Concrete Wall Shrinkage Cracks
Concrete Wall Shrinkage Cracks
(OP)
My boss and I have been going back and forth on where a concrete shrinkage crack propagates from. Our concrete wall is 8" thick and 100 ft long and 4 feet high. It is poured on a footing that is assumed to have no further shrinkage... i.e. fixed. The wall is heavily reinforced, but we still have some cracks, which we know are shrinkage cracks. Some start from the bottom, some start from the top.
My theory is that if the wall is restrained on the bottom and the top is free, the crack should propagate from the top and work its way down. My boss' theory is that the crack propagates from the bottom, since that is where the highest stress from resisting shrinkage would occur.
I see both sides but am unable to come up with a definitive answer, at least this late in the day. Anyone ever thought about this question? I think that starting from the top, each succesive layer as you go down, has more restraint against movement and is less likely to crack. On the other hand, if the concrete is trying to shrink, and it is resisted by the base, the stress will increase and a crack would eventually have to occur at the bottom..... Any ideas?
My theory is that if the wall is restrained on the bottom and the top is free, the crack should propagate from the top and work its way down. My boss' theory is that the crack propagates from the bottom, since that is where the highest stress from resisting shrinkage would occur.
I see both sides but am unable to come up with a definitive answer, at least this late in the day. Anyone ever thought about this question? I think that starting from the top, each succesive layer as you go down, has more restraint against movement and is less likely to crack. On the other hand, if the concrete is trying to shrink, and it is resisted by the base, the stress will increase and a crack would eventually have to occur at the bottom..... Any ideas?






RE: Concrete Wall Shrinkage Cracks
RE: Concrete Wall Shrinkage Cracks
RE: Concrete Wall Shrinkage Cracks
RE: Concrete Wall Shrinkage Cracks
RE: Concrete Wall Shrinkage Cracks
Shrinkage cracks are caused by restraint. If the wall were free to move, had an appropriate w/c ratio and mix design, and was cured properly, there would be little reason for cracks.
The bottom restraint and shrinkage result in cracking near the base, while the top goes into tension because the bottom cannot move as the top tries, like a curling slab.
RE: Concrete Wall Shrinkage Cracks
The cracking is due to the combined stresses from both curing shrinkage (very high early) and temperature (due to the weather conditions that vary daily or seasonally). The tops of walls may see higher temperature extremes than the lower portions where the temperature of the soil is a moderator. Once a critical stress is met the crack will begin propagating.
It is not impossible for the initial shrinkage to occur due to the curing shrinkage early and then later cracks due to the combined stresses, including temperature due to the exposure later in the life of the wall.
Are there any sections that have the generally alternating or sequential cracking between the primary (assumed) cracks?
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Concrete Wall Shrinkage Cracks
An unrestrained piece of concrete will normally shrink without cracking unless there is a large differential in temperature..
I would tend to agree with your boss that the cracks should start from the bottom.
RE: Concrete Wall Shrinkage Cracks
RE: Concrete Wall Shrinkage Cracks
In the middle of the wall, at the bottom, the stress would be highest and I would anticipate finding a crack starting just above where the bottom of the wall meets the base. A crack could be expected to propagate from this point. I would then expect the crack to propagate upwards, following the stress concentration at the tip of the crack until it reaches the top of the wall. When the crack reaches the top, both sides of the crack retreat to their "shrunken" position. This is what confused me. On paper, the crack would appear to be wider at the top, which made me believe that the crack must have propagated from this point.
On the other hand, I think that a weak point at the top of the wall could be supported by a relatively rigid layer below, which could cause a crack to form at the top as well. This crack could have even been instigated by plastic shrinkage in the early stages, causing an extremely weak point.
Definately not an exact science, but interesting to think about. Thanks.
RE: Concrete Wall Shrinkage Cracks