Almost all concrete cracks a little. The fine cracks you may see on a precast part are shrinkage cracks — they’re cosmetic, not structural. The steel reinforcement inside the part carries the load right across them, so the pad performs exactly as it was engineered to. This post explains why that’s true, how to tell a cosmetic crack from one worth a second look, and why our precast process keeps cracking to a minimum in the first place.
Why Concrete Cracks at All
Concrete is a composite: coarse and fine aggregate locked together by cement paste that hardens through a chemical reaction between cement and water. That reaction is what builds strength, but it also consumes water, and as water leaves the mix, the concrete shrinks slightly.
Here’s the catch. Concrete is enormously strong in compression and comparatively weak in tension, only about a tenth as strong. So when shrinkage tries to pull the material inward and something resists that movement, tension builds. The moment that tension exceeds concrete’s modest tensile strength, a small crack forms to relieve it. That’s not a defect or a failure. It’s concrete doing exactly what concrete does, and engineers design for it from the start.
The shrinkage cracks you're most likely to see
- Plastic shrinkage happens while the concrete is still fresh — in the first few hours, before it sets. If the surface loses moisture faster than “bleed water” can rise to replace it, the top layer contracts while the concrete underneath is still wet and holds it back. The result is shallow, hairline cracks at the surface. (See Figure 1.) These are driven by hot, dry, windy conditions during casting — the kind of job-site exposure a precast plant is built to control.
- Drying shrinkage is the long game. Over weeks and months, hardened concrete slowly gives up moisture to the air and contracts. If it were free to shrink evenly, it simply wouldn’t crack. But concrete is almost always restrained — by friction underneath it, by the reinforcing steel inside it, or by whatever it’s connected to. That restraint turns shrinkage into tension, and where the tension wins, a crack opens. (See Figure 2.)
A third type, autogenous shrinkage, shows up in high-strength, low-water mixes where the water is consumed internally during curing. It produces only very fine micro-cracks — even more benign than the two above.
Cosmetic crack or real concern? How to tell
This is the part worth knowing, because not all cracks mean the same thing. Shrinkage cracks look and behave a specific way, and a genuinely structural crack comes from a different cause entirely (overload, poor bearing or support, impact, or settlement), not from shrinkage.
Shrinkage cracks (cosmetic) | Cracks worth a closer look | |
Width | Hairline to narrow, and stable over time | Wide, and actively growing |
Pattern | Short parallel lines, or random “map” cracking | Follows a load path; one side displaced or offset |
Behavior | Tight, quiet, no movement | Spalling edges, vertical offset, or tied to visible settlement |
Effect on strength | None — reinforcement carries the load across them | Different cause; have it evaluated |
If a crack is a tight hairline that isn’t moving, it’s cosmetic. If one side has dropped relative to the other, the crack is opening up over time, or the edges are breaking away, that’s a different conversation, and it points to something other than shrinkage.
Why surface cracks don't weaken the part
Concrete handles compression; steel handles tension. Every reinforced part is built on that division of labor. Reinforcing steel is placed precisely where tensile forces occur, and its job is to carry those forces across any crack that forms, holding the crack tight, keeping it from widening, and stopping it from propagating. A hairline shrinkage crack removes essentially none of the part’s load-bearing cross-section, and the steel bridges it regardless. The part carries the load it was rated for, and it keeps carrying it.
That’s why, in the vast majority of applications, shrinkage cracks are classified as non-structural. They may not be pretty, but they do not reduce strength, capacity, or service life.
The Precast Advantage
With the traditional pour-in-place concrete, curing happens outdoors, on the job site, at the mercy of the weather; which is exactly where the worst plastic shrinkage comes from.
Our parts are different. Every EZ-CRETE product is poured and cured under controlled conditions in the plant, then shipped fully cured, ready to install and build on. That means the temperature, moisture, and wind exposure that drive surface cracking are engineered out before the part ever leaves the building, and the reinforcement handles whatever minor drying shrinkage occurs over the part’s life.
Bottom line
A fine crack in a precast part is a normal characteristic of concrete — not a sign of weakness and not a defect. It’s cosmetic, it’s controlled by the mix, the curing, and the steel inside the part, and it does not compromise the part’s ability to do its job. Built right, reinforced right, and cured right, your part will perform and last as intended.