Foundation Cracking
Cracking associated with drying shrinkage in concrete block foundation walls. The shrinkage of concrete block walls during the drying process will often result in patterns of cracking similar to that caused by differential settlement: tapering cracks that widen as they move, resulting in a diagonal upward pattern. These cracks usually form during the building's first year, and in existing buildings will appear as "old" cracks and exhibit no further movement. Although such cracks are often mistaken for settlement cracks, shrinkage cracks usually occur in the middle third of the wall and the footer beneath them remains intact. Shrinkage cracking is rarely serious, and in an older building may have been previously repaired. If the wall is unsound, its structural integrity can sometimes be restored by pressure-injecting concrete epoxy grout into the cracks or by adding pilasters.Backfilling can affect foundation walls.
How horizontal cracks relate to foundation wall movement.
Sweeping or horizontal cracking of the foundation walls. The sweeping or horizontal cracking of brick or concrete block foundation walls may be caused by improper backfilling, vibration (from the movement of heavy equipment or vehicles close to the wall), or by the swelling or freezing and heaving of water
saturated soils adjacent to the wall. Like the shrinkage associated with drying, sweeping or horizontal cracking may have occurred during the original construction and been compensated for at that time. Such distress, however, is potentially serious, as it indicates that the vertical supporting member (the foundation wall) that is carrying a portion of the structure above is "bent" or" broken." It may be possible to push the wall back into place by careful jacking, and then reinforce it with the addition of interior buttresses or by pressure-injecting concrete epoxy grout into the wall. If outside ground conditions allow, the wall can be relieved of some lateral pressure by lowering the ground level around the building. When expansive soils are suspected as the cause of the cracking, examine the exterior for sources of water such as broken leaders or poor surface drainage. Frost heaving may be the culprit if the damage is above local frost depth or if it occurred during an especially cold period.
Brick wall cracking associated with thermal and moisture movement. Above ground brick walls expand in warm weather (particularly if facing south or west) and contract in cool weather. This creates stress in the walls that may cause a variety of cracking patterns, depending on the configuration of the wall and the number and location of openings. Such cracks are normally cyclical and will open and close with the season-they will grow wider in cold weather and narrower in hot weather. Look for cracking at the corners of long walls, walls with abrupt changes in cross section (such as at a row of windows), walls with abrupt turns or jogs, and in transitions from one to two-story walls. These are the weak points that have the least capacity for stress.
Common moisture and thermal movement cracking includes:
·Horizontal or diagonal cracks near the ground at piers in long walls due to horizontal shearing stresses between the upper wall and the wall where it enters the ground. The upper wall can thermally expand but its movement at ground level is moderated by earth temperatures. Such cracks extend across the
piers from one opening to the next along the line of least resistance. This condition is normally found only in walls of substantial length. Vertical cracks near the end walls due to thermal movement. A contracting wall does not have the tensile strength to pull its end walls with it as it moves inward, causing it or the end walls to crack vertically where they meet.
Vertical cracks in short offsets and setbacks caused by the thermal expansion of the longer walls that are adjacent to them. The shorter walls are"bent" by this thermal movement and crack vertically.
· Vertical cracks near the top and ends of the facade due to the thermal movement of the wall. This may indicate poorly bonded masonry. Cracks will tend to follow openings upward.
· Cracks around stone sills or lintels caused by the expansion of the masonry against both ends of a tight-fitting stone piece that cannot be compressed.Cracks associated with thermal and moisture movement often present only cosmetic problems. After their cause has been determined, they should be repaired with a flexible sealant, since filling such cyclic cracks with mortar will simply cause the masonry to crack in another location. Cracks should be examined by a structural engineer and may require the installation of expansion joints
Brick wall cracking associated with freeze-thaw cycles and corrosion. Brick walls often exhibit distress due to the expansion of freezing water or the rusting of embedded metals. Such distress includes:·Cracking around sills, cornices, eaves, chimneys, parapets, and other elements subject to water penetration, usually due to the migration of water into the masonry. The water expands upon freezing, breaking the bond between the mortar and the masonry and eventually displacing the masonry itself.The path of the water through the wall is indicated by the pattern of deterioration.
·Cracking around iron or steel lintels caused by the expansive force of corrosion that builds up on the surface of the metal. This corrosion exerts great
Pressure on the surrounding masonry and dis¬places it, since corroded iron can expand to many times its original thickness. Structural iron and steel concealed within the masonry, if exposed to moisture, can also corrode and cause cracking and displace¬ment of its masonry cover. Rust stains usually indicate that corrosion is the cause of the problem. Check to make sure the joint between the masonry and the steel lintel that supports the masonry over an opening is clear and open. If the joint has been sealed, the sealant or mortar should be removed. These conditions can usually be corrected by repairing or replacing corroded metal com¬ponents, and by repairing and pointing the masonry. Where cracking is severe, portions of the wall may have to be recon¬structed. Cracks should be examined by a structural engineer.
Structural Failure-Wall cracking or displacement associated with the structural failure of building elements.
Problems related to the structure, aside from those caused by differential settlement or earth-quakes, are usually found over openings and (less commonly) under roof eaves or in areas of structural overloading. Such problems include:
·Cracking or displacement of masonry over openings, resulting from the deflection or failure of the lintels or arches that span the openings. In older masonry walls with wood lintels, cracking will occur as the wood sags or decays. Iron and steel lintels also cause cracking as they deflect over time. Concrete and stone lintels occasionally bow and sometimes crack.Masonry arches of brick or stone may crack or fail when there is wall movement or when their mortar joints deteriorate. When such lintel deflections or arch failures occur, the masonry above may be supporting itself and will exhibit step cracks beginning at the edges of the opening and joining in an inverted "V" above the opening's midpoint. Correcting such problems usually means replacing failed components and rebuilding the area above the opening.
·Occasionally masonry arches fail because the walls that surround them cannot provide an adequate counterthrust to the arch action. This sometimes happens on windows that are too close to the corners of a wall or bay. In such cases, the masonry arch pushes the unbraced wall outward, caus¬ing it to crack above the open¬ing near or just above the spring of the arch. When this occurs, the end walls must be strengthened.
·Cracking or outward displacement under the eaves of a pitched roof due to a failure in the horizontal roof ties which results in the roof spreading outward. The lateral thrust of the roof on the masonry wall may cause it to crack horizontally just below the eaves or to move outward with the roof. In this case the roof will probably be leaking as well. When this occurs, examine the roof structure carefully to ascertain whether there is a tying failure. If so, additional horizontal ties or tension members will have to be added and, if possible, the roof pulled back into place. The damaged masonry can then be repaired. The weight also can be transferred to interior walls. Jacking of the ridge and rafters is another possible solution.
·Cracking due to overloading (or interior movement) is fairly uncommon, but may be caused by a point load (often added during an alteration) bearing on a wall of insufficient thickness. If the member has been concealed, such a problem will be difficult to investigate. The addition of interior wall supports
or bracing, however, may correct the source of the problem by relieving the load.
·Cracking due to ground tremors from nearby construction, heavy vehicular traffic, or earthquakes. This cracking is roughly vertical in direction and occurs more toward the center of the building. Buildings exhibiting such cracking should be treated on a case-by-case basis, since serious structural damage may have taken place. Recommend a structural engineer experienced in such matters.
Bulging of walls. Masonry walls sometimes show signs of bulging as they age. A wall itself may bulge, or the bulge may only be in the outer withe. Bulging of ten takes place so slowly that the masonry doesn't crack, and therefore it may go unnoticed over a long period of time. The bulging of the whole wall is
usually due to thermal or moisture expansion of the wall's outer surface, or to contraction of the inner width. This expansion is not completely reversible because once the wall and its associated structural components are "pushed" out of place they can rarely be completely "pulled" back to their original
positions.The effects of the cyclical expansion of the wall are cumulative, and after many years the wall will show a detectable bulge. Inside the building, separation cracks will occur on the inside face of the wall at floors, walls, and ceilings.
Bulging of only the outer masonry withe is usually due to the same gradual process of thermal or moisture expansion; masonry debris accumulates behind the bulge and prevents the course from returning to its original position.
In very old buildings, small wall bulges may result from the decay and collapse of an internal wood lintel or wood-bonding course. This can cause the inner course to settle and the outer course to bulge outward.When wall bulges occur in solid masonry walls, the walls may be insufficiently tied to the structure or their mortar may have lost its bond strength.Large bulges must be tied back to the structure; the star-shaped anchors on the exterior of masonry walls of many older buildings are examples of such ties
(check with local building ordinances on their use). Small bulges in the outer masonry course often can be pinned to the inner course or dismantled and rebuilt.
The V3 rule for wall stability.-Leaning of walls. Masonry walls that lean (invariably outward) represent a serious if uncommon condition that is usually caused by poor design and construction practices, particularly by inadequate structural tying or poor foundation work. When tilting or leaning occurs, it is often associated with parapets and other upper wall areas, especially those with heavy masonry cornices cantilevered from the wall. Leaning can produce separation cracking on the end walls and cracking on the interior wall face along floors, walls, and ceilings. Leaning walls can sometimes be tied back to the structure and thereby restrained. In such cases, the bearing and connections of interior beams, joists, floors, and roof should be examined.
A bowed brick veneer wall. When large areas or whole walls lean, rebuilding the wall and possibly the foundation may be the only answer.
Test: A wall is usually considered unsafe if it leans to such an extent that a plumb line passing through its center of gravity does not fall inside the middle one-third of its base (called the V3 rule). In such an event, recommend a structural engineer.
Brick Veneer Walls
Problems associated with brick veneer walls. Brick veneer walls are subject to the forces of differential settlement, moisture and thermal-related cracking, and the effects of freezing and corrosion. Common problems peculiar to brick veneer walls are:
·Cracks caused by wood frame shrinkage, which are most likely to be found around fixed openings where the independent movement of the veneer wall is restrained. These cracks are also formed early in the life of the building and can be repaired by pointing. Bulging, which is caused by inadequate or deteriorated ties between the brick and the wall to which it's held.
·Vertical cracking at corners or horizontal cracking near the ground, which is caused by thermal movement of the wall. This cracking is similar to that in solid masonry or masonry cavity walls, but possibly more pronounced in well-insulated buildings because of the reduction in the moderating effect
from interior tempera¬tures. Thermal cracks are cyclical and should be filled with a flexible sealant. In cases of severe cracking, expansion joints may have to be installed.
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