Buyers ask about diameter, tensile strength, and price per kilogram. Almost nobody asks about fibre sizing, because it is invisible on a finished bar and it is not a decision the pultrusion line makes at all — it is applied at the glass manufacturer, before the roving ever reaches India. That makes it easy to overlook and hard to inspect, which is exactly why it deserves more attention than it gets, since it is one of the two or three things that actually decide whether a bar survives inside concrete.
What sizing actually is
Freshly drawn glass fibre is coated, within seconds of formation, with a thin chemical layer called sizing — typically a silane coupling agent, a polymer film former, and a lubricant, applied at a thickness measured in tens of nanometres. The lubricant lets the fibre survive winding and weaving without abrading itself into dust. The film former holds the individual filaments together as a manageable strand. The coupling agent does the part that matters most structurally: one end of the silane molecule bonds to the glass surface, the other end bonds to the resin, and that chemical bridge is what turns a bundle of glass filaments sitting inside cured resin into a composite that actually transfers load between the two.
Without that bridge, fibre and resin are just two materials in physical contact. Under load, or under years of moisture cycling, physical contact separates. A chemical bond does not — or does not as easily, and how well it resists separating is most of what a rebar's long-term durability comes down to.
Why concrete is a harder fight than most composites face
Water sitting inside cured concrete is not neutral. Pore water in hydrating cement typically sits above pH 12, held there by the calcium, sodium and potassium hydroxides that hydration produces, and it stays that alkaline for the life of the structure. Bare glass is vulnerable to that environment — hydroxide ions attack the silica network in the glass itself, not just the surface, which is why unprotected E-glass loses a large share of its tensile strength after sustained exposure to alkaline solution in lab testing. The sizing layer, and the resin it lets adhere properly, is what stands between the glass and that pore water. IS 18256 tests exactly this: bars go into a simulated concrete pore solution at elevated temperature for a sustained period and have to come out having retained most of their original tensile strength. A bar with poor fibre-resin adhesion fails that test even when the glass and resin are individually fine, because the failure path runs straight through the interface between them.
Why the glass grade alone does not settle it
E-glass is the standard, economical fibre used in most pultruded GFRP rebar, and alkali-resistant glass formulations exist as an alternative with better inherent resistance to hydroxide attack. It is tempting to treat that as the whole decision — pick the more resistant glass and the durability question is answered. It is not, because the sizing determines how much of the glass surface is actually protected by resin in the first place, and a poorly wet-out bar leaves microscopic paths where pore water reaches bare glass regardless of which glass grade you started with. A well-sized E-glass roving, fully impregnated, out-performs a poorly wet-out alkali-resistant one in practice, which is why manufacturers cannot treat glass selection and impregnation quality as separate line items — they only work together.
What this means for impregnation on the line
This is also why impregnation quality is not a cosmetic processing step but the point where sizing chemistry either gets to do its job or does not. A coupling agent bonded to fibre that never gets properly wet by resin is a bond with nothing on the other side of it. Dry fibre inside a bar — the defect a multi-stage impregnation bath exists to prevent — is not just a void that weakens the bar mechanically. It is a length of sized glass with no resin bonded to it, sitting inside alkaline concrete with its protective chemistry doing nothing.
What to actually ask a supplier
Sizing is not something a buyer can inspect visually or verify at the plant gate, which means it comes down to paperwork and testing rather than a factory walk-through. Ask for the glass roving's mill certificate and sizing type from the fibre supplier, not just from the rebar trader. Ask to see the alkali-resistance test report for bar made from that specific roving batch, not a generic datasheet. And treat a supplier who cannot answer which sizing chemistry their fibre uses, or why, as a supplier who has not looked into the one property that decides whether their bar is still doing its job twenty years into a structure's life.