There are a couple of angles we can take when looking to answer the question of how long should concrete cure before putting weight on it.
First, there is the traditional industry answer and all the wrinkles that entails.
Next, there is the homeowners angle, which doesn’t really apply to our business here at Columbia Precast Products but is worth exploring in some detail.
And finally, there is the opinion and advice of industry leaders and experts, who work hard to expose and deflate certain myths about how long concrete should cure before putting weight on it.
Let’s take a quick look at each of these angles.
Concrete Curing: Traditional Industry Advice
Right off the bat, we should say that concrete begins to harden almost immediately after pouring and sets in a few hours. And yet, concrete never stops curing. It just keeps getting stronger and more durable as time goes by. That’s one of the reasons so many concrete structures from ancient times remain standing to this day.
Still, we need to have a practical standard by which to measure the appropriate timeframe for curing. Thus, traditional industry advice suggests that it takes 28 days for most industrial concrete mixes to cure.
Variations in the concrete matrix and mix, however, along with climate (ambient temperature, hot or cold weather, humidity, etc.), placement, and finishing on the surface of the concrete, make that 28-day number a ballpark figure.
Requirements from government agencies take this into account by recommending that concrete reach at least 75% of its compression strength before building things on top of it. It can sometimes take a week to reach this capacity.
More expensive concrete mixes reach that point much faster, sometimes in as little as a day or two. There are other factors, too. For example, ready-mix concretes, curing compounds, cure seals, and the plastic sheeting placed atop newly poured concrete can all affect the time it takes for concrete to dry and cure to full strength.
Do-it-yourselfers often construct driveways, sidewalks, concrete patios, basketball courts, and other concrete surfaces on their properties. They need to know when they can tread upon their newly laid concrete, too.
For projects of this type, homeowners can usually walk on newly poured concrete slabs between 24-48 hours. As long as one has followed the manufacturer’s recommendations to the letter, it should also reach 75% compression strength in a week and full capacity in 28 days.
What’s happening in the interim — i.e., after that initial 24 to 48 hours? We call it hydration, a process during which we must ensure that moisture remains in the concrete.
“Otherwise,” writes Bob Vila, “water evaporating too quickly from the surface — which can happen easily outdoors and in direct sun — will weaken the finished product with stresses and cracking.”
As we mentioned, the 28-day number is a ballpark figure for the reasons specified above. But it’s proven to be a pretty good ballpark.
A piece written by precast concrete expert Eric Barger and published by the National Precast Concrete Association (NPCA) goes into some detail on this point.
“Specifying concrete strength is normally done with a minimum compressive strength (psi) at a certain age (days),” Barger writes in a 2013 post. “Specified concrete compressive strength is the minimum compressive strength at which the concrete should fail in standard tests of 28-day-old concrete cylinders.”
Barger continues: “A typical concrete compressive strength specification requires 4,000 to 5,000 psi at 28 days. Some go a step further and mandate that concrete products cannot be installed or used until 28 days after the date of manufacture. This, mistakenly, has given concrete a reputation among some specifiers as being weak or inferior if it has not cured for the full 28 days.”
The whole truth is that 28 days allows for a consistent measuring standard, not necessarily having to do with a specific piece of concrete’s curing time or subsequent compressive strength.
“So, while curing does help the hydration process, ‘28 days’ is not an inclusive rule dictating a specific time to produce minimum compressive strengths,” Barger writes for the NPCA. “Simply stated, as the concrete cures and hydration takes place, the concrete gets stronger — and hydration may continue long after the minimum required compressive strength is reached.”
So how long should concrete cure before putting weight on it? It depends.
Follow those manufacturers’ recommendations. Make sure you’ve met the product’s minimum compressive strength requirements. Your concrete will perform, whether you’ve built a tennis court in your backyard or a tennis stadium in your hometown.
Questions about curing concrete? Comments about the curing process? Get in touch with the precast concrete experts at Columbia Precast Products.