Adding steel fibers to ready mixed concrete improves the concrete's impact strength, flexural strength, fatigue strength, toughness, and its ability to resist cracking and spalling.

ADDING THE FIBERS

To disperse them throughout the concrete mix and to prevent them from clumping together, steel fibers should be added to a fluid mix. This can be done in two ways. The steel fibers may be added to transmit mix trucks after all the other ingredients have been added and mixed. In a plant set up to charge a central mixer or transit mixers, steel fibers can be added along with the fine aggregate.

HOW TO PREVENT FIBERS FROM CLUMPING

Fibers usually clump together before they are added to the concrete mix. If no clumps of fibers are added to the mix, the mix nearly always stays clump-free. Adding too many fibers to a mixture or adding fibers too fast to a harsh mixture can cause clumping too.

DON'T ADD WATER; USE A VIBRATOR

Though steel fiber reinforced concrete looks stiff and unworkable, when a vibrator is used, it usually can be placed very easily. Batch plant operators and truck drivers should not add additional water to the mix based on the concrete's appearance and their experience with conventional concrete. Workability of fiber reinforced concrete placed by vibration is best measured using the inverted slump cone test method, ASTM C 995. The time that it takes an inverted slump cone full of fiber reinforced concrete to be emptied after a vibrator is inserted into the concrete is called the inverted-slump-cone time. For steel fiber reinforced concrete, the inverted-slump-cone time should be between 10 and 30 seconds.