Question on slab cracks.

Hey guys,
I’ve got a question for you. I did a partial inspection for a 1 year warrantee. The client was concerned with some cracking on his foundation walls, as well as the floor of the basement.

The foundation wall cracks were hairline, shrinkage cracks. Nothing unusual.

The slab cracks are what I need input on. They formed a large uneven X across the basement meeting at a steel column. One branch of the X was about 1/16 of an inch. (I could insert the tip of my screwdriver.) The other was surprisingly large. 1/4 along most of it, with some very minor displacement of about 1/16 in at one part. The cracks themselves followed plastic/rubber inserts that were placed under the surface, and can be seen in the pictures. Any ideas on causes, and recommendations? I don’t want to defer this, because he called me to look at them and advise him.

I told him I had access to over 10,000 experts world wide that I was sure could give me some additional information to tell him. So the pressure is on :D.

Thanks in advance






I’m not familiar with your hydrology and soils but there appears to be movement at the sealed expansion/control joint materials. Overstretched/delaminating sealant or this may be general shrinkage of an early application. They could do a crack monitor, for further expansion identification and or consult an engineer now. Either way an engineer may be the course of action in the long run.

A very simple expansion gauge is made from Popsicle sticks or similar glued perpendicular to the crack on opposing sides, right next to one another. Once the glue has dried draw a line anywhere across both sticks…monitor for further expansion or contraction at regular intervals…another 1/8"-1/4" and you’ll know the rest of this story.

Can’t open these pictures, either. HELP!

I would defer it to at least a foundation expert if not an engineer. Too new to be doing this. I would be very careful in what I say and especially what I put in writing. Getting the average contractor to voluntarily make good on this will be very hard, so consider this case going to court. Document every thing that you do and say, your client does and says and the contractor does and says. Just to be on the safe side. I usually write a letter of “In my professional opinion” than make the recommendation of an expert should be consulted. If concrete a year old is doing this already, I guarantee it will do a lot more in the near future. The contractor may argue it is not structural but you and your client can argue the basement can not be finished. This is the best way you can serve your client because you know since we
are generalist, a contractors lawyer will have his own concrete expert to trump what we say.

Looks like footing is heaved. Cold in your area? How does the beam look above the column? What is the plastic inserts? under-slab vapor barrier? How high is the ceiling? A wood sub floor can be installed over slab. Slabs are non structural, I would report it as that.

Ditto, that looks like way to much differential settlement between the perimiter wall footers and the column support footing.



Appears to be more something along these lines.

Expansion Joints

Expansion joints permit volume change movement of a concrete structure or member. These are usually constructed by installing pre-formed, or pre-molded elastic/resilient material of approximately 1/4" to 1/2" thickness as wide as the concrete is thick, before the concrete is placed. Expansion joints should never be less than 1/4" wide. Pre-molded expansion joints for installation in residential, commercial, or industrial slabs may be of fiber, sponge rubber, plastic, or cork composition. Such materials must be highly resilient, and non-extruding in hot weather, or brittle in cold weather.
An expansion joint should always be utilized where a concrete member will join or abut an existing structure of any type. This would include a junction of sidewalks, sidewalk with a driveway, building, curb, or other similar members, as well as where a floor slab joins a column, staircase, etc. The square formed by the intersection of two sidewalks should have pre-molded expansion material enclosing the perimeter. Normally, expansion joints are not provided in sidewalks other than where the walk abuts an existing structure.

Expansion joints should also be provided in a building floor slab where the slab abuts walls or footings. Sealing of expansion joints is desirable in many outdoor or industrial/commercial applications.

What do control joints or “expansion joints” look like? The photograph at page top and the photo just above where Andy is walking away from the camera show expansion joints in a garage floor slab in Arizona. Even in a climate where we do not anticipate freezing, control joints are needed to prevent random shrinkage cracks that would otherwise occur in a large concrete floor slab pour like this one. Notice that we do not see other cracks in this slab. Control joints are likely to appear as straight lines at regular intervals across a poured concrete slab (if they were used in the construction of the slab) such as we show in the sketch below, at the lines marked (G) at 4’ intervals or larger depending on the concrete materials and slab design used. Shrinkage cracks that occur at control joints such as shown in the pair of close up concrete slab control joint crack photos here, are occurring where they are supposed-to. The fine crack shown in the left-hand photo of a concrete slab control joint is normal - this crack would have occurred in a random pattern instead of along the control joint if this floor slab (the same floor shown at the top of this page) had been poured without any control joints. In a different building, the width of the control joint crack in the right-hand photo above was surprisingly large. These cracks are not normally a defect in the slab but may be a source of water or radon gas entry into the building and may need to be sealed.

Quite possible that it is somewhere down these lines.

Concrete floor contractor in the area would be recommended to find the actuall reason.

Different methods and products for different geographical areas.

Hope this helps a little.

Marcel :slight_smile: :smiley:

Thanks Marcel.
Your picture on the bottom right is very similar to what I’ve got. The strange thing was that there wasn’t a control “joint”. Just the pre-molded expansion material (that wasn’t laid straight). It was actually sub-surface, and when it cracked, it was pretty ugly, pretty wide, and the edges chipped easily.

I had to get the report out last night, but here’s what I said…

  1.   Observed some cracking in the basement floors.  There appears to be movement at the sealed expansion/control joint materials. Gaps ranging from 1/16th of an inch to over ¼ inch.  There is also some minor (1/16 in.) displacement in one section of the larger crack.  This does not appear to be affecting the structural integrity of the house at this time; however it is an indicator of settlement.  Hairline cracks are typical in any concrete slab.  While it is normal to find small hairline cracks in most if not all concrete, these cracks should be monitored for further movement/changes and or consult an engineer now. If these cracks worsen over time, then I would definitely consider getting a engineer to further evaluate your situation. At the very least you may want to have all of these properly sealed to prevent additional moisture from seeping into these cracks which will rust the embedded rebar and worsen the situation.  As the cracks are now, you will not be able to finish the basement until it is determined that the floor has stabilized and the cracks are repaired. Major Concern.