IS 15607 : 2005Fibre Reinforced Concrete - Code of Practice

IS 15607:2005 is the code of practice for fibre-reinforced concrete (FRC) — concrete with discrete steel, polymeric, glass or other fibres dispersed through it to provide post-cracking ductility, crack control and toughness. It is used where the value is in *what the concrete does after it cracks*: industrial floors, pavements, tunnel linings (shotcrete), precast, and slabs-on-grade.

It sits with the concrete stack:

• IS 456 — plain & reinforced concrete (FRC supplements, rarely replaces, conventional rebar) · IS 10262 — mix design
• IS 9103 — admixtures (workability with fibres) · IS 383 — aggregate · IS 1608 — tensile/fibre testing

Plain concrete is brittle in tension: it cracks and the section is then dependent on conventional rebar. Fibres change the post-crack behaviour:

• Post-cracking residual strength & toughness — fibres bridge cracks, so the concrete carries load *after* cracking instead of failing brittlely (the headline benefit, quantified by residual flexural strength)
• Crack-width control — distributed fibres limit shrinkage/thermal crack widths in slabs and pavements
• Impact, fatigue and spalling resistance — valuable in floors, pavements, tunnel linings (fire spalling), precast

What fibres generally do not do: meaningfully raise first-crack/compressive strength, or replace primary structural reinforcement for flexure/shear in normal members (FRC complements rebar; it is not a rebar substitute except in specific slab-on-grade/shotcrete design methods). Performance depends on fibre type, aspect ratio, dosage, and uniform dispersion — and on avoiding balling (clumped fibres) and the workability loss fibres cause. The engineering point: FRC is bought for *toughness and crack control after cracking*, designed on residual strength, not for a higher cube number.

Scenario: a heavy-duty jointless-tending industrial floor where crack control and post-crack capacity govern.

Step 1 — pick fibre by function: structural steel fibres for load-carrying post-crack residual strength (slab-on-grade design), or polymeric fibres for plastic-shrinkage/crack control — driven by the design requirement, not generic 'add fibres'.

Step 2 — design on residual strength: size the slab using the specified residual flexural strength (post-crack), not the compressive cube — that is the property FRC actually provides.

Step 3 — mix & dispersion: design the IS 10262 mix for the dosage, add fibres to avoid balling, and adjust workability (IS 9103 HRWR — fibres stiffen the mix); verify uniform dispersion.

Step 4 — verify toughness: test residual/flexural toughness on beams (the FRC acceptance), plus normal strength.

Step 5 — detail & cure: FRC controls crack *width* but doesn't abolish movement — keep necessary joints, and cure as any IS 456 concrete.

Designed on residual strength with good dispersion, FRC gives a tough, crack-controlled floor; specified as 'concrete + some fibres' it balls, loses workability and delivers none of the toughness it was paid for.

1. Expecting higher compressive/first-crack strength. Fibres give *post-crack* toughness and crack control, not a bigger cube — design on residual strength.

2. Treating FRC as a rebar replacement. It complements primary reinforcement; replacing structural rebar with fibres outside validated slab-on-grade/shotcrete methods is unsafe.

3. Fibre balling / poor dispersion. Clumped fibres create weak zones and lost workability — control mixing sequence and dosage.

4. Ignoring workability loss. Fibres stiffen the mix; not compensating (IS 9103) gives unplaceable, poorly-compacted concrete.

5. Wrong fibre for the goal. Polymeric (plastic-shrinkage control) vs structural steel (post-crack load) are not interchangeable — match fibre to function.

• IS 456 — plain & reinforced concrete (FRC supplements rebar) · IS 10262 — mix design
• IS 9103 — admixtures (workability with fibres) · IS 383 — aggregate
• IS 516 Part 1 — flexural strength (toughness test basis) · IS 9284 — abrasion (FRC floors)
• IS 1608 / ASTM A820 / EN 14889 — fibre tensile & classification (referenced for fibre properties)

IS 15607 is reaffirmed and FRC is now standard for industrial floors, pavements, shotcrete tunnel linings and precast — but it is one of the most *over-claimed* materials in practice. The honest framing: fibres buy post-cracking ductility, toughness and crack-width control, not a stronger cube and not (in general) a substitute for structural rebar. Design FRC on residual/post-crack strength, choose the fibre by the function (structural steel vs anti-shrinkage polymeric), and respect the two practical enemies — balling and workability loss. The recurring disappointment is 'we added fibres so it won't crack/we removed the rebar' — FRC controls crack width and adds toughness, it does not abolish cracking or replace primary reinforcement. Used for what it actually does, and dispersed and cured properly, FRC is excellent value for crack-sensitive and impact/fatigue-loaded concrete.