TRAVELLING CONCRETE SLABS

By Michael King 

It is amazing, if not mind boggling, that something as big and as heavy as a concrete slab can travel. One would think that something of this sort, when constructed would stay put. However, slabs do move. Sometimes just a little with only slightly noticeable effects, and other times by several inches or more, where they become a visual annoyance and perhaps a hazard. This is due to ground movement under the slab and by the time you notice, it’s too late because it should have been built properly in the beginning.

Expansion Joints have stopped this cracking! The reasons slabs move are varied, and all the reasons are built on the understanding that the concrete slab is not moving alone. If a slab moves, it is likely that the soil underneath has moved as well. The ground must be thought of as a slow moving fluid not solid. We have all noticed soil can appear to grow over concrete that is both adjacent and slightly lower in than the body of the soil.

If the soil under the slab is located on a level plane, the movement will be generally downward, thus the soil overtime will become more compacted. If the soil is on a slope, the soil will move with the force of gravity down the slope.

To complicate matters, much of the soil is made up of expansive clay which moves seasonally. The volume of this soil is constantly being changed as it takes on and gives off moisture. Basically the soil is going through constant cycles of growth and reduction. Some soil is considered unusable for structural purposes, without special engineering, as it will not support the weight of heavy objects. Normally the weight of objects is transmitted down and outward from its base, but some soil, those with a high plasticity index, acts like small ball bearings, and moves out from under the object causing it to shift and sink.

These factors can generally be overcome, if they are known in advance of the construction process. This is why I recommend that before any building be considered, particularly where it represents a substantial investment, that the soil be tested professionally to determine its makeup and weight bearing capacity. If the condition of the soil is known and problems exist, they usually can be remedied. Possible solutions include compacting loose soil, removing bad soil, and replacing with compactable soil, proper foundation/footing sizing and placement, and to reach down to stable soil, piers and pilings.

Slab is Falling Away from the pool - Where the slab has fallen away from the edge of a pool, the cause is likely the supporting soil was not compacted correctly. The picture on the right shows a concrete deck on reasonable level soil that over time settled away from the pool. The edge furthest from the pool had the most freedom and fell by several inches. The edge along the pool was likely tied structurally to the pool. The result created a hinging effect where the deck now leans. Note that on the accompanying raised deck section the deck movement is such that the brick and tile fascia have broken loose and and are no longer level with the pool structure.

Wire Mesh Reinforcement - In a recent renovation project we found the deck was moving away from the pool, sometimes horizontally. In some sections, the deck sections had separated from each other where one section was 2″ higher than the adjacent section. In this particular case there had been little or no attempt to tie the sections together structurally. In fact, the only reinforcement existing in the deck was a wire mesh, insufficient in this case.

If this was not actually the case and the supporting ground settled beneath the concrete, then the concrete would then have to span across the gap created by the settled earth, a condition it was not designed to withstand.

Lastly, I should point out there is movement of decks and other concrete work which is not related to the soil. It is expansion and contraction. In this case the slab is not so much moving, as in away from or towards something else, as much as it is getting ever so slightly larger when it warms and then smaller when it cools.
This type of movement is anticipated by the integration of isolation joints or expansion joints in the concrete. These joints are actually separations in the concrete works, usually filled with a flexible material. The concrete sections should not be structurally tied together unless designed by an engineer. Keeping them from from moving apart will be accomplished by properly addressing any soil concerns. Failure to install these isolation joints at appropriate locations can often cause their own problems, such as cracking where two sections, both expanding moved against each other.