The performance of residential structures, built on ground supported concrete foundations depends not only on proper design and construction, but also on proper foundation environment maintenance performed by the occupant or owner of the property. Many residential foundations have experienced foundation problems because of improper installation, maintenance or alterations of the drainage system and landscaping. A properly designed and constructed foundation will experience distress from soils, which under go non-climactic moisture changes. The majority of the problems can be traced to leaky or damaged plumbing, or over irrigating the surrounding lot. Over irrigating, your surrounding property is comparable to no foundation maintenance; they both create undesirable distress to the slab. Initial site grading shall provide positive drainage away from the foundation perimeter. The site drainage plan developed by the civil engineer should be maintained throughout the entire life of the foundation. Positive drainage, to prevent ponding next to the foundation, is imperative if you want to minimize foundation problems. Drainage from gutters or down spouts must be maintained such that water run off is directed away from the foundation perimeter, and will insure that the water does not collect directly near the foundation. Poor drainage will cause a change in the moisture content of the soil surrounding the foundation. As the soil absorbs excessive amounts of moisture, it will swell or rise, causing unwanted foundation movement. A 3% to 5% slope should be maintained for a minimum distance of 10 feet from the edge of the foundation. Pay close attention to landscaped beds around the foundation to insure that water does not collect or dam in between shrubbery and the edge of the slab. It will be necessary at times to re-establish a positive grade away from the foundation due to climactic deterioration. Should the drainage surrounding the foundation become inadequate, compacted, select fill should be applied to the surrounding area. Improper fill material and/or fill compaction may result in the appearance of positive drainage; however, upon closer inspection the repairs made may be ineffective. The use of the improper fill for repairs to the drainage can be detrimental to the foundation. Refer to the soil evaluation of the property, to determine the proper fill and compaction that is recommended by the testing soil engineer. The use of subsurface drains can be used to control an undesirable water table, but the use of such drains will not stop water from migrating under the foundation. The geotechnical engineer can also recommend types of waterproof barriers that can be applied surrounding the edge of the foundation if further moisture control is required. The homeowner should be aware of the conditions surrounding the foundation at all times. Flooding produced by excessive water, from swimming pools, surrounding drain down spouts, and plumbing leaks can damage the foundation if corrective measures are not taken immediately. The moisture content of the soil surrounding the foundation is the KEY to the life and performance of the slab, and must be maintained year round.
The objective of a proper maintenance program is to maintain a constant moisture content for the soil, both, around the perimeter of the slab and under the foundation.
The following is a list of items to be considered when planning proper foundation maintenance: 1) Maintain positive drainage away from the foundation and install gutters, if applicable. Never allow water to pond near or against foundation slabs. 2) Replace and compact any loose fill adjacent to the foundation with native soil. DO NOT USE sand or granular material. 3) Incorporate a foundation maintenance program into your weekly schedule. Check the gutters and down spouts to be sure that they are clear and the water is discharged away from the foundation area at all times. Down spouts should drain directly onto concrete such as walkways, driveways, or concrete flood ways. 4) Avoid seasonal drying around the perimeter of the foundation. Install a sprinkler system with a timed irrigation for the best results. Sprinkler mains should be located a minimum of five feet from the building line. Check with the Soil Engineer to determine the proper moisture content desired for your area. DO NOT OVERSATURATE. The soil in the southwest is comprised mainly of clays that easily expand, when too much moisture is applied. Expansive soil will swell causing distress to the foundation. 5) Existing vegetation near the foundation typically draws added water from the adjacent soil towards the foundation, thus causing added soil movement. Remember: Any changes in the exterior layout of the property, flower beds, decks, patios, fences, trees and shrubs, must be planned such that positive drainage is directed away from any foundation structure and off the property. This must be maintained at all times through out the life of the structure. 6) Trees located near a foundation can cause potential foundation distress and possible failure. Experience has shown that the presence of or the removal of large trees can create serious damage to the foundation. Depending on the type of tree, proximity to the edge of the foundation and its size may produce vertical movements between 3 to 5 inches. Proper research is required prior to planting or removing trees. Tree removal with out proper re-compaction of the surrounding area can be detrimental to the foundation. Planting trees that are closer than 10 feet to the foundation will cause future foundation problems, depending on the root structure. Root shields should be installed to prevent root penetration under the foundation. Do the proper research before planting and advise the foundation engineer, of tree removal compaction, prior to the design phase. Large trees and shrubs should not be allowed closer to the foundations than a horizontal distance equal to roughly their mature height due to their significant moisture demand and root growth, upon maturing. 7) The homeowner must conduct a thorough, yearly survey of their foundation and perform regular maintenance throughout the year as required to insure the performance of the foundation. Immediate corrective measures must be taken should problems arise. 8) The homeowner should be informed by the builder of the precautions that should be taken prior to any modification to the foundation with respect to the post-tensioned tendons that provide the support required for the foundations performance. Any modification to the foundation, without written approval from the design Engineer, is dangerous and could possibly be fatal to the life of the structure, and will not be warranted under the limitations.
COMPATIBILITY CONSIDERATIONS Post-Tensioned slab foundation as well as other conventionally reinforced shallow foundation systems constructed on compressive or expansive soils is expected to deform. The flexible foundation primarily distributes localized soil movement (edge lift and center lift) to a more uniform slab shape. The builder should consider suppliers that offer compatible products that offer flexible properties to work with the foundation movement that is anticipated. In particular, the compatibility for roof trusses, load concentration, brittle exterior siding, and areas, which slope to drain and utility connections, should be addressed prior to the architectural design phase. North Texas is noted for highly expansive soil, and some areas should be considered extremely expansive. The builder should select construction materials that will accommodate movement and homeowner should be educated and anticipate a degree of structure deformation will occur during the seasonal climatic changes. Again, yearly preventative maintenance programs will limit the amount of structural movement, but foundation movement will occur and the property owner should be prepared for foundation movement, and problems that will eventually develop.
CONCRETE CRACKING & ELIMINATING POST CONSTRUCTION TENSIONS Concrete exhibits two inevitable properties as it cures, it will eventually become solid and it will eventually crack. Cracks that develop and run parallel to the perimeter of the foundation will cause the majority of foundation problems. If these types of cracks are encountered, the design engineer should be contacted and a serviceability inspection should be scheduled and completed. Many problems can be fixed with minor repairs and possibly eliminated, if the problem is recognized at an early stage. All other cracks, in the central portion or dormant region, of ground supported slabs are insignificant and are not considered when evaluating the performance of a foundation that exhibits seasonal movement due to the expansive soil properties. Post-Tension reinforcement is considered an ACTIVE foundation support system, in which the foundation is designed to accommodate the expansive soil movement, thus creating cracks during the movement process. The cracks may be visually distracting to the property owner, but the foundation is performing and functioning exactly as the design specifies, thus preventing foundation failure. Cracks of this type are common and should be expected by the homeowner, but it should be understood that cracks of this nature do not affect the structural integrity of the foundation. Educating the property owner about the Post-Tension process will eliminate the inevitable call from a homeowner that does not understand that their foundation is actually functioning per the design requirements, thus preventing the stress and worry associated with foundation cracks. Pre-stressing the foundation to 30 percent, 24 hours after the concrete pour is recommended to help prevent shrinkage cracks, but will not eliminate surface cracks. You won’t have to stress, if you know your foundation functioning properly.