paste quantity has to increase to cover the additional surface area of the smaller aggregate....with increased cement paste, comes increased shrinkage.
Almost all cracks in concrete can be controlled. If the thickness of the concrete is kept uniform (+/- 1/4 inch) and sawcuts are made at the proper time (same day as placement) and to the proper depth (1/4 of slab thickness), then "random" cracking will most likely be controlled. If you don't do ALL of these things, then "random" cracking will occur.
RE: Cracks in the Slab-on-grade for Warehouse building
Poor curing practices.
Moisture migration from below, destabilizing the subgrade.
Loading patterns.
Poor reinforcement layout and installation.
RE: Cracks in the Slab-on-grade for Warehouse building
RE: Cracks in the Slab-on-grade for Warehouse building
You may have lost moisture through the granular base. Did you have a PEVB between the slab and fill?
As noted, adding water is bad, and the use of small aggregate increases shrinkage.
Dik
RE: Cracks in the Slab-on-grade for Warehouse building
RE: Cracks in the Slab-on-grade for Warehouse building
RE: Cracks in the Slab-on-grade for Warehouse building
Dik