Foundation problems can be very costly for homeowners. Cracks in exterior brick veneer and/or interior ceramic tile could be the result of an unstable foundation. Repairs to such defects will only need to be repaired again in the near future if the foundation issues are not resolved. Jade Home Inspection has several years of experience in detecting and solving foundation problems.
It is nearly impossible to prevent cracks from developing in concrete slabs on grade, especially exterior slabs. The reasons are explained herein.
First of all, whenever concrete changes from a semi-liquid or plastic (i.e. from fresh, ready-mix concrete) to a hardened state (i.e. cured-out), it undergoes a volume shrinkage. This volume shrinkage is primarily related to loss of water. Concrete generally consists of sand, portland cement, gravel and water. The water combines (chemically-reacts) with the portland cement to create the rock-like, hardened mass we call concrete. During this process, much of the water is used up. In addition, a significant amount of water is lost to either evaporation or seepage into the subgrade. To help prevent rapid loss of water in freshly placed concrete, especially during warm/hot/windy days, the finished surface of the concrete should be kept moist and covered with plastic or wet burlap for several days. (This helps to maintain moisture to slow down the moisture loss process.)
In spite of extended curing [in a moist state], the eventual loss of water in fresh concrete will cause a significant volume change (reduction) from the ready-mix to the final, hardened state. In relatively thin concrete slabs on grade, this volume change results in a significant dimension change in the surface (plane) of the slab. As the slab shrinks, frictional forces can be generated along its bottom face (due to contact with the subgrade); this can set-up tensile stresses in the concrete. Also, if the concrete loses moisture through the surface only, due to the presence of an underlying plastic vapor barrier, the top face of the slab will shrink more than the bottom. This causes the slab to curl up around the perimeter edges. Both of these conditions can lead to cracking, because both (dimension shrinkage and curling) set-up tensile stresses in the slab.
Tensile stresses are detrimental because, while strong in compression, concrete is very weak in tension; i.e., it is relatively easy to “pull-apart” plain (unreinforced) concrete. Therefore, steel (wire mesh) is embedded in concrete to provide it with the tensile strength needed to control crack formation. Effective steel reinforcement, however, is more easily attempted than accomplished. Unless the wire mesh is held up by closely-spaced chairs, it is not likely to remain properly positioned near the top-center of the hardened slab (the position at which it is most effective). The success of wire mesh in crack control depends upon whether the contractor/finisher systematically reaches down through the concrete mix with a hooked bar and pulls the steel mesh off of the ground and up into the plastic mix. Unfortunately, as is well documented, test cuts consistently show that these precautions are not taken; more often than not, the wire mesh is found lying on the ground, barely embedded in the concrete. Under these circumstances, it serves no purpose. It is also necessary to realize that larger slabs require larger steel for crack control. Engineers may be mindful of this, but few concrete finishers are.
Once an exterior concrete slab on grade is placed, finished, and cured, it is subjected to perpetual, direct weather exposure, including periodic (and sometimes drastic) changes in temperature and humidity. Because concrete tends to absorb and/or give off moisture according to the surrounding weather conditions, weather changes affect the concrete’s moisture content; and, along with any moisture content change comes a volume (dimension) change. Concrete also expands/contracts with changes in temperature. Hence, once hairline cracks form in concrete, the effects of Mother Nature usually start to take their toll: initially small, invisible shrinkage cracks later enlarge. Crack enlargement facilitates direct moisture penetration into the crack, which can lead to isolated subgrade saturation or freezing expansion of trapped water during extremely cold weather. (Everyone knows that water expands upon freezing; hence, trapped water inside concrete cracks can pry cracks apart during freezing weather.)
In addition, moisture that seeps into the subgrade beneath a slab on grade can affect the behavior of some soils–especially highly plastic or silty-clays. Highly plastic clay soils exhibit an electro-chemical affinity for water, actually incorporating water molecules into their chemical structure (crystalline lattice). Thus, these soils are typically called expansive clays. During the winter, which is the normal rainy season in North Alabama, these soils take on moisture and expand in volume. Soil expansion can lift up or heave a slab-on-grade. During the summer, when droughts and hot weather frequently occur, the soils desiccate and shrink in volume, causing slabs to sink or settle. Hence, changes in the subgrade moisture content can cause slabs on grade to “move” throughout the year. Fine-grained soils, like silts and clays, are also very weak when wet or saturated and easily yield (deform) under a load. Hence, placing concrete slabs directly on top of highly plastic clay soils is a bad idea–especially for driveways which support heavy vehicle wheel loads. There are many types of highly plastic silty and expansive clays in North Alabama. They typically occur above/around limestone bedrock deposits. Contact the county extension agent at the nearest USDA Soil Conservation Service to learn more about the locations of expansive clay soils in your area.
In addition to the adverse effects of shrink-swell soils beneath driveway slabs, consider the problems that water beneath slabs can cause. Water that penetrates slab cracks can puddle beneath slabs; then, whenever a heavy vehicle crosses the slab, it will deflect under the wheel load and force water from the underlying puddle through the cracks (this is called “subgrade pumping”). The water usually carries fine-grained soil particles from the ground with it. After a sufficient amount of erosion has occurred, a void typically forms beneath the slab; then, when a heavy wheel load crosses the slab, it could cause an isolated portion of the slab to collapse into the eroded hole, resulting in pavement failure.
Generally, closely-spaced control joints are installed in slabs to prevent/conceal unsightly cracks. Either pre-formed or saw-cut, control joints create pre-planned planes of weakness in concrete slabs, thereby predetermining straight (as opposed to random, jagged) lines in the slab surface.
Whenever an engineer or architect is left out of a concrete slab construction project, the concrete contractor is responsible for providing proper control joint spacing. A rule of thumb states that control joints should be spaced at intervals (measured in feet) equal to three times the slab thickness (measured in inches). In other words, in a 4 inch thick slab, control joints should be spaced at (4” x 3 = 12′) 12 foot intervals. Moreover, control joints should be placed in areas of abrupt slab dimension change. In order to be effective, saw-cut control joints must penetrate ¼ of the slab thickness. Hence, a 4 inch thick slab must have 1 inch deep, saw-cut control joints. In narrow pavements, like sidewalks, control joint spacings should range from what to no more than twice the width of the sidewalk. If wider control joint spacings are desired, steel reinforcement can be utilized to prevent random shrinkage cracking.
Another cause of concrete cracking is structural overload. Concrete slabs should be placed on well-compacted beds of gravel. This helps to provide uniform bearing pressure beneath the slab whenever it is exposed to surface loads. Heavy wheel loads, for example, are transferred through the slab and onto the subgrade. If the subgrade consists of weak or saturated soil, it will likely deform under the pressure. This can set up bending stresses in the concrete slab which, in turn, can generate adverse shear and tensile stresses. Under some heavy loads and poor subgrade bearing conditions, concrete slabs crack. As stated, this lets water seep into the subgrade and the application of future heavy wheel loads leads to a worsening condition (see the preceding discussion of “subgrade pumping”).
In summary, large, crack free, unreinforced concrete slabs on grade are rare. To have a long-lasting, aesthetically pleasing concrete slab on grade: utilize a high quality concrete mix with proper air-entrainment (see section on surface defects), place the slab on a well-compacted layer of gravel (subgrade), saw cut control joints at a close-spacing (throughout the slab) within 12 – 24 hours of placement (in order to provide pre-planned planes of weakness for crack control), then properly cure the slab for several days. You’ll find that, in a relatively short time, cracks will form at each of the control joints. You can seal these cracks with a commercially-available concrete crack sealer/caulking to preclude water entry. Other construction precautions include utilizing a quality concrete ready mix during placement and adding the least amount of water possible to the delivered concrete prior to placement. Proper curing entails wetting the surface of the concrete after finishing, then covering the slab with plastic or wet burlap. The concrete should be kept moist in this manner for as long as possible–at least three to seven days. If this is impossible, spray the slab with a commercial concrete sealer (according to directions of the sealer manufacturer). You’ll generally find that a very generous application of sealer is required to form a thick layer of surface sealer, i.e., one capable of preventing excessive moisture loss through the slab. Lastly, finished slabs should drain freely onto the adjacent ground or designated areas which direct runoff away from the slab.
For more information you can check out the web site called Concrete Basics from the Portland Cement Association.[/learn_more]
[learn_more caption=”Foundation Movements I”]
A home-buyer transferring into the Huntsville metropolitan area once asked me, “What is the most common residential foundation problem that you see in your home inspection business?” I answered, “Differential foundation settlements (movements) due to expansive clay soils.” “Not so much expansion,” I explained, “but the opposite actually-or what we commonly call ‘shrinkage.’”
The residential foundation problems are related to volume changes in the clay soils which underlie the house. These particular clay soils have what engineers call a ” high plasticity”. This is a term that is used to describe a clayey soil which remains “plastic” (or in a “moldable” state), neither turning liquid nor crumbling apart, over a wide range of moisture contents. This is unlike other fine-grained soils (mixtures of sands, silts and clays) which either liquify or crumble apart at the extreme ends of a fairly small range of plasticity (moisture contents.) Liquifying or fluid flow occurs at the wet end of the moisture scale, whereas crumbling occurs at the dry end. Many of you might compare a highly plastic clay soil to the popular child’s toy, “Playdough®” in that highly plastic clays can be molded into various shapes or, when worked in between the thumb and index finger, one can form a long, thin “ribbon” of clay.
The reason why these soils remain plastic over such a wide range of moisture contents, is because they have an “affinity” or attraction for water. Actually, they have an attraction for the H2O water molecule. In order to understand this, you must understand that unlike silts or sands, clay particles are microscopic in size. We cannot see them with our naked eye. Walk along the beach and grab a handful of sand and you can easily sort out individual sand particles. You can’t do this with clay. The clay particles (actually platelets) are microscopic in size. They are, in essence, ions or molecules and they bond together (to one another). Most clay ions also chemically react or interact with the H20 water molecule. They usually want to bring in (attach) water molecules to their crystalline lattice (or molecular structure/chain).
Clay soils usually evolve from the weathering of minerals and, therefore, take on the mineral’s crystalline structure or chemical make-up. Typical clay minerals in our area are -silicates of either iron, magnesium or aluminum. Some clay soil minerals have a very high attraction for water. Instead of wanting to add one or two water molecules to their structure, they want to add many more. These are the expansive clays. When this happens, the individual clay ion’s crystalline structure or lattice dramatically grows in size. When all the millions and billions of clay ions/molecules underlying a foundation footing absorb large numbers of water molecules, the soil dramatically grows in size. In the real world, where we live, this volume change can be significant. The reverse is also true. And here is where the foundation problems seem to arise. Whenever the clay ions give up attached water molecules (from evaporation or plant root uptake) they shrink in volume. Extreme examples of this type of shrinkage are the formation of large cracks in the ground surface and/or the ground pulling away from a home/foundation during the hot, dry summer months or during droughts. Examples of foundation problems are differential settlements caused by shrinkage of the clay soil.
A simple analogy for this phenomena would be, “clay soils are like sponges”. Whenever sponges soak up water they swell in volume. When left on the counter to dry, however, they shrink back down to a very small size…only to puff-up again when placed into contact with water. Clay soils are also like sponges in that when they become wet, they become soft or softer. When dry, they become very hard. Place a heavy load on dry clay soil and it goes nowhere or settles very little. The ground will support a sky-scraper! When wet or saturated however, don’t even think about driving a riding lawn mower across the ground. It will sink up to its axle.
Hence, when living in a house constructed on highly plastic clay soils (particularly swelling clay soils) it is very important that the owner make sure that the drainage conditions around the home are monitored and maintained annually, so that surface water runoff always flows away from the home and never stands or ponds beside it. If this ever occurs, and the ground alongside and beneath a home (foundation footing) becomes wet or saturated during heavy rains or throughout the rainy season (due to poor drainage conditions, overflowing gutters, lack of gutters, etc.), then the house may slowly settle into the ground in the areas where the clay soil has become softest. This, alone, can lead to differential (or uneven) foundation settlement. But another, and often more serious problem occurs during the following summer or fall after the hot, dry weather we typically experience in the North Alabama geographic area. The soils that stayed extremely wet throughout the year, and swelled accordingly, shrunk dramatically by the time July and August rolled around. The portion of the house resting on this formerly wet ground also settles (dramatically) along with the shrinking clay.
Believe it or not, however, this common problem can always be avoided during foundation construction if builders would simply admit that these soil conditions exist and recognize that they need to have each building site evaluated by a professional soil scientist or geotechnical engineer prior to house construction. If not, it is likely that the builder will utilize the standard shallow-bearing foundation, and place the footings or foundation just below or within a few feet of the ground surface. Soil shrinkage-related settlements or movements will eventually lead to foundation settlement cracking and/or the formation of large cracks in brittle construction materials such as brick veneer and sheetrock. This is almost a guaranteed.
In my next article, I’ll discuss why plastic clay soils can cause 20 to 30 year-old houses to suddenly develop foundation settlement damage. I’ll also explain how to live in and/or deal with a home constructed on highly plastic clay soils by controlling the seasonal movements associated with shrinking clays, and I’ll tell how to go about repairing severely cracked and damaged houses.[/learn_more]
[learn_more caption=”Foundation Movements II”]
It has been my experience that whenever an older home (say 15 to 20 years or more) that is constructed on highly plastic clay suddenly experiences cracking in brick or sheetrock, after many years of crack-free service, and there’s no other obvious answer to the problem, it’s probably related to drying shrinkage of underlying expansive clay subsoils. These subsoils probably remained wet each year, throughout the life of the home. Or, at least, the moisture change that did occur each year in the underlying massive clay soil was not sufficient to cause enough building distortion to, in turn, cause building material cracking. If the home suddenly develops cracks, however, there’s probably been some major change in the drainage conditions or landscape surrounding the home. For example, if the house originally had gutters and these served their purpose for many years, then, for some reason (frequent leaf/debris clogging hassles and subsequent rusting deterioration?), the owner removes them, the moisture conditions around the house suddenly change. My personal experience suggests that the effects of water falling beside the home will eventually begin to take its toll by eroding more and more surface soil– thus exposing the foundation footing and/or the underlying plastic clays to more and more solar radiation. At some time, say 20 years after construction, the effects of normal summertime drying shrinkage may finally cause soil shrinkage sufficient to create enough foundation distortion to bring about enough building distortion to in turn cause sudden building material cracking, such as cracks in brick or sheetrock.
Another problem that I’ve noticed over the past twelve years of inspecting houses, is that if a particular part of the crawl space or foundation remains moist or under standing water during most or part of the year, as evidenced by dark, water/algae stains on the concrete or concrete block foundation system, then the adverse effects of a summer drought are most pronounced on the underlying bearing soils in these wet areas. In other words, these soils go from the extreme wet end of the moisture scale to the dry end. Imagine: the larger the moisture change, the more the soil shrinkage and, therefore, the greater potential for foundation settlement.
A third problem is related to large trees. If hardwoods are planted near a home soon after construction, it is likely that in 20 to 30 years, the trees will overhang the house-which means that their roots underlie the house. These roots are going to contribute to soil desiccation during a drought by robbing the soil of all available moisture. Hence, whenever a hardwood or pine tree overhangs a home/roof built on a highly plastic clay soil site, the tree should probably be removed.
Regardless of trees and poor drainage conditions, if a house is initially built on a shallow foundation system which bears on expansive clay soils, then the house will likely move and distort from day one. Moreover, it is unlikely that this movement will be uniform because the moisture content around and beneath the home will not be uniform. It will vary because of different drainage conditions that exist, the varying degree of solar exposure and the presence of large trees or shrubs. As stated, trees are especially capable of desiccating (completely drying) soil during drought periods. In time, the effects of uneven foundation movement, or differential foundation movement/settlement, or simply building distortion, will take its toll on the home and cracks will begin to form in the more brittle materials. The cracks form to relieve the built-up stress of distortion.
What can be done to control the distortion?
The most economical way to deal with expansive clay soils underlying a home is to try and maintain a uniform moisture content in the ground (clay soil) throughout the year. This is best accomplished by letting nature takes its course throughout the wet winter, when rainfall is greatest and evaporation is lowest — then adding water via irrigation, as required, during the dry periods of the year. Drip irrigation tubing laid alongside the foundation footings, covered with thick beds of mulch, typically works well to do this. By applying water slowly, via the drip tube, the ground can absorb water previously lost or given up to evaporation and plant root uptake. One has to be careful, however, not to over-water or saturate the ground because as stated, this can soften the bearing soils and lead to foundation settlement. Any irrigation contractor should be able to come up with an automated drip irrigation system. In other words, moisture detectors can probably be buried below/beside the foundation and utilized to activate (automate) a drip irrigation system using a conventional lawn irrigation control panel. The buried sensors tell the irrigation control panel when to turn valves on and off. The irrigation contractor should work closely with a professional soil scientist to determine the range of soil moisture content readings that should be utilized to activate/deactivate the irrigation system.
In some instances, when the amount of differential foundation movement has been so severe that a foundation component has cracked and faulted, when doors and windows start to bind, or when the brick veneer has cracked, faulted and/or pulled away from the home, it might become necessary to underpin the distorted foundation so that future movements do not make matters worse. Foundation underpinning can also be used to uplift and/or relevel a settled (distorted) foundation.
Underpinning typically entails digging or drilling a vertical shaft into the ground, alongside and beneath an isolated part of the foundation, then filling the excavation with concrete. These shafts are usually spaced several feet apart so as to not undermine the entire foundation, which would obviously lead to collapse. In some instances the concrete is simply placed into the shaft until it contacts the footing, thus preventing future or additional settlement. In other cases, the concrete placed into the excavation is stopped a foot or more beneath the footing and allowed to harden or cure for several days. Large hydraulic jacks are then placed beneath the footing, on top of these concrete piers or shafts, and the footing is carefully jacked upward, extending the jacks in proportion to the amount of settlement at each point. It’s a real art (and danger) to relevel a settled house/foundation, so this type of repair should be left only to professionals.
A similar method of foundation repair entails the use of helical augers which are turned or twisted into the ground and stopped whenever the driving torque reaches a certain level — indicating that the blades on the auger have reached a sufficient bearing capacity with the deeper soil strata to provide a substantial amount of vertical support. Steel brackets are fitted over the auger shaft and under the foundation. The auger shaft is usually threaded, so large nuts placed below the bracket are turned in order to lift the footing upward.