Photograph of a basement floor slab crack Fiber Reinforced Concrete
Types of fibres, research citations

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Fiber-reinforced concrete:

Fibres of various sorts have been used to improve the crack resistance of concrete for thousands of years as a way to strengthen the concrete in effect by making it less brittle.

This article presents research articles and their abstracts discussing the types of fibres used to reinforce concrete and its performance, followed by a field report of cracks in a new fibre-reinforced concrete garage floor.

This article series describes the types of cracks that occur in poured concrete slabs or floors and explains the risks associated with each, thus assisting in deciding what types of repair may be needed.

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Fiber-Reinforced Concrete Foundations, Walls, Floors, Slabs

Severe cracks in new concrete garage floor slab (C) InspectApedia BHResearch on Fiber-Reinforced Concrete

Photos of cracks in a garage slab that might be due to improper concrete mix or due to concrete inclusions as well as a discussion of this cracked slab are provided later in this article at FIBER REINFORCED CONCRETE SLAB CRACKS following this list of research articles on types of fibers used in concrete reinforcement and the strength, features, and crack resistance of fiber reinforced concrete.

Reader Anon provided the photo at left and below. [Click to enlarge any image]

Question: Causes of severe cracking in new concrete garage floor?

Severe cracks in new concrete garage floor slab (C) InspectApedia BHI have a disagreement with our construction company’s warranty department over the cracks in our garage’s floating slab.

1. Poured Oct-Nov 2016 (Southeastern Virginia)

2. Reported to builder Feb 2017 one day prior to closing

3. Attached Pictures taken in May 5th ( I added water to cracks to better show the number of cracks)

[Click to enlarge any image]

Home is a new two story end condo:

4. Our home inspector (well respected-have used him several times) reported “Excessive cracks for a new slab”

5. Builder’s response: “concrete does two things – gets hard and cracks, it’s normal – will look at it in six months” (got same answer July 20th – not warranted )

6. Builder’s warranty states cracks must be 3/16 inch to qualify for repair and ours doesn’t meet that criteria (would say ours are around 1/16th or less)

7. We have visited several other homes nearing completion and found one 3 foot crack (map, mud, etc.)

The cracks [have these properties]:

8. Several cracks run across length of slab; meandering.

9. Some stop and start again a ½ inch over then continues on same direction

10. Some cracks go right through the control point as if it wasn’t there

11. Control points are not v-shaped. One inch deep channel. Quarter inch wide.

12. No shrinkage separation space between slab and foundation

13. No water intrusion except from car’s a/c drainage or rain dripping off a wet car

The builder says their warranty doesn't apply to these cracks

Our position is that the builder’s published warranty does not apply to our slab because the cracks we have are the result of a poor quality workmanship related to the mix/curing process.

State of Virginia holds that a contract between buyer and seller contains an “implied warranty” that the buyer can expect quality workmanship and if quality is missing – Breach of Contract. We don’t want to pursue this option unless last resort and maybe not then.

Our “man on the street” version: If you ordered bacon and eggs and the cook burnt the bacon it wouldn’t be the bacon’s fault, it’s the cook’s.

Appreciate any comments/conclusions you may have. I understand difficulty in assessing email reported problems. - Anonymous by private email 2017/08/03

Reader additional remark:

I just found out the slabs reinforcement is 3500 Fiber and 4” thick. I believe there is more than one kind of fiber (4000 years ago it was straw) but I don’t know what was used here.

Reply: this slab is cracked more than "typical" and may be due to inclusions or due to improper mix

Severe cracks in new concrete garage floor slab (C) InspectApedia BHOPINION: I agree that the slab looks terrible and that what look like shrinkage cracks crisscross the work in a pattern suggesting improper mixing at the least, possibly inadequate or no reinforcement.

Where fiber-reinforced concrete is poured, often the builder may be permitted to omit steel reinforcement mesh or re-bar.

However, with or without steel reinforcement, if there is a mix problem more-severe cracking can be one of the results. Adding confusion to terminology, some concrete is steel-fiber-reinforced - not the same as steel mesh or steel re-bar reinforcement.

Also I'm not sure that fiber-reinforced concrete is as crack-resistant to some forces (such as settlement) as is steel re-bar-reinforced concrete, particularly if the mix is improperly prepared. (Mirmiran 2000)

Residential fiber-reinforced concrete probably uses synthetic or plastic fibers, though some mixes may use steel fibers. Straw, hemp, and elephant grass have indeed been and is still used in concrete in some countries but not commonly in the U.S. (Merta 2013).

Your contractor is right that concrete will crack. At a conference I organized on this topic years ago several concrete installers led a panel who commented that "every concrete truck has a bunch of cracks in it" a jokey way of saying the concrete they poured would crack as it cured.

Their point was that control joints were essential to manage the stresses in concrete that make it want to crack, by allowing the stress to be relieved in or crack along the control joint.

A point made by some of the engineers and architects in the room was that from a civil or structural engineer's point of view, they don't want to see cracks in concrete, and that in many cases if there are other than trivial cosmetic cracks they're probabaly going to characterize the concrete as having suffered a failure.

There are few standards about crack failures in new residential pours, and the crack-width argument was intended to distinguish between a cosmetic shrinkage crack and a crack that causes more-significant breakage in the concrete. Similarly a difference in elevation across a crack points to settlement - a diagnostic observation.

Nevertheless, the amount of cracking, extent, pattern, location in the work in your photos of concrete and in concrete work less than a year old is certainly not "normal concrete work" and is in my OPINION a poor job.

When I see cracks all over relatively-small areas of concrete and crossing the control joint I'm pretty sure something's wrong with the pour.

Too much water, wrong mix, not enough concrete, not enough gravel, no reinforcement, over-tooling, or pouring in wrong weather conditions, too hot or too cold, are example causes.

When I see cracking in concrete that in pattern resembles shrinkage cracks - somewhat random cracks that vary in direction, cross one another, often varying in width - but that are also much wider or more severe than the usual shrinkage cracking, I suspect an inclusion or mix problem in the concrete such as we describe at FOUNDATION DAMAGE by MATERIAL or INCLUSIONS where I discuss Iron sulfide mineral (pyrrhotite) cracking.

I have found references that note that inclusions of shale in the concrete mix can cause cracks in the pattern of shrinkage cracks but larger and more disruptive - that look like your photos.

I also found that some areas of Virginia suffer from concrete mix that sometimes includes shale, but I don't know about your specific case.

If you can find out where the concrete came from and if you have photos of the job that'd be informative. It might be possible to ID inclusions in fragments of the broken slab as well.

From what I can make out in your photos (which is of course incomplete compared with a visit from an onsite expert), these are not settlement cracks, SETTLEMENT vs. SHRINKAGE CRACKS which would have pointed to pouring over soft fill or a similar snafu.

In my opinion, the work you show would be unacceptable to most customers first on a cosmetic basis (for which I generally tend to be rather forgiving) and second on a functional basis (meaning slab failure, need for replacement).

I don't know the thickness nor composition of your slab, but when we see so much cracking in new work you can pretty much figure that the slab has no decent predictable future life.

When a contractor says about a defect in her work, "They all do that" we often find that she is right, all of her work "does that" because all of her work is done the same, improper way.

Or if in your contractor's work, "they don't all do that" that is if you can see other of her or his jobs that don't look like yours, then that too would be evidence that something was different, and wrong, with your installation.

Garage or basement floor sloped or semi-uniform settlement may also produce a tipped floor even if the concrete is not cracked, or the floor may settle uniformly. This condition occurs if the concrete was reinforced by steel or fiber cement, but was poured inside of a separate concrete or masonry block foundation.

We see this condition more often in garages in which the slab was reinforced but poured on poorly-compacted soil. The problem may be worst if in addition to poor compaction, water runs under the slab, causing additional or more rapid soil settlement.

In a garage where the slab has settled you can often spot the original level of the slab and thus can measure the amount of settlement.

Look for a concrete line above the level of the top of the slab and found along the masonry block or poured concrete foundation wall. we have seen this line ranging from a fraction of an inch to six to eight inches above the current level of the slab!

Readers whose garage or other floor slab is settling, tipping, etc. may also want to see SINKING BUILDINGS where we include case histories of both building settlement and slab cracking, heaving, settling: diagnosis and repair.


Continue reading at CONCRETE SLAB CRACK EVALUATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.





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Concrete Crack Diagnosis & Repair Articles

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