Question:

I was interested in finding out about the possible causes of concrete street expansion, other than heat itself. We have experienced numerous problems of over-expansive joints, causing the individual sections of concrete to force one another to push up and crack. I was wondering if you had heard of any reasons or studies aiding in this phenomenon involving any chemicals in the concrete or anything similar to this. Any information about expansive concrete or street blowups/blowouts would be great.

Answer:

I do not have formalized studies about concrete paving blowups to share with you, try the Federal Highway Administration in Washington D.C. But, I intend to share with you the experiences I have.

Slab on ground concrete construction with proper jointing in the transverse direction, usually perform very well even in hot conditions. Pavement abutting bridges is a separate matter and will not be addressed here.

You are correct, there are situations that bring about blow ups. I have seen the following causes:

  1. Contraction joints are crammed full of incompressible matter, sand, soil, calcium efflorescence, etc. This occurs when the joint filler is absent or not in good repair. The slab or panel of concrete is bearing directly upon the abutting panel. The original design did not permit that. The original design allowed for a clear space to develop between the panels due to shrinkage during setting. The contraction joint was detailed to be filled with bitumen sealant and to remain free to move horizontally.
  2. The freedom of horizontal movement is denied by the joint's fill of incompressible material. The free joint space designed in by the owners staff is no longer available and the two or more weather heated panels begin to push against one another. The more heat or the longer the heat spell the more the expansion of the flat panels.
  3. All the above is taking place and then we can become site specific, an intersection is very vulnerable, due to multiple directions of the expansive forces. A slab sloping down a hill and jamming against an other set of panels going in the 90 degree direction from the down hill slope will cause a lot of energy to build at the interface between the slope panel and the flat intersection panel.
  4. Last, dowels and other embedded items cause point stresses that contribute to the build up of energy in a particular place. Dowels are supposed to permit freedom of movement in a horizontal direction and of course prevent vertical dislocation at a particular joint detail. Dowels must be dead level to perform this function, and one directional end of all the dowels needs to be bonded in a panel and the opposite side needs to be isolated to allow freedom of horizontal movement. Real world, dowels are installed not level, dowels are disturbed in the paving process, and sadly, dowels are not isolated on one end and they actually transfer the expansion energy through to the abutting panels.

Result, the heated panels push directly upon one another and rare up at the joint interface and "blow up." Sometimes violently. The 6:00 p.m. news makes a big deal out of it, some pitiful little old lady's Ford Falcon is tittering on top of the center of the blow up and everybody says Wow! Bad concrete! We know, it is not bad concrete it is poor joint maintenance or a bad dowel installation, or maybe a set of man holes in an intersection and the man holes are arranged in a wedge, and the wedge shape is blocking the natural movement of the concrete panels, but it is not bad concrete.

The thing I did not mention is the subgrade under the concrete. there are times when expansive clay will bulge or uplift a section of concrete and it is blamed upon heat, but in fact is a subgrade related problem.