January 28, 2020

Did You Know Concrete Can Be Soft?

I bet you thought all concrete was rock hard and suitable for concrete polishing? Unfortunately that is far from true, let us show you why and how to get around the problem.

an example of a soft concrete floor which will be difficult to polish

What is soft concrete? Does it actually exist? Can you polish soft concrete floors? In this article we’re going to explore the subject of soft concrete and try to show you that it is something we come across far too often. We’ll also take a look at what causes it and what can be done to avoid it. Let’s have a look at why soft concrete is hard to polish.

Concrete flooring is difficult enough to polish as it is; why make the process more difficult? It is an incredibly time consuming, labour intensive and expensive task to get a satisfactory polished concrete floor from a good, strong and hard concrete slab. When we add a troublesome soft concrete into the mix the task becomes even harder to complete and an acceptable floor finish even harder to achieve.

Often we find builders and end clients struggle to understand why their concrete is soft, hopefully this article will make things clearer. It seems like such a contradiction in terms doesn’t it? How can something like concrete, that gives the impression of being strong, hard and resistant to damage be soft?

We class concrete as soft when it has a compressive strength of less than C25(25MPA) or a MOHs scratch reading of less than 4. When a concrete slab has been laid and had long enough to cure, we can perform tests on the slab which will let us know how strong it is and how resistant it is to scratching. We use a hammer test to determine the compressive strength and a scratch test to determine the MOHs rating.

On our recommended specification, we state that in order to be polished and for optimal hardness a concrete slab should be a C35 mix. What this means is that is should be able to withstand 35 newtons of pressure before the surface can be damaged.

There are a few factors which lead toward soft concrete. Excess water is the biggest culprit but we also have to consider the following; poor or insufficient curing, too little cement, carbonation, incorrect proportion of trace ingredients such as pozzolans or shrinkage additives.

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How Concrete Works – In A Nutshell

So let us briefly explain in simple terms. Decorative concrete surfaces such as flooring, walls, concrete countertops and furniture are all essentially large heavy, rigid slabs comprising of cement, sand, aggregates and water. These ingredients are mixed together either in a ready mix truck for delivery to a building site or locally onsite in a smaller concrete mixer, to form a paste that can be placed into moulds or shuttering or laid to form a floor surface.

When all of these ingredients are combined together and mixed thoroughly, a process of hydration occurs whereby the cement particles begin to react with the liquid present in the mix. Initially a gel like substance is created; this gel readily binds to and will coat the sand and aggregates and where possible coats them. This phase usually gives the mix a greenish hue. A green slab or mix is widely understood as being a freshly laid slab or mix.

The constant rotation of the concrete mixing machine ensures that the gel is blended thoroughly with the sand and aggregates. This gel begins to stiffen as time passes and the new concrete begins to set, later forming crystalline bonds which essentially glues everything together.

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It is important to realise that all concrete has aggregates incorporated into the mix, whether you want to see these aggregates in your finished flooring or not.

The amount of cement and water is critical to producing a quality concrete, these two ingredients determine the strength, porosity, shrinkage and overall abrasion resistance of the concrete slab. Too much liquid and the surface will be soft, dusty and highly porous; too little and concrete will be very difficult to put in place and smooth out before it sets.

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 liquid 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.

Too Much Water Added To The Concrete Mix Is A Recipe For Disaster

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. Now, we get it - installing a dryer, strong concrete mix is quite a difficult task, especially when what is being laid is actually going be the finished flooring. Normally when concrete is laid it’s not going to be seen and therefore it’s not a big concern how it’s laid or how it looks. It will regularly be covered with carpet or tile.

It’s much easier for a contractor to put plenty of water into the mix so that the concrete flows into place without the need to be encouraged to get into the nooks and crannies by hand. Unfortunately, it’s this mindset that leads to a sub standard floor and less than successful results being achieved at the polishing stage.

One thing the concrete installer can utilise to aid his task is 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. At CARRcrete, we really do think they’re a pretty good idea!

Curing the concrete correctly is also incredibly important to ensure you end up with a strong hard concrete slab that is less porous and less likely to crack. We give full instructions on curing your concrete on our recommended concrete specification which you can request from us anytime.

Curing the concrete properly is not difficult. Simply wait for the surface bleed-water to evaporate then once it has, cover the slab with plastic sheeting that is secured at the edges to ensure no creases appear. What this does is stops the moisture within the slab from escaping too quickly. When this happens, the concrete is quite dry and has a sandy appearance, not the best substrate for us to polish.

If you prefer, you can also wet cure the slab which involves saturating the slab over a two week period. There are also certain curing membranes on the market but please do speak to us in advance if you’re thinking of using these. Some do present a problem to us when it comes to working on the floor.

One final point on curing. Concrete doesn’t react well to low temperatures so in the winter months it’s worth waiting until your premises are weatherproof and can be heated. A minimum temperature of 10 degrees C will produce good results.

Interior concrete will always have a superior hardness to concrete that is left to cure outside. This very rarely ends up optimal, even if it’s covered over.

Soft Concrete Floors Are More Expensive To Polish

A soft concrete floor is far more expensive to grind and polish for many reasons. Aesthetically, it will only yield an aggregated terrazzo like finish, which is not good news if you’ve chosen our InfinityFloor Cream finish as many do. The reason for this is that, with the InfinityFloor Cream finish, we simply want to polish up the top laitance layers that hold the swirly, cloudy patterns that client’s love. Sadly, any laitance layers on a soft concrete are incredibly weak and will disappear without much effort, leaving behind hints of aggregate and sand pebbles.

Concrete floors that are soft require exponentially more concrete densifier (hardener) over floors that meet our specification. Sometimes, in order to increase a slab’s compressive strength, we may even need to treat the concrete with significant amounts of densifier and leave it for a week or two before we are able to get ahead with any further work.

Soft Concrete Floors Wear Down Diamond Tooling Far More Quickly

Grinding and polishing concrete is a labour intensive procedure where we need to use metal bond tools for grinding and resin bonded tool for polishing. Each tool has a grit rating dependant on how abrasive it is to the flooring we’re working on. We need to follow a process step by step. Additional costs for excessive diamond wear due to a soft floor’s inherent abrasiveness should also be factored in. It’s not unheard of for a full set of diamond tools to be worn away within 50 square meters of polishing, when they should last over 500 square metres.

The lack of strength in a concrete floor poses problems for a concrete polisher as there is often not enough abrasion resistance in the surface. Soft concrete has a rough sandpaper like appearance and feel which, when we work on it forces lots of additional loose, granular particles onto the floor, particularly when using metal tools. Along with wearing down our diamonds, these particles scratch and wear away the surface of the concrete. A concrete slab will never give a polished appearance while it’s shedding sand.

Imagine rubbing a precious diamond ring across a sheet of sandpaper, your instinct just screams a big no! Soft concrete can result in us using over five times more tooling than we normally would. For best results, our floor grinder responds better to a smooth surface

Other Special Requirements

We have previously mentioned that working on an abrasive floor causes the surface to scratch and deteriorate. In a concrete slab that is full of aggregate that often means that stones are lost, leaving holes in the surface that need to be filled. Soft concrete requires excessive grouting which adds considerable working time to your project as we need to factor in drying times for the grout itself.

Along with necessary grouting, filling of aeration holes and missing stones, additional sealer will also be needed and all of these elements cost time and money. Sealed concrete floors can still be soft however, the use of a sealer is more about protecting if from staining. To keep it dust free you would still need to embark of a low stress maintenance routine.

It has to be said, having a slab laid to our concrete specification is incredibly important and by following it, you are laying the foundations not only for a beautifully polished floor but also for a stress free project.

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. At the very least they are faced with increased costs and potential movement in their timescales.

Top Tips For Avoiding Soft Concrete

  1. Keep water:cement ratio at 0.40
  2. Do not add additional water to the mix
  3. Ensure bleed water has evaporated before curing begins
  4. Always fully cure the slab
  5. Follow our concrete specification

When a concrete floor is too soft to be polished

As you have read in this article, soft concrete is incredibly troublesome and should be avoided at all costs. If you find yourself in the situation where you have been left with a soft concrete slab and are wondering what to do with it, then read ahead.

It may be possible to apply a concrete product such as our CARRcrete PROtop overlay to the surface. This will provide you with a strong, hard and durable surface to use but we do need to take into consideration if what you have is strong enough to take the weight of an overlay, given that overlays are only laid to quite a thin depth.

It’s often the case that we need to treat your soft concrete with densifiers over a number of days prior to application. We also recommend that additional base coats of overlay are applied; the base coats are denser and stronger than the finer top coats and can much needed additional strength.

As mentioned previously, this does of course have time and cost implications but we’ll take care to look at each case individually and create a schedule of works that benefits you and your property.

CARRcrete are here to help

The aim of the team here at CARRcrete is to guide you every step of the way, so you end up with the stunning, dust free, durable, functional and clean concrete floors you’ve planned. We have a wealth of experience plus the most up to date polishing machines, diamond tools and chemicals.

Without a good concrete installation, power trowel work is almost impossible to get right and this is specified for all of our InfinityFloor finishes. Finally achieving a strong, clean concrete floor that is low maintenance is rarely achieved with a soft concrete, as the surface is dry and resists a grinder. We don’t offer any acid stain services but imagine trying to do that on a soft concrete; the results would be incredibly patchy and not favourable to most clients. More often than not, we have to work wet on a soft surface as dry grinding and dry polishing is just not possible.

We also commit to be there to offer aftercare and helping you maintain a clean concrete floor. Dust and grit will only deteriorate the surface over time and continue to reduce the hardness of the concrete. We’ll be there through every step to ensure you end with a cleaner concrete. We’ll explain how the occasional use of a high-speed burnisher can restore a well used floor.

Our concrete grinding equipment has been designed and engineered by us here at CARRcrete to enable us to get the best results when grinding and to get the most from our diamond tools.

Get in touch with CARRcrete today and we’ll help you to make your polished concrete floor a reality!

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