Raft - Top surface cracking

[slickdeals]

We have a project where the top of a 1.5m to 2m thick raft is unidirectionally sloped to drain and serves as a parking slab. Are there any best practices to help with shrinkage/temperature cracking on the top. I am providing a closely spaced outermost layer of T20@150 (#6@6") to help keep the cracks from opening up.

Does anyone have similar experience to share?

 

[aaronPTeng]

All depends on restraint, slab size etc. Dont forget about restraint from elements above the slab, Ie if you are placing walls on top.
I would always recommend Post tension raft with a nominal mat on top to meet your local codes based on crack control and environment
Document the reinforcement to follow the slope of the slab. As you do not want excess cover at the high points, It also gives the concrete a guide to follow.

I looked at a raft (Designed by others) recently that was only 0.4m thick with N20@ 200 cts e/w top with large cracks everywhere, There are several items that need to be considered not just the final mat you place near the top. Do you have a sketch?

Regards

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[Structcon]

It will get pretty hot due to the curing and you will get cracks no matter what rebar you spec, but ... I would suggest a mix design that has retarders that slow down the cure rate, and I would suggest a misting or water spray to cool the mat slab as it cures to minimize those drying cracks.

 

[oldestguy]

Top shrinkage means higher W//c ratio there. Don't let the crew do a lot of vibrating or it will bring water up. Also don't let the finisher get on there too early wait as long as possible. Once struck off stand clear for as long as possible.

 

[hokie66]

The amount of reinforcement you have in the top layer sounds about right. The mix design is important, and also the curing.

I disagree with oldestguy about the vibration. If the concrete is not consolidated well, you could well get plastic settlement cracking around the bars. With a mat that thick, a lot of vibration, and sometimes revibration, is required to guard against these reflective cracks.

 

[Ron]

Use the largest nominal size coarse aggregate you can, provided you can get good consolidation and encapsulation of rebar. Keep w-c ratio as low as practicable. Reinforcement looks ok.