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It Starts with the Foundation

It Starts with the Foundation

Article written by Jack Carr, P.E., R.S., LEED AP, Criterium Engineers
Published in Condo Media, March 2011

Deep into winter, thoughts turn indoors, and unit owners start noticing the drywall cracks and basement foundation walls. So now is a good time to go over some of the common foundation issues faced by association managers and building committees this time of year. The more common types of perimeter foundation walls found in Maine condos include concrete, block and stone. But, let’s first eliminate the common complaint of cracked basement or garage slabs.

When Slab Cracks are Cause for Concern

Most basements and garages have four- to six-inch concrete slabs. And, unless this is a slab-on—grade foundation, the slabs were poured independently of the foundation walls. Often the construction joint between the slab and wall can easily be seen. The common complaint is hairline cracks often in spider web-like patterns. These cracks can show up shortly after construction, and are normally caused by shrinkage during the curing process. The key point here is that this type of slab cracking is rarely a structural problem, for after all, the slab could be completely removed leaving a dirt floor and the foundation walls and columns with footings would easily maintain a stable building.
 
Slab cracking is often more of a cosmetic problem. Cracks are repaired with a variety of grout, caulk or epoxy products primarily to prevent ground-water penetration, insect entry or radon gas infiltration. However, cracks that show differential movement on opposing surfaces aren’t only a tripping hazard, but also can be a sign of more serious subsurface conditions needing further investigation.

Walls and Ceiling Repair Issues

Regarding foundation walls, the most typical problem with concrete walls are vertical hairline cracks, often starting at the top of the wall and traveling down to the floor slab. A subset of these types of cracks are those that propagate, often in a diagonal direction from stress concentration points, such as the bottom corners of basement window openings. The key point to remember is that, for the most part, these types of cracks, even when they penetrate the entire thickness of the wall, do not constitute a structural problem as the loads from above pass unobstructed to the footings below. This general rule is not to be followed when the walls on both sides of the crack are moving out of plane or the structure above shows stress in the form of movement or cracking walls and ceilings.
 
Typically, these cracks are repaired when periodic Water infiltration occurs. Repairing cracks from the outside is often the best method, but due to the excavation costs involved, repairing the crack from the interior by injecting a crack filling material through drilled holes on both sides of the crack has become a routine solution.

When horizontal wall cracks, multiple closely spaced vertical cracks, or large diagonal cracks in basement corners are observed, these conditions may indicate more serious problems related to settlement or other structural problems. Similarly, a single vertical crack that is much wider at the top of the wall may indicate foundation settlement problems stemming from poor soil conditions, hydrostatic groundwater pressures or frost heaving, A knowledgeable consultant should handle this.
 
Regarding concrete block foundation walls, most of the guidance above can be used with some exceptions. By their nature, concrete block walls are often not well reinforced and are subject to inward movement from various soil pressures. This can even occur from a vehicles weight being too close to the foundation such as an oil delivery truck. When horizontal cracking is observed in block walls, steps should be taken quickly to prevent further movement. These types of walls also are very susceptible to water penetration even when foundation drains are present often requiring serious water proofing repairs from the outside.

Many urban condominiums converted from old multifamily apartment buildings have mortared or un-mortared stone foundations some with brick foundation walls above. These have stood the test of time and are more than 100 years old and, if well maintained, can last another century. They are more likely to allow the entrance of ground water due to their porous nature; the necessary steps should be taken to protect the structural elements and indoor air quality of the building if high moisture is a problem.

To maintain a sound stone/brick foundation, ensure loose or dislocated stones are not ignored. Any observed bulges, horizontal movement or cracks should be quickly addressed. 
 

 
© 2011 CRITERIUM ENGINEERS