Fresh concrete is not strong simply because it dries. It gains strength because water reacts chemically with cement in a process called hydration. Curing is the job of keeping the concrete moist and at a suitable temperature so these reactions can continue.
Good curing matters because it affects the strength, durability, and crack resistance of roads, floors, bridges, and building foundations.
During hydration, cement particles form tiny interlocking crystals that bind sand and gravel into a hard solid. If the slab loses water too quickly, the crystal network stops developing near the surface and the concrete becomes weaker. Construction crews use curing blankets, water sprayers, plastic sheets, curing compounds, and temperature control equipment to slow moisture loss.
Strength usually rises quickly during the first week, then continues increasing more slowly for weeks or months.
Key Facts
- Hydration is a chemical reaction between cement and water that forms strength giving crystals.
- Curing keeps concrete moist and warm enough for hydration to continue.
- Concrete does not harden well by drying alone because water is needed for the reaction.
- Compressive strength is measured as stress: stress = force / area.
- A common design reference is 28 day strength, when concrete has gained most of its specified strength.
- Too much evaporation can cause plastic shrinkage cracks and a weak surface layer.
Vocabulary
- Hydration
- Hydration is the chemical reaction between cement and water that produces solid crystals and binds concrete together.
- Curing
- Curing is the controlled process of keeping concrete moist and at the right temperature while it gains strength.
- Compressive strength
- Compressive strength is the maximum squeezing stress a concrete sample can withstand before failing.
- Cement paste
- Cement paste is the mixture of cement powder and water that coats aggregates and later hardens through hydration.
- Plastic shrinkage
- Plastic shrinkage is early cracking that can happen when fresh concrete loses surface water too quickly before it hardens.
Common Mistakes to Avoid
- Saying concrete gets strong because it dries. This is wrong because concrete strength comes mainly from hydration, which needs water to continue.
- Removing curing covers too early. This is wrong because fast evaporation can stop surface hydration and cause cracks or a dusty weak layer.
- Adding extra water to make concrete easier to place. This is wrong because a higher water to cement ratio usually lowers final strength and increases porosity.
- Assuming concrete reaches full strength overnight. This is wrong because concrete gains strength over days and weeks, with 28 days often used as a standard reference point.
Practice Questions
- 1 A concrete test cylinder has a loaded area of 0.018 m2 and fails under a force of 540000 N. What is its compressive strength in pascals and megapascals?
- 2 A slab is designed for a 28 day strength of 32 MPa. If it has reached 70 percent of that strength after 7 days, what is its 7 day strength?
- 3 Two identical slabs are poured on a hot day. One is covered with wet burlap and plastic, while the other is left uncovered in the sun. Explain which slab is likely to become stronger near the surface and why.