Blog
Blog
Walking through any construction site, you may see workers spraying water on newly poured concrete slabs or covering them with wet burlap bags. Some people may wonder why concrete should be kept moist in a timely manner when it should harden. The answer lies in the fundamental chemical properties of concrete itself. If concrete is not properly wetted and kept moist during its early life, it will never be able to unleash its full potential, and the consequences can be serious: weak structure, unsightly cracks, premature deterioration, and expensive maintenance costs.
This article explains why concrete must be wetted and kept moist promptly, why does concrete need curing? The science behind the curing process, the best methods for proper handling, and what happens when proper curing is ignored.
Concrete curing refers to the technical measures taken to manually control temperature and humidity after pouring concrete to ensure that the cement hydration reaction proceeds to completion, allowing the concrete to harden normally and reach the design strength.
Curing is different from simply “drying” and hardening concrete. In fact, concrete does not harden due to drying, but rather due to a chemical reaction called hydration. Appropriate maintenance can ensure that the reaction proceeds as expected, transforming the wet mixture of cement, water, and aggregate into a dense, sturdy, and durable material.
Concrete gains strength through hydration, a series of exothermic chemical reactions in which water combines with cement particles to form calcium silicate hydrate (C-S-H) and other compounds, binding the aggregates into solids. This reaction begins immediately after adding water to cement and lasts for months or even years.
However, in order to carry out hydration, there must be free water in the concrete matrix. If water evaporates too quickly – due to heat, wind, or low humidity – the reaction will stop prematurely. Unreacted cement particles remain unhydrated, and concrete can never reach its design strength or durability.
A common misconception is that the water added during the mixing process is sufficient for complete hydration. In fact, during the placement and adjustment process, most of the mixed water will be lost due to evaporation, especially in hot or windy conditions. In addition, as hydration progresses, concrete will absorb water from its internal pores. In low-water-cement-ratio concrete (less than about 0.40), the mixed water may be completely consumed during hydration, so additional water must be supplied from external sources if hydration is to continue.
That’s why simply pouring concrete and walking away is not enough. If water is not replenished, concrete will dry from the surface inward, leaving a thin, weak, and porous surface layer that is prone to cracking, dust, and scaling.
In terms of concrete curing, time is everything. The ideal time to start the curing process is after the concrete pouring is completed and before the surface begins to dry. Once the concrete is poured, the hydration process begins immediately, and the possibility of water loss increases accordingly.
Even just a few hours can cause the surface to dry out, leading to shrinkage cracks and weakening of the surface layer. Under ideal conditions, wet coverings should be placed within 10-15 minutes after completion, or within 6-10 hours after pouring, when the surface is sufficiently hard to avoid damage. Concrete must be kept continuously moist, not intermittently, as allowing it to dry between soaking can disrupt hydration and promote cracking.
Laboratory tests have shown that dry concrete without proper curing may lose up to 50% of its potential strength compared to wet-cured concrete of similar strength. The strength of wet-cured concrete is 50% higher than that of concrete without wet curing.
Well-maintained concrete has greater surface hardness and better resistance to surface wear and abrasion. Concrete curing can also make concrete more waterproof, preventing moisture and water-based chemicals from entering the concrete, thereby improving durability and service life. Properly maintained concrete is less prone to cracking, shrinkage, and other long-term durability issues.
Rapid drying can cause uneven shrinkage and lead to surface cracks. Curing ensures uniform surface temperature and humidity, thereby reducing this risk. If handled properly, curing can reduce cracking, improve durability, and increase strength. On the contrary, improper maintenance of concrete can lead to insufficient strength increase, surface dust, surface cracking, plastic shrinkage cracks, excessive curling of concrete slabs, reduced freeze-thaw resistance, and scaling.
Covering concrete with wet burlap bags, cotton pads, or burlap is an efficient and low-tech method. These materials are kept continuously moist through regular spraying; when kept moist, they will provide excellent results. Care must be taken not to let the covering dry out, and not to absorb water from the concrete instead of supplying water.
Waterproof construction paper or polyethylene film can prevent moisture from evaporating from concrete. Plastic film is impermeable, lightweight, and can be reused under good conditions. This method is particularly useful for small projects or situations where continuous water curing is not feasible.
Liquid film-forming compounds can be sprayed onto concrete to form a protective film that seals moisture out. These methods are applicable to large areas such as highways and bridges, although they are usually not as effective as water maintenance in achieving maximum strength.
The general rule of thumb is that concrete should be cured for at least 7 days to reach a significant portion of its potential strength, and it is best to keep it moist for 14 days. According to ACI guidelines, concrete should be wet-cured for 5 days at 21°C or 7 days at 10°C.
Concrete reaches about 70% of its design strength within 7 days and reaches full design strength within 28 days. However, sufficient hydration never truly stops – concrete can maintain its strength for months or even years with proper maintenance.
Strength reduction: Concrete can reach only 50-60% of its potential strength when it is dry and uncured.
Cracking and shrinkage: Rapid drying can cause plastic shrinkage cracks and surface cracking.
Poor durability: uncured concrete is more permeable, allowing water and harmful chemicals to penetrate and cause deterioration.
Surface defects: Dust, scaling, and poor wear resistance are common in poorly maintained concrete.
Long-term cost increase: Fragile and cracked concrete needs to be repaired or replaced in advance.
Concrete needs to be kept moist promptly because the hydration process requires a continuous water supply. If there is insufficient moisture, the reaction will stop, and the concrete will never reach its designed strength, durability, or crack resistance.
Proper curing is a fundamental requirement for high-quality concrete construction. Whether you are pouring small lanes, building foundations, or large industrial panels, spending time keeping the concrete moist will yield a stronger, more durable, and longer-lasting structural form.
