Homeowner’s Guide on Concrete Foundation Cracks

From homeowners to buyers on the market, foundation cracking can be unsettling and comes in a wide variety of shapes, sizes, directions, and severities. Cracks in concrete are typical. The issue is really how severe are the cracks? This article is an attempt to help homeowners and buyers identify the different types of foundation cracks, in the hope of giving some comfort that these problematic issues are easily repairable.

Basement foundation walls are most commonly built by pouring concrete into molds to contain the fresh material until it dries (i.e., cures). The molds are made of fiberglass, metal, or wooden formworks. The most common molds are wooden formworks. After 24-28 hours the concrete is strong enough to support its own weight, and its intended designed bearing loads, so the formworks are removed. For slabs (i.e., floors) the formworks can be removed after 3-4 days.

There are foundation walls built with other materials such as stone, brick, clay tiles found in vintage structures, precast concrete, and concrete masonry units (CMU), also known as masonry blocks. This article will address concrete and block walls.

Soil

The City of Chicago was built over a swamp. At the end of the last ice age, 10,000 years ago, Chicago was the low spot in the great lakes area and as the ice melted it flowed across the Chicago area in a river the size of the Amazon. This is the origin of Chicago being a swamp and also the very mixed soil types.

In the 1850s it was raised from four feet above Michigan Lake to approximately 14 feet to make room for the sewer pipes under the streets. The city was able to control street flooding and drain the water away. The majority of Chicago land is composed of up to 85% clay and some loam soils.  Generally, clay soil tends to hold water or absorbs moisture much slower, while loam holds water. Basement foundation designs should take into account the soil conditions to avoid moisture damage around the perimeter walls.

Soils in the Midwest are mostly clay types. Therefore, they are vulnerable to water runs off, flooding, pooling, and slow draining. That means foundation walls throughout this region should be properly waterproofed and contain an underground drainage system. Roofs also need large gutters and downspouts to move stormwater away from the building quickly to avoid flooding. The soil near buildings should slope away and not be nearly flat where water pooling can occur.

Note: Penetrating moisture in clay particles in the soil attracts water molecules. Since water is incompressible, the particles in clay soil are pushed apart, causing an expansion of the clay volume. Simply, the water fills in the gaps in the soil causing it to swell. When water evaporates, the opposite occurs, it shrinks.  This occurrence happens regularly in rainstorms and snow melts (i.e., freezing and thawing cycles). Foundation walls and slabs should be constructed to withstand these natural soil pressures and conditions.

Ground Movement

There are three common types of ground movement in the soil. They are heaving, settlement, and subsidence. Please note, there are other types of movements, in this article, I’m addressing the three most common for simplicity.

Many in the concrete construction industry who are not familiar with soil movement identify the three as common settlement issues. Without the contractor’s clear understanding of heaving, settlement, and subsidence, their evaluation could result in an incorrect method of repair.

Heaving: Uplift earth movement from the addition of moisture to an unsaturated expansive soil. When clay soil becomes wet, the dirt swells. Since the earth cannot expand down or from opposite sides, the direction of pressure is upward. This condition may occur during rainstorms or the melting of snow and the ground becomes oversaturated with moisture.

Settlement: Settlement is downward movement due to soil weakening from excessive overhead loads. Settlement is caused by a variety of reasons, such as inadequately designed foundations for the type of soil, the excavated soil was not properly compacted to bear the building’s load, or the depth of the foundation was insufficient to bear the structure.

Subsidence: is downward movement due to soil’s decrease in moisture. Subsidence may occur due to several reasons, such as the soil being weakened from an underground water pipe that has been leaking for years, fracking excavations, sinkhole, clay shrinkage, mining, or decomposing organic matter.

It is worth noting that cracks in concrete are very common. Many homeowners assume the worst when they witness cracks in their concrete walls and slabs. However, that may not be the case. The most a general contractor (GC) can implement during the concrete pour and after, is to attempt to control the locations and amounts of cracks wherever possible. Some cracking is to be expected. The GC can control the cracking by ensuring the soil is properly compacted, the gravel amount is sufficient for the new slabs, controlling the amount of water in the concrete mix, installation of steel reinforcement, spacing of control joints, expansion joints, and avoiding cold joints as much as possible. And yet, some cracking will still develop.

Note: The American Concrete Institute 302.1R-04: Guide for Concrete Floor and Slab Construction mentions the following “Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor’s design or the quality of its construction (Ytter-berg 1987; Campbell et al. 1976).”

Types of Foundation Cracks

Vertical: Professionals around the industry have identified these cracks as typically “non-threating” and the most common types of cracks. With that being said, vertical cracks can have different inferences based on the size and locations of the cracks within the foundation wall. Vertical cracks in the foundation wall are usually the result of the building’s concrete settling one to two years after the concrete pouring. This occurs because concrete is very strong under compression but easily cracks under tension due to the adhesive properties of concrete. These types of cracks are not a real structural concern and can be fixed with the proper equipment and products.   

Causes of vertical and diagonal cracks are due to settlement, shrinkage, and temperature. Generally, they begin from the corners of beam penetrations and window and door openings. They are typically not an issue as long as there is no water infiltration. These cracks are known in the industry as re-entrant cracks.

Re-entrant cracks are tensile stress concentration loads at the corners. As concrete cures linearly any shrink travels in two directions and opposing sides. This will further occur due to temperature changes (expansion and contraction). The cracks can be vertical, horizontal, or diagonal in direction. The anomaly occurs in most materials. To alleviate this condition and before the pouring of concrete, steel rebars should be installed in the corners and at critical areas.

Horizontal: Cracks typically found in the middle of the foundation walls are the most severe. An architect or structural engineer should be contacted immediately to evaluate the condition, determine the gravity of the problem, and develop a repair scope of work. The wall may appear bulging or tilting and cracked. The wall surface between the cracks might appear out-of-plum.  The soil condition that creates these cracks can be lateral pressure loads, poor backfill compaction, and/or hydrostatic pressure on the exterior side of the foundation wall.

If the wall bows closer to the top, frost conditions might be the culprit. The root system of large trees that grow near basement foundation walls creates enormous structural stresses and create havoc on walls. Horizontal, vertical, and diagonal cracks will develop slowly as the tree matures and the root system increase in size. 

Hairline and Shrinkage: Generally, hairline cracks appear early in concrete from plastic shrinkage. Concrete begins in a plastic liquid state prior to curing (i.e., before drying, hardening, etc.), it’s mostly water. As the water evaporates, the concrete decreases in size whether it’s a slab, wall, or another shape. This is the curing, drying, or hardening process. Concrete is a stiff material, as it decreases or shrinks, tension stresses develop throughout. As it relieves the tensions hairline gaps are created. As you may know, concrete is good in compression and not in tension. To gain strength in tension, steel reinforcements are introduced to make up for this weakness.

When there is much evidence of shrinkage cracks, there is a possibility that too much water was added to the mix solution. Therefore, excessive water mixed into the concrete will shrink further than if the correct amount of water was used. More tension stresses are created, consequently, more gaps and a weak mix solution is developed. When crews add water to a pre-designed concrete from a cement truck-mixer, they are doing so to make the work easier. Concrete is manufactured with a specified designed strength and transferred to the job site in a mixing truck. The mix solution is designed with an equilibrium of strength and workability in mind. Therefore, it’s not too stiff or too wet. Pouring and working the concrete into shape does take some effort. To reduce this effort crews will add more water to the pre-designed mix to expedite their efforts. Unknowingly, they reduce the strength in the concrete and pour a lower strength concrete batch into the forms.

Diagonal: Typically caused by building settling unevenly. Another source of this problem might from the soil expanding and contracting underneath the structure from freezing and thawing. The lack of steel reinforcements inside the foundation wall can also be the issue. As mentioned before, concrete is weak in tension and if the soil forces are great, diagonal cracking will be created. Differential settlement can develop when structures are built on a cut and fill slope. This is where a section of the land where the soil is excavated from the top of a slope and moved to a lower area to make a nearly flat grade to build on. Cracking occurs if there is no proper compaction of the soil, pouring a foundation, and then building above. The loadbearing stress into the soil can create a differential settlement and create cracks on the foundation.

Stepped: Cracking also known as stair-step mostly occurs in masonry such as brick, stone or concrete masonry units (i.e., CMU, concrete block, cinder block, etc.) walls. The cracks follow the mortar joints in horizontal and vertical directions. On occasion, the crack might run through masonry itself. The causes of these cracks can be from the expansion of soil settlement, heaving due to hydrostatic pressure, and lack of reinforcing in the CMU. The majority of CMU, brick, or stone walls have hairline cracks within the mortar joints that are not major issues. The problem is when the cracks expand, and the wall is bowing.

Repair Methods

There are two different and common materials used to repairs cracks in concrete foundation walls.  They are epoxy and polyurethane resins. Epoxy applications are used for structural strengthening, while polyurethanes are used to stop water infiltrations. Both are manufactured as two-part or single blends. Each should be researched to determine which is preferable for the particular type of repair.

Epoxy: For the 2-part blend epoxies, one component is the epoxy resin and the second is harder in equal balance measurements. Epoxy applications are used when cracked walls are creating structural issues and it is necessary to strengthen them. It is imperative to keep the 50-50 mix balance amount equal. The hardener does not perform as a catalyst, rather as the component of the reaction.  They are available from very thin viscosities to a thick paste-like substance. The tensile strength when epoxy cures is, on average, 7,000 pounds per square inch (psi) and more - depending on the manufacturer’s specifications. The bonding strength of epoxy exceeds the concrete foundation wall. A typical concrete foundation wall will range between 2,500 psi to 3,000 psi. When the wall is cracked and under tension from the soil’s pressures, the cured epoxy repair will not yield. This means epoxy is an excellent choice for the structural repairs of foundation walls. 

The wall and cracks must be dry and free of moisture. Epoxy injection is a surface port injection. The injection ports must bond to the foundation wall’s cracked surfaces. The epoxy adhesion capabilities to the cracked walls need to be solid and strong. Moisture on the crack’s surface negatively affects the adhesive properties of the epoxy and its repair installation. If the anchored epoxy is not fully adhering to the cracked wall, the repair can fail. The epoxy repair will not withstand the soil’s fluctuating pressures from thermal expansion and contraction from temperature changes

Polyurethanes (AKA: urethane): As mentioned, they can be 2-part or 1-part blends. Generally, most foundation wall repairs in the Midwest are performed with 1-part blends. They are used to eliminate or mitigate water intrusions through concrete foundation walls. The one-part resin blend is formed by reacting a polyol (i.e., alcohol with reactive hydroxyl properties per molecule). This includes diisocyanate or polymeric isocyanate with compactable catalysts and other supplements. A variety of building materials are made with various types of polyurethanes. Such as polyurethanes for the repair of cracked foundation walls with water infiltration issues. Polyurethanes come in a variety of viscosities (i.e., fluid’s thicknesses) for each intended chemical formulation application. The manufacturer’s consistency of each of these products can vary from powdery-like surface feel, gel, rigid, rubbery, and others.

The installation method for polyurethane is very similar to epoxy, they are both injected into the cracked walls of the concrete foundation. The injection can be accomplished with low or high pressure.

Before you begin the application in the cracked areas, remove any coating and surface contamination from the wall with a grinder or a wire brush. It is recommended to drill holes at a 45-degree angle to intersect the crack halfway through the concrete wall and flush out the drilled holes with a water pump.

For high pressure use an electric injection pump. Drill the hole, flush out the opening, install injection ports, flush the crack again, inject the resin, and allow it to cure. After the polyurethane has dried, remove the plastic ports and grind the excess resin off the wall.  The polyurethane form is flexible and allows movement in the cracked joints that may be occurring from soil thermal pressures.

Conclusion

There is a process to discovering and making a diagnosis of water intrusion or determining if there are structural issues. Too many times, we have found that owners might contact a company, the person starts making assumptions of the problems based on what they see or think. This is like rolling the dice in Vegas and hoping for a win. Your home is your investment to be kept at its highest value. It should not be left for someone to makes it into a guessing game of problems. This costs more money and the probability is that it will not correct the problems.

Recommendation on diagnosing these issues are:

1) Interview the owners and ask questions, such as:

• When did you first notice the problems?

• When there is a rainstorm, does it always leak?

• How often do you witness water infiltrating through walls?

• Has the wall appeared to get worse during each winter?

• Have you attempted to make repairs? Did they slow down the deteriorations?

2) After the professional has interviewed the owners, they should have a fairly good understanding of how to proceed with their evaluation and testing process. 

It is usually a good idea when selecting products for a particular specific repair to contact the manufacturer and speak with their technical representative. US-BES/US-CES contact technical manufacturer representatives and ask them to come out to the job site and make mockup test samples on the wall. This further helps in determining which product is preferable for the specific issue. Basically, it is narrowing down which product works better than the other, for each unique water infiltration or structural problem.

To this end, homeowners and buyers of properties should consider reaching out to professionals who are specifically educated, experienced, and trained in troubleshooting building problems. This will require the professionals to step back, perform testing, evaluate issues, and develop a repair scope of work, before executing the repairs. This may require developing a field report describing observations, testing methods, finding issues, and providing repair recommendations. After this, the construction and restoration of the foundation wall can begin.

Many thanks to Raymond M. Lemming, PE, JD for his suggestions and comments about this article. 

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Frank V. Gonzalez, AIA, ALA, GGP is a restoration and preservation architect and contractor. He is also a public adjuster specializing in building science failures. Frank is the founder of U.S. Building Efficiency Solutions, Inc. and U.S. Construction Efficiency Solutions, Inc. He has worked on historical buildings, landmark structures, vintage properties, and contemporary buildings. He has over 30 years in the construction industry and is a licensed architect in Illinois, New York, and Florida. In Illinois, he is a licensed Roofing Contractor, Home Inspector, Public Adjuster, and in Chicago a licensed Concrete Contractor and General Contractor. He can be reached at frank@us-bes.com.