MasterFiber M 687 is classified as a next-generation fibrillated polypropylene fiber for hydraulic cementitious matrices. The product is supplied as a collated fibrillated bundle that opens into a three-dimensional network when exposed to normal concrete mixing shear. Under ASTM C1116/C1116M, the material falls within the Type III synthetic fiber category for fiber-reinforced concrete. Primary engineering intent is control of plastic shrinkage cracking and distribution of localized microcracking in slabs, pavements, precast units, wet-mix shotcrete, overlays, and repair mortars. It is not specified as a one-to-one substitute for primary structural reinforcement unless the structural design explicitly accounts for the fiber contribution. Published data for this specific configuration are limited outside the supplier batch certificate; where product-specific values are not published, class-typical fibrillated polypropylene data are used with explicit identification.

Typical commercial documentation for this fiber class specifies a cut length of 19 mm, a density of 0.91 g/cm³, and water absorption below 0.1% under ASTM D570. The polypropylene base polymer exhibits a melt temperature of 160–170 °C under ISO 11357-3:2018. Tensile strength for oriented fibrillated polypropylene is typically 400–700 MPa under ISO 527-2:2012, with elongation at break 15–25%. These values are quality-control benchmarks for the fibrillated PP class and should be confirmed against the MasterFiber M 687 batch certificate.

What Limits the Dispersed Fiber Count at High Addition Rates?

Addition rate, rather than fiber count alone, controls the transition from plastic shrinkage crack suppression to measurable residual strength contribution. The working dosage window for a 19 mm fibrillated polypropylene fiber is commonly 0.9–1.8 kg/m³. Below 0.9 kg/m³, fiber distribution across a slab placement can become discontinuous, particularly when discharge time exceeds 45 min and fibers segregate near the top surface. Above 1.8 kg/m³, the limiting factor is not fiber saturation but water demand and entrapped air. Slump measured under ASTM C143/C143M may decrease by 20–40 mm at the upper boundary, and the air void system can shift by 0.5–1.0% unless the mix is re-engineered.

The dispersed fiber count is governed by bundle opening efficiency rather than by fiber mass loading. In a 9 m³ twin-shaft compulsory mixer, sufficient shear is generated at 14–18 rpm during 60–90 s of post-fiber mixing. A free-fall drum mixer at the same dosage may require 90–120 s and can still exhibit balling when the drum is loaded above 75% of nominal capacity. The practical upper addition rate in any production unit is established by a washout test or visual inspection of the discharged slump sample, not by the fiber datasheet alone.

When high addition rates are specified for crack control, the water-to-cement ratio should not be raised to compensate for slump loss. Instead, polycarboxylate ether superplasticizer is adjusted in increments not exceeding 0.1% by cement mass until the target slump is restored. A site trial under ASTM C1579-21 is required to verify plastic shrinkage crack reduction. MasterFiber M 687 cannot compensate for a deficient mix design or for inadequate finishing and curing.

Fibrillated polypropylene differs from monofilament material because the bundle opens into a reticulated network under shear. This network increases mechanical anchorage within the cement paste and arrests plastic shrinkage cracks that propagate through the first 24 h after placement. The fibrillation process creates a higher effective surface area than smooth monofilaments of equivalent denier, which explains the slightly higher water demand at equal dosage. In field trials, the network is observed as a three-dimensional fibrous web in the plastic concrete; if the web is absent and individual filaments are discharging separately, the mixer shear is insufficient to open the bundles fully.

Low-paste mixes with cementitious material below 300 kg/m³ create a specific processing conflict. The fibrillated network requires paste to coat the fibrils; without sufficient paste, the network remains partially exposed and can produce surface fuzzing under power trowel finishing. This is not a fiber defect but a paste-volume limitation. The addition of supplementary cementitious materials does not automatically correct this because fly ash and slag increase paste volume only when total cementitious content is raised. Silica fume at 5–8% by mass can improve coating but also increases water demand. Site trials under the specified finishing equipment are required.

Values in the table reflect class-typical published data. MasterFiber M 687 batch certificates control product-specific values, and the comparison is not a substitute for project-specific qualification.

When Fibrillated Polypropylene Replaces Welded Wire Mesh in Slab-on-Grade Placement

In interior slabs on ground, the comparison between fibrillated polypropylene and welded wire mesh is specific to crack-control function, not structural load transfer. Welded wire reinforcement under ASTM A1064/A1064M provides defined yield strength and continuous bond; fibrillated polypropylene at 0.9–1.8 kg/m³ is non-structural in most design codes. Under ACI 360R-10, fibers may control initial shrinkage cracking but do not replace wire mesh for joint stability, load transfer at control joints, or structural flexural reinforcement. In a slab with truck traffic or heavy point loads, the fiber is not a substitute for steel reinforcing bars or dowels.

Comparative batching under ASTM C1399/C1399M and ASTM C1609/C1609M can be used to measure residual flexural strength. For micro fibrillated PP at dosages below 1.8 kg/m³, the residual strength values are typically low, and published data for this specific configuration are limited. The fiber is therefore specified for plastic shrinkage crack reduction and not for post-crack flexural toughness. In applications such as residential slabs, parking lots, industrial floors, and shotcrete tunnel linings, the product is used to reduce early cracking; joint spacing, dowel placement, and slab thickness remain governed by structural design.

For wet-mix shotcrete, fibrillated PP can reduce plastic shrinkage cracking and may reduce rebound, but pumpability trials are required because the opened fiber network can accumulate in sharp bends and reducers. Nozzle wear and air-loss characteristics should be evaluated with the specific pump type, usually a piston or rotor-stator gun operating at 50–80 bar outlet pressure. Published data for MasterFiber M 687 in this specific configuration are limited outside site-specific shotcrete qualifications. In pervious concrete with high void content, the use of fibrillated PP is not recommended because the open-graded aggregate matrix does not generate the paste shear required to open the bundle and the fiber can be lost through the void network.

Relative to monofilament polypropylene, MasterFiber M 687 relies on fibrillated strand separation rather than discrete filaments. The opened network provides greater mechanical anchorage within the cement paste because the fibrils create a larger number of crack arrest points per unit mass. Relative to macro synthetic fibers, which are typically 38–60 mm long and dosed above 4 kg/m³, the product does not provide equivalent post-crack flexural toughness. Relative to steel fibers, the polymer fiber eliminates corrosion staining and magnetic disturbance, but its modulus is approximately 40–60 times lower, so it cannot compensate for primary tensile reinforcement. Under EN 14889-2:2006, the product is classified as a polymer fiber; microfiber and macrofiber categories are distinguished by diameter and performance class, not by raw material alone.

Batch Sequencing and Shear Input Requirements for Ready-Mix Operations

The addition sequence is critical for fibrillated fiber dispersion. In a central-mix batch plant, MasterFiber M 687 is charged after the coarse aggregate and 50–70% of the mixing water, but before the remaining water and chemical admixtures. This order allows the bundle to wet and open under aggregate shear without floating on the surface of the initial water. In a 9 m³ twin-shaft compulsory mixer, 60–90 s of additional mixing at full speed after fiber addition is sufficient; in a single-shaft mixer, 90–120 s may be required. The fiber should not be added as the first dry material onto the mixer blades because static charge and clumping can occur.

For dry-batch truck mixing, the fiber may be pre-blended with aggregate before cement addition. The discharge time should be kept below 45 min when the dosage exceeds 1.2 kg/m³, because prolonged agitation can promote fiber flotation in high-slump mixes. Automatic fiber dispensers should be calibrated within ±5% of target mass, and manual bag additions should be recorded for each batch. The most common production failure is fiber balling caused by worn mixer blades or hardened concrete accumulation; clumps are visible in the slump sample and require mechanical mixer cleaning, not just extended mixing time.

Fiber addition does not require pre-drying at relative humidity above 60%, because the polypropylene surface is hydrophobic and water absorption is below 0.1% under ASTM D570. However, storage should avoid temperatures above 50 °C and direct ultraviolet exposure. Bags that have been punctured or exposed to rain should be quarantined because surface contamination can alter fiber opening and create clumps.

Quality documentation should include a batch certificate referencing EN 14889-2:2006, CE marking under the Construction Products Regulation (EU) No 305/2011, and REACH registration under EC 1907/2006. For North American projects, the supplier declaration usually confirms classification under ASTM C1116/C1116M and provides density, cut length, and tensile test data. The following matrix summarizes the test methods used for release of fibrillated polypropylene fiber of this class.

Where project specifications require a different standard or additional testing, the batch certificate is not sufficient; a concrete trial using the actual mix design and placement equipment is mandatory.

Alkali Resistance Does Not Imply UV Stability or High-Temperature Compatibility

Polypropylene is resistant to the alkaline environment of portland cement, with no known degradation at the pH range 12.5–13.5 typical of concrete pore solution. This performance is independent of the cement type and is not improved by coating the fiber. However, alkali resistance does not translate to ultraviolet stability or high-temperature compatibility. Unprotected storage in direct sunlight or surface exposure of unmolded fiber bundles should be avoided because polypropylene undergoes photo-oxidative embrittlement. Bulk storage should be in closed, shaded conditions below 50 °C.

The melt temperature of polypropylene is 160–170 °C under ISO 11357-3:2018. Sustained exposure above 150 °C can soften or fuse individual fibrils, which destroys the opened network and may create small polymer agglomerations in the concrete. Autoclave curing cycles in the 180–190 °C range are therefore outside the operational boundary for this product. Precast heat curing at 60–80 °C is within established practice, but sustained exposure at 90 °C has limited published data for this specific configuration.

Chemical compatibility in concrete is broad; the fiber is inert to cement, silica fume, fly ash, slag, and common superplasticizers. It should not be exposed to strong oxidizing agents or organic solvents during storage. The product is insoluble in water and is not biodegradable; disposal should follow local plastics waste regulations. These boundaries are more restrictive than those for monofilament PP only in that the fibrillated network must remain mechanically opened to function, so thermal softening that only reduces filament stiffness in a monofilament can entirely eliminate the three-dimensional anchorage of a fibrillated product.