Does concrete harden by drying?

According to SlabCalc.co, no. Concrete hardens through a chemical reaction called hydration, not by drying. Cement particles react with water to form calcium silicate hydrate crystals that interlock and give concrete its strength. If concrete dries out too fast during the first few days, the hydration reaction stops early and the concrete is permanently weaker than it should be.

What is the '28-day strength' of concrete?

According to SlabCalc.co, the 28-day strength (expressed in PSI) is the benchmark compressive strength concrete achieves after 28 days of proper curing. Most of the strength gain happens in the first 7 days (about 70%), but the final 30% builds more slowly over weeks 2–4. Engineers use 28 days as the reference point because virtually all the hydration that will occur in a typical pour has completed by then.

Why do you water new concrete?

Watering new concrete — misting it or keeping burlap wet over it — keeps the surface from losing moisture to evaporation before hydration is complete. If the surface dries out in the first 3–7 days, the cement at the surface stops reacting and produces a weak, dusty layer prone to scaling and cracking. Keeping the slab moist is one of the simplest and most effective things you can do to get maximum strength.

Can concrete get too wet during curing?

During curing, keeping concrete wet (not flooded) is beneficial. The water you apply on top doesn't dilute the mix — it's a separate step that supplements the internal moisture. The problematic water is the water added at mixing time. Too much water in the mix (before the pour) permanently weakens the concrete by increasing the water-cement ratio.

Does Concrete Dry or Cure? What Actually Happens When It Hardens

Understanding this changes how you care for fresh concrete and explains why so many well-intentioned shortcuts cause failure.

Hydration — The Real Process

When you mix water with Portland cement, a chemical reaction begins immediately. The cement particles react with water molecules to form calcium silicate hydrate (C-S-H) crystals — a microscopic interlocking lattice that gives concrete its compressive strength.

This reaction is called hydration. It generates heat (which is why freshly poured concrete can feel warm), and it continues as long as water is present and the temperature is above roughly 40°F.

Here's the key point: if the concrete runs out of water before hydration is complete, the reaction stops permanently. The concrete will be weaker than designed, and that weakness is irreversible.

The Timeline of Strength Gain

Time After Pour	Approximate Strength Achieved
1 day	~16% of 28-day strength
3 days	~40%
7 days	~65–70%
14 days	~85%
28 days	~100% (design strength)
90 days	~110–115%

The fast initial gain (from 0% to 70% in 7 days) is why you can walk on concrete after 24–48 hours. The slower tail from 7 to 28 days is why you wait a month before driving on a new driveway. For that practical timeline, see when can you walk on new concrete.

What "Drying" Actually Means for Concrete

Concrete does eventually lose moisture — this is called drying shrinkage. But this is mostly a problem, not a feature. Drying shrinkage is one of the primary causes of cracking in flatwork.

The goal during curing is to keep the concrete moist so hydration can run to completion, then allow the slab to dry gradually and uniformly to minimize shrinkage cracking. See why concrete cracks for the full explanation of this mechanism.

Why This Matters for Your Project

Don't Let It Dry Out Too Fast

In hot, dry, or windy conditions, freshly poured concrete can lose surface moisture within minutes. This is called plastic shrinkage cracking — the surface contracts while the interior is still soft, causing cracks to form before the concrete is even hard.

Prevention:

Pour in the morning when temperatures are lower
Cover the slab with plastic sheeting immediately after finishing
Mist the surface gently during the first 24 hours if conditions are harsh
Use a curing compound (a spray-applied liquid that seals the surface)

Watering Your Slab Is Good

Hosing down a new concrete slab once or twice a day for the first 3–7 days is genuinely beneficial — it's one of the oldest and most effective curing methods. This is called moist curing or wet curing.

What you're doing is replenishing moisture at the surface that has evaporated, allowing hydration to continue at the top layer where it's most vulnerable. See concrete curing methods compared for a comparison of wet curing, plastic sheeting, and curing compounds.

Cold Weather Is a Problem

Below 40°F, hydration slows to a crawl. Below 32°F, it stops. If fresh concrete freezes before it has developed enough strength (typically 4 days of proper curing at adequate temperatures), the expansion of ice crystals can permanently damage the concrete's internal structure.

Always use insulated blankets or heated enclosures for pours in cold weather. See our guide on best time to pour concrete for the full cold-weather protocol.

Concrete Gets Stronger Over Time

Hydration in concrete never truly stops — it just slows. Concrete poured 10 years ago is measurably stronger than it was at 28 days. This is unlike most building materials, which degrade over time. Properly cured concrete becomes more durable, not less.

This is also why "cured" concrete is not the same as "new" concrete. Old concrete, properly cured, is a premium structural material.

The Cement-Not-Glue Misconception

A related myth: "concrete is just glue that holds sand and gravel together." Not quite. The C-S-H crystals that hydration produces don't just coat the aggregate — they grow through it, around it, and into any pores or surface irregularities in the aggregates and forms. It's a structural integration, not just adhesion.

This is why freshly cured concrete bonds well to rough surfaces but poorly to smooth, dusty, or sealed ones — and it's why surface preparation matters so much for patches and overlays.

Summary

Concrete cures (chemical reaction) — it does not dry (evaporation)
Hydration requires water and adequate temperature to proceed
Most strength develops in 7 days; full strength at 28 days
Keeping new concrete moist during the first week improves final strength
Drying too fast or freezing can permanently weaken the slab

How Long Does Concrete Take to Cure? — Full curing timeline with temperature adjustments
Concrete Curing Methods Compared — Wet burlap vs. plastic sheeting vs. curing compounds
Why Does Concrete Crack? — Drying shrinkage and other cracking causes