Help: concrete information

[flatbutt]

Some of you will remember that I had a major renovation done last summer which involved a new basement.

There was excavation down to virgin soil and footings installed. Then I had an 8 inch concrete slab poured over the virgin soil to form the basement floor. I just noticed a small crack running the entire width of the slab. I'm just amazed that this slab cracked.

But WTF!!! Is this a structural issue? Must I have this dug up? Given that the footings are separate from the slab is the foundation OK? WTF do I do?

I've contacted the contractor but no response so far.

Help please

[widebody911]

How big (wide) is the crack?

Concrete cracks - that's a fact of life.

[Zeke]

Where is the crack? Do you mean by the entire width than it is in the center? That would be important to know. If so, possibly they should have placed a gravel base over the soil. If it's not below a water table and has good perimeter drainage, I'd say no big deal except for the cosmetics.

[Dantilla]

A large slab will crack. Period. As long as it's olny a crack, no worries. If the slab starts shifting, lifting or sinking, then you need to take a closer look

[TimT]

Reality is that concrete cracks, not if...when..

The basement slab is not a structural element.

Was there any reinforcing steel or mesh in the slab? What sort of curing was done ? Or was the slab poured, finished, and no curing agent was applied (could be spray on membrane, plastic, or flooding with water)?

I had a similar thing happen at my house in Vermont... New extension and basement added to the house... a few months after the house was completed I noticed a crack in the basement slab..

Just monitor the crack, If you start getting water coming up through the crack you could saw and seal the crack.

I'd do nothing about it... just monitor it

[flatbutt]

It is very small, may be 1/16th of an inch? Dead center, end to end. No curingthat I'm aware of.
Bear in mind this slab is only 1 year old.

[red-beard]

Everything else everyone else said. Concrete cracks, it is not structural and it may not even be re-inforced concrete.

[vash]

is the slab reinforced with steel?

[flatbutt]

Originally Posted by vash

is the slab reinforced with steel?

I cant swear to it as I wasn't present at all stages, but I did pay for re-bar.

Well if it isn't structural then I feel better but, if not so then why did they design it to be so thick?

[Dantilla]

Because they charged you for how many yards of concrete used.

[Dantilla]

Originally Posted by flatbutt

Bear in mind this slab is only 1 year old.

I would expect a slab to have hairline cracks within a couple weeks. Yours did good.

[masraum]

Weird, that sounds REALLY thick to me. I think my parents house is 6" and was an inch more than code minimum. I think their house is 4400sqft and one story.

[javadog]

It's probably cracked from shrinkage, which is normal. Concrete doesn't have much strength in tension. Large slabs are ordinarily sawn to a shallow depth every x feet in both directions to try and control where the cracks occur but this doesn't always work. Rebar in the slab won't matter; it will still crack.

I wouldn't worry about it. The important thing is how well the water in the soil is controlled. You want the area around and underneath a basement well drained.

JR

[gr8fl4porsche]

The 2 axioms of construction

Concrete cracks
Steel rusts

[VINMAN]

Whats the dimensions? Length and width? 8 inch is pretty damn thick for a basement floor.

[tops911]

My Dad worked concrete for years, he said the biggest slab you could pour without cracking was 10'x10'. Seems like the stuff he poured around his house that size or smaller has not cracked.

[URY914]

8"? Why so thick?

Did they cut any control joints? (Surprized you experts hadn't asked this already)

[Steve W]

Regarding the cracks, you shouldn't have much to worry about. It is a slab on grade and not structural. Concrete cracks and 1/16 is acceptable. The norm for slab on grade with a properly compacted soil is 4". 6" and thicker with #5 rebar 16" o.c. w/3000 psi or higher concrete becomes structural, and does not depend on the underlying soil, although it can crack too. Although you say the slab was poured on virgin soil, that does not mean that it is of proper compaction of 90% or better. You need to have performed borings and soils tests to determine that, unless it was obviously bedrock. If the virgin soil does not meet proper compaction, it needs to be excavated to a layer that is, and backfilled in lifts until you meet your subgrade elevation. I am guessing that if your contractor poured an 8" slab, no soils testing was performed and your contractor was not confident of the soils conditions. Was this ever performed? Typically a city will not allow a building permit unless a final soils report was submitted. Take a look at it and read the analysis and design recommendations. Did the contractor tie a mesh of rebar across the entire subfloor prior to the pour (not wire mesh)? Was the mesh lifted off the ground with little concrete blocks over a properly prepped subgrade -typically 6mil visqueen under 2" of sand? If your underlying soil is clay, it will expand and shrink dramatically as the moisture content changes, causing cracks even in 8" concrete. You can only mitigate and control the cracks visually by having saw cut expansion grooves in the slab the day after the pour about 5' o.c. In any event, I would be more concerned about proper waterproofing of your basement exterior walls than visual cracks on the floor. That is not a simple or inexpensive process to do properly.

[dad911]

In NJ they should have put 4" stone, then poly, with the seams overlapped and sealed, and 4" concrete. AND A RADON VENT! which is perforated pipe, venting the stone. That is the code.

Crack is probably not a problem, no stone or radon vent is.

Per code, there should also be control joints.

[911Rob]

agree.