Q: I am having a driveway replaced and am concerned about the use of those glass fibers that my contractor is recommending. What do they really do and shouldn’t I be concerned about breathing them in over time?
A: With increasing frequency, we are hearing from homeowners about the presence of fibers in concrete. While it is somewhat surprising that this issue could actually be increasing in its frequency, it is also disconcerting that it is an issue at all. In our most recent contact, the homeowner had in mind fiberglass batt insulation and the warnings heard over time about not wanting to inhale the fibers when working in the attic. So from this perspective, it was clear that homeowner and proposing contractor were on different pages, perhaps even different books entirely when it came down to the use of a product.
In the new ACI 332 Guide to Residential Concrete Construction, available later this year, ACI 332 Committee members offer a direct discussion of the use of fibers in residential concrete. There are two major classifications for fibers, micro and macro and in each classification the fiber can be described as being monofilament or fibrillated. Micro-fibers are intended to provide pre-crack stress control for plastic shrinkage and lessoning stress relief for settlement or relaxation of the concrete during its plastic stage. These are the more common fibers seen throughout the residential industry today. The macro-fiber, when offered provides similar performance for intended use to the micro-fiber but at a greater degree of control and they provide post-crack control for temperature and shrinkage when the dosages are high enough to provide adequate tensile capacity.
In the case of the homeowner asking this most recent question, what was in mind was that “fibers” were the fine, spun-glass fibers found in the insulating blankets commonly placed between studs for a wall or laid between the joists in an attic. It should be clearly identified to any homeowner that the synthetic fibers added to concrete, even when micro in nature, are thicker and stronger than the fiber elements that get in your skin from insulation. While there are glass fibers, they are not very common. Although the fibers may be present at the surface of the concrete to some extent they will neither become air-born nor be cause for concern of brittle impalement or puncture.
When asking of the contractor why glass fibers might be present in the driveway slab, the response was given that they would control cracking in the slab and would be more economical than placing all that steel wire. To the homeowner, the evaluation of steel versus small randomly dispersed fibers in a concrete mix did not equate. It is true enough that the concern is warranted and this is largely due to the proposing contractor’s misconception of the behavior of fiber-reinforced concrete and the intended purpose of reinforcement in concrete as a whole.
ACI 544.1R and NRMCA CIP24 are the recognized authoritative documents for the performance of fibers in concrete. In both documents, synthetic fibers are limited in their benefit to hardened concrete as lowering permeability and increasing resistance to shattering, abrasion and impact. In other words, synthetic fibers strengthen the surface of concrete and lessen the formation of surface cracks that lead to water penetration. This is definitely a positive for driveway slabs that suffer over time from the presence of de-icing salts brought in from the streets or applied by the homeowner as well as spalling from the effects of freezing and thawing. However, these documents also indicate that crack control is not an attribute of the commonly used synthetic fiber.
A contractor in the concrete industry, you are fully aware of two truths with concret:
- It gets hard.
- It cracks.
Concrete cracks, and we are powerless to do anything about that. That is, we must assume that concrete will crack and therefore it must be augmented in its design to control the significance of that cracking. This is where steel reinforcement is essential. The homeowner was told that glass fibers would be an economical way of eliminating the steel necessary to control cracking. The result of this decision would most certainly be replacing that driveway in a few short years, especially since the contractor was not likely to properly joint the slab in small enough sections to control the cracks that would soon develop.
NRMCA CIP24 cautions the contractor to make sure a manufacturer supplies documentation to prove that any fiber can function as a nonstructural temperature or secondary reinforcement. There are few options out there that can produce such evidence. Instead, the quality-minded contractor should consider a two-part system that implements steel reinforcement bars along with the surface durability protection of a glass fiber to deliver an optimum slab solution.
What cannot be overlooked in the process of delivering a quality slab to a customer is the base preparation, elimination of active water below the slab, properly located and spaced steel reinforcement bar pattern, control of the ratio of water to cementitious material in the mix and properly spaced control joints cut early in the slab. These are the best methods to ensure a long-lasting concrete slab.
Ed. Note: Want to know more? Contact CFA Managing Director, Jim Baty at 866-232-9255 or by email at firstname.lastname@example.org. The CFA is a national association for professionals with the mission to support the cast-in-place contractor as the voice and recognized authority for the residential concrete industry. ACI 332 is the Residential Concrete committee for the American Concrete Institute and as a code committee is seeking professionals from all aspects of this industry with an interest in participating in the development of expanding and strengthening this concrete code.