Concrete is difficult enought to polish; why make the process more difficult? It is an incredibly time consuming and expensive task to get a satisfactory polished concrete floor from soft concrete. Often we find builders and end clients struggle to understand why their concrete is soft; hopefully this article will make things clearer.
We class concrete soft when it has a compressive strength of less than C25(25MPA) or a mohs scratch reading of less than 4. There are a few factors which lead toward soft concrete (excess water, poor or insufficent curing, too little cement, carbonation, too much pozzolan and too little cement).
Good concrete is like an espresso coffee, strong and rich. Soft concrete is like an over watered coffee, of poor strength and quickly regretted
When water is added to the dry concrete mix of sand, aggregates and cement, the cementitious particles begin to gel, coating the remaining ingredients. During this green/grey plastic looking stage the cement is slowly beginning to activate, sticking together neighbouring particles. Over the next 24 hours this gel will begin a transformation into a crystalline spines, looking not to dissimilar to icicles under a microscope. Depending on how much water is used during the mixing largely depends on how concentrated these crystals are; lower water additions promote a denser formation leading to strong hard concrete with lower porosity and reduced cracking. Conversely high amounts of water promote a dispersed crystalline structure with higher porosity suffering from poor structural strength, more cracking and possibly carbonation.
In our experience, the most common cause for soft concrete in the UK is too much water being added to the mix and lack of/or refusal to cure the concrete after installation. These factors could be improved by the concrete installer simply by requesting a super plasticiser or water reducing agent which offer extra workability without the need for more water, they also promote stronger floors and less cracking. I’d say they’re a pretty good idea. Curing is also incredibly important to ensure a strong hard concrete slab that is less porous and less likely to crack. Curing is not difficult; simply wait for the surface bleed water to evaporate then cover the slab with plastic or wet cure or spray on a curing membrane; it is worth the effort!
A soft concrete floor is far more expensive to grind and polish and will only yield an aggregated terrazzo like finish, any laitance layers will behave sacrificially due to their weakness and disappear without much effort. Concrete floors that are soft require exponentially more concrete densifier (hardener) over floors that meet our specification, additional costs for excessive diamond wear due to a soft floors inherent abrasiveness should also be factored in, along with necessary grouting and filling of aeration holes and missing stones. Additional sealer will also be needed, all of these elements cost time and money; so having a slab laid to our concrete specification is incredibly important.
We have lost count of how many times we have had to walk away from a soft concrete that should have been laid to our specification; yet for whatever reason it hasn’t. Often this forces the end client to either accept a different finish to what they had originally anticipated or faced with increased costs and movement in their timescales.
The lack of strength in a concrete floor poses problems for a polisher as there is often not enough abrasion resistance in the surface, leading to sand and stones falling out during the polishing process. This leaves a rough sandpaper like appearance and the additional loose media on the floor actively works like an additional granular abrasive which helps to break down the already soft surface further still. Whilst the slab is shedding sand you will not get a satisfactory finish.
Top Tips For Avoiding Soft Concrete
- Keep water:cement ratio at 0.40
- Do not add additional water to the mix
- Ensure bleed water has evaporated before curing begins
- Always fully cure the slab
- Follow our concrete specification