Welcome to this comprehensive guide on the drying and hardening process of concrete. As someone with years of experience in the field of construction and concrete, I am thrilled to present this well-researched article on how long concrete takes to dry and the reasons behind this timeline.

This guide is aimed at addressing frequently asked queries related to concrete drying and hardening, ways to accelerate this process, and acquaint you with some key terminologies such as curing, drying, and setting.

Delving into the Basics: What is Concrete?

To start, let’s define concrete. It’s a sturdy, resilient material resulting from mixing fine and coarse aggregates with liquid cement, which, over time, transforms from a pliable substance to a solid state. Cement acts as a binding agent when mixed with water and aggregates. After being poured and smoothed out, it solidifies into a durable material used in numerous construction projects.

A typical concrete mix involves water, Portland cement, and aggregate which can include sand, rock, or gravel. Hence, timing is essential during the production of concrete – it needs to be used immediately after it’s ready to prevent premature hardening.

Cement Vs. Concrete: Clearing the Misconception

A common misunderstanding that many have is equating cement with concrete. However, cement is actually a constituent of concrete, primarily used for its production. The creation of concrete involves combining cement with water, resulting in a paste that binds the aggregates together. The mixing process typically happens at high speeds within concrete tanks to form the final product we refer to as concrete.

Accelerating the Hardening Process of Concrete

Contrary to popular belief, concrete never really stops hardening. This continuous hardening occurs as the cement particles in the mix react with water, forming molecular connections. These connections continue to solidify over time, causing the concrete to harden further. This is why even after reaching “full strength,” the hardening process subtly continues due to the microscopic moisture bubbles present.

One practical way to expedite the setting and drying process of concrete is to use warm water in the mix. Doing so significantly accelerates the reaction, leading to quicker bond formation.

Understanding Concrete Curing

Curing is a multifaceted process that involves more than just keeping the concrete damp. It’s about providing the concrete with the necessary conditions to develop strength. The strength of concrete hinges on the development of crystals within the concrete matrix. This curing process effectively prevents drying shrinkage until the concrete is strong enough to resist shrinkage cracking, thereby boosting its strength, durability, water-tightness, and wear resistance.

Putting  finishing touches on a newly poured cement sidewalk.

Why Does Concrete Crack?

Let’s shed some light on why concrete tends to crack:

  1. Hydration: Hydration is a crucial chemical process that leads to crystal formation. The absence of sufficient water hinders crystal formation, thereby weakening the concrete. On the other hand, an adequate water supply enables these crystals to form rock-hard structures that intertwine with the sand and gravel in the mix.
  2. Temperature: Concrete curing is temperature-sensitive; it shouldn’t be too hot or cold. Cold concrete slows down the hydration reaction. While considering temperature, focus on the concrete temperature, not the ambient air temperature. The hydration process decreases significantly below 50 degrees Fahrenheit and stops completely below 40 degrees Fahrenheit.
  3. Exothermic Reactions: These heat-producing reactions can cause temperature differentials inside the concrete, leading to cracking. When cement reacts rapidly, the crystals do not get enough time to form properly, resulting in weakened concrete.

Deciphering Construction Terminology: Curing, Drying, and Setting

The construction industry, like any other, has its specific terminology, some of which we’ve already discussed. From the structure of your material, it’s essential to understand the importance of hydration, temperature, and exothermic reactions. This knowledge isn’t just a ‘nice-to-have’; it’s vital in preventing the concrete from cracking, which could have significant implications for the integrity and durability of a structure.

Now, the construction industry, like many others, has a unique vocabulary that might seem confusing to an outsider. Three of these terms are curing, drying, and setting, and understanding them can vastly improve your understanding of concrete.

Curing of Concrete

The process of curing isn’t just about keeping the surface of the concrete damp. Instead, it’s a complex process that determines the strength of the resulting structure. Curing is essentially the process of keeping the mixture above 50°F for three to fourteen days after pouring. If moisture is available, the curing process is continuous.

Concrete is an ever-evolving material, and it will continue to respond even after 28 days. This constant change is a testament to its impermanence. As the hydration process unfolds after pouring, the final product becomes stronger over time.

Drying of Concrete

The drying process is the one that ensures the concrete attains the moisture level required for its intended usage or the installation of floor finishes. For freshly poured concrete, it can take up to 48 hours to dry enough for foot or vehicular traffic. But for it to be considered completely dry, it takes around 28 days.

For an official dry status, it needs to have a moisture vapor emission rate (MVER) of 3lb/1000sqft/24 hours or less. This is especially important for materials like wood, vinyl, and linoleum floors, as well as epoxy coatings.

Setting of Concrete

Setting refers to the slow transition of concrete from a liquid to a solid state after it has been laid. It is due to the production of Calcium silicate hydrate and ettringite and is influenced by the water content in the mix. After setting, the concrete is still soft and needs time to cure and dry thoroughly before it is deemed fully hardened.

Concrete begins to set within 30 minutes of placement and is considered complete after about 10 hours. A Penetrometer measurement of 3.43MPa or 500 psi generally marks the beginning of hardening, while a reading of 26.97MPa usually signifies the end of hardening.

A construction worker is Smoothing wet cement

Factors Influencing Concrete Drying Time

Concrete drying timeframes can be unpredictable. The duration it takes for concrete to dry is dependent on several factors, including:

  • Moisture: The less water there is, the quicker the concrete will cure. But remember, too little water might compromise the concrete’s strength due to less bonding. Conversely, if there’s too much water, the curing process will take longer, and you may notice flaking on the surface.
  • Temperature: Higher temperatures expedite moisture evaporation, helping your concrete to harden quicker. If you’re working in colder conditions, a concrete blanket can be a valuable tool to keep the concrete warm and speed up the curing process.
  • Mix Design: Including an accelerant in your concrete mix can hasten the curing process. However, it’s important to note that concrete cured this way might not be as sturdy as concrete that’s allowed to cure to full strength naturally.

Quickening the Drying Process of Concrete

There are several ways to speed up the hardening process of concrete for quicker utilization, including using additives. Both pre-mixed and post-mixed concrete can benefit from various additives. But regardless of the method or type of concrete used, it typically takes 28 days for concrete to reach 99% of its strength.

Before Pouring

Before you pour the concrete, consider using additives in the mix to speed up the hardening process. Rapid-setting cement is an option that can set in about 15 minutes when mixed with water, which can be ideal for urgent repair and maintenance jobs. Another option is calcium chloride, a widely used accelerator for concrete that reduces the initial and final setting times.

However, bear in mind that using an accelerator might make the concrete harder to work with, particularly in large pours, as it will begin to set more quickly. Also, in cold temperatures, heated mixed water or live steam is often used to raise the concrete temperature, making it set and gain strength faster.

After Pouring

After pouring the concrete, curing plays a critical role in hastening its hardening. The most effective method of curing is water curing, either by immersion, ponding, or continuous sprinkling. Alternatively, you can use curing compounds, which are typically sprayed onto the concrete after it has been set to create a barrier that prevents the water from evaporating.

On the other hand, in conditions where the concrete is exposed to extreme heat, it is crucial to avoid rapid drying. Techniques like fogging or misting can be used to increase the relative humidity around the concrete and reduce the evaporation rate. If the temperature is high, it can also be beneficial to erect temporary shades to prevent direct sunlight exposure.

Using curing blankets or insulated formwork can also help control the temperature of the concrete, especially in cold climates. These methods trap heat produced by hydration and prevent rapid cooling, which could cause the concrete to crack.

Tips to Consider

  • It’s recommended to start with a well-drained subgrade so that there is less moisture in the concrete when it is placed.
  • Minimize the amount of water used in the mix. A low water-to-cement ratio will not only result in stronger concrete but will also reduce drying time.
  • The use of curing compounds can reduce the need for water curing, which can also speed up the drying time.
  • Remember, a balance should always be maintained. While it’s tempting to try to speed up the drying process, rushing it can weaken the structure, compromising its longevity and performance.

Hopefully, this guide helps you better understand the variables at play when working with concrete. Patience is often a concrete worker’s most valuable virtue. With it, you can ensure the concrete has the time it needs to reach its maximum strength and durability, leading to a better and longer-lasting final product.