Having a cracking time – wish you were here!
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.
![IMG_5557[1].JPG](https://pewpetblog.com/wp-content/uploads/2016/11/img_55571.jpg?w=595)
Surface Cracking caused by rapid water loss.
![IMG_5554[1].JPG](https://pewpetblog.com/wp-content/uploads/2016/11/img_55541.jpg?w=8064)
A crack along the top of a ramp – not really a concern though.
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.
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Think I might stay quiet too! No chance! The shrinkage cracks in your standard slabs are just poor curing – a concreting/workmanship issue. They are easily avoided and shouldn’t be there. They will let air, water and other contaminants into your reinforcement sooner than should have been the case leading to earlier onset of deterioration. Client has every right to be upset.
The prestress crack I would have ignored and said this should close up as you stress if it was a minor shrinkage crack mid span albeit that it might be a significant source of prestress loss. This one however appears o be in the anchorage zone – remember end blocks and anchorage zones? Lots of stray stresses likely to lead to failure hence reinforcement in several directions… This crack looks interesting – I’m half hopeful I will see the photo of it doubling in width as the slab undergoes elastic shortening and the tensile force in the anchorage zone develops (which in this region are perpendicular to the crack so acting to open it up). Exciting (from several thousand miles and no liability perspective!).
Richard, I think the key thing here is the 80 MPa concrete mix. I have done some digging with the new structures site engineer. This transfer slab has been a real mess, for some reason they poured the beam mix in the slabs in these two locations. They used aliphatic alcohol and curing agents but this mix is a nightmare to work with and should never have been put in the slab. The cracks first appeared when they were applying the broom finish.
The pt subcontractor has stressed the anchor and I have seen no difference in the size of the crack. The PT runs in 2 directions and despite the engineer’s advice I was sceptical on the crack closing.
All the cracks will need patching. Will the PT crack need anything special, now that it has been stressed?
I think your transfer slab would have enjoyed a spray applied curing film or wet hessian but it’s in and done now.
The PT stressed two ways wouldn’t help that crack and it won’t close up. If it hasn’t opened up that’s a good thing. It won’t need anything special in terms of repair, in fact that particular area of concrete, although prone to cracking resulting from stray tensile forces, doesn’t do much beyond protect the steel in the anchorage block.
With regards to patching, that was my understanding. The new PM was on about using high strength epoxy but, my initial thought is that the majority of the strain has happened therefore high strength expo you is not required.
A curing agent was used but the concreter complained about a hard crust and soft centre. They don’t seem to use the hessian over here. Given the multitude of chemicals that they put in this 80 MPa it’s a nightmare to deal with. Only surpassed by the 100 MPa concrete.
I cannot believe you got your credit card out!
No mate you know me, that is the brisbane equivalent of an Oyster card.