Did you know that to build bridges, builders pour concrete foundations into a river or the bottom of the ocean? A mix of sand or gravel, water, and cement forms concrete, and cement is the component that holds this mixture together. But have you ever wondered why concrete doesn’t get diluted or get washed away underwater?
Concrete can dry underwater even much better than it does in air. This happens when cement particles hydrate. The cement reacts chemically with water, binding together the sand and gravel. This curing (hardening) process takes almost a month and causes the concrete to set.
So, for your concrete to achieve optimum strength and durability, you need to keep it moist during the curing period. To learn more about how concrete sets underwater, read on.
How Does Concrete Set Underwater?
When concrete is poured underwater, one of its compounds reacts with water to form an outer coating. This coating prevents a lot of water from either seeping in or worse, diluting the cement. Next, another compound in the concrete reacts at a slower pace and sets to its final hardness 28 days later.
While most people believe that the reason cement sets is that the water mixed in evaporates, this is not quite correct. As seen above, cement sets due to the chemical reaction that occurs. After that, most of the water in the mixture gets used up by reacting with compounds within the cement to form new compounds that are very hard.
When concrete sets, it doesn’t “dry” per se since this would weaken the chemical reaction. This is why concrete that sets underwater tends to be stronger than its counterpart that sets in air.
However, for non-hydraulic types of cement — these use lime and gypsum plaster as a binder— this is true; their curing process depends on water evaporation. These kinds of cement are useful for special projects like historical restoration.
How Do You Get Concrete To Set Underwater?
Most concrete structures — whether buildings or bridges — are based on hydraulic cement. What this means is that most common concrete aggregate mixtures use hydraulic cement as a binder. This then uses water, which allows for the chemical reaction that forms the basis of the setting process.
The concrete sets over time, so long as you maintain the correct amount of water in the concrete mixture.
So, how do you ensure that the drying process goes smoothly? How do you get the concrete into its final position while preventing it from falling through the water?
One way to do this is by tremie or using a concrete pump. You can also control the unset mixture from spreading either by using specific concrete specifications or a special framework laid on a water bed.
How Do You Prevent Cement Washout Underwater?
One way you can prevent cement washout while drying concrete underwater is by increasing the setting time. You can do this by adding admixtures that will aid in this. Admixtures are concrete ingredients — other than aggregate, portland cement, and water — that are added either before or during mixing.
They help to assure quality concrete during the processes of mixing, transporting, placing, and curing or to alter the features of hardened concrete.
How to Ensure Durability of Your Concrete Projects
Concrete is among the most enduring and appealing construction materials, and its usage is much higher than that of steel or wood. Yet, what you do after pouring determines its strength as much as the mixing process you use.
During curing, it’s important to keep the concrete moist. If the water evaporates from the surface too fast, the final product will get weak due to stresses and cracking. This occurs when the mixing occurs outdoors, under the direct sun.
What happens next? The first several days are crucial. This is because you need to control the new concrete’s moisture content as well as its temperature. Giving the concrete mix additional attention during the curing period helps to increase the concrete’s structural integrity. This allows it to become more resistant to cracking in the future.
Below are additional tips to help ensure that your concrete structures remain strong and long-lasting:
Spray New Concrete With Water
Hosing down concrete with water is a commonly used method of curing it. Do this as often as five to ten times a day during the first week.
This “moist curing” allows moisture within the concrete to slowly evaporate. Besides, concrete cured using this method is almost 50% stronger than concrete cured without moisture. Nevertheless, for concrete poured during chilly weather, this method is not recommended.
Make Sure to Cover the New Concrete
If for some reason, you are unable to spray your concrete with water, you can use a cover to trap and slow down the rate of moisture evaporation in the mix. Use polyethylene sheeting or a curing blanket. The sheeting should be at least 4mm (0.157 inches) thick.
After wetting the concrete thoroughly, cover it with the sheeting, and use something heavy such as bricks or rocks to secure the sheeting. For the next seven days, remove the sheeting, wet the concrete, and replace the sheeting every day. You can also use this technique on concrete columns and walls.
Use Pond Cure Concrete Slabs
This is yet another method that you can use to cure your concrete. You do this by creating temporary berms around the new concrete slab, then flooding the inside area with water up to a foot (30.48cm).
Pond curing is an activity that takes three days. No daily attention needed—just ensure that you maintain the water level above the concrete slab.
Note that it takes a lot of soil to form berms around a big concrete slab, such as a foundation slab. As such, this method is often used by large-scale constructors as it helps speed up the construction process. It allows them to quickly pour the structure’s foundation slabs and move on to framing.
Make Curing Easier by Applying a Curing Compound
Finally, curing compounds provide a much simpler solution. This is because they boast soluble emulsions that form a protective coating on newly poured concrete slabs or walls. When sprayed on the wall surface directly, this coating offers a protective film that prevents water from evaporating. This allows the wall to cure at a consistent pace.
While some curing compounds disintegrate completely after some weeks, others get scrubbed out after curing is complete. Still, others penetrate the concrete to form a permanent sealer that waterproofs the concrete while giving it a freshly-poured look.
Conclusion
To create a high-quality, crack-resistant product, consider drying your concrete underwater. Note that some concrete slabs might still crack because concrete shrinkage occurs during hydration — water gets used up, and the temperature fluctuates.
To preserve your concrete slab and maintain its beauty, do the following:
- Place control joints at specific locations within a day of the pour. Use a metallic jointing tool to cut smoothly into the concrete surface, and these joints will guide the inevitable cracks
- Don’t allow the new concrete to get too cold as this interrupts the chemical hardening process. If it gets chilly, keep the concrete warm using a concrete insulating blanket
- Don’t subject new concrete to excess weight. Concrete reaches its full strength in approximately 28 days, so don’t allow foot traffic on a newly poured slab within the first 24 hours.
Follow these tips to ensure the longevity of your concrete projects.
Sources
- Wikipedia Concrete
- Indiana Public Media: Pouring Foundations Under Water
- Quora: Does Cement Set Underwater?
- Quora: How To Set Up Concrete Underwater
- Hunker: How to Quikrete an Underwater Post
- Concrete Construction: Placing Concrete Under Water
- Quora: How Can We Prevent Cement Washout While Doing Concrete Underwater?
- Bob Vila: Curing Concrete Dos and Don’ts
- Nbmcw: Significance of Curing of Concrete for Durability of Structures
- Civiltoday: Hydraulic Cement-Definition-Composition-When to Use
