Cracks in the Slab-on-grade for Warehouse building 1

[gosai]

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.

 

[Ron]

gosai...yes, concrete strength influences cracking. Your photo shows what appears to be a drying shrinkage crack; however, there is not context for the photo.

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.

 

[TXStructural]

In addition to Ron's input, a few items that could do this:
Poor curing practices.
Moisture migration from below, destabilizing the subgrade.
Loading patterns.
Poor reinforcement layout and installation.

 

[hokie66]

I'd like to add a couple of things to the comments by Ron and TX. Cracks like this which are said to have occurred months after the placement are usually there very early, just not observed. And irregular cracks are often made worse by the regular cracks (sawcuts) not working as wished. One more: the crack shown in the picture appears to be propagating perpendicular to a construction joint, which may indicate that dowels or reinforcement through the joint restrained the shrinkage of the section cast last. There is a crack on the other side of the joint a little further up, which may be just a nondirect extension of the first one. Just having joints at a given spacing gives no guarantee that these locations will be the positions where the movement occurs, particularly if due attention is not paid to how these joints are made.

 

[dik]

You don't mention the specified slump, when the slab was sawcut, the size of the sawcut, or what curing was used.

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

 

[gosai]

The specified slump was 5". No vapour barriers underneath. Fibres were used to prevent cracking.

 

[Ron]

Fibers don't prevent cracking if the other crack mitigation procedures are not followed. At best, fibers will only cause the distance between cracks to be slightly greater...but the actualy crack width will be greater as well.

 

[dik]

When was it sawcut, and how deep? What curing? A 5" slump is high and permits a slump of nearly 6". Water was added at the site? I usually spec 4" max for SOG...

Dik