huangyhg

cracks in new cmu basement wall
anyone have any suggestions on what would cause numerous vertical (9) cracks in a 10' poured solid 12" cmu wall. i originally was thinking shrinkage, but they are spaced too close together in my opinion for it to be that. some of the cracks (2) do increase in size as they go up the wall indicating settlement, but most (7) are hairline. the wall is 65 feet long, floor framing is perpendicular to the wall, anchor bolts are at 4' and don't seem to coincide with the cracks. i know asce states that walls over 8' should be designed for 60psf equivalent fluid pressure, which many ignore. any suggestions?
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are there any control joints (vertical caulked spaces with a rubber filler) in the wall?
chipb,
is this reinforced masonry? are the cracks concentrated in a specific area? any moisture problems? what is the age of the structure? are the original geotech reports available, if any? please give us some more information so we can get an idea of what is going on.
no vertical control joints, however, the length of the wall is relatively short.
yes it's reinforced but can't verify to what extent.
cracks dont't seem to have a specific pattern. not concetrated in any one area.
it's for a residence, so no geotech report.
differential fill of approx 9'.
residence was completed late 2002
you don't indicate the width of the cracks, and i must assume that there is no horizontal reinforcing in the assembly. however, i would expect them to be hairline width to maybe 1/16", or no more than a total (aggregate) width of about 1/2" or so, dependent on the circumstances. and yes, the cracks could very easily be due to shrinkage (drying and thermal shrinkage).
following is an excerpt from a report i prepared (after a considerable literature search) for a client a couple of years ago. you may find it interesting.
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the most common cracks observed in basement walls are vertical cracks that range from hairline to almost 1/4". these cracks are generally found at locations within the middle half of the length of a wall, are widest at the top of the wall, and narrow slightly, or in many cases disappear before reaching the footing. vertical cracks are generally more prevalent on longer walls (25 feet or more) with no disjunctions, such as horizontal offsets or expansion/contraction joints. another common location from which these cracks emanate are the corners of openings through the masonry (for windows, etc.), as these are re-entrant corners, or "stress-raisers", and are also a location of a change in mass in the cross-section of the wall. if these cracks are not accompanied by a vertical, shear-type differential movement between the two sides of the crack, and if there are no out-of-plane offsets, then these cracks are most likely due to the relieving of shrinkage and thermal stresses. this type of crack typically narrows near the footing due to the fact that the footing restrains the shrinkage, thereby decreasing the severity of the crack.
shrinkage stresses are created in basement walls by post-construction drying shrinkage in the concrete block, as well as in the mortar joints. the asce publication, guidelines for failure investigation, states that, "for concrete masonry units, shrinkage after construction is a major factor." one shrinkage effect is non-reversible, this being the loss of moisture occurring between the time the block is manufactured, and the time the block is laid in the foundation wall. the more common cause of shrinkage in a basement wall results from the environmental conditions around the wall after erection. typically, a thirty foot long basement wall may have to accommodate nearly ¼" drying shrinkage resulting from average conditions after erection. a basement wall (before being damp-proofed and before enclosure by the superstructure may be subjected to rainstorms, allowing the block to be wetted from the exterior face, the interior face, and maybe even the inside of the cells. although air-dried block may retain as much as 20% moisture, a saturated block may retain as much as 40% moisture (by weight). this can cause even more extensive shrinkage cracking (than that due to average conditions) after the basement has been enclosed in its permanent environment, and has undergone a proportionately larger decrease in moisture content.
subsequent to the wall reaching the so-called stable permanent environment, the wall will still undergo cyclical volume changes due to the moisture in the weather, soil, basement, etc. these cyclical volume changes will cause opening and closing of existing cracks, to some degree.
thermal stresses (also a cyclical phenomenon) build-up in basement walls, and vertical cracking is exacerbated by this volume change. the extent to which thermal stresses affect the basement wall is usually less than other causes, primarily because the temperature variations are mitigated by the relatively constant temperature of the earth into which the wall is embedded. basement walls that are fully protected by the soil are minimally affected by thermal stresses. conversely, basement walls that are fully exposed to the south (such as on a sloping site) are impacted more by thermal stresses.
if vertical cracks are combined with a vertical differential across the interface, it is usually a sign that differential settlement is the cause. this type of vertical crack can usually be identified by the fact that it continues through the footing, even though the crack may narrow in width.
if the vertical crack is associated with an out-of-plane offset on one side of the crack, it is usually a result of a localized lateral force, such as a construction mishap (impact), or a tree root pushing against the wall.
the vertical cracks described above would normally be visible on the exterior face of the wall also. however, if the crack is not visible, and the wall is bowed horizontally inward at the top, the anchorage at the top of the wall to the superstructure may be deficient or non-existent.
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after reading this, imagine what would happen with a 65' length of wall, constructed with moist block during the warm season. now measure the vertical cracks after the masonry has dried, and the temperature has fallen during a cold winter. the nine cracks that you mention could each measure nearly 1/16" quite easily.
check the national concrete masonry association's web site for their tek manuals for movement control (10).

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