Movement Restraint And Cracking In Concrete Structures Pdf
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There are a variety of potential causes of cracking. Understanding the cause of potential cracking allows the designer to incorporate appropriate design procedures to control it. The most common causes of cracking in concrete masonry are shown in Figure 1 and are discussed below.
Various building materials may react differently to changes in temperature, moisture, or structural loading. Any time materials with different properties are combined in a wall system, a potential exists for cracking due to differential movement. With concrete masonry construction, two materials in particular should be considered: clay brick and structural steel.
Differential movement between clay brick and concrete masonry must be considered when the two are attached since concrete masonry has an overall tendency to shrink while clay brick masonry tends to expand. These differential movements may cause cracking, especially in composite construction and in walls that incorporate brick and block in the same wythe.
Composite walls are multi-wythe walls designed to act structurally, as a single unit in resisting applied loads. The wythes are typically bonded together using wall ties at prescribed intervals to assure adequate load transfer. When the composite wall includes a clay brick wythe bonded to a concrete masonry wythe, ladder-type joint reinforcement, or box ties are used to provide some degree of lateral movement between wythes. In addition, expansion joints are installed in the clay brick wythe to coincide with control joints in the concrete masonry wythe.
When clay brick is used as an accent band in a concrete masonry wall, or vice-versa, the differential movement of the two materials may result in cracking unless provisions are made to accommodate the movement. To reduce cracking, slip planes between the band and the surrounding wall, horizontal reinforcement or more frequent control joints or a combination thereof can be used to control cracking. See Crack Control for Concrete Brick and Other Concrete Masonry Veneers (ref. 6) for more information on these approaches.
In addition to the proper design strategies discussed above for structural capacity and differential movement, the following recommendations can be applied to limit cracking in concrete masonry walls.
Traditionally, crack control in concrete masonry has relied on specifying concrete masonry units with a low moisture content, using horizontal reinforcement, and using control joints to accommodate movement. Prior to the 2000 edition of ASTM C90 (ref. 8), low moisture content was specified by requiring a Type I moisture controlled unit. The intent was to provide designers an assurance of units with lower moisture content to minimize potential shrinkage cracking. However, there are several limitations to relying on moisture content alone since there are other factors that influence shrinkage which are not accounted for by specifying a Type I unit. Additionally, Type I units were not always inventoried by concrete masonry manufacturers. Most importantly, Type I units needed to be kept protected until placed in the wall, which was proven to be difficult on some projects. Because of the above problems associated with the Type I specification, ASTM removed the designations of Type I, Moisture-Controlled Units and Type II, Nonmoisture Controlled Units from the standard.
Control joints are essentially vertical separations built into the wall to reduce restraint and permit longitudinal movement. Because shrinkage cracks in concrete masonry are an aesthetic rather than a structural concern, control joints are typically only required in walls where shrinkage cracking may detract from the appearance or where water penetration may occur. TEK 10-2C (ref. 4) provides much more detailed information on control joint details, types and locations.
Studies have shown that reinforcement, either in the form of joint reinforcement or reinforced bond beams, effectively limits crack width in concrete masonry walls. As indicated previously, as the level of reinforcement increases and as the spacing of the reinforcement decreases, cracking becomes more uniformly distributed and crack width decreases. For this reason, a minimal amount of horizontal reinforcement is needed when utilizing the NCMA recommended maximum control joint spacings (refs. 3 & 4).
Cracking is a common observation in concrete structures. Cracks can be a result of plastic shrinkage, or constructional movements. Moreover, cracks can happen as a result of overloading concrete elements, or creep. Chemical reactions such as alkali-aggregate reactions and corrosion can induce cracking. Material and Structural engineers are interested in the main reasons behind concrete cracks, and determine the extent and severity of existing cracks. Two parameters are often used by engineers to characterize cracks: crack width, and crack depth. In this article, we will review 3 methods for evaluating crack depth in concrete.
In a concrete element, the crack (shrinkage, thermal, and service loads) width and distribution is mainly controlled by steel reinforcement. In fiber-reinforced concrete, fibers help control cracking. Cracks that are caused by internal or external chemical reactions, or a result of accidental loads (i.e. blast, or impact load from accidents) are different in their nature and require further investigation to assess their impact on structural integrity and durability performance of the element.
Hairline cracking within concrete block walls, often referred to as stair-step cracking or mortar joint cracking, is an example of an imperfection or distress but does not typically compromise structural integrity. Hairline cracking within concrete block walls is the result of internal stresses resulting from shrinkage, creep, and thermal expansion and contraction; all of which are anticipated, can be predicted, and need to be accounted for in design and construction.
In order for concrete masonry to structurally perform as intended, to transfer vertical loads and to resist lateral loads, the walls must be restrained. This restraint is accomplished by structurally connecting the wall to the foundation as well as other components such as pilasters and bond beams. In addition to connecting the walls with the foundation and bond beams, walls are typically constructed integrally at corners and at changes in geometry. All of these locations, although necessary for the proper structural performance of the wall, result in restraints within the wall which induce stresses as the wall experiences shrinkage. As with plain concrete, concrete masonry is strong in compression but weak in tension. Therefore, restrained tensile forces often lead to cracking as the wall acts to relieve the stress.
When concrete masonry shrinks the cracking that results will form different patterns depending on where the wall acts to relieve the stress. Typically, shrinkage cracks manifest themselves at changes in material, changes in geometry (such as openings for windows or doors), and adjacent to corners. Their patterns can be either in a stair-step, horizontal, or vertical configuration. Cracking can also occur along the interface of different components within the wall such as the foundation-to-wall interface or the wall-to-bond-beam interface. Cracks at these locations are typically horizontal in nature (refer to Figure 1).
REINFORCED CELLS: Vertical cracks typically occur within the field of a wall or alongside reinforced openings. This occurs when internal stresses associated with shrinkage causes cracking between the internally reinforced grout filled cells and the adjacent unreinforced sections. Varying material properties relate directly to varying material strengths. In the case of concrete masonry assemblies, a typical concrete block has a compressive strength of roughly 2000 psi as do most common mortars. Grout, however, can range in compressive strength from 3000 psi to more than 5000 psi. These varying strengths result in varying behavior and performance. It is this fluctuation and resulting change in volume that creates internal stresses. As the volume of the wall changes and shrinkage stresses build, cracking occurs between the much stronger reinforced grout-filled cell and the adjacent unreinforced sections. 1e1e36bf2d