My new house, built on a concrete slab, is about 60% done, framing and drywall are in. My concern is I have several hairline cracks throughout the slab. I purposely had the slab made thicker (6 inches) with heavier rebar and a fiber- type additive to hopefully minimize this from happening.
Is there anything I should do prior to having my flooring installed or something I can do to minimize what’s already happened? My contractor did tell me it was going to crack even though I made it thicker. – Larry B.
A crack in a slab of 1/8 inch or less is typically a normal shrinkage crack and not a cause for concern. If the crack is larger or growing larger (an “active” crack), or one side of the crack is higher than the other, then you may need to have the work reviewed by a structural engineer.
Crack Types & Solutions
There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize.
Plastic shrinkage cracks. These occur during the first few hours when the concrete is still in a “plastic” state. They are caused when the surface moisture evaporates too quickly, usually during hot or windy weather. These range from barely visible “hairline” cracks to 1/8 inch in width and from a few inches to a few feet in length. They typically have an irregular pattern or do not reach the edge of the slab. They are usually stable over time and non-structural.
Synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.
Plastic settlement cracks. These are similar in width to plastic shrinkage cracks, but are caused by uneven settlement of the wet concrete. They appear within several hours of concrete placement and usually stop at the rebar. They typically form at a “restraint” such as a pipe or post in the middle of the slab, or at an inside corner. Causes include weak concrete mixes, rebar too close to the surface, or a change in slab thickness — for example near a concrete beam. These tend to follow the grid of the rebar. They are usually not structural unless they are excessive in size or number, but may need to be sealed to protect the rebar from corrosion.
Drying shrinkage cracks. These occur as moisture leaves the concrete after the slab has hardened. The main cause is concrete that is too wet, referred to as a “high-slump” mix. The best solution is to use less water in the concrete mix. Concrete suppliers sometimes add water to make the concrete easier to work with, but this weakens the concrete.
Welded wire mesh can also help reduce shrinkage cracking, but only if it is placed in the middle or upper half of the slab, but at least 2 inches below the surface. Wire mesh also helps keep small cracks from growing. In too many cases, however, the wire mesh ends up on the bottom of the slab where it does nothing.
Shrinkage cracking can be managed by the use of control joints placed in the slab. Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid, which looks better than random cracks, but functions the same way. If you are placing tile on the slab, it’s important that the control joints in the slab line up with control joints in the tile – easier said than done. So random cracking might be a better approach for tile, except on very large floors.
Structural cracks. Concrete can support a lot of weight in compression (squeezing forces), but is weak in tension (pulling apart). For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab.
A water leak in the piping under a slab can also lead to soil movement and hydrostatic pressure that results in cracking. Wet spots in and around the slab and foundation water leakage are other symptoms of underground water leaks.
The best protection against structural cracking in residential structures is good compaction of the soil and gravel underneath the slab. In addition, rebar should be placed in the footings around the perimeter of the slab and at post bases within the slab.
Rebar is generally not needed in the field of the slab in residential projects and is difficult to place properly in a slab less than 6 inches thick. However, it can be useful in garages and other areas subject to structural loading. Rebar may also be required in areas with problem soils or in seismic zones. To be effective, rebar should be placed near the bottom of the slab, about 1/3 of the way up. The rebar must be fully embedded in concrete, with sufficient coverage, to prevent corrosion. To keep the rebar in place during the pour, it is wired together are joints and supported by metal or plastic “chairs”.
Underslab Vapor Barrier
I strongly support the use of a vapor barrier below the slab to prevent moisture migration into the building. Concrete might look solid as rock, but it is highly permeable to moisture. This can introduce excessive moisture into the home and can wreck havoc with many types of flooring. Some contractors like to place a layer of sand over the vapor barrier to control warping and shrinkage cracking — especially on large commercial projects. However, most concrete contractors have adapted to placing the concrete directly on top of a heavy-duty vapor barrier. Read more on vapor barriers in concrete slabs.
Flooring Installation
If you are installing ceramic tile on the slab, you will definitely want to install a crack-isolation membrane on the slab before installing the tile. There are a number of excellent membranes on the market. If you are installing wood flooring, carpeting, or resilient flooring, no extra precautions should be necessary.
Symptoms of Foundation Problems
Once the slab is covered with flooring, it may be difficult to detect problems with cracking or uneven settlement. During the first few years, every new home experiences some degree of adjustment as the framing and other materials acclimate to the moisture and temperature conditions of the home, and react to daily and seasonal changes.
In the first year, nail pops in the drywall and small joint separations at ceiling corners are normal and should be fixed by the contractor. Similarly, you may see some small gaps in hardwood flooring during the first heating season.
If you start seeing more extensive or systemic problems, however, you may need to call in the experts. Problems might include:
- Growing “active” cracks in concrete slabs or walls
- Sticking or gaps in doors and windows
- Cracks in tile floors
- Cracks in the drywall or in stucco or brick at window corners
- Diagonal or “stair-step” cracks in drywall, stucco, or brick
- Floors that are sloped, humped, or out of alignment
- Displacement of cabinets, interior trim, or other building elements
Any of these problems could indicate excessive or uneven settlement or other significant problems with the foundation or soil under the building. Get a structural engineer or other qualified professional to take a look. These problems can be difficult and expensive to address, but the sooner you start, the better. – Steve Bliss, BuildingAdvisor
Related articles: Building a Dry Basement Roof & Yard Drainage Waterproofing & Dampproofing Foundation Insulation Building Site Evaluation
F. Aydo says
Are Cracks in New Slab A Concern?
Hi.
We just poured a new slab for a single-story accessory dwelling (granny flat). Several cracks appeared the next day (see photo). Is there anything we should be concerned about?
buildingadvisor says
To my eye, these appear to be plastic settlement cracks, caused by uneven settlement of the wet concrete. These often occur at “restraints” in the concrete – like the steel post in your photo. As described in the article above, these are usually not structural, but you may want to seal them to prevent corrosion of the rebar (if used in the field of your slab).
As with any concrete crack, it is a good ideal to observe them over time to make sure they are stable and not growing in length or width – or showing any variation in height across the crack.
It’s hard to know what the cause is, but this type of crack is often related to too much water in the concrete or over-vibration of the wet concrete. If the cracks form a grid, it may be due to rebar that is too rebar that is not embedded deeply enough.
Not sure what you are planning for the finish flooring, but make sure you use a good isolation membrane if you are planning to use ceramic tile. The membrane prevents cracks in the slab from telegraphing though to tile surface.
S.A. Bhat says
Why Did Concrete Crack In Every Room?
I hired a contractor to build a small house (1,365 Sqft), but within a year the first-floor slab developed cracks in all the rooms (minimum 3-4 per room) across the length and width. Some are visible from below also. What is the likely cause for these problems. Please advise me and I will take up the issue with the contractor.
buildingadvisor says
There is not enough information here to diagnose the cause or severity of the cracks you have experienced. However the extent of the cracking is certainly a cause for concern.
Crack Causes
Shinkage Cracks. The most common cause of slab cracking is too much water in mix. This makes it easier to place the concrete from tends to cause excessive shrinkage cracks when the slab cures. Shrinkage cracks are generally non-structural, but can be unsightly and can cause problems with finishes. These usually occur within the first 6 months of placing the wet concrete.
Cutting or forming control joints will not eliminate shrinkage cracks, but can help keep them in an organized grid, where they are less noticeable.
Crazing. Too fast drying of the surface in hot weather is another common cause of non-structural cracks. This can cause hairline cracks or crazing, a network of random cracks across the slab. Crazing occurs when the surface dries before to slab has developed adequate strength, creating a random pattern of cracks across the slab, Crazing can also be caused by excessive floating, or intermittent wetting and drying during curing. Crazing is non-structural, but can result in a flaked and dusty surface that needs resurfacing.
Improper Curing. Improper curing can also lead to cracking and other problems. The goal of curing is to make sure the slab retains enough moisture over the first few weeks to gain full strength, water tightness, and durability. Curing is typically complete within 28 days of placement, but can handle foot traffic in 24-48 hours and lightweight machinery and traffic in about 7 days.
Other causes of cracking include:
• Concrete mix is too weak for the job
• Inadequate reinforcement of the slab
• Poor substrate under the slab: poor soil, uncompacted fill, frozen soil, etc.
If the soil under a building settles more in one area than another, you will often see settlement cracks, which may grow over time.
Types of Cracks
Hairline cracks, shrinkage cracks, and other non-structural cracks are mainly a cosmetic issue, but should be repaired if they create a hazard or are unsightly.
Hairline cracks at the surface should be repaired if they are widening with time, are a hazard, collect dirt or are unsightly.
Shrinkage cracks occur while the concrete is curing and can be reduced or prevented with proper joint placement.
Settlement cracks happen when the ground beneath the slab wasn’t compacted properly and part of the concrete sinks.
Structural cracks wider than a credit card, or that run through the entire slab require repair or replacement of the concrete.
Cosmetic (non-structural) cracks are usually easy to fix with fillers or sealants, or they can be left as is if they do not concern you. Structural cracks are more serious. These are typically 1/8-inch or wider and run all the way through the slab. Cracks that continue to grow are the most serious and represent an unstable situation where the loads are too great for the concrete or the soil underneath is too weak to support the structure. More strength or steel reinforcement in the slab may have enabled it to span over the weaker areas of the soil.
Remediation can take many forms, from sealing cracks or resurfacing the slab to demolishing and replacing the slab in the worst case.
Since this happened a year after construction, it suggests to me that some uneven settlement may have taken place. In this case, you will probably also have problems with windows and doors sticking from going out of square, and possibly cracking in wall areas.
If the settlement is complete the cracks can be repaired and the repairs should remain stable. If the cracks continue to grow, you may need to do more serious stabilization of the foundation. This may involve soil additives that stabilize the soil, injecting mortar under the slab (slab jacking), adding concrete piers or supports, or using specialized hardware such as helical piers to stabilize the foundation.
It may also be that the concrete mix was too weak for the load imposed and is cracking due to normal building usage.
If you are unsure about how to proceed, I would recommend having a structural engineer take a look.
You can read more at these links:
Building on Poor Soils
Building on Filled Land
Soil Washed Away Under Foundation
Helical Pier Foundations
Louis Carbone says
Are Cracks All Over New Slab OK?
Thank you for a very helpful, well-informed article. I have a new build. Walls are not even up yet. Many spider cracks throughout entire slab. Some run entire length of house. The slab does not have any expansion joints. Is it industry standard to have control joints? My concern is that cracks are just going to become wider and deeper. If the cracks are consider normal should I have them repaired? Please advise
buildingadvisor says
While expansion joints are a good idea, they are not a standard detail in residential construction. For best results, a 4-in. slab should have control joints every 8 to 10 feet in each direction.
Cracking throughout the slab, as you describe, is not a good sign. Since these occurred shortly after the slab was poured, they are most likely due to concrete shrinkage. Plastic shrinkage cracks are typically tight and narrow, but extend all the way through the slab. They are often found around posts or other interruptions in the slab caused by pipes, drains, etc.
Too much water in the mix is the most common cause of excess shrinkage cracks. Contractors add water to make the concrete flow better. This makes the concrete easier to place, but weaker when it cures. Assuming the subsurface was prepared properly, the cracks may remain stable, but could telegraph through ceramic tile or other finishes. Also, water or radon could enter through the cracks. If there is uneven settling, the cracks can grow over time.
A few shrinkage cracks are normal, but cracks throughout the slab are a sign of substandard concrete that may be weaker and less durable than needed.
If the cracks are very fine and just on the surface, they may be “crazing” cracks. These often have the appearance of spider webs. They occur when the top layer of the slab dries too fast due to hot or windy weather. These are not structural, but can be unsightly.
Depending on the severity of the problem, you may want to get the opinion of a structural engineer before proceeding. Shrinkage cracks can be repaired, and crazing cracks can be resurfaced, but these are done mainly for the appearance. If, on the other hand, the concrete does not meet industry or building-code standards, the optimal solution may be to demolish the slab and start over — an extreme fix, but better than building on a weak foundation.
Jon says
SHould I Be Concerned About Tight Crack in Slab?
Hi. I purchased a home in Florida 5 years ago and just noticed a crack in the slab about ten feet with no space. Is that OK? The house was was build in 2004 and I am the second owner.
buildingadvisor says
Tight cracks are common in concrete slabs. In general, if the crack is stable and not leaking water, it does not indicate a structural problem. In most cases, these are shrinkage cracks which formed when the concrete cured.
Keep an eye on the crack to make sure it is not growing, which would indicate a more serious problem. For a narrow crack like this, you can use a self-leveling concrete crack filler to seal the crack before painting or finishing the surface. You can also use polyurethane or other types of sealants formulated for concrete. With any sealant, first test on a small area to see if the appearance is acceptable.
Also, if you plan to install ceramic tile over the slab in this area, make sure your contractor uses a “crack-isolation membrane” over the slab. It is important to closely follow the manufacturer’s instructions for bonding the membrane to both the concrete and tile.
Jeannine Ott says
Cause of Cracked Slab & Sand in the Ductwork?
Our home built in 1959 was built on a slab. It was built by the original owner who was at the time a commercial builder. We are the second owners for about 14 years. We have a crack in the middle of the house that is getting bigger and uneven. We are also getting sand come up through the duck work (vents) -forced air heating. I’m very concerned that this is a structure problem. Are my worries warranted?
buildingadvisor says
These are both serious problems that need to be addressed. They may or may not be related.
If the crack in the middle of the house is over 1/8 in. wide and growing, and the two sides of the crack are not at the same level, this is an active structural crack. This is typically due to uneven settling of the materials supporting the slab that the slab is not strong enough to resist. The settling or other soil movement may be due to subsidence (subsurface erosion), or building on uncompacted fill, expansive clay, or other problem soils. The crack may stabilize or keep growing depending on the subsurface conditions that are causing the problem.
I would recommend getting a structural engineer or geotechnical engineer to take a look and specify a solution. If required, there are numerous ways to stabilize soils with additives and raise fallen slabs by injecting grout or other materials under the slab, called slabjacking or mudjacking.
The ductwork problem suggests that the metal ductwork has separated or rusted through, allowing the sand base under the slab to leak into the ducts, probably assisted by water. A water problem under the slab could also be contributing to the cracking problem. For example, if you have a seasonally high water table, or a leak in a subsurface water pipe, this could be the cause of erosion or excessive soil expansion and contraction, especially with clay soils. If this is the case, you would probably see water entry through the crack.
At a minimum, you have openings in your ductwork, which can provide entry points for rodents, soil gases including radon, termiticide, and other pollutants. Water in the ductwork can also be a source of mold, which is then distributed throughout the house. You can sometimes get a good look at the inside of the ducts with a standard camera, inserted in the ductwork after removing the floor register. Professionals can do a more thorough investigation with special cameras that can be snaked through the ductwork.
Once you’ve assessed the damage, you can decide whether to repair the existing ductwork or relocate it to the attic. If the damage is minor, repair may be the better option. But relocating the ductwork to the attic (under the insulation for energy efficiency) might provide a better long-term solution.
Michael Leibowitz says
Concerned About Hairline Crack in Slab and Tile
We bought a foreclosure that seems to be in relatively good condition. There’s a 8 ft. hairline crack across the master bath tile floor. Also a 14 ft. crack in the master bedroom found when we removed the carpeting. Both are less than 1/8 wide”. We’re removing the tile and carpeting throughout the house and installing 8mm waterproof vinyl board throughout the house. The house is 20 years old.
Do we need to worry and consider adding an under-slab support system?
buildingadvisor says
In general, a small number of tight (less than 1/8 inch wide), stable cracks are considered normal shrinkage cracks. If they are larger or growing wider, then you probably have uneven settling taking place and the slab is not strong enough to tolerate the movement. This requires tensile strength provide by rebar placed in a grid in the slab. More commonly, contractors use just wire mesh, which keeps cracks from separating, but does not prevent cracks.
A key factor is whether these are new cracks that are growing in size or stable cracks that have been there for years and are not likely to increase in length or width.
To keep small cracks for telegraphic through ceramic tile, most tile contractors now apply a crack-isolation membrane on top of the slab before applying the thinset mortar.
I’m not familiar with the flooring product that you are using, but would advise contacting the manufacturer about installation over concrete cracks. You will probably be fine, but it never hurts to ask and is always wise to follow the manufacturer’s recommendations in case there are warranty claims down the road.
Best of luck!
buildingadvisor says
To my eye, these appear to be plastic settlement cracks, caused by uneven settlement of the wet concrete. These often occur at “restraints” in the concrete – like the steel post in your photo. As described in the article above, these are usually not structural, but you may want to seal them to prevent corrosion of the rebar (if used in the field of your slab).
As with any concrete crack, it is a good ideal to observe them over time to make sure they are stable and not growing in length or width – or showing any variation in height across the crack.
Hard to know what the cause is, but it is often related to the concrete mix (too much water), over-vibration of the wet concrete, or rebar that is not deep enough.
Not sure what you are planning for the finish flooring, but make sure you use a good isolation membrane if you are planning to use ceramic tile. This prevents cracks in the slab from telegraphing though to tile surface.
Michael Stutzke says
What Is Industry Standard For Concrete Cracks?
What is considered industry standard when it comes to cracks in the cement on a new build. My new home is near completion and on the wrap around porch I have a crack which I am concerned with. Please advise.
buildingadvisor says
Is the porch floor a concrete slab? Is this the location of the crack that is concerning you?
How wide and long is the crack? Straight or jagged? Also, when was the slab poured and when did the crack appear? Can you send a photo?
In general, a small number of tight (less than 1/8 inch wide), stable cracks are considered normal shrinkage cracks. If they are larger or growing wider, then you probably have uneven settling taking place and the slab is not strong enough to tolerate the movement. This requires tensile strength provide by rebar placed in a grid in the slab. More commonly, contractors use just wire mesh, which keeps cracks from separating, but does not prevent cracks
Read more on Shrinkage Cracks in Concrete
Michael S. says
Steve,
Thank you very much for your response. Attached please find photo of crack. My thought has been the contractor should have placed a joint similar to what he did on the second photo.
Michael
buildingadvisor says
Thanks for the pics. The grooves in the concrete are “control joints,” put in place at regular intervals to control the location of shrinkage cracks, which are hard to avoid. The idea is that the cracks occur at the weak points in the slab created by the control joints, rather than randomly across the face of the slab.
You are correct that the angled control joint in the photo would probably have helped here, since stresses tend to concentrate at inside corners of slabs and other masonry surfaces (like window corners in a wall). Either an angled control joint or extra reinforcement is needed to prevent this type of cracking.
If the cracks are stable and less than ¼ inch wide, you can repair them with flexible sealant made for concrete, such as Sikaflex Concrete Fix. This is a polyurethane sealant that will outlast cheaper acrylic sealant. It will not give you a perfect color match, but will keep out water and dirt and help stabilize the crack.
You can find additional technical info at this link
buildingadvisor says
Do you happen to know the thickness of your slab and whether it has rebar? Did they place the concrete over compacted gravel/crushed stone?
A slab-on-grade foundation, like you have, usually has rebar running both ways at about 18 inches on-center. Slab foundations typically run 4 to 6 inches thick depending on local codes and practices.. They may also use a stronger mix with lower slump (less water).
Basement floors, on the other hand, rarely use rebar and typically run 3 to 4 inches thick. Often they use wire mesh as reinforcement, which keeps concrete pieces together but doesn’t prevent cracks. That may explain why your slab has fared so well vs. the typical basement floor. There are a lot of variables.
L. Russel says
No Cracks in Our Slab
Our house is built with a concrete slab foundation. I don’t see any cracks or other defects. We have lived here for almost 10 yrs. now.
buildingadvisor says
Do you happen to know the thickness of your slab and whether it has rebar? Did they place the concrete over compacted gravel/crushed stone?
A slab-on-grade foundation, like you have, usually has rebar running both ways at about 18 inches on-center. Slab foundations typically run 4 to 6 inches thick depending on local codes and practices. They may also use a stronger mix with lower slump (less water).
Basement floors, on the other hand, rarely use rebar and typically run 3 to 4 inches thick. Often they use wire mesh as reinforcement, which keeps concrete pieces together but doesn’t prevent cracks. That may explain why your slab has fared so well vs. the typical basement floor. There are a lot of variables.