What heat does to concrete
Concrete does not dry: it hydrates. The mixing water is the reagent that lets cement form the crystals that give strength, and that process takes time. When ambient temperature exceeds 35 °C, when dry wind blows or when relative humidity is low — the three normal conditions of a Mexicali summer — the water leaves by evaporation before it can react.
The result has two faces. The visible one is plastic shrinkage cracking: fine fissures that appear in the first hours while the concrete is still soft, and that grow later. The invisible one is loss of final strength, because part of the cement never got to hydrate. That second part is not noticed on pour day; it is noticed years later.
Rule 1: planning — pour in the cool hours
Schedule pours for the coolest hours of the day: early in the morning or in the afternoon. This is not cosmetic advice. The difference between pouring at 7 am and pouring at one in the afternoon can be fifteen degrees in fresh concrete temperature, and every degree shortens the time you have to place and finish.
Scheduling includes the crew. If the concrete arrives at seven but the team starts at nine, the benefit is gone. The work front has to be ready — formwork checked, rebar placed, tools laid out — before the first truck enters the site.
Rule 2: shade and protection
Shade the materials and shield the area from wind to prevent rapid evaporation. Timber formwork in the sun reaches temperatures that steal water from the concrete by contact; dampening it before the pour costs minutes and prevents that theft. The same goes for the rebar: a bar that has been in the sun all morning heats the concrete from within.
Wind is the most underestimated factor. Dry air at 20 km/h over a freshly poured slab evaporates more water than direct sun with no wind. Screens, shade cloth or simply pouring on the sheltered side of the site change the outcome.
Rule 3: fast, efficient placement
Transport, place and finish the concrete in the shortest possible time. The clock starts when the mix leaves the plant, not when it reaches the site. That is why in hot weather pumping stops being a luxury: it keeps a continuous flow, avoids barrowing under the sun and lets several trucks discharge back to back onto the same front.
And here is the mistake that costs the most strength: adding water on site to make the mix run. It raises the slump, yes, but it also raises the water-cement ratio and with it the final strength collapses. If the front is difficult, the answer is pumping or an appropriate mix design, never the hose.
What comes next: immediate curing
The three rules cover the pour. What remains starts the moment finishing ends: curing. In hot weather curing must be immediate, not next day. Curing membrane, continuous watering or plastic sheeting, depending on the element and what the site allows.
Fibre-reinforced concrete helps in this scenario: the fibres are distributed uniformly through the mass and control plastic and drying shrinkage microcracking. It does not replace curing or protection, but it reduces the damage when conditions get hard. On industrial floors and sidewalks poured above 40 °C, the difference shows.
