I am crouched at the edge of a slab we poured three days ago, a crack map in my left hand and the batch certificate for this load of Dramix fibers in my right. The surface has gone into a fine web of hairline cracks, and the man beside me is already talking about adding more fiber to the next truck. That is the wrong lever. When a Dramix slab cracks early and sounds brittle under a tap hammer, the first suspect is not the dosage number on the pour record. It is the anchoring, the bents and hooks on the fiber, and how much of that anchoring survived the mixer.
Why Early Cracks Look Like a Dosage Problem
When hairline cracks appear a few days after the pour, the counts on the delivery docket look like the obvious place to start. So what should you actually read first, the crack pattern or the dosage figure? Read the crack. Concrete has an inherently low strain capacity, and reinforcement is what compensates for it and keeps cracking under control. Dramix fibers sit throughout the section rather than only at the steel mat, so they pick up small cracks the moment they open, which is exactly why the pattern tells you something a bag count cannot. Map cracking spread across a whole panel points to restrained shrinkage and early-age movement. One clean crack running across a joint or out of a re-entrant corner points to a flexural or detailing issue at that location, and no amount of extra fiber fixes a missing movement joint. Before anyone touches the dosage number, log the width, the spacing and the pattern. A 0.15 mm web at 200 mm centres and a single 0.4 mm crack are two different investigations, and only one of them is likely to be a fiber problem at all.
Reading Bents and Hooks After Mixing
Suppose the crack map really does look like a fiber underperforming, and the wash-out sample comes back with fibers evenly spread, no balls, no clusters. What then? If dispersion is clean, why can the same fiber still fail to hold a crack closed? Because dispersing a fiber is not the same as anchoring it. The bents and hooks at the ends of each fiber are what grip the cement matrix, and they drive the fiber's anchoring performance and the ductility that follows. A hook that arrived from the plant in perfect shape can leave the mixer flattened, kinked or snapped. A high-shear cycle, an overfilled drum, or fibers dumped in as a clump instead of broadcast into the flow will all do it. I have pulled fibers from a wash-out sample that looked flawless in a photograph, even count and good spread, then bent them straight between my fingers with almost no resistance; the anchoring was gone. So the check is not how many fibers per kilogram. It is the shape and length of the hook, compared against the spec sheet for the grade you specified. Take twenty fibers from a fresh sample, lay them on the drawing, and look at the ends.
The Check Order Before You Blame the Mix
In what order should anchoring, dispersion and dosage be ruled out so the site check does not stall? Work it in a fixed sequence so nobody restarts the argument every morning. First, confirm the fiber type and hook geometry against the batch certificate: the grade on the bag, the length and the hook form all have to match what the design assumed. Second, check dispersion and balling in a wash-out sample taken from the middle of a discharged load, not from the top of the drum where the fibers float. Third, verify dosage per cubic metre against the pour record, using the actual bags divided by the actual volume placed; a mixer that ran short on water and a truck that went out half-full both show up here. Only when those three line up do you go after the mix design, the admixtures and the curing.
The order matters because the fixes cost different amounts of money and time. Re-tempering a load or adding fiber is slow and expensive, while finding a mismatched bag on the pallet is cheap and quick. For orientation, Dramix 4D slab work is commonly specified in the 25 to 40 kilogram per cubic metre band, so a docket reading 45 is not a sign of generosity; it usually means someone upstream misread the specification. Confirm the band against your own project document, not against a number a supplier quoted at the gate.
When a Crack Means Switch the Fiber
At what point does a returning crack pattern stop being a site problem and become a fiber-selection problem? When dispersion and dosage have already been confirmed against the pour record, nothing about the mix has changed, and the next pour cracks in the same places in the same way. That repeat signature points at the hook type or the fiber length not fitting the application. A slab on grade with tight joints and heavy point loads behaves nothing like a sprayed tunnel lining or a precast element, and the geometry that anchors well in one can be the wrong anchorage length in the other. Two things justify picking up the phone to Bekaert instead of reworking the pour on site. One is a load whose fiber grade does not match what the design assumed, a stiffer or shorter fiber substituted because the specified one was short at the plant. The other is a crack that keeps opening past the width the design allowed, after every check on site has cleared. In both cases, more fiber on site will not buy back what the wrong fiber is losing.
What to Request Before Signing Off the Pour
What should an inspector be holding before accepting the pour? Four documents and one matching check at the gate. The batch certificate with the fiber type identifier, the dosage records per load, the wash-out dispersion result for at least one sample per pour day, and the declared anchoring and ductility data for the grade specified. Hold the delivery until the batch number on the certificate matches the number printed on the bag sitting in front of you. I watched a contractor accept a pallet on a verbal assurance that it was the same fiber as last time, then spend a week chasing a crack pattern that turned out to come from a different hook form off a different line. The certificate is not paperwork for the file; it is the only thing tying the fiber in your slab to the performance the design counted on.
Who This Check Is Not For
This sequence is not for the buyer shopping purely on price per kilogram and willing to live with cracks that keep moving after the pour. For that buyer, comparing dosage is the wrong instrument, and no certificate changes the outcome. It is for the inspector or contractor who has to put a signature against crack control and ductility, and who needs evidence that the hooks are still working inside the concrete rather than in a brochure.