How to Choose Concrete Fiber: Micro vs Macro, Glass, Basalt

To choose concrete fiber, start with the job: micro fiber controls shrinkage cracks in fresh concrete, screed or plaster, while structural macro fiber reinforces hardened concrete in place of steel mesh. Then choose the material: polypropylene micro fiber for screed and concrete, glass fiber for plaster, dry mixes and gypsum, and basalt fiber for polished floors and heat-resistant concrete.

Concrete fiber consists of short fibers added to concrete, screed or mortar so that they crack less and carry loads better. But different fibers do different jobs. Choose the wrong one and you pay for it either in money or in cracks.

Step 1. What problem are you solving?

Cracks in fresh concrete, screed or plaster. In the first hours, the mix loses water quickly and shrinks. This is where micro fiber works: millions of fine fibers hold the mix together while it is still plastic.

Loads on hardened concrete. A warehouse floor, an outdoor pad, a slab on grade. Here you need macro fiber: thick, stiff fibers that “stitch” a crack together and keep it from opening. This is the fiber that replaces steel mesh.

Step 2. Fiber material

Material	Strengths	Limitations
Polypropylene (micro)	Low cost, fully alkali-resistant, protects concrete in a fire	Does not reinforce hardened concrete
Polypropylene (macro)	Replaces steel mesh, does not rust, lightweight	Does not replace structural rebar in load-bearing structures
Glass fiber	Stiff, invisible on the surface, non-combustible	Not for facade glass fiber-reinforced concrete (GRC)
Basalt	Does not float to the surface or settle, non-combustible, suited to polished floors	Added to dry components only

Step 3. Match the fiber to the job

Job	We recommend
Replace steel mesh in a floor or slab on grade	ReinStrong or ReinFlex
Floors with forklift traffic and impact	ReinFlex
Shrinkage cracks in screed, concrete, plaster	ReinLite
Plasters, dry mixes, gypsum	ReinGlass or ReinLite
Polished floors, high temperatures	ReinBasalt
Facade glass fiber-reinforced concrete (GRC)	requires alkali-resistant AR glass

Step 4. Calculate the quantity

Fiber is dosed in kilograms per cubic meter of mix. Micro fiber usually needs less than 1–1.5 kg/m³, and macro fiber 2–8 kg/m³, depending on the load. Example: a 50 m² screed 5 cm thick is 2.5 m³, and at a dosage of 0.9 kg/m³ you will need about 2.3 kg of micro fiber. The dosage tables for all five products are collected in concrete fiber dosage per m³.

Polypropylene or basalt fiber?

For ordinary screed and concrete, polypropylene micro fiber is usually enough to control shrinkage cracks, while basalt fiber is the choice when surface grinding, high temperatures or an even fiber distribution through the slab thickness matter.

	ReinLite, polypropylene	ReinBasalt, basalt
Density	0.91 g/cm³, lighter than water	2.7 g/cm³, close to concrete
Tensile strength	450 MPa	1200 MPa
When heated	Melts at about 160 °C and protects concrete against explosive spalling in a fire	Non-combustible, service temperature up to 600 °C
Works best in	Screeds, underfloor heating, concrete, plasters, dry mixes	Industrial and polished floors, roads, heat-resistant concrete
Dosage in screed	0.9–1.5 kg/m³	0.6–1.5 kg/m³

Basalt fiber has a modulus of elasticity about 10 times higher than polypropylene, and because its density is close to that of concrete, it neither floats nor sinks. When a floor is ground and polished, basalt fibers don’t pull out or make the surface fuzzy. ReinLite, on the other hand, costs less and is fully alkali-resistant. Basalt fiber goes into the dry components only and is never added to water or wet ready-mixed concrete, because it clumps. The two fibers are often combined.

Polypropylene fiber or glass fiber?

Both work against shrinkage cracks, so the choice depends on the layer: glass fiber suits plasters, skim coats, gypsum and finishing coats where fibers must not show, while polypropylene micro fiber suits concrete, screed and jobs that need full alkali resistance or fire protection of concrete.

• Stiffness. ReinGlass glass fiber has a modulus of elasticity of 72 GPa, about three times higher than polypropylene. That’s why it resists plastic shrinkage and settlement of fresh mortar better.
• Surface. The 17 µm glass filaments don’t stick out or make the surface fuzzy. Glass fiber doesn’t interfere with painting, skim coating or tiling.
• Alkalis and gypsum. ReinLite is fully resistant to cement alkalis. ReinGlass fibers are coated with an alkali-resistant sizing, and in gypsum, which contains no alkalis, glass fiber retains its strength without any limitations.
• Fire. ReinGlass is non-combustible. ReinLite melts in a fire and leaves channels in the concrete through which steam escapes, so it is the fiber used for fire protection of concrete, at 1–2 kg/m³.
• Mixing. After adding water, mix ReinGlass for 2–3 minutes and no longer, or the fibers start to clump. After adding ReinLite, mix for at least another 3–5 minutes.

For screeds, the ReinGlass dosage is 0.6–1 kg/m³, and the ReinLite dosage is 0.9–1.5 kg/m³.

Which fiber is best for plaster?

For plaster, use micro fiber: ReinGlass glass fiber or 3 mm ReinLite polypropylene fiber, and for gypsum plasters and skim coats, choose glass fiber.

ReinGlass is designed for cement and gypsum plasters and skim coats. The fine fibers are barely visible after troweling, so the surface stays smooth for paint, skim coat or tiles. Gypsum contains no alkalis, so glass fiber works there without limitations. For plasters and dry mixes, it is dosed at 0.1–0.3% of the dry mix weight, which is 25–75 g per 25 kg bag.

ReinLite in the 3 mm length suits plasters, masonry mortars, tile adhesives and dry mixes. Its dosage is 0.6–1 kg/m³ or 0.1–0.2% of the dry mix weight, which is 25–50 g per 25 kg bag.

Add either fiber to the dry mix or together with the sand, mix until it is evenly distributed, and only then add the water. There is no need to change the mix design or the amount of water. Wear gloves and safety glasses when working with glass fiber: the fibers can cause temporary itching of the skin.

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