Holdfast Training Services Trial Blog

There have been a lot of posts about concrete recently so I thought I would throw my hat into the ring with some issues I have been having with reinforced and post-tensioned concrete.  The client was walking around site and went mad about some surface cracking that we are experiencing in the car park.  This sent the PM into a flap and I was sent out to investigate.  I took these photos of the area she was concerned with.

If you are having problems seeing them then you are not alone.  These cracks are in a reinforced concrete slab and are confined by reinforcement and other than cosmetic reasons are nothing to worry about.  The slab was made from 80 MPa Concrete poured in 37 C heat and a crack free finish is difficult to achieve.  Netherless I have to speak to the concreters and consultants to determine that they are not structural and to determine what QA can be done to prevent other slabs cracking.

However, while on my crack patrol I came across this crack- that hadn’t been spotted by the client or the PM.  Now it is big enough to fit a credit card into so I was concerned.  I immediately checked the propping and found it to be in good order.

These cracks are in a post-tensioned slab that has not been tensioned yet and are a very different beast.  Normally PT slabs are initially stressed 24 hrs after pour and then stressed again after 72 hrs.  The advice I had from a consulting engineer from another company was that you should expect big cracks to form because PT slabs are not as heavily reinforced.  The small cracks turn into one big crack, so don’t worry about cracks until the slab is fully stressed.  I called the engineers working on this slab and other than a visit yesterday they have slipped into radio silence.  We are leaving these tendons until last but, we are stressing the rest of the slab now.  If we have still heard nopthing back form the engineers our intention is to isolate the area and then stress the tendons.  I will let you know how it goes.