SikaFiber P/P 18 mm is a polypropylene fiber for secondary reinforcement of concrete and mortars. Manufacturer-published typical properties include nominal length 18 mm, density 0.91 g/cm³, tensile strength in the range 300 MPa to 400 MPa, and modulus of elasticity between 3.5 GPa and 4.0 GPa. The raw material is 100% virgin polypropylene, which is chemically inert in the alkaline pore solution of normal portland cement concrete where pH values typically range from 12.5 to 13.5. The fiber can be specified under ASTM C1116/C1116M as a Type III synthetic fiber and may be CE marked under EN 14889-2:2006 for polymer fibres for concrete.

The 18 mm length places the product between short polypropylene microfibers commonly used for plastic shrinkage and fire-spalling control and longer synthetic macrofibers or steel fibers used for post-crack flexural toughness. The product is positioned for slab-on-grade construction, precast concrete elements, repair mortars, and non-structural shotcrete where early-age crack control, corrosion resistance, and low fiber weight are selection factors. Published performance data for this exact product configuration in fire spalling or structural residual strength applications is limited; qualification testing is required for project-specific acceptance.

What mechanisms explain the 18 mm length effect in synthetic fiber reinforcement?

In freshly placed concrete, the critical period for plastic shrinkage cracking begins when surface bleeding stops and capillary tension in the pore water exceeds the still-low tensile strength of the cement paste. A polypropylene fiber of 18 mm length intersects a larger representative area of the developing microcrack network than a 6 mm or 12 mm product at equivalent fiber count because the longer embedment length increases the force required to pull a fiber out of the paste. This crack-bridging mechanism is evaluated under ASTM C1579, which subjects panel specimens to an evaporation rate sufficient to induce plastic shrinkage cracks. In such tests, the benefit is influenced by fiber dosage, concrete w/cm ratio, aggregate surface area, and curing start time; published comparative studies on fibrillated polypropylene fibers show reductions in crack area relative to unreinforced control panels, but the result is not linear with dosage and should be verified for each mix design.

Because the fiber modulus is approximately 3.5 GPa to 4.0 GPa, it does not significantly stiffen hardened concrete at low strain. Its engineering function is not to replace structural reinforcement but to bridge microcracks in the millimetre-width range during the first 24 hours after placement and, to a limited degree, to distribute shrinkage strain after hardening. The fiber is hydrophobic, so it does not absorb mix water; unlike cellulose or alkali-sensitive glass fiber, it does not lose cross-section in the alkaline cement pore environment. These properties support its use as a low-density secondary reinforcement, provided the dosage is controlled and dispersion is verified.

Production-scale addition trials in twin-shaft compulsory mixers of 1.0 m³ to 3.0 m³ capacity indicate that fiber introduction through the aggregate weigh hopper is more reproducible than direct addition to the paste. The fiber should be fed during aggregate charging, not after water and cement have formed a paste, because aggregate shear separates the collated bundles before the fiber surface becomes coated with cement slurry. In a pan mixer operating at 45–60 rpm, visual dispersion is commonly achieved after 60–90 s of wet mixing; in a truck mixer at 12–18 rpm, 10–15 minutes after all ingredients are charged may be required. These values are equipment-specific and are not a substitute for a wash-out test. A fresh concrete sample washed over a 4.75 mm sieve provides a direct check for undispersed fiber agglomerates; any visible clumps retained after washing indicate that the addition point or mixing time must be corrected.

Rheological evaluation under ASTM C143/C143M often shows a slump decrease of 0 mm to 20 mm at 0.9 kg/m³ addition, depending on aggregate angularity and initial slump. If flow is restored with water, the w/cm ratio increases and all durability assumptions change. The correct adjustment is with a high-range water reducer meeting ASTM C494/C494M or EN 934-2, and the adjusted mix must be rechecked for air content, setting time, and plastic shrinkage. Because polypropylene has a density of 0.91 g/cm³, fiber addition can marginally alter air-void stability; air content should be measured under ASTM C231/C231M or EN 12350-7, not assumed from previous non-fiber production data.

Differential response of polypropylene and steel fiber reinforcement at 18 mm length

Steel fibers meeting ASTM A820/A820M or EN 14889-1 provide post-crack flexural toughness because their modulus of elasticity is approximately 200 GPa and their tensile strength is typically in the range 1000–2000 MPa. SikaFiber P/P 18 mm has a modulus between 3.5 GPa and 4.0 GPa and tensile strength between 300 MPa and 400 MPa. At the low dosage used for plastic shrinkage control, the product does not produce significant residual flexural strength under ASTM C1609/C1609M or EN 14651. It should not be treated as a direct replacement for steel fiber where a structural designer has specified a minimum post-crack residual strength. Its differences are operational: density is approximately one-eighth that of steel, the fiber is non-corrosive in cracked concrete, and it does not create a conductive path in floors used for electrical-sensitive equipment.

Compared with shorter polypropylene microfibers, the 18 mm length gives a larger embedded length and therefore more effective bridging of plastic settlement cracks in deeper sections, but it also increases the potential for surface fuzz and for fiber balling in low-slump mixtures below 40 mm. Compared with longer synthetic macrofibers of 40–60 mm length, the product has lower post-crack capacity and is not typically used at the 3–6 kg/m³ dosages that may be required for residual strength. Published product documentation for SikaFiber P/P 18 mm focuses on early-age crack control at relatively low addition rates, while project-specific qualification is required for flexural toughness applications.

In fire exposure, polypropylene melts at 160–170 °C and may form pressure-relief channels in the cement matrix; this mechanism is used by shorter PP fibers in high-strength concrete to reduce explosive spalling. The 18 mm product can provide similar channels if distributed uniformly, but the larger fiber diameter and length may require a higher dosage to achieve an equivalent channel count. Published data for this exact product under ISO 834-1 or EN 1992-1-2 fire tests is limited, so fire spalling performance must be validated by panel testing rather than inferred from polypropylene chemistry alone. Chemical resistance of the fiber in concrete is based on the inert nature of polypropylene in alkaline media; continuous exposure of the hardened composite to temperatures above 80–100 °C may alter long-term creep properties of the synthetic fiber.

When slab depth falls below 120 mm and saw-cut timing is shortened

In ground-supported slabs of 100 mm to 150 mm thickness, the fiber length-to-depth ratio becomes a finishing consideration. For a 100 mm slab, an 18 mm fiber occupies nearly one-fifth of the total thickness, and surface protrusion or fuzzing may occur if the dosage exceeds 1.8 kg/m³ or if the finishing window is missed. Trial panels placed under the same ambient conditions and tooling sequence as the production floor are required to evaluate finishability. The presence of fiber does not allow saw-cut spacing to be extended beyond the value determined by the slab design method in ACI 360R-10 or TR 34. Saw cutting should proceed when the concrete has gained sufficient strength to resist raveling, typically 6–12 hours after final setting under normal curing conditions, but the exact window depends on concrete temperature, ambient evaporation rate, and mix design.

In this thickness range, the product is used primarily for plastic settlement cracking over reinforcement or over slab penetrations. A dosage of 0.9 kg/m³ is introduced at the plant and documented on the delivery ticket under ASTM C94/C94M. If on-site addition is unavoidable, the drum must be charged at mixing speed for an interval established by a trial batch, and visual inspection must confirm that no fiber balls are present at discharge. Because the 18 mm fiber is hydrophobic, it does not increase water demand, but it can reduce pumpability through lines smaller than 100 mm or through restrictive reducers. Pump trials should include the planned line length, vertical rise, and concrete pressure levels; published data for this specific product in high-rise pumping is limited.

SikaFiber P/P 18 mm is not intended as a replacement for structural steel reinforcement, for structural macro-synthetic fiber in load-bearing ground-supported floors, or for steel fiber in suspended slabs where design flexural residual strength is mandatory. It should not be exposed to strong oxidizing acids or aromatic hydrocarbon solvents before or during storage; in hardened concrete, the fiber is encapsulated and not in contact with external fluids unless the concrete is cracked and penetrated. The product should be stored in the original packaging in a dry covered area. Fiber that has been wet or contaminated with oil or diesel on site should be segregated, because hydrocarbon contamination can alter the fiber surface and impair dispersion in the fresh mix. For project compliance, the fiber supplier’s delivery documentation should be checked against the specified dosage and the standard designation EN 14889-2:2006 or ASTM C1116/C1116M as required by the specification. No design value for residual flexural strength is assigned here; it must be obtained from project-specific beam tests under EN 14651 or ASTM C1609/C1609M.