It Starts with the Foundation

Article written by Jack Carr, P.E., R.S., LEED AP, CRITERIUM ENGINEERS

Deep into the Maine winter our thoughts turn inward and indoors. Unit owners start noticing the drywall cracks and basement foundation walls with a cabin fever intensity. So I thought this would be a good time to go over some of the common foundation issues faced by property managers and building committees this time of year.

There are of course many types of foundations but lets focus on the more common types of perimeter foundation walls found in Maine condos such as concrete, block, and stone. But before we get into discussing walls, let us first eliminate the common complaint of cracked basement or garage slabs.

Most basements and garages have 4 to 6 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.

Therefore, slab cracking is often more of a cosmetic problem. Cracks are often repaired with a variety of grout, caulk, or epoxy products primarily to prevent groundwater penetration, insect entry, or radon gas infiltration. Cracks that show differential movement on opposing surfaces can not only be a tripping hazard but also be an indication of more serious sub-surface conditions needing further investigation.

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 sub-set 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, are 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 if 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 indicated foundation settlement problems stemming from poor soil conditions, hydrostatic groundwater pressures, or frost heaving. These problems should be directed to a knowledgeable consultant. 

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 causing these types of walls to move inward. This can even occur from a vehicles weight being too close to the foundation such as oil delivery truck. When horizontal cracking is observed in block walls, steps should be taken quickly to prevent further movement. These types of walls are also very susceptible to water penetration even when foundation drains are present often requiring serious water proofing repairs from the outside. 

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

The key to maintaining a sound stone/brick foundation is periodic vigilance to ensure loose or dislocated stones are not ignored. Any observed bulges or horizontal movement as well as cracks should be quickly addressed. But then, we all are getting a little older and need extra care from time to time.  

© 2011 CRITERIUM ENGINEERS