What is a pop-out in concrete?
A pop-out is a small, cone-shaped crater in a concrete surface, usually between 6 mm and 50 mm across, often with a fractured aggregate particle at its base. It forms when a porous or reactive particle near the surface expands and breaks the thin layer of mortar above it. Pop-outs are a near-surface event and do not indicate that the concrete is weak through its full depth.
What causes concrete pop-outs?
The common causes are porous or expansive aggregate such as chert, shale, and clay lumps; freeze-thaw expansion of saturated particles near the surface; alkali-silica reaction (ASR) in certain reactive aggregates; foreign or contaminated material in the mix; and over-finishing that draws reactive fines to the top. Every one of these sits with the aggregate, the mix, the finish, or the weather rather than with the formwork.
How do you fix pop-outs in concrete?
Clean the crater of the fractured stone and any loose material down to sound concrete, dampen the cavity, and apply a bonding agent. Pack a cementitious or polymer-modified patching material into the void, fill it flush, feather the edge into the surrounding surface, and cure it like new concrete. On exposed or architectural slabs, match the colour and trial the patch on a sample area first. Where pop-outs are widespread, a thin overlay or a grind-and-seal may be more practical than individual patches.
Are concrete pop-outs a structural problem?
Most pop-outs are cosmetic. The concrete below is sound and the section keeps its strength, so a scatter of small craters is usually a surface repair. Look closer when there is a dense field across the slab, pop-outs paired with map cracking (a sign of alkali-silica reaction), or craters on a thin topping or wear slab. Those cases point to a material issue worth investigating, and persistent or structural concerns belong with the engineer of record.
What is the difference between a pop-out and spalling?
A pop-out points at a single piece of aggregate: one reactive particle expands and lifts a small cone of paste. Spalling is broader flaking and chipping of the surface, often driven by reinforcement corrosion, freeze-thaw, impact, or fire, and it can run deeper and expose aggregate or steel. In short, a pop-out is a localized aggregate event while spalling is a wider surface-layer failure.
