I’m in the process of changing the InterNACHI Narrative Library so that it’s not just narratives. I’m adding a lot of reference material, the idea being that if an inspector sees something he’s not sure about, he’ll have reference material in his software that he can refer to. Ignore the formatting, it got squirrelly when I pasted into this thread and wouldn’t take correction.
Here’s what I’m thinking of including for the foundation section. Comments?
When forces act on a foundation that create stress, cracking is one of the methods that nature uses to relieve that stress. The concern is always whether the stress/ damage will continue to worsen, or the condition is stable.
Here are some of the forces that create stress in foundations:
Differential Soil Movement
Differential soil movement happens when:
- One part of a building remains stable while an adjacent part of the building moves. Heaving, settling and creep can all cause differential movement.
Heaving can be caused by:
- Saturated soil expanding as it turns to ice. You might see this where an exterior hose bib leaks where plumbing supply pipes have leaked, or where roof drainage discharges to the foundation. Damage is most likely to appear as cracking in foundations with shallow footings
- Expansive soils. Soils that expand in volume with increases in moisture content are typically clays. Clay deposits can be cover large areas and small deposits may exist randomly and affect only scattered homes in a small area. Expansive soil has to ability to crack foundation walls and slabs.
a. It may exist for years beneath a structure without causing damage until something happens to introduce water to the soil.
i. Additional construction that reroutes surface drainage or makes changes to the plumbing system, or
ii. Plumbing pipes that eventually leak.
Settling can be caused by:
- Inadequately compacted fill. Soil disturbed by the excavation process must be compacted to a density equal to that of the surrounding, undisturbed soil. If soil is inadequately compacted, the weight of the structure will force air out of the spaces between soil particles and settling will occur.
• Across an older home, the inspector feels a hump in the middle of the floor. The perimeter foundation has settled, but the a girder supporting floor structure joists and bearing walls as not.
• Concrete flatwork has settled, creating trip hazards.
• A concrete porch has settled away from the home foundation.
a. Compression: occurs when the weight of a structure forces air out of the spaces between soil particles leaving only pore spaces filled with water.
b. Consolidation occurs when the weight of the structure will force air and water out of the pore spaces between soil particles.
Compaction will continue until Soil beneath the foundation reaches a density that will support the load. Soils testing would be required to determine the extent of the problem and likelihood that settling will continue.
Fill impurities can consist of wood that will eventually decay, leaving a void in its place, or any other material that deterioration will reduce in size over time. Once soil has settled into voids and reached density commensurate with eh surrounding soil, the condition should become stable.
Collapsible soils: Collapsible soils are those in which a significant portion of soil particles are loosely stacked and held together by thin binding agents like clay or other materials, all of which are water-sensitive. The particle structure is stable as long as it remains dry. Once water is introduced, the binder dissolves and the particle structure collapses, causing settling.
Soil Creep: Soil creep happens when gravity slowly moves soil downhill. This can happen when, in order to create a buildable lot from a sloped lot, the builder brings in fill to level the lot. If the original lot was too steeply sloped, the fill may start sliding slowly downhill. Especially if the rate of creep beneath a structure foundation varies among different parts of the foundation, stresses may develop that are relieved by cracking. Soil creep is typically continuous and cracking will worsen unless stabilization measures are taken.
Hydrostatic pressure is created by the weight of water in saturated soil next to the foundation. Water in soil is moved downward by gravity and outward by capillary action. Pressure builds against anything that impedes its flow. Pressure is created not only by the flow of water in contact with the foundation, but also the weight of the water above. Water weighs 8 lb. per gallon and saturated soil is capable of holding a lot of water! Deeper foundations will be exposed to higher pressures. The concern is that damage will worsen with time.
Hydrostatic pressure can cause cracking or bowing of foundation walls:
• Cracking of often horizontal, sometimes vertical, depending on the nature of the forces acting upon the wall.
• Bowing is caused by creep, meaning the gradual deformation of concrete and masonry walls over the long term. Bowing may be worse toward the middle top of the wall because movement of the bottom of the wall will be restricted by the concrete floor slab.
Damage may or may not continue. Recommend evaluation by a qualified foundation contractor (free) or a structural engineer (not free). Neither is a neutral third party since the contractor would get paid for work and the engineer would get paid for correction design. In many areas, the best course of action is to develop a trust relationship with a local foundation contractor.
Rocks in backfill:
Large rocks in the backfill can damage foundations during either placement or compaction. Damage is most likely to occur at the time of backfilling operations when concrete is still young and more vulnerable. Other forces aside, the foundation is damaged but often stable, depending on the seriousness of the damage. Through cracking may allow moisture entry and freeze damage can widen the crack over time, depending on individual conditions.
Inside and outside corners develop stresses as they cure and cracking is more likely there. Outside the structure at exterior corners cracking or corner detachment is common in some areas.
Inside corners, also called “re-entrant corners” are common places for cracking to occur.
These types of cracks are seldom a structural problem.
Resistance to Shrinkage:
The surface of a poured concrete wall is directly exposed to air and loses water relatively quickly through evaporation. The main body of the concrete wall is not exposed to air, but is drawn toward the surface by the concentration gradient as water at the surface evaporates. Diffusion happens more slowly than evaporation. This means that concrete at the surface loses water, dries, and shrinks at a faster rate than the underlying concrete. This difference in shrinkage rates creates stresses that are relieved by cracking.
Shrinkage cracks can appear differently depending on the way concrete dries/cures:
• Diagonal cracking.
• Craze cracking is more common with concrete floor slabs that walls.
Shrinkage cracks are a cosmetic issue only.
Cracking caused by soil movement is typically visible lower in the walls.
Stepped cracking in mortar is typically the result of foundation movement caused by differential soil movement. It will follow the rules for crack closures but is typically oriented diagonally. Cracks follow mortar lines because the mortar is weaker than the brick.
Differential soil movement can also cause straight cracking. If a type of mortar has been used that is stronger than the brick, cracks will propagate (grow) through the brick and mortar in relatively straight lines (not stepped cracking).
Cracks should be described in terms of “closures”.
Descending closure: crack is wider at the top.
Ascending closure: crack is wider at the bottom.
Both ascending and descending closures can be caused by either heaving or settling, depending on to which part of the foundation walls the vertical force is applied; at the corners or between corners.
• Heaving at a corner will cause an ascending closure (wider at the bottom).
• Heaving between corners will cause a descending closure (wider at the top).
• Settling at a corner will cause a descending closure (wider at the top).
• Settling between corners will cause an ascending closure (wider at the bottom).
Horizontal cracks may develop where mortar loses its bond to brick. Raking and re-pointing would be required for correction.
OTHER FOUNDATION ISSUES
Although we may think of concrete as being brittle, it will actually bend instead of breaking under certain conditions. Concrete creep is the deformation of a concrete structure under sustained load. Long-term pressure or stress on concrete can make it change shape. This deformation typically happens in the direction in which the force is applied. Inspectors are most likely to see creep in retaining walls, or in foundation walls that are not reinforced by a floor structure. Creep does not necessarily cause concrete to fail.
White stains are typically efflorescence. As water moves through masonry it dissolves mineral salts that it carries to the face of the masonry. As water evaporates from the face, the white, powdery residue accumulates on the surface. Efflorescence is typically a cosmetic issue, but is also a red flag that moisture moving through masonry may cause problems other than efflorescence.
Deposits of mineral salts located in the body of the brick but near the surface (called “subflorescence”) can cause spalling as the salt crystals expand with wetting. This process is influenced by the Saline Gradient: water moves from less salty areas toward saltier areas. In other words, salt attracts water. Larger deposits of mineral salts will have a stronger attraction for water. As water moves faster and faster through masonry, hydrostatic pressure develops. Water is moved through masonry by three different forces:
- Diffusion pressures are 3-5 pounds per square inch (psi).
- Capillary pressure reaches 300-500 psi.
- Osmosis is a stronger force, reaching 3,000-5,000 psi.
If the hydrostatic pressure exceeds the compressive strength of the brick, spalling may occur. Compressive strength of brick varies from about 3,000 psi to about 6,500 psi depending on clay characteristics and the manufacturing process.
When either of these conditions is visible, try to identify the source of moisture and recommend correction. The Standards of Practice do not require inspector to identify the source of problems, and it can increase your liability of you’re wrong, but sources are often obvious, and identification makes an inspector look good.
Cone of Compression:
In a basement or crawlspace, digging too close to the foundation footings can compromised the Cone of Compression. The Cone of Compression is the area of soil beneath a foundation that supports the weight of the foundation and the home structure above. This soil should remain undisturbed to avoid compromising its structural integrity. The profile of the weight-bearing soil beneath the foundation is roughly cone-shaped, sloping down and out from the bottom corners of the foundation footing at an angle of approximately 45 degrees. This condition can lead to undermining of the foundation and loss of foundation support leading to structural failure in the affected areas.
Inspectors should identify such areas and mention whether they observe signs of failure connected to this condition, and that the possibility for future damage exists… Recommend that the client consult with a structural engineer or qualified foundation contractor to determine the necessity, options and costs for stabilization.
Foundation hardware consists of hold-downs, bolts or straps designed to connect the structure above to the foundation. In homes it typically consists of anchor bolts that connect to wood framing.
In areas in which seismic activity or high winds are a potential problem, additional hardware, sometimes extensive, may be required in newer construction but missing in older homes. For example, in southern California, where major earthquakes are not uncommon, special hardware is often combined with specific framing elements (like posts) that connect each floor and the roof to the foundation. In these areas, interior shear walls may also require anchor bolts connecting them to the concrete slab foundation.
Although inspectors are not required to confirm proper engineering and construction, inspectors should take the time to learn about basic requirements in the areas in which they work to better protect their clients.
Cold joint are parts of the wall where, during the time of original construction, concrete placement was stopped before completion, and concrete was allowed to set up before placement continued to completion, resulting in limited bond between concrete placed at different times. Cold joints are avenues for moisture intrusion, and areas of structural weakness. Inspectors should mention evidence of moisture intrusion or structural concerns.
Honeycombing is caused by incomplete consolidation of the concrete at the time it is originally placed. Although this condition reduces the load-bearing capacity of the wall to a small degree, unless is extreme and widespread, it is more of a cosmetic issue than a structural problem.