Building Problem Solutions
Building Problem Solutions
Cracked Foundation Walls - Causes & Repair
by John F Mann, PE
Cracks often occur in foundation walls. Proper structural evaluation is required to determine if repair work is warranted.
See report (April 2012) in "Foundation Wall Design" about defective design and construction of foundation walls in New Jersey.
There is much incorrect and misleading information on various web site about this subject. Essentially all such erroneous information is being offered by persons who are not licensed professional engineers.
Costs for bracing cracked foundation wall are discussed near the end of this article.
Cracks Due to Settlement
Downward movement (settlement) of foundation walls is one basic problem that can cause cracks. Masonry walls (brick, concrete block, stone) are easily cracked by minor settlement. Typically, settlement cracks are mostly vertical or "step" pattern (in mortar joints).
However, the step pattern can also be an indication of excessive lateral soil pressure (pushing inward).
If foundation settlement is the cause of cracks, there will almost always have to be other evidence such as sloped floors and crqcks in finish materials (drywall) inside the house at or near the location of foundation settlement.
Cracks Due To Lateral Soil Pressure; Block Walls
In addition to supporting the weight of the house, foundation walls around a basement must also resist lateral (horizontal) soil pressure, pushing inward against the wall. Essentially, these foundation walls must act as retaining walls.
Water in the soil is often a contributing factor, increasing lateral soil pressure. Clay soils (which tend to retain water) cause greater lateral pressure than sandy soils.
Other sources of lateral pressure are tree roots (growing along and against the wall) and the "surcharge" from weight of anything placed on top of ground outside the wall (such as vehicles on a driveway or a new raised patio).
For concrete block walls around a full basement, horizontal cracks in mortar joints, on inside face of wall, have been caused by excessive lateral pressure against the wall, from soil and other sources (water, tree roots, surcharge weight).
To resist lateral pressure, the wall acts like a slab, with lateral supports provided along bottom edge (by basement floor slab) and top edge (by first floor of house, if available and effective). The wall-slab resists lateral force by bending in both directions (vertical & horizontal).However, away from corners and other intersecting walls, bending occurs almost entirely in the vertical direction.
Except near intersecting walls, bending due to inward-acting pressure results in tension stress on inside face of wall which is the direct cause of cracks. Since mortar is very weak in tension, these cracks tend to occur mostly in mortar joints.
Lateral soil pressure often causes vertical curvature (bulging) that can be measured with a 4-foot level held plumb (vertical) against the wall. Typical amount of inward movement (at or near mid-height of wall, relative to base of wall) is 3/8-inch to 3/4-inch. Inward movement exceeding 1 inch indicates that remedial work is necessary as soon as feasible.
Although this type of damage can remain unchanged for many years, the wall has been structurally weakened. Basement walls have completely collapsed inward, especially when movement occurs behind finished walls (and is therefore not seen).
Photo 1 - July 2009
Wide horizontal crack in concrete block foundation wall, with large inward movement. Condition was discovered only when remodeling contractor noticed uneven drywall on finished wall. Sloped wood braces installed by contractor as precaution, before complete reconstruction of this severely damaged wall. Structural Support provided design plans and report for owner to submit to insurance company.
Note that the block walls are built with concrete block, not "cinder" block which has not been manufactured in the US since before anyone alive today was born. The incorrect "cinder" terminology has persisted however, apparently since the concrete used to make concrete block has a gray coloration (not off-white seen in sidewalks) and a porous-looking texture.
Photo 2 - March 2010
Little Egg Harbor, NJ
"Step" cracking in block wall.
Sealant was installed to fill some cracks, which were caused by excessive lateral earth pressure due to height of backfill and saturated soil conditions. Pressure from tree roots is also a likely contribution.
Note that floor joists are parallel to the wall, such that top of wall does not have effective lateral bracing from first floor. Without lateral support along top of wall, the wall resists lateral pressure in the horizontal direction and the vertical direction, as a structural plate with supports on three sides. When length of wall is relatively short, these conditions often cause flexural bending cracks in horizontal and vertical mortar joints ("step" cracking).
Reinforced block piers are to be installed to brace this wall and another wall with horizontal cracks. Evaluation and design of repairs by Structural Support.
Photo 3 - April 2010
Wide crack in horizontal mortar joint (second from top) caused primarily by tree roots pushing against wall (see Photo 4). Relatively high soil backfill also contributes to overload condition.
In addition to a large tree near foundation wall, tipoff to tree root damage is limited and somewhat unusual conditions. In this case, length of cracking was limited.
Location of crack near top of wall would also be considered somewhat unusual if the wall were one thickness. However,this wall is also an example of a more general problem caused by use of two different block thicknesses (8-inch at top, on 12-inch below). Although bending stress may be less near top of wall, reduction in thickness causes much greater decrease in strength. Many such walls end up cracked just above the change in thickness, even without a tree root problem. Use of two different block thicknesses is generally not a good idea.
Photo 4 - Large tree near foundation wall caused wide horizontal cracks in mortar joints (see Photo 3).
Cracks In Concrete Foundation Walls
In general, concrete foundation walls have much greater strength than concrete block foundation walls.
Concrete foundation walls develop numerous, very narrow, mostly vertical cracks that are caused by normal shrinkage that occurs during the curing process (water reacting with cement). Many of these cracks are only visible by close inspection using a light. Very narrow ("hairline") and narrow shrinkage cracks are generally not a structural problem. Wide shrinkage cracks (such that thickness of a quarter fits into crack) may indicate structural distress. Such wide cracks often begin as shrinkage cracks and then become wider due to settlement or inward lateral soil pressure.
Basic Methods of Repair - Settlement
If cracks are due to settlement, the key issue is to determine if settlement is an ongoing problem. The owner may already know this to be the case based on observations over time, such as increasing slope of floors and increasing cracks in finish materials (drywall, flooring). Further engineering analysis based on soil borings may be warranted to determine character of soils underlying the house.
If settlement is determined to be a continuing problem, the basic method of repair is to provide additional support under the foundation wall or walls. Such work is termed "underpinning".
Prior to about 1980, the primary method of underpinning was to install concrete underneath an existing footing. However, this method can only be used effectively if the cause is compressible soil directly under the foundation (not deep under foundation). Cost of underpinning with concrete is also very labor intensive and therefore expensive.
Since at least 1980, the preferred method of underpinning is now installation of helical steel piles, also termed "auger piles". These piles, manufactured by a few companies, are installed using hydraulic equipment around the perimeter of a building, usually with L-shaped brackets that fit under the footing.
In New Jersey and eastern Pennsylvania, Danbro Distributors is a contractor with extensive experience installing helical steel piles;
Soil borings are sometimes warranted (before developing repair method) to obtain detailed information about soils deep under the foundation.
In 2009, soil borings were recommended for a residential project in Woodbridge NJ where a large void (sinkhole) occurred under front corner of a house. Underpinning was eventually performed using helical steel piles, per design plans prepared by Structural Support.
Basic Methods of Repair - Inward Movement & Cracking
To repair foundation walls that have been pushed inward due to excessive lateral pressure, the two basic repair concepts are to; (1) Brace the wall from inside, and (2) Reinforce (strengthen) the wall.
However, if the wall (especially block wall) is pushed inward more than 1-1/2 inches (relative to base of wall), consideration must be given to complete replacement of the severely damaged wall. In addition to cost, factors to consider include amount of vertical load the wall is supporting (number of floors), potential for water infiltration after repairs, and finished appearance (especially for resale).
Methods of bracing include; (1) Vertical steel posts, (2) Reinforced block or concrete piers on concrete base, and (more recently) (3) Steel-rod tiebacks installed through the wall, into soil outside.
For block walls, additional methods of reinforcing the wall include; (1) Vertical steel reinforcing bars grouted into hollow cores of block, and, most recently, (2) Vertical polymer (carbon-fiber) strips glued to inside face of wall or glued into slots made on inside face of wall.
For each repair method, design should be performed by a qualified professional engineer.
Reinforced Block Piers
For bracing a block wall, reinforced block piers provide secure, durable bracing that has a good appearance. Piers very often appear to have been built with the original foundation.
Piers should be spaced not more than 8 feet center-to-center. Lesser spacing may be warranted, especially for 8-inch block and relatively high lateral soil pressure.
For the pier to be effective, vertical reinforcing bars must extend at least 9 inches into the concrete base. Hooked bars should be considered. Bars must be installed in hollow cores nearest to the block wall. All hollow cores (including cores without bars) must be filled completely with grout or pea-gravel concrete, not mortar.
The concrete base (at least 12 inches thick) can be placed on top of the concrete floor slab. Short steel dowel bars should be grouted into the floor slab to resist sliding. However, the base, typically about 2'-6" square, will take up space. Installing the base flush with floor slab requires cutting the slab and excavating soil. Excavation must not undermine the existing footing or foundation wall (if no footing).
In the northeastern US, current (2010) costs for each reinforced block pier are typically in the range of $1,000 (concrete base on existing floor slab) to $1,600 (concrete base flush with existing slab). Of course accurate costs can only be provided by qualified contractor bidding on well-defined scope of work. Cost can be greater for conditions such as difficult access and obstructions (pipes, HVAC).
Steel Brace Posts
Vertical steel brace posts are recommended for a finished basement (current or proposed) so that brace posts can be installed behind or within the finished wall. Otherwise the braces posts are usually considered to be somewhat unsightly.
Various shapes can be used, including wide-flange (I-beam), tube and angle (L-shape). Stiffness is more important than strength. A relatively flexible post will not be as effective as a stiff post. The block wall will crack again even for small additional inward movement.
Brace posts should be spaced not more than 6 feet along the wall.
Key issue for brace posts is the connection requirements at each end. Base of post can be embedded in the concrete floor slab. If the slab is at least 4 inches thick, the base of post (with welded base plate) can be bolted to the slab.
Top of post must have a very secure connection to first floor framing (joists). Nailed connections are certainly not adequate. Generally, some arrangement of steel plates or L-shaped angles are required, with substantial bolts. When floor joists are parallel to the foundation wall, lines of wood blocking must be installed.
In northeastern US, current (2010) cost for each steel brace post is typically in the range of $800 to $1,200. However, since lesser spacing is required (compared to block piers), total cost is likely to be only slightly less.
Reinforcing Bars Grouted Into Block Wall
Vertical reinforcing bars can be grouted into hollow cores of a block wall. This method has the advantage (and disadvantage!) that the repair is not seen after work is completed.
Installation of the reinforcing bars is not easy and must only be performed by a qualified contractor with experience. Inspection of work is difficult at best.
Inside face shells of block must be broken out to allow for installation of bars and grout (not mortar!). This work must be performed carefully to avoid further damage to the block wall.
Unless bars can be installed in one length, two bars must be installed. Two bars must be lapped at least 24 inches (near mid-height of wall) to be effective.
Cost for internal reinforcing bars is typically less than for block piers or steel brace posts. However, the key requirement for this method is an experienced contractor, so that a proposal with very low cost should be a warning sign.