Rebound of Fiber in Shotcrete Operation

Shotcrete is special type of concrete; the difference is conveying and placing. As in concrete; steel fibers are added to shotcrete to improve its properties.

Steel fibers improve durability of concrete; as it helps to improve resistance against cracks thus increase ductility of concrete. Same performance can be expected from shotcrete with the addition of steel fibre. It have added advantage over welded mesh reinforcement as their require

• Less labor
• Less time of construction

In special work like lining in tunnels and in industrial floors for the former one it is suggested to use small fibers and later one to use longer fibers.

In case of welded mesh/ or conventional reinforcement, we can increase tensile strength in some particular direction; whereas fiber shotcrete offer us a multidirectional reinforcing effect.

While projecting shotcrete material (at a high speed) on to a target (say backup surface), it is not usual that all portion of projected materials to adhere to expected target surface. The fraction of materials rebounds or falls down. In our previous post we learnt that coarser particles are rebounded.

Here we will learn about steel fibers in steel fiber reinforced shotcrete, also known as SFRS, their performance in shotcreting in respect of rebound.

We know coarse particles rebound more and in case of SFRS steel fibers rebounds more than any other aggregates of shotcrete materials.

So in a hardened concrete surface we will have richer concrete than that is batched. ‘Rich concrete’ term is used in respect of cement content but the dose of fiber becomes less in hardened surface than expected in batching.

That is the purposes of introduction of fibers are not served. Again we will be billed for total amount of concrete materials and fibers too. Thus our exception will be lowest possible rebound.

Rebound, as expressed as percentage, depends on many parameters. The factors those are important-

• Parameters of concrete pump
• Distance of nozzle
• Spraying angle
• And most important is skill of nozzzlemen
• Method of shotcreting

Again fiber size and shape have influence on rebound and direction of shotcreting has also influence. Say

• More rebound in shotcreting to overhead than shotcreting in horizontal direction
• Regarding shape, flat ended fibers have less rebound than other shape
• Regarding size, very light and very fine fibers have more rebound than heavier fibers.

An overall reduction in rebound can be achieved by using micro-silica in concrete mix both of fibers and aggregates. This also reduces dust during shotcreting.