MIX DESIGN OF SFRC
As with any other type of concrete, the mix proportions for SFRC depend upon the requirements for a particular job, in terms of strength, workability, and so on. Several procedures for proportioning SFRC mixes are available, which emphasize the workability of the resulting mix. However, there are some considerations that are particular to SFRC. In general, SFRC mixes contain higher cement contents and higher ratios of fine to coarse aggregate than do ordinary concretes, and so the mix design procedures the apply to conventional concrete may not be entirely applicable to SFRC. Commonly, to reduce the quantity of cement, up to 35% of the cement may be replaced with fly ash. In addition, to improve the workability of higher Fiber volume mixes, water reducing admixtures and, in particular, superplasticizers are often used, in conjunction with air entrainment. The range of proportions for normal weight SFRC.
Steel fiber reinforced concrete is a composite material having fibers as the additional ingredients, dispersed uniformly at random in small percentages, i.e. between 0.3% and 2.5% by volume in plain concrete.
SFRC products are manufactured by adding steel fibers to the ingredients of concrete in the mixer and by transferring the green concrete into moulds. The product is then compacted and cured by the conventional methods.
Segregation or balling is one of the problems encountered during mixing and compacting SFRC. This should be avoided for uniform distribution of fibers. The energy required for mixing, conveying, placing and finishing of SFRC is slightly higher.
Use of pan mixer and fiber dispenser to assist in better mixing and to reduce the formation of fiber balls is essential. Additional fines and limiting the maximum size of aggregates to 20mm occasionally, cement contents of 350 kg to 550 kg per cubic meter are normally needed.
Steel fibers are added to concrete to improve the structural properties, particularly tensile and flexural strength. The extent of improvement in the mechanical properties achieved with SFRC over those of plain concrete depends on several factors, such as shape, size, volume, percentage and distribution of fibers.
Plain, straight and round fibers were found to develop a very weak bond and hence low flexural strength. For a given shape of fibers, flexural strength of SFRC was found to increase with aspect ratio (ratio of length to equivalent diameter).
Even though higher ratios of fibers gave increased flexural strength, workability of green SFRC was found to be adversely affected with increasing aspect ratios. Hence the aspect ratio is generally limited to an optimum value to achieve good workability and strength.
Grey suggested that the aspect ratio of less than 60 are best from the point of handling and mixing of fibers, but an aspect ratio of about 100 is desirable from a strength point of view. Schwarz, however suggested aspect ratio between 50 and 70 is more practicable value for ready mix concrete.
In most of the field applications tried out of date, the size of the fibers varies between 0.25 mm and 1.00mm in diameter and from 12mm to 60mm in length, and the fiber content ranged from 0.3 to 2.5 percent by volume. Higher contests of fiber up to 10% have also been experimented. Addition of steel fibers up to 5% by volume increased the flexural strength to about 2.5 times that of plain concrete.
As explained above, mixing steel fibers considerably improves the structural properties of concrete, particularly tensile and flexural strength. Ductility and post cracking strength, resistance to fatigue, spalling and wear and tear of SFRC are higher than in the case of conventional reinforced concrete.
How Workability is affected?
If you opt-in to add fiber to your concrete mix, be aware that there will be some changes in the way you manage this concrete. First of all, the slump will be affected, and it is recommended to add a super plasticizer to enhance the slump and make the concrete a little more fluid. Not all steel fiber can be used as a substitute for steel reinforcement, so make sure that your structural engineer has reviewed and analyzed the loads before proceeding.