Steel Fiber in Precast Concrete: Demoulding, Lifting and Thin Walls

Article

June 15, 2015

A precast plant does not want the highest strength, it wants to strip the mould today. Three constraints, demoulding time, lifting points and wall thickness, decide the fiber.

What a Precast Plant Wants Is to Strip Today

Cast-in-place sites care about final strength; precast plants care about turnover. One strip per day versus one every two days doubles capacity and changes how plant depreciation and labour are absorbed. The first purpose of steel fiber in precast is therefore usually early crack resistance rather than ultimate capacity, so an element can be lifted, turned and stripped while the concrete is still young without spalling at the corners. That also explains why precast plants are more sensitive to dispersion than to tensile strength: one fiber ball can take the corner off an element during stripping.

Early Strength and Stripping Time

Stripping time follows early strength, usually 50% to 70% of design. Steel fiber does not raise early compressive strength, which is cement and curing, but it raises early tensile strength, which is the tolerance for not cracking or chipping at stripping. In practice, an element with 30 to 40 kg/m3 of fiber can be stripped four to eight hours earlier under the same curing, or can be stripped on the same day with a much lower corner damage rate. For thin walls the difference is larger still.

Lifting Points: Fiber Does Not Replace Lifting Loops

A dangerous misconception is that steel fiber removes the need for lifting loops. Lifting applies a concentrated force, and the concrete around the point is in local bearing and punching shear. Fiber improves punching toughness but cannot replace cast-in loops or lifting holes. Keep the lifting points, and locally densify fiber or add mesh around them. One detail does change: the lifting stress field in fiber concrete is more uniform, so points can sit slightly further out than in plain concrete, but only after a check, never by eye.

Thin Walls: Only 0.12-0.3 mm Micro Wire Makes Sense

Below about 60 mm wall thickness, ordinary steel fiber simply does not fit. A 50 mm fiber in a 40 mm wall spans the entire section, forms no three-dimensional network, and gets pushed to the surface during vibration. This is where stainless micro wire applies: 0.12 to 0.3 mm diameter, 6 to 20 mm length, 2000 to 2850 MPa tensile strength. Micro wire forms a dense network inside a thin section, raising crack resistance without hurting forming accuracy. Decorative elements, sunshades and thin UHPC panels all belong here.

Glued Bundles: Where the 40% Dispersion Gain Comes From

Loose fiber enters the mixer in clumps and depends on the mixer to separate them; what does not separate becomes a ball. Glued fiber is bonded into a row along its length with a water-soluble adhesive. The row is fed as a unit, the glue dissolves within 30 to 60 seconds, and each fiber separates individually. Measured uniformity is over 40% better than loose fiber and the balling rate drops sharply. It costs a little more per tonne, but on thin walls, high dosage and fair-faced concrete that premium is far below the cost of one rejected panel.

UHPC and RPC: Why Two Fiber Types Are Often Blended

Ultra-high performance matrices are extremely dense, with aggregate of only a few millimetres and very low porosity. In that matrix steel fiber provides post-crack bridging, while plastic shrinkage cracking happens earlier and needs a finer fiber. UHPC therefore commonly blends steel and synthetic fiber: steel at 1% to 3% by volume for load carrying, synthetic at 1% to 3% for early shrinkage control and to reduce explosive spalling at high temperature. Synthetic fiber has a density of only 1.41 g/cm3, adds no weight to the element, and at 99% alkali resistance stays stable in a strongly alkaline cementitious matrix.

Moulds, Release Agents and Steam Curing: Three Hidden Losses in Precast

Mould life on steel fiber precast runs thirty to fifty percent shorter than on plain concrete, for a simple reason: fibers scrape the mould wall during vibration and demoulding, and hooked wire acts like tiny barbs. Three measures contain the wear. First, do not economise on draft, at least 1 in 20 for deep sections and 1 in 15 for thin walls. Second, choose a film-forming release agent rather than an oil, because a film keeps a continuous barrier between wall and paste while oil is scraped away exactly where a fiber presses against it. Third, switch to high frequency low amplitude vibration and keep the poker at least 50 mm from the mould face so fibers do not concentrate there. Glued fiber bundles add a specific benefit here: dosing in bundles prevents tangling at the placing stage, improves distribution uniformity by roughly 40 percent, and removes the local overload on the mould. Steam curing needs temperature discipline, no faster than 20 degrees Celsius per hour on the ramp, a hold below 60 degrees Celsius, and an equally slow decline. Overshoot and thin sections develop fine surface cracks located precisely at fiber ends, which costs one class of impermeability. Demoulding can be brought forward, but lifting cannot: fiber improves flexural behaviour and toughness, not early compressive strength, so lifting loops and lifting points all stay. For a precast concrete steel fiber supplier, the useful question is not the price per tonne but the demoulding time they will guarantee in writing for your section thickness, because that number sets the mould count and therefore the whole production plan.

The Precast Plant's Purchasing List

Precast purchasing is more detailed than site purchasing: wall thickness and minimum clear spacing, stripping time requirement, whether the face is fair-faced, whether steam curing is used, and whether EN 14889-1 is needed for export. Steam curing is the item most often missed: above 60 C the stability of a copper coating is affected, and stainless fiber should take priority. To match grades and recommended dosage by element type, the precast section of the steel fiber product page lists the relevant specifications, including diameter and length ranges for glued grades and stainless micro wire.