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.

Fiber-Reinforced Concrete Foundations, Walls, Floors, Slabs

Research 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]

• Bouhicha, M., F. Aouissi, and S. Kenai. "Performance of composite soil reinforced with barley straw." Cement and Concrete Composites 27, no. 5 (2005): 617-621.
  Harvard
• Brandt, Andrzej M. "Fibre reinforced cement-based (FRC) composites after over 40 years of development in building and civil engineering." Composite structures 86, no. 1 (2008): 3-9.
  
  Abstract:
  
  Fibres have been used since Biblical times to strengthen brittle matrices; for example straw and horse-hair was mixed with clay to form bricks and floors. In modern technology, steel fibres were for the first time proposed as dispersed reinforcement for concrete by Romualdi in his two papers in 1963 and 1964.
  
  Since that time, the concept of dispersed fibres in cement-based materials has developed considerably: hundreds of books and papers, many dissertations, and also applications in building and civil engineering structures all over the world.
  
  After over forty years, it is interesting to review the present state of knowledge and technology of FRC.
  
  The balance of achievements and shortcomings is certainly positive. Our knowledge, based on theoretical solutions and experimental findings, is rich and quite large. Test methods that are transferred from the so called high-strength composites are very effective. However, practical applications are not so numerous as it was initially expected with developments not exactly in the foreseen directions.
  
  In this paper the main fields of application of FRC composites are examined and future perspectives discussed. After a brief review of various kinds of fibres and applied techniques, some attention is paid to computation methods and composite materials’ design approaches.
  
  Large practical application of FRC in construction is mostly hampered by insufficient development of relevant standards, based on performance concepts. It should also be admitted that the cost of fibre reinforcement and related technological operations is certainly an obstacle for use of FRC in ordinary structures. On the other hand, in successful applications in demanding structures very special requirements are satisfied; probably future developments will go in this direction.
• De Lorenzis, Laura, and Ralejs Tepfers. "Comparative study of models on confinement of concrete cylinders with fiber-reinforced polymer composites." Journal of Composites for Construction 7, no. 3 (2003): 219-237.
• Khuntia, Madhusudan, Bozidar Stojadinovic, and Subhash C. Goel. "Shear strength of normal and high-strength fiber reinforced concrete beams without stirrups." Structural Journal 96, no. 2 (1999): 282-289.
  Abstract:
  
  This paper presents a rational and unified procedure for predicting the shear strength of normal and high-strength fiber reinforced concrete (FRC) beams. A design equation is suggested for evaluating the ultimate shear strength of FRC beams based on the basic shear transfer mechanisms and numerous published experimental data on concrete strength up to 100 MPa (14,500 psi).
  
  In addition to concrete strength, the influence of other variables such as fiber factor, shear span-to-depth ratio, longitudinal steel ratio, and size effect is considered. The modeling approach is similar to that applied for conventional reinforced concrete beams, except for some modifications suggested in this paper, to account for the effect of the fibers.
  
  The comparison between computed values and experimentally observed values is shown to validate the proposed analytical treatment.
• Li, Zhijian, Xungai Wang, and Lijing Wang. "Properties of hemp fibre reinforced concrete composites." Composites part A: applied science and manufacturing 37, no. 3 (2006): 497-505.
• Merta, I., and E. K. Tschegg. "Fracture energy of natural fibre reinforced concrete." Construction and Building Materials 40 (2013): 991-997.
  
  Abstract:
  This paper reports on an experimental study of the fracture energy of concrete reinforced with natural fibres of hemp, elephant grass, and wheat straw. Concrete specimens containing 0.19% of fibres by weight and of 40 mm of length were uniaxially tested with the wedge splitting test (WST) method. The addition of fibres was found to improve the fracture toughness of plane concrete.
  
  The most distinctive increase in the fracture energy has been observed by hemp reinforced concrete, up to 70%, when comparing with non reinforced concrete, whereas for straw and elephant grass reinforced concrete this increase was moderate, up to 2% and 5%, respectively.
  
  The beneficial effect of hemp fibres is believed to be the result of the fibre’s high tensile strength and the fibre’s fineness, resulting in a better bonding between fibres and concrete matrix. The presence of fibres in concrete decreased minimally the tensile strength of concrete, for 4%, 7%, and 8% for hemp, straw and elephant grass reinforced specimens, respectively.
• Mirmiran, Amir, and Salam Philip. "Comparison of acoustic emission activity in steel-reinforced and FRP-reinforced concrete beams." Construction and Building Materials 14, no. 6 (2000): 299-310.
  Abstract
  
  In order to compare the acoustic emission signature of concrete beams reinforced with fiber reinforced polymer (FRP) or steel re-bars, a total of 16 203×203×1320-mm FRP-RC and steel-RC beams were tested under bending. The FRP-RC beams emitted higher activity and peak amplitude than their steel-RC counterparts.
  
  They also had lower felicity ratios, showed higher activity at each load drop, and emitted signals even during the unloading process. These characteristics were attributed to the lower stiffness, larger deflections, and brittleness of FRP re-bars, as well as their lower bond strength with concrete.
• Park, Jong-Hwa, Byung-Wan Jo, Soon-Jong Yoon, and Seung-Kook Park. "Experimental investigation on the structural behavior of concrete filled FRP tubes with/without steel re-bar." KSCE Journal of Civil Engineering 15, no. 2 (2011): 337-345.
  Abstract:
  
  This study evaluates the mechanical performance of Reinforced Concrete Filled Fiber Reinforced Tube (RCFFT) through compressive and flexural tests for the purpose of applying RCFFT as strut members for a Prestressed Concrete (PSC) Box Girder bridge.
  
  Firstly, the compressive behavior of Concrete Filled Fiber Reinforced Plastic Tube (CFFT) and RCFFT members was investigated to examine their confinement effects.
  
  Secondly, based on the experimental results, an equation for estimating the ultimate compressive strengths of CFFT and RCFFT was proposed.
  
  In addition, the degree of improvement on the flexural performance due to the reinforcement by Fiber Reinforced Plastic (FRP) was analyzed from flexural tests on CFFT and RCFFT. It could be confirmed from the result of this research that the structural performance of RCFFT members were improved.
  
  Furthermore, the result of this study might be utilized in the design of the RCFFT strut members of a real PC BOX Girder bridge.
• Ritchie, Philip A., David A. Thomas, Le-Wu Lu, and Guy M. Connelly. "External reinforcement of concrete beams using fiber-reinforced plastics." (1990).
• Saafi, Mohamed, Houssam A. Toutanji, and Zongjin Li. "Behavior of concrete columns confined with fiber reinforced polymer tubes." ACI materials journal 96, no. 4 (1999): 500-509.
• Song, P. S., J. C. Wu, S. Hwang, and B. C. Sheu. "Assessment of statistical variations in impact resistance of high-strength concrete and high-strength steel fiber-reinforced concrete." Cement and Concrete Research 35, no. 2 (2005): 393-399.
• Song, P. S., and S. Hwang. "Mechanical properties of high-strength steel fiber-reinforced concrete." Construction and Building Materials 18, no. 9 (2004): 669-673.
  Abstract
  
  The impact resistance variations of high-strength steel fiber-reinforced concrete (HSFRC), versus those of high-strength concrete (HSC), commanded this research. The impact resistance of the high-strength steel fiber-reinforced concrete improved satisfactorily over that of the high-strength concrete; the failure strength improved most, followed by first-crack strength and percentage increase in the number of post-first-crack blows.
  
  The two concretes resembled each other on the coefficient of variation values, respectively, on the two strengths, whereas the high-strength concrete was much higher in the value on the percentage increase.
  
  The Kolmogorov–Smirnov test indicates that the high-strength concrete was approximately normally distributed in first-crack and failure strengths, high-strength steel fiber-reinforced concrete was poorly normally distributed in the two strengths, and both concretes were hardly normally distributed in the percentage increase.
  
  Finally, for both concretes, failure strength regression models were developed, and then, the accompanying 95% prediction intervals for the strength were established.
• Stang, Henrik, Victor C. Li, and Herbert Krenchel. "Design and structural applications of stress-crack width relations in fibre reinforced concrete." Materials and Structures 28, no. 4 (1995): 210-219.
  
  Abstract: The stress-crack width relationship has been shown to be the key to an understanding of fracture propagation in and mechanical behaviour in tension of fibre reinforced concrete materials and structures.
  
  A model is derived for the stress-crack width relationship for randomly oriented short fibre composites which takes hybrid fibre systems and possible fibre rupture into account. It is shown how this stress-crack width relationship can be included in a structural model for the prediction of crack widths in reinforced concrete structures.
  
  With this combination of models a rational design tool for the design of composite materials and structures has been established. It is shown how different fibre systems can be tested for structural applicability and how combined material and structural optimization can take place.
• Yost, Joseph R., Shawn P. Gross, and David W. Dinehart. "Shear strength of normal strength concrete beams reinforced with deformed GFRP bars." Journal of composites for construction 5, no. 4 (2001): 268-275.
  
  Abstract
  This paper evaluates the shear strength, Vc, of intermediate length (2.5 < a/d < 6) simply supported concrete beams subjected to four-point monotonic loading and reinforced with deformed, glass fiber-reinforced polymer (GFRP) reinforcement bars.
  
  Six different overreinforced GFRP designs, ρ > ρb, were tested with three replicate beams per design. All samples failed as a result of diagonal-tension shear. Measured shear strengths at failure are compared with theoretical predictions calculated according to traditional steel-reinforced concrete procedures and recently published expressions intended for beams reinforced with GFRP.
  
  Recommendations are made regarding the adequacy of shear strength prediction equations for GFRP-reinforced members.
  
  The study concludes that shear capacity is significantly overestimated by the “Building Code Requirements for Structural Concrete and Commentary” (ACI 318-99) expression for, Vc, as a result of the large crack widths, small compression block, and reduced dowel action in GFRP-reinforced flexural members.
  
  Shear strength was found to be independent of the amount of longitudinal GFRP reinforcement. A simplified empirical equation for predicting the ultimate shear strength of concrete beams reinforced with GFRP is endorsed.

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

I 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

OPINION: 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 the closely-related articles below, or see the complete ARTICLE INDEX.

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• CONCRETE COLD POUR JOINTS
• CONCRETE CORROSION DUE TO HYDROGEN SULFIDE
• CONCRETE DELIVERY & MIXING
• CONCRETE FOUNDATION, WALL, SLAB DEFECTS
• CONCRETE REINFORCEMENT, FIBER
• CONCRETE SHRINKAGE CRACKS
• CONCRETE SLAB CRACK EVALUATION

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