By Tim Long
It is amazing, if not mind boggling, that something as big and as heavy as a concrete deck can move. One would think that something of this sort, when constructed would stay put. However, decks can and do move. Sometimes just slightly, with only slightly noticeable effects, and other times by several inches or more, where they are at the least a visual annoyance and perhaps a hazard.
Wide Expansion Joints
The reasons decks move are varied. All the reasons are built on the understanding that the deck is not moving alone. If a deck moves, it is likely that the soil underneath the deck has moved as well. The soil, must be thought of not as a solid material, but as a slow moving fluid. We have all noticed soil can appear to grow over concrete that is both adjacent and slightly lower in elevation than the body of the soil. Generally the soil, which is loose combinations of particles in varied size, is bound and held together by only gravity and the root systems of it’s plant life. If the soil 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 things, much of the soil in north Texas is made up of expansive clay. 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 compactible soil, proper foundation/footing sizing and placement, and to reach down to stable soil, piers and pilings.
Deck Falling Away
Where the deck has fallen away from the pool, the cause is likely the supporting soil was not compacted correctly. The picture shows a 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 just 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.
Structural Cracks
In some cases, where the soil has moved, the deck is unable to move, it responds differently, by cracking. A structural deck crack is one that exist all the way through the concrete structure and indicates a complete break. This type of crack is a failure of the concrete to resist external forces to bend. In some cases there was insufficient or incorrectly laid steel reinforcement within the concrete to withstand normal, expected forces. Other times, however, the deck was engineered with the assumption the ground would be stable and sufficiently compacted . 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 a movement of decks and other concrete work which is not related to the soil. It is expansion and contraction. In this case the deck 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.
