The forklifts and material handling equipment traversing many warehouses and distribution centers at times may rival a big city’s rush hour traffic. These vehicles are most effective when the facility’s concrete floor is perfectly flat and level.
The best way for a building owner who needs a concrete floor to meet flatness and levelness requirements for this equipment is to specify the floor tolerance requirements at the very beginning, at the conception phase of the project.
In fast-track construction of new big box facilities, floors sometimes are placed and finished in pours of 50,000 to 80,000 square feet or more with laser screeds and large riding trowels with pans. Then, the surface is remediated to a specific tolerance by grinding the material handling equipment wheel paths only or simply the width of the aisles between the racks to increase throughput or material handling efficiency. Both depend on the owners’ necessary F-min tolerance.
For example, a FF 50/FF 35 floor is specified in a building which sits empty for six months or more before it is occupied. Depending on how the facility is proposed to be laid out by the new tenant/owner and depending on the amount of slab curling and settling that occurred while it was vacant, it will likely require remediation to meet F-min tolerances of at least F-min 50 to F-min 100 or higher if required by forklift manufacturers’ recommendations or the architect/engineers’ tolerances. (See ACI 360-10, Guide to Design of Slabs-on-Ground.)
In lieu of Type II concrete, Type K shrinkage compensating concrete is placed and finished in large square pours by experienced contractors. Construction joints or form boundaries are set about 100 feet apart. These methods may allow for quicker installation of the floor and could reduce the amount of grinding at joints due to the lack of curling because of the expansive properties of Type K cement.
Some owners and contractors prefer to get high-tolerance F-min floors using the “strip pour” method. This placing and finishing method uses forms set to rigid tolerances of plus or minus 1/16-inch, milled wood form tops or thin steel edge forms, and vibrating truss screeds with hand tools (check rods and bump cutters) used for strike-off. Some contractors prefer laser screeds instead of truss screeds. Floors installed this way usually require little or no remediation, as tolerances are more stringently controlled with the narrow strips.
What if owners want to expand or move, and must buy or lease an existing facility? What if they have an existing facility and want to upgrade, move, or reconfigure the current material handling equipment, racking layout and conveyors, but the floor isn’t up to par?
There are usually three options. But first, they should hire a qualified firm to measure the existing slab with the proper floor flatness and levelness equipment to assess its current condition. This could save time and money if deciding to lease a nearby facility that could be better than the one the owner is seeking to rent.
There are three common methods for upgrading:
1. Remove and replace, which is expensive, invasive, and time consuming in an operational facility or one that must be operational soon.
2. Apply a self-leveling or epoxy topping, which often requires scabbling or scarifying the existing surface for an underlying mastic layer. If not applied by an experienced contractor, this can be challenging, time consuming, and have a high probability of failure (blister or delaminate) due to moisture content of the subgrade and high alkalinity at the surface.
3. Usually the most cost-effective fix entails grinding, but don’t be afraid, read on.
Grinding might have its pitfalls. People immediately think of loud noises (engines and blades), dust (silicosis), and downtime. However, relatively quiet and dust-free concrete floor resurfacing equipment capable of creating superflat F-min tolerances for very narrow aisle (VNA) applications does exist and is used every day with great success.
Some grinding equipment was designed to work in operational VNA buildings, order picker paths, crane rows, or even wide open areas while the facility continues to operate with little or no interruption. Hundreds of building owners that had their floors ground as long as 25 years ago are currently operating today with great success.
Instead of grinding, it should be called planing or skimming, which does not sound so harsh. Think of it this way: If the American Concrete Institute says an F-min 100 floor is approximately plus or minus 1/16-inch in any one location and a floor is currently plus or minus 1/8-inch, then only 1/16-inch needs to be removed.
It does not take much to throw off the performance or longevity of material handling equipment in warehouses. That is true for the amount of concrete that must be removed to make an ordinary Random-Traffic FF/FL floor a Defined-Traffic Superflat Floor. (See ASTM E 1155, Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.)
However, the key to this is doing it planar or kept to a high degree of levelness. That’s the tricky part, but there is equipment that can do the job. Many concrete grinders are on the market, but they are usually manually operated (although there are a few laser-guided models.)
The problem is one will almost always encounter an area that needs 1/2 to 3/4 inches removed. That is a lot of concrete to remove, but some equipment can move right through it with a single pass with results well above F-min 100.
In conclusion, there are different ways to design a high-tolerance concrete floor with a specific use, and there are several qualified architects, engineers, and consultants who can assist owners and developers with their projects. There are many experienced contractors who can install these types of floors every day.
David Fudala is the founder and CEO of ALLFLAT Consulting, San Diego, which provides professional services for concrete slabs from design phase and construction to testing and inspection. Visit www.allflat.com.