When building with cement/concrete, the most widely used building material in the world, it is all about timing and the right timing at that.
This article covers why you may need a rapid setting or rapid strength gain “R” cement over a normal one noted as “N” on the cement bag. It points out what applications tend to need rapid setting/strength gain cement and for good measure, adds in some tips on how to mix fast setting/high early strength gain concrete.
N refers to a class of cement with normal strength development and R (rapid) denotes cement that achieves better/higher early strength. The main difference in the composition of “N” versus “R” is that rapid-set cement will have certain admixtures which speed up hydration and it also tends to have higher cement contents than standard mixes.
”R” rated cement (if compared in same strength class) is more suited for accelerated concretes as it is normally milled finer and therefore reacts quicker to admixtures normally used in fast setting concretes. Its speedy hardening properties make it useful for applications that include concrete, rendering and mortar but the reasons why would you choose R over N depend largely on what you need to make with the cement/concrete.
It is also important to differentiate between rapid strength gain “cement” and “concrete”. Rapid strength gain cement has a slightly shorter setting time when compared to “N” class cement but with a relatively big difference in the strength gain curve.
Comparatively rapid setting/strength gain concrete can be made with “any” type of cement but accelerating admixtures changes the behaviour of the cement used in the concrete. This provides a faster setting time and possible strength gain but drastically changes the concrete placing time and methods of finishing.
One source says that rapid setting/strength gain cement is the “simplest way to add early strength and structure to any tiny project, and it requires no tools.”
Total Concrete UK explains it well saying, “As the name suggests, rapid-set concrete is a concrete mix which hardens faster than standard mixes. Whereas traditional concrete will harden within 48 hours, rapid-set concrete will harden within an hour. This significant difference makes it highly beneficial for certain jobs, but unsuitable for others.”
Yet its big advantage is not just speed because it is also a strong and durable cement that can reach up to 40 MPa after a month. This is stronger than the foundation of most houses.
Rapid setting concrete is however not suitable for construction work that has the project team building load bearing columns and walls or large-scale infrastructure as it has certain disadvantages:
“Because the concrete sets so quickly, it becomes harder to make any changes to the concrete after pouring. Finishing, for example, can be more difficult, as there’s less leeway for errors.
With a standard mix, you can take your time to ensure a smooth finish — avoiding air bubbles or uneven levelling — which helps to provide long-term durability for the finished construction. For this reason, it’s important not to use rapid-set concrete on load-bearing constructions.”
HERE ARE SOME TIPS FOR WHEN YOU ARE WORKING WITH RAPID SETTING CONCRETE:
- Have your materials ready: Organise all your tools and materials so that they are easily at hand before your mix the concrete.
- Have fine product: Mix the dry concrete before you add any water to ensure all the material in the mix is loose. Make sure that the cement mix has no chunks/lumps in it.
- Use clean and cold tap water: Do not use hot water because the concrete will set so fast that you will have a bucket or wheelbarrow mould of concrete before you can use it.
- Pour the water in a bit at a time: This pouring approach enables you to mix the concrete in a measured way so that it gets to the right consistency.
- Be thorough in your mixing: You don’t want to have dry pockets in your concrete mix. The wetter the mix is, the better, because the drier it is the faster it will set.
A handy tip is that if you get to the bottom of your concrete mix as you work through your project, and it is setting, start another batch. Do not just add water to the dregs and try to use that mix.
With that advice in hand – quick as a blink – you are ready to get building with rapid setting concrete.