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What is the best time of day to pour concrete?

Plan to pour concrete before 10 a.m. or after 8 p.m. in order to avoid the hottest part of the day. If you are concerned about the temperature while working on a summer project, consider pouring concrete at night to give the mixture the best chance to harden and cure before the sun comes up.

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Whether you are looking to put in a new basketball net for the kids or install a workshop in the backyard, concrete is an important part of the process. However, concrete doesn't immediately become hard and resistant to damage after pouring. It takes at least 4 to 8 hours before the concrete hardens, then a further 24 to 72 hours to cure, so it's important that the concrete be poured and cure at the ideal temperature.

How Concrete Works

Concrete is an incredibly useful substance when it's poured and cured properly. It's typically made up of an aggregate and a paste. The aggregate is essentially a combination of sand and broken gravel, mixed together in precise amounts, while the paste is a mixture of water and cement. This mixture of aggregate and paste becomes a fluid concrete substance that can be poured and formed into different shapes or custom molds. After pouring, the concrete mixture begins to chemically react with the water molecules in a process known as curing. This bonds the components together, creating an incredibly strong, cohesive substance. However, both low- and high-temperature extremes can affect the curing process, so it's important to keep the concrete at the right temperature while it cures.

The Risks of Pouring Concrete at the Wrong Temperature

Pouring concrete in hot weather can reduce the amount of water in the concrete mixture, due to the process of evaporation. This can create abnormalities in the shape, but it can also cause the final product to be weaker, putting the concrete at risk of flaking, chipping, and cracking under pressure. However, if the concrete mixture is poured when it's too cold outside, then the curing process can take much longer. While this doesn't seem like a big deal at first, the potential repercussions are hard to ignore. Slow curing concrete can shift over time, causing severe structural damage to any buildings or materials that are being supported. Freezing temperatures can also cause the water in the concrete to freeze and expand, breaking the concrete. These breaks may be immediately noticeable or they can form in the middle of the concrete, going unseen until a heavy structure is placed on top of the concrete and the weight causes it to crumble.

The Ideal Temperature for Pouring Concrete

In order to avoid these issues, it's advised to pour concrete at a temperature between 50 to 60°F. Temperatures below 50°F will begin to slow the curing process, and the water in the cement is at risk of freezing at temperatures below 40°F. If the temperature is higher than 60°F, the water can begin to evaporate while the concrete is curing, affecting the structural integrity of the poured concrete, though this effect can be mitigated by regularly watering the concrete after it has hardened. Lightly spray the concrete down between 5 and 10 times a day for the first 72 hours to help the curing process with additional hydration. Just keep in mind that concrete shouldn't be sprayed until it has hardened, which takes about 4 to 8 hours.

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The Best Season for Pouring Concrete

While summer may seem like an appropriate time for these projects, the unceasing heat can become a problem, given that the best temperature for pouring concrete is between 50 and 60°F. So, the ideal season for pouring concrete is during the cool to moderate months of the year. Depending on where you live, this could be the spring, fall, or even the winter for individuals living in very warm locations. Ideally, the temperature should remain within 50 to 60°F for most of the day. Also, make sure that nighttime temperatures don't drop below 40°F, as this may cause the water in the concrete to freeze, putting it at risk of chipping and breaking.

Does Time of Day Matter?

The temperature is one of the most important factors for getting the right consistency and finish for your concrete, but the time of day and weather should also factor into your decision. Plan to pour concrete before 10 a.m. or after 8 p.m. in order to avoid the hottest part of the day. If you are concerned about the temperature while working on a summer project, consider pouring concrete at night to give the mixture the best chance to harden and cure before the sun comes up. Also, avoid pouring concrete in rainy weather. Even light rain can damage newly poured concrete, though after about 4 to 8 hours the concrete is hard enough that rainwater can actually help the curing process by providing additional hydration, similar to watering the concrete with a garden hose. Mixing, pouring, and curing concrete for home DIY projects is a relatively simple process with a few deceptively complex considerations to improve the physical bonding between the aggregate and the paste. If done correctly at the ideal temperature, concrete can become a strong, durable material that lasts for years with minimal maintenance, so it's worth the time to learn how and when to properly mix, pour, and cure concrete for the best results.

thespruce.com - The Best Temperature to Pour Concrete - The Spruce