Concrete Deck Construction
Concrete Decks in Expansive Soils
Minimizing Lifting and Cracking of Concrete Decking in Expansive Soils
By Ron Lacher, P.E.
Pool Engineering, Inc.
It is well known that expansive soils and poor drainage are a potentially damaging combination. Poor drainage results in standing water, which can penetrate into the ground causing various problems to structures (such as concrete pool decking) supported on and in the ground.
It is imperative that adequate yard drainage be provided as soon as possible after (or before) installation of the concrete flat work. Delay may result in moisture migration into the pool concrete deck sub-grade with the consequence of damage in the form of lifted pool decking. Most standard swimming pool contracts require that the owner is responsible for yard drainage away from the pool site.
In addition to establishing and maintaining proper yard drainage, the following is highly recommended to minimize lifting of concrete pool decking:
The most important practice in reducing the potential for lifting of concrete decks due to expansive soil is the pre-saturation of the soil prior to pouring concrete. A common specification used by soils engineers is to attain a 110% to 120% of optimum moisture content to a depth of at least 18″. This moisture penetration should be verified by test holes prior to the pouring of concrete. Of course, without the installation of crushed rock as discussed in No. 2 below, pre-saturation will create a muddy sub-grade making it extremely difficult to place and finish the concrete.
Water migrates very slowly through most expansive soil, therefore, obtaining the recommended pre-saturation is often a tedious process. Experienced decking contractors often spread an ample amount of a surfactant such as laundry detergent on the ground during pre-saturation to aid in obtaining the recommended moisture penetration. Another technique is to utilize a post hole auger to create a grid of small diameter holes (2 to 4 inches in diameter), 2 to 2 ½ feet deep that would be filled with crushed rock (see No. 2 below). These holes fill with water during pre-saturation to further aid in obtaining the recommended moisture penetration. As an added benefit, expansive soil could expand latterly into the augured void helping to relieve upward soil expansion pressure.
2. Crushed Rock
Crushed rock can be installed (at additional cost to the pool purchaser) under the concrete pool deck to retard the heaving of concrete pool decking. A minimum of four to six inches of crushed rock is highly recommended as a cushion (the expanding soil will partially push in between the voids between the rocks) and also to permit pre-saturation of the of the deck sub-grade soils without creating a muddy sub-grade.
3. Perimeter Footings
Perimeter footings can be installed (at additional cost to the pool purchaser) at the perimeter of all slabs exposed to planters or other sources of moisture. Perimeter footings will slow the infiltration of moisture into the concrete decking sub-grade and retard the soil expansion and resultant heaving of the concrete pool decking.
4. Cut off of Water Following Plumbing Trenches
Water, normally, is very slow to penetrate or flow under compacted expansive soils. Pool plumbing and drain line trenches frequently run under concrete swimming pool decking and these trenches are usually not properly compacted when back filled. Surface water from outside the deck area usually finds these loosely compacted trenches and will follow them back under the pool deck where the water can penetrate into the expansive soil and be a cause lifted decking. A cut off wall of properly compacted expansive soil can be placed at the point where the trenches extend under the pool decking. This cut off wall will stop surface water from following the plumbing trenches under the pool deck, which will decrease the potential for lifted pool decking. Carefully attention should also be given to properly sealing the drain lines extending under the pool deck.
5. Reinforcing Steel
The addition of reinforcing steel in the concrete pool deck would minimize cracking but would not stop lifting of the concrete slab due to water infiltration into expansive sub grade soil. With regard to the addition of reinforcing steel in concrete flat work, a frequent recommendation in soil reports prepared for pool and landscape construction in expansive soil areas is the installation of #3 re-bar at 12 to 24 inches on center in each direction or 6×6-W2.9xW2.9 welded wire fabric.
6. Control Joints
Cracking in concrete flatwork is unavoidable due to shrinkage upon drying and expansion and contraction due to temperature change. However, joints are installed in concrete flatwork in an effort to limit cracking and to control the crack locations. Experience and proper trade practice dictates the location of joints to minimize cracking in concrete flatwork.
Guidance on proper trade practices can be found in the American Concrete Institute Manual, “Slabs on Grade”. I highly recommend that contractors involved in installing concrete slabs on grade obtain a copy and follow this manual. The following is a brief summary of the recommendations in this manual:
To avoid random cracks, contraction joints are used to create straight-line planes of weakness in the slab. As the slab shrinks and tends to curl, the joints open slightly and cracks occur at the predetermined locations instead of randomly over the slab. The planes of weakness may be established by jointing tools, by insertion of joint forming strips while the concrete is still plastic, or by sawing after the concrete has been finished.
Where to put contraction joints
Contraction joints should be placed between joints at 24 to 36 times the slab thickness. The resulting panels should be as nearly square as practical, dividing a large floor area into relatively small panels. Avoid elongated and L-shape panels. Never make the long side more than 1½ times as long as the short side. All contraction joints should be continuous, not staggered or offset. Diagrams depicting contraction joint locations are shown below.
7. Owners Maintenance Responsibilities
Maintenance of Joints
Contraction joints, when cracked and open, may allow moisture to flow through the crack and penetrate into the sub-grade soil. If these joints open, it would usually occur well after the project is complete. The owner should be informed of the possible consequences (lifted and cracked concrete flatwork) of leaving these joints unsealed. Sealing of contraction joints as well as maintaining the seal on the joint between the brick coping and the concrete pool decking is an ongoing maintenance responsibility of the owner.
Maintenance of Proper Yard Drainage
The proper yard drainage established at the time of concrete installation is frequently modified during landscape installation or is simply not maintained over time. The owner should be informed of the possible consequences of not maintaining proper yard drainage.
Following these recommended practices will minimize concrete lifting and cracking and will provide quality results. If you have any further questions please do not hesitate to contact us.
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