Project Description
It is relatively common to add short fibers to concrete. When present in very small amounts (less than 0.5% by volume), fibers reduce the tendency of freshly cast concrete to crack when exposed to hot, dry, or windy conditions and provide hardened concrete with an increased ability to absorb energy during deformation, manifested by its ability to remain intact and carry load after cracks form. These properties are of particular importance for the durability of concrete pavements, bridge decks, airport runways, and parking garage floors. Although commonly used fibers are made from synthetic materials (e.g. polypropylene or glass), recent studies have shown that natural fibers may provide similar benefits.
An experimental investigation was undertaken to determine whether addition of flax fibers to concrete in short lengths could impact improvements in prope1ties to an extent similar to that obtained by other commercially available fibers. In particular, the study investigated the ability of flax fibers to reduce the amount of cracking associated with exposure of fresh concrete to hot, dry, and windy conditions, and to provide increased levels of load carrying capacity to a fibre-reinforced beam after cracks had formed. The effects of different amounts of flax fibre (varying from 0.05 to 0.5% by volume) and different fibre lengths (varying from 6 to 38 mm) were investigated. For comparison, tests were also conducted on concrete containing commercially available polypropylene and glass fibers in similar amounts and lengths. In addition, the influence of fibre addition on other standard properties of fresh and hardened concrete was measured. Finally, a preliminary study was conducted to determine the susceptibility of flax fibers to deterioration when embedded in concrete.
Results indicated that addition of flax fibers reduced the ease with which the concrete could be handled and placed when fresh (i.e. workability) to a greater extent than the other types of fibers investigated. This resulted in higher air contents and difficulties in obtaining well-compacted specimens, which in turn produced lower compressive strengths. The greatest reduction in compressive strength (22%) was obtained by specimens containing the longest (38 mm) and greatest amounts (0.5% by volume) of flax fibers. Smaller reductions were seen when smaller amounts of fiber were used. Greater attention to proper mixing and placement practices are required to obtain a good quality finished product when flax fibers are incorporated into concrete. This is true when other types of fibers are used as well, but it is even more important with flax fibres due to their capacity to absorb water. The use of a high range water reducing admixture (superplasticizer) is required to achieve an adequate level of workability.
Flax fibres were found to be very effective in reducing the maximum size and total area of cracks produced when freshly cast specimens were exposed to hot, dry, and windy conditions. In amounts of 0.1 % by volume, flax fibres reduced the total area of cracks on specimen surfaces by at least 95% relative to concrete specimens without fibres; the maximum width of cracks was reduced by more than 90%. When the amount of fibres was increased to 0.3%, crack areas and maximum widths were reduced by more than 99.5% and 98.5%, respectively. Fibre length did not appear to have a significant influence. Flax fibres were as effective as other types of fibres investigated.
On the other hand, addition of flax fibres was found to produce very small improvements in the ability of specimens to absorb energy during deformation-much smaller than the improvements produced by addition of polypropylene fibres. This was attributed to the excellent bond between the fibres and hardened concrete and the relative inability of flax fibres to stretch before breaking. It is believed that additional research could be successful in identifying a method of improving the effectiveness of flax fibres for this application.
A preliminary study found that flax fibres are susceptible to chemical attack when embedded in concrete. This type of degradation has been reported in the literature for other types of natural fibres. The long-term effects of this attack on the durability of concrete containing flax fibres are not known. It is important that these effects be determined and methods of overcoming them be identified.
Because of the unanswered questions regarding the long-term durability of flax fibre reinforced concrete, additional research is required before the use of flax fibres in concrete can be promoted. Further study is warranted by the potential benefit of flax fibre addition, particularly in reducing early age cracking. Should further research identify methods of dealing with the durability issues, there is much potential for positive impact to flax producers in terms of providing a new market for flax straw and possibly new industries to process the fibre. Furthermore, if methods of improving the energy absorption properties of flax fibre-reinforced concrete can be identified, an increasing number of applications for the material become available. It is believed that a substantial market exists in the construction industry for a low-cost natural fibre reinforcement, provided contractors, suppliers and engineers can be convinced of its effectiveness, economy and durability.
