By Adrian Henry, Artisan Award-winning General Manager of Polished Concrete at NEx Systems
Sooner or later every commercial architect is asked to renovate a building that requires concrete floor repair.
Often when an owner buys an old property, they envision turning an industrial-era facility into a sleek open office space… or converting a warehouse into a trendy food court with retail boutiques.
The problem is that while polished concrete floors are in high demand now, when most of these old structures were built, it was assumed that the concrete slab would be covered with wood, carpets or tile. So builders often made multiple pours, resulting in a patchwork quilt effect… or left exposed rebar or bolt ends, assuming no one would ever see it. And of course, over time, as buildings settle or earthquakes shift the ground, long fissure lines in concrete slabs would appear. Especially here in California where we do most of our work.
At NEx we’re often asked by architects how to repair cracked concrete floors – and just as often we’re asked how to keep a crack from reappearing once it’s been repaired.
It turns out there are multiple approaches to take to turn an old, damaged floor into a gleaming new-looking one – and much of it depends on the current condition of the floor before you start. So before you draw a gleaming new epicenter of cool and show it to your client, it’s a good idea to get some idea of what materials and costs are required to make that vision a reality. This article will cover the following topics:
Different types of cracked concrete floors and their causes
Different approaches for concrete floor repair
Different materials used for different types of cracked concrete renovation
When to specify which repair option in your design
Approximate costs to expect for different options
Assessing the damage - different types of damaged concrete
While there are a lot of reasons why a slab might be damaged, for our purposes we’ll look at a few of the most common types of concrete floor cracking.
Fine, spider-web crazing cracks
Typically resulting from the original pour, this type of cracking occurs when the cement is mixed incorrectly. It’s often a result of an installer trying to stretch their material too far by watering down the mixture too much.
Large fissures from settling or excess weight
If your building is close to a fault line or the building has been settling for a few decades it’s not uncommon for the ground beneath the structure to have shifted slightly, putting pressure on the slab, which eventually cracks to relieve pressure. Often though, this kind of crack is also a result of incorrectly mixed concrete. Many contractors who don’t specialize in hard surface flooring assume that adding more hardener to a cement mix is always better. However, hardeners act to make concrete more brittle. Just as a living tree can move with shifting winds while a dead, hardened tree will snap, concrete is a porous, living substrate that needs enough hardener for structural integrity but not so much that it becomes brittle and unyielding to its environment. If you are asked to specify the concrete mix for your project, we advise a maximum of 3500 psi in most environments – but if you’re uncertain, ask us (or your concrete floor specialist) to recommend a specification.
Patchwork Poured or uneven floors
As I mentioned above, often in older buildings the slab installation was never intended to be seen. Because of that, builders would make multiple pours at different times during the building or a subsequent renovation process. Because every batch of concrete is always a unique blend of sand, cement, aggregate, water and other additives (and can change depending on humidity), it is nearly impossible to match one pour of concrete to another – and typically when these older buildings were built, no one bothered to try.
Scuffed or stained floors
If your renovation is an old factory or garage, it’s very likely you have oil or chemical stains as well as scuffed up areas where old industrial equipment once stood. Concrete is very porous and when older buildings were built there were no reliable sealers as there are today to protect surfaces from absorbing stains. But don’t worry; typically these kinds of concrete floor damage are the easiest to repair.
Recommended Options for scuffed, stained or spider-web cracked floors:
For floors that are scuffed, stained or have crazing cracks, most owners opt to keep the original floor but to grind away imperfections and seal it, often with a darker stain to hide small defects. These floors can be ground, polished and sealed to varying levels of reflectivity.
The first thing to ask yourself when considering what to specify for your concrete floor renovation is how it will be used.
If your end usage will be as a warehouse, loading facility or other non-consumer-facing application, most clients choose the least costly option. This kind of finish – what we call a ‘NEx Systems #1’ finish (also called a “Grind and Seal”) is very inexpensive (about $2 - $3 / square foot) but is not suitable for consumer-facing applications because it is not stain-resistant and typically has higher maintenance costs to keep it looking new.
The process involves grinding the top surface with a low (approximately #80 #100 grit) grind and sealing it so it can be mopped and will not continue to release dust as the concrete is walked on. This does not protect it from stains, but a stain-resistant coating can be applied for an additional cost. It’s a great solution for quick and inexpensive renovations in areas where floor imperfections are tolerable.
For public-facing environments, we recommend either a NEx Systems #2 (medium polish) or NEx Systems #3 (high polish) application. These finishes are really just a difference in how highly reflective the desired end product is.
A NEx Systems #2 (Medium Polish Concrete) is typically seen in restaurant flooring, retail, and office flooring and grocery stores because it’s moderately priced (around $3.95 - $6.25 per square foot) but has stain-resistant coatings and a higher gloss, more finished look and low maintenance costs.
For hotel lobbies, conference rooms, designer showrooms or other design-centric spaces, clients usually opt for the NEx Systems #3 (High Polish Concrete) finish. Because each level of finish requires more rounds of grinding and polishing with progressively finer grit bits to produce a glass-like surface, the labor required to produce this finish is more costly (typically in the $9 to $15 per square foot range), but maintenance costs are low.
To increase the reflectivity as well as floor slip resistance of polished concrete floors, NEx uses a proprietary non-toxic finish coating called NEx P.A.H. that can preserve the appearance of a 1500 grit grind mirror-finish floor with traction similar to that of a typical sidewalk. It’s a slip resistant floor coating that can be added to most hard surfaces to create slip resistant flooring.
Recommended Options for uneven, patchwork-poured floors or concrete with large cracks or fissures:
If you have a large fissure, patchwork-poured floors, rusted rebar or bolts or unevenly poured floors, your best option is using a concrete overlay. While this sounds like a limiting option, it actually opens up a whole new world of design possibilities, because you essentially are going to be laying a thin veneer surface over the (repaired) slab… so you can specify that new overlayed surface to look any way you’d like.
Before pouring any new concrete overlay it is critical to repair and resurface the concrete floor beneath to keep new cracks from forming. Failure to fix concrete cracks or properly address unevenly poured slabs will result in concrete overlay that cracks easily or doesn’t have a uniform look when poured.
Crack repair:
Expert floor specialists know that the best way to repair large fissures is by actually widening the cracks (called “routing out the cracks” with a v-shaped diamond blade) so that a flexible repair material can be poured inside of the crack to adhere to the sides (not the bottom) of the slab. Epoxy used to be used for this, but in recent years, more advanced materials designed to withstand moisture seepage, drying and maintain flexibility to keep your floor from re-cracking have become available. NEx uses a proprietary polyurea mixture specified to the environment of the building and its intended usage to fill the cracks before then resurfacing the whole slab to create an even overlay substrate.
Slab repair:
Once any large cracks have been repaired, bolts, rebar and high spots can be ground down and low spots in the slab can be levelled with new pours. When the floor is even, we do a rough grind of the surface to make certain the overlayment materials will adhere to the slab completely. Then it’s ready for the concrete overlay to be poured and polished.
Design options for Concrete Overlay:
The best thing about concrete overlay is that it can look like almost any poured surface. If you want something that looks like newly-poured polished concrete floors that’s simple. Stain can be added to the overlay mix to create a permanent uniformly tinted or colored floor. If you want a more designer look, you can select aggregates to be broadcast into the overlay and then ground to reveal interesting rock designs.
A well-kept secret of the hard surface flooring world is that concrete overlay is an inexpensive way to get the same look of terrazzo flooring at less than half the typical price. It’s also a great solution for terrazzo floor restoration. Designers can create patterns using Schluter strips and specify colored glass to be broadcast in a mix that match a clients brand colors. Nearly any high-end aggregate effect is possible.
Want to know more?
You can download our free Polished Concrete Buying Guide as well as design case studies and detailed information on polished concrete, concrete overlay and epoxy (resin) flooring here.
If you want to see design options and examples of materials used in a wide variety of commercial flooring, be sure to check out our website here.