For concrete to cure properly, all components need to be mixed correctly. Too much aggregate, and the concrete won’t bond properly; too much cement and the concrete will be brittle and flake away. But how would the absence of water affect things, a method called dry pouring?
Pouring dry concrete is used by some to dry-fill holes and fence posts in the ground allowing the ground moisture to seep in and harden the concrete, however water is still recommended. You should not dry pour concrete on any project that needs a solid foundation. Pouring dry concrete with the absence of water, when a solid foundation is required will result in a weakened structure.
We will take a closer look at the one occasion where you can get by using only a small amount of water at the end of your project and then explore the importance of using wet cement when mixing concrete.
Dry Pouring Holes with Concrete
Mixing concrete from scratch can be quite a procedure, particularly if you are following each step meticulously. This can be a pain when all you need is a small amount to secure something lightweight, meaning it would not require a large amount of concrete to hold it in place.
The most common applications for cases like these are when holes must be filled, such as when placing a laundry pole in the ground, erecting fences, or planting a mailbox post.
As a shortcut, they utilize a method called dry-filling, where you fill the hole with concrete or cement, pour water on it, and compact it to encourage it to solidify.
Most homeowners opt to use concrete in these cases, as opposed to simply refilling the hole with the soil they moved since this will hold much more firm than freshly dug soil.
Dry Pouring With Cement
Dry-filling a hole with only cement saves considerable time and does not require aggregate since the cement partially binds itself to the soil around it, as well as the item being fixed into the ground.
In very rare cases, where the soil already contains enough moisture, water doesn’t even need to be added. The cement would draw out the water content from the soil around it while it sets.
Dry Pouring Concrete Pitfalls
As with all shortcuts, though, many DIY projects have been reported to fail when relying on dry-filling to hold their items in the ground.
This is simply because concrete’s real strength is only drawn out when mixed correctly — simply pouring water on cement does not utilize the full strength of concrete.
The correct ratio of water to cement and then cement to aggregate is important for the strength of the concrete bond. We unpack this ratio in the section below, where we look at the correct procedures for mixing concrete.
The reason dry-filling fails in these cases is that the cement does not have anything to bond to but itself and the soil around it, and neither of these is very stable, especially soon after the hole has been dug.
Wet cement becomes grainy as it dries and, since it’d likely be exposed to the elements, is not hardy like properly cured concrete. This would mean the cement wouldn’t set firmly and could give way, particularly in strong winds.
Aesthetically speaking, it’s also incredibly difficult to get a dry-filled hole to look nice. As has been mentioned, wet cement lumps together — a far cry from the smoothly finished concrete that you might picture.
With all of that being said, though, it is very handy to know that dry-filling is an option for when you’re in a hurry, lack the full equipment, or are just looking to cut a corner to get a job done. Mixing concrete properly takes time and requires precision, which can be painstaking.
While it has some risks attached to it and could be called inconsistent, it is still a viable option for those looking to quickly secure a pole or something similar and might be something for you to consider.
When working on a task where you cannot risk failure or afford to redo at a later stage, you’ll want to ensure you use a properly mixed batch of concrete.
The Correct Mix of Concrete Components
Mixing concrete properly involves the careful measurement of its three components, namely, water, cement, and aggregate. Each of these components plays an important role since they interact with each other on a molecular level.
Importance of the Water-to-Cement Ratio
First and foremost, wetting the cement starts a chemical reaction that begins the curing process. This is why, when done on a larger scale, cement mixers are utilized to keep the mixture in a constant state of motion, thus putting off the curing process.
There is a misconception that only the ratio between cement and aggregate matters since that is the macro-reaction binding the mixture together. However, water plays an integral part in determining the overall strength of the concrete (source).
Water is, therefore, the catalyst that begins the process of creating concrete. It reacts with the cement on an elemental level, producing the highly alkaline calcium hydroxide that can reach a pH level of 12 or higher very quickly (source).
This is why one needs to be very careful when mixing concrete, as there is a risk of chemical poisoning.
A noteworthy example of this can be seen when using Portland cement specifically. The cement reacts with water in such a specific way that it can even harden underwater.
The Correct Concrete Water-to-Cement Ratio
Concrete mixers recommend a ratio of 6:1, using 6 gallons of water for every bag of cement added. This is the maximum ratio, however, so it is important to observe your mixture while you add the water — it may require less than that.
This is important to note in light of dry-filling. When mixing large amounts of concrete, normally on an industrial scale, such as at a construction site, water containers are used to measure the amount of water prior to adding it to the cement.
This precision is what was missing from dry-filling, rather than pouring water haphazardly onto the cement in the ground. It would be extremely difficult to achieve the correct ratio in this way.
To the untrained eye, aggregate simply looks like a pile of rocks or gravel — possibly dug up while leveling land or laying foundations. Experienced builders know this is certainly not the case since aggregate possesses a wide range of properties that can drastically affect the overall result.
Graduation, maximum size, unit weight, and moisture content are all taken into consideration when selecting the right aggregate for the job.
For the most part, those looking to do some DIY projects around the house don’t need to research this too deeply — hardware stores often sell concrete premixes that only require water to get going.
Outside of that, you may struggle to find a wide variety of aggregate since the full scope is mainly available on a wholesale level to contractors and other construction companies.
Where the ratio of cement and water dictates the strength of your mixture’s bond, the aggregate you use will determine the structural integrity once your concrete mixture has cured.
These variables all play a part in how much cement you’d need to add to your mixture to ensure it cures properly; however, this delicate ratio is something we discuss more in-depth in another article, “What Happens if You Put too Much Cement in Concrete?”
It’s important that the aggregate is clean. If there were any other material in the aggregate, it would affect the molecular makeup of your concrete mixture, and this could have an impact on how it cures or its strength in the long term.
The most common “stowaway” material is clay, as it would simply look like dust on the pieces of aggregate. This is why aggregate is cleaned on strainers to wash off other material and safeguard the concrete mixtures that they’re used in.
This opens the door to another problem, though — aggregate needs to be dry when added to the concrete mixture.
Wet aggregate would be adding further moisture to the wet cement, which, after measuring the ratio of water-to-cement precisely, should be avoided. A change in the water-to-cement ratio would weaken the chemical bond between the cement and the aggregate.
This is why making concrete isn’t something that should be rushed. It’s a good idea to wet your cement separately first before introducing aggregate.
This will allow you to inspect the aggregate prior to pouring the cement onto it, as well as limiting the chance of accidentally getting it wet and affecting the composition of your mixture.
Alternatively, purchasing premixed aggregate from your hardware store may be simpler, or at least the aggregate you purchase from them would be washed already and bear the right characteristics that you’d need for your project.
Cement vs. Concrete
It’s important to distinguish between cement and concrete, as they’re really not interchangeable terms.
Cement is a construction material used to bind other materials together, and it is rarely used on its own; instead, it is mixed with aggregate and sand components. It is a key component for making concrete and mortar — both of which get used regularly on construction sites (source).
Concrete is a composite construction material combining cement, aggregate, and sand. It hardens as it cures, making it a very durable surface for flooring. You can purchase premixed concrete, which comes with the right amount of each component since the ratios of each are important (source).
It wouldn’t be right to say there are no cases when you could get by using dry concrete since it is possible in very certain situations. However, this should be seen as a shortcut that may not always perform the way you hoped it would.
Instead, the safer option would be to properly mix your concrete, which involves very carefully measured rations of water content, cement, and aggregate. Once this ratio is met, you can count on your concrete to get the job done!