Differential settlement
Differential settlement, is when a building's piers or foundation settles unequally. Differential settlement can result in damage to the structure, and is therefore, of concern. Differential settlement occurs when the soil beneath the structure expands, contracts or shifts away.
The purpose of a foundation is to distribute the weight of a structure securely into the ground. Geo technical engineering data regarding soil, rock, and water conditions are used to design foundations with structural integrity. When foundation failure does occur, it is usually the result of differential settlement or heaving of the soil that supports the foundation. In any instance, foundation repair is necessary to prevent further damage to your house or building. Not only this, but crack repair will prevent moisture and insects from entering your house.
The results of foundation settlement may be one or more of the following problems subject to soil and structural testing:
Windows and doors sticking
Roof or basement leaking
Bricks or walls cracking
Walls bowing, bulging or leaning
Drywall separating
Stair step cracks in masonry mortar joints
Space between wall baseboard and floors
Chimneys tilting or leaning
Garage leaning
Caulking tearing at windows or doors.
If you’ve noticed any of these indications of foundation settlement, your house may require foundation repair services (as well as crack repair services).
Drought conditions – Typical clay-based soils are susceptible to changes in moisture content. Wet conditions cause clay-based soils to expand. Dry conditions cause clay-based soils to contract. As soils contract, they pull away from the foundation allowing settlement to occur.
Undermining of the foundation – Poor drainage can cause soils to be eroded away allowing settlement to occur.
Sinkholes – A naturally occurring phenomenon resulting in the supporting soil under the foundation to collapse allowing settlement to occur.
Weak soils or foundations
Erosion or scour effects to foundations and supporting piles;
Movement and deterioration of substructure elements such as floor beams, joists, sagging or overloaded beams, footings
and other detrimental effects as determined by the engineer upon structural analysis and calculations;
Under designed foundations and effects of surge, flooding or high wind hurricane forces including seismic effects and earthquakes;
Blasting effects such as underground blasts by rock quarries
Erosion for properties near or adjacent to sea walls, soil and water erosion behind sea walls, causing building settlement
Other causes that may be attributable to structural movement and settlement.
United Dynamics, Inc. Resistance Piers are hydraulically advanced through soil to reach bedrock or load bearing strata. The steel pier transfers the weight of the building from the failing foundation system to the pier itself, thus preventing any future additional settlement. In some instances, lifting of the structure is possible allowing for the correction of problem doors or windows and the closure of existing cracks.
To facilitate the installation of the steel piers, our skilled crews will carefully excavate the failing foundation to properly prepare the footing for the acceptance of the steel bracket that attaches the pier to the structure. Once the bracket is installed, the crew will begin foundation repairs by advancing high strength steel tubing through a collar located on the bracket via a hydraulic cylinder and hydraulic pump. When refusal is reached, the bracket is then proof tested to verify its holding capacity and locked off to prevent any further movement of the structure.
Depending on the structure, minimal site disturbance should be expected during the course of the foundation repair procedure. Our trained crews take great pride in putting everything back in place whether it is mulch, shrubs, plants, sidewalks etc. They will also typically repair cracks in the masonry as well as retooling caulk joints that may need to be repaired.
United Dynamics, Inc. Helical Piers are hydraulically advanced through soil to reach bedrock or load bearing strata. The helical pier technology utilizes foundation repair techniques allowing for a passive installation not dependent on the weight of the structure to drive the pier. To facilitate the installation of the steel piers, our skilled crews will carefully excavate the failing foundation to properly prepare the footing for the acceptance of the steel bracket that attaches the pier to the structure. Once the helical is installed, the crew will then mount the bracket to the steel helical, effectively transferring the load off of the failing foundation system to the helical pier. The bracket is then proof tested to verify its holding capacity and locked off to prevent any further movement of the structure.
Depending on the structure, minimal site disturbance should be expected during the course of the foundation repair procedure. Our trained crews take great pride in putting everything back in place whether it is mulch, shrubs, plants, sidewalks etc. They will also typically repair cracks in the masonry as well as retooling caulk joints that may need to be repaired.
Expansion and Contraction- Thermal Expansion of Materials
Thermal Expansion and Contraction. Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled. When free to deform, concrete will expand or contract due to fluctuations in temperature.
Imagine yourself in an open space, chairs laid out in a certain area. All the chairs fill up,and everyone sits side by side. Everyone coexists in a very harmonious way, occupying that set space. Now imagine a skunk popping up from underneath the chair right squat in the middle. What's going to happen? You guessed it, everyone is going to jump out of the chair and scatter everywhere! The crowd will expand.
This is very much what thermal expansion is all about. At a certain temperature, atoms within a material are going to occupy a set space. This set space defines the boundaries of that material. Atoms are in constant motion, bouncing off of each other, but at that specific temperature they are moving and bouncing against each other in that set space. When you heat that material, the atoms get agitated. They move faster. The hotter the temperature, the faster they move. They start bouncing off of each other more frequently and with more force. They start to need more space to coexist in, and they get it. They expand the space they exist in; expand the boundaries of the material they make up. What ends up happening is the material itself expands.
Solids, liquids and gasses all expand when they are heated. Different materials expand at different rates although a general rule of thumb is that liquids and gasses expand at higher rates than solids.
Thermal contraction is the opposite of thermal expansion. When the temperature drops, atoms calm down and shrink. They aren't bouncing so aggressively off of each other and don't need that much space to coexist. When they shrink, the boundaries of the material shrink. The material contracts.
Most solids, liquids and gasses contract when they are cooled. One notable exception is water. Water is a magical element that expands at freezing point. Liquid water contracts when cooled, and you would expect that it continues to contract as it gets colder. When it hits freezing and turns into a solid, it expands. That's why water pipes may burst when the water inside freezes over. Ice takes up more space than liquid water.
There are three basic types of thermal expansion:
Linear thermal expansion
Area thermal expansion
Volumetric thermal expansion
Linear thermal expansion is only used for solids and indicates an expansion in one dimension. Imagine a measuring tape pulled out to measure half a meter. Grab the measuring tape and pull it out to measure one meter.
Materials expand or contract when subjected to changes in temperature. Most materials expand when they are heated, and contract when they are cooled. When free to deform, concrete will expand or contract due to fluctuations in temperature. The size of the concrete structure whether it is a bridge, a highway, or a building does not make it immune to the effects of temperature. The expansion and contraction with changes in temperature occur regardless of the structure’s cross-sectional area.
Concrete expands slightly as temperature rises and contracts as temperature falls. Temperature changes may be caused by environmental conditions or by cement hydration (the exothermic chemical process in which the cement reacts with the water in a mixture of concrete to create the calcium silicate hydrate binder and other compounds). An average value for the coefficient of thermal expansion of concrete is about 10 millionths per degree Celsius (10x10-6/C), although values ranging from 7 to 12 millionths per degree Celsius have been observed. This amounts to a length change of 1.7 centimeters for every 30.5 meters of concrete subjected to a rise or fall of 38 degrees Celsius.
Thermal expansion and contraction of concrete varies primarily with aggregate type (shale, limestone, siliceous gravel, granite), cementitious material content, water cement ratio, temperature range, concrete age, and ambient relative humidity. Of these factors, aggregate type has the greatest influence on the expansion and contraction of concrete.
Severe problems develop in massive structures where heat cannot be dissipated. Thermal contraction on the concrete’s surface without a corresponding change in its interior temperature will cause a thermal differential and potentially lead to cracking. Temperature changes that result in shortening will crack concrete members that are held in place or restrained by another part of the structure, internal reinforcement or by the ground. For example, a long restrained concrete section is allowed to drop in temperature. As the temperature drops, the concrete tends to shorten, but cannot as it is restrained along its base length. This causes the concrete to be stressed, and eventually crack.
Joints are the most effective way to control cracking. If a sizable section of concrete is not provided with properly spaced joints to accommodate temperature movement, the concrete will crack in a regular pattern related to the temperature and restraint directory. Control joints are grooved, formed, or sawed into sidewalks, driveways, pavements, floors, and walls so that cracking will occur in these joints rather than in a random manner. Contraction joints provide for movement in the plane of a slab or wall, and induce cracking caused by thermal shrinkage at pres elected locations. One of the most economical methods for making a contraction joint is by simply sawing a continuous cut in the top of the slab with a masonry saw.
Concrete Cracking
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A common adage is that there are two guarantees with concrete. One, it will get hard and two, it will crack. Cracking is a frequent cause of complaints in the concrete industry. The Concrete Foundations Association has produced a new flyer to help contractors educate their customers about the causes of cracks and when they should be a concern. A more detailed explanation of cracking is presented in this article.
Cracking can be the result of one or a combination of factors such as drying shrinkage, thermal contraction, restraint (external or internal) to shortening, subgrade settlement, and applied loads. Cracking can not be prevented but it can be significantly reduced or controlled when the causes are taken into account and preventative steps are taken.
Another problem associated with cracking is public perception. Cracks can be unsightly but many consumers feel that if a crack develops in their wall or floor that the product has failed. In the case of a wall, if a crack is not structural, is not too wide (the acceptable crack of a crack depends on who you ask and ranges from 1/16” to 1/4”) and is not leaking water, it should be considered acceptable. It is in the best interest of you, the wall contractor, to educate your customers that the wall will crack and when it should be a concern to them.
Cracks that occur before hardening usually are the result of settlement within the concrete mass, or shrinkage of the surface (plastic-shrinkage cracks) caused by loss of water while the concrete is still plastic.
Settlement cracks may develop over embedded items, such as reinforcing steel, or adjacent to forms or hardened concrete as the concrete settles or subsides. Settlement cracking results from insufficient consolidation (vibration), high slumps (overly wet concrete), or a lack of adequate cover over embedded items.
Plastic-shrinkage cracks are most common in slabs and are relatively short cracks that may occur before final finishing on days when wind, a low humidity, and a high temperature occur. Surface moisture evaporates faster than it can be replaced by rising bleed water, causing the surface to shrink more than the interior concrete. As the interior concrete restrains shrinkage of the surface concrete, stresses can develop that exceed the concrete's tensile strength, resulting in surface cracks. Plastic-shrinkage cracks are of varying lengths spaced from a few centimeters (inches) up to 3 m (10 ft) apart and often penetrate to mid-depth of a slab.
Cracks that occur after hardening usually are the result of drying shrinkage, thermal contraction, or subgrade settlement. While drying, hardened concrete will shrink about 1/16 in. in 10 ft of length. One method to accommodate this shrinkage and control the location of cracks is to place construction joints at regular intervals. For example, joints can be constructed to force cracks to occur in places where they are inconspicuous or predictable. Horizontal reinforcement steel can be installed to reduce the number of cracks or prevent those that do occur from opening too wide.
The major factor influencing the drying shrinkage properties of concrete is the total water content of the concrete. As the water content increases, the amount of shrinkage increases proportionally. Large increases in the sand content and significant reductions in the size of the coarse aggregate increase shrinkage because total water is increased and because smaller size coarse aggregates provide less internal restraint to shrinkage. Use of high-shrinkage aggregates and calcium chloride admixtures also increases shrinkage. Within the range of practical concrete mixes – 470 to 750 lb/yd3 (5- to 8-bag mixes) cement content – increases in cement content have little to no effect on shrinkage as long as the water content is not increased significantly.
Concrete has a coefficient of thermal expansion and contraction of about 5.5 x 10-6 per °F. Concrete placed during hot midday temperatures will contract as it cools during the night. A 40°F drop in temperature between day and night-not uncommon in some areas-would cause about 0.03 in. of contraction in a 10-ft length of concrete, sufficient to cause cracking if the concrete is restrained. Thermal expansion can also cause cracking.
Structural cracks in residential foundations usually result from settlement or horizontal loading. Most (but not all) structural cracks resulting from applied loads are nearly horizontal (parallel to the floor) and occur 16” to 48” from the top of the wall. They are much more prevalent concrete block construction. They can be brought about by hydrostatic pressure or heavy equipment next to the foundation.
Diagonal cracks that extend nearly the full height of the wall are often an indication of settlement. In either of the above conditions, an engineer should be consulted. Diagonal cracks emanating from the corner of windows and other openings are called reentrant cracks and are usually the result of stress build-up at the corner. Diagonal reinforcement at the corner of openings can reduce the instance of crack formation and will keep the cracks narrow.
Other procedures which can reduce cracking in concrete include the following practices.
Minimize the mix water content by maximizing the size and amount of coarse aggregate and by using low-shrinkage aggregate.
Use the lowest amount of mix water required for workability and placement; do not permit overly wet consistencies.
Use calcium chloride admixtures only when necessary.
Prevent rapid loss of surface moisture while the concrete is still plastic through use of spray-applied finishing aids or plastic sheets to avoid plastic-shrinkage cracks (more important in slabs)
Provide contraction joints at reasonable intervals, 30 times the wall thickness is a good “rule-of-thumb”.
Prevent extreme changes in temperature after placement and initial cure.
Properly place and consolidate the concrete.
Cracks can also be caused by freezing and thawing of saturated concrete, alkali- aggregate reactivity, sulfate attack, or corrosion of reinforcing steel. However, cracks from these sources may not appear for years. Proper mix design and selection of suitable concrete materials can significantly reduce or eliminate the formation of cracks and deterioration related to freezing and thawing, alkali-aggregate reactivity, sulfate attack, or steel corrosion.
For more information, refer to Design and Control of Concrete Mixtures, EB001, and Diagnosis and Control of Alkali-Aggregate Reactions in Concrete, IS413.
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