At first sight, the floor appears to be the very simplest element of the warehouse. After all, what does it take to make a floor – lots of concrete and a bloke with a spirit level. Except, of course, it doesn’t work like that. Modern warehouse floors are miracles of engineering. It’s the flatness that matters – particularly for very narrow aisle applications.
To provide the degree of flatness necessary for a modern VNA operation means working to tiny tolerances across huge expanses of concrete and needs careful planning.
Twintec points out that it is crucial to understand the potential end-user needs prior to the construction of a floor slab for warehouses and distribution centres alike. “Many operators agree that this is the most critical area of the building.”
Early identification of specific client requirements regarding product storage systems, likely vehicle types and movements is essential when considering suitable flooring solutions at design stage. All floor loading conditions and vehicular activity must be considered and also any potential changes of client use in years to come should be taken into account wherever possible.
These needs can then be used to specify the most appropriate flatness tolerance and construction methodology correctly. By producing the flattest floor surface tolerances dictated by the worst case usage, the operator will be provided with unhindered access throughout the full floor footprint, especially down very narrow aisles for his selected mechanical handling equipment.
Another benefit of careful pre-planning and early liaison with all parties is that post production problems can be substantially reduced or even completely eliminated. Long-term maintenance issues, which can be deemed an expensive and time-consuming headache by the end-user, can also be remedied before handover with careful consideration and forethought.
Twintec says a major issue over the years for users of concrete floors has been the frequency of joints in slabs. These can be subject to constant breakdown caused by forklift truck movements. However, it says, floor damage can be substantially reduced by designing-out joints, and proposing ‘jointless’ floor construction.
“Use the latest developments in materials, equipment and methodology, large areas of concrete can now be cast daily without the need for induced joints – the only joints being leave-in-place steel sections, which give exponentially higher performance when subject to constant forklift traffic. A floor with vastly reduced long-term issues can also be one that allows a client full functionality of floor use, which can remain unaltered over the course of years.
“It is also important to ensure a client can benefit from a single point of responsibility, achieved by employing a reputable specialist flooring contractor who not only specialise in the casting of large area “jointless” floors reinforced with steel fibres, but who are also able to act as a one-stop-shop in floor design, and able to provide on-going customer support as well.”
Upgrading a standard floor for very narrow aisle operation needs careful consideration.
Kevin Dare, managing director of Concrete Grinding, points out that there are two methods of grinding that are commonly used: manual floor grinding and mechanised grinding using a laser grinder.
Manual floor grinding using trolley-mounted grinding machines is a method that has been used for over 20 years. This method uses walk behind, diamond headed grinders that have until a few years ago provided the most effective method of upgrading the floor flatness in a very narrow aisle warehouse. However, this method has a number of drawbacks.
The manual grinding operation is labour intensive and the time and effort needed to treat even the smallest of flatness problems can be significant – and soul destroying for the grinding operative. Frequent checking of the manual grinding process is necessary to ensure that the correct amount of concrete is being removed in the correct areas. This checking, using optical levels, straight edges or Profileographs, is disruptive to the grinding process as the floor needs to be cleaned and cleared of equipment before each check is made. It might then be necessary to re-grind the same area, and then re-clean and re-check many times.
The grinding blades have a smaller diameter than the width of most VNA fork truck wheels. Therefore, to create a wide enough running path, the grinding machine needs to be moved from side to side along the wheel tracks. This action can create a “dished” profile. A badly dished ground path can affect the forklift truck’s ability to drive in a straight line, as the wheels naturally try to run into the base of the ground path. This can create excessive loading on rail guidance systems and, with wire guidance systems, the trucks can actually ‘come off the wire’ and lose the guidance signal.
Dare points out that the third edition of the Concrete Society’s Technical Report 34 has picked up on this problem and now states that any remedial grinding must result in the wheels having full contact with the floor surface. Depending upon the amount of dishing and the exact wheel spacing of the Profileograph’s sensors, the compliance check survey can represent a significantly different floor profile to that upon which the wide wheels of the VNA forklift truck will operate.
The left and right wheel tracks are treated entirely separately making it extremely difficult to make them follow identical profiles. A VNA truck feels every bump in its defined wheel paths and if these bumps occur in different places and on opposite sides of an aisle to each other, the VNA truck is going to feel them even when they are ‘within tolerance’
Dare says the most serious disadvantage of the manual grinding process is the general tendency to work as close as possible to the limits of the floor flatness specification.
Many of the drawbacks of the manual grinding methods can be overcome with laser grinding, he says.
Throughout the grinding process, the Laser Grinder uses water from its on-board water tanks to prevent airborne dust. The waste product – a mixture of finely ground concrete dust and water, is collected in a mobile waste container by the Laser Grinder’s vacuum system.
Although the resultant ground path created by the Laser Grinder is flat across the aisle allowing full wheel contact, the grinding process will not form a ‘flat’ profile along the aisle as this would usually require grinding to excessive depths.
The Laser Grinder uses the allowable gradients determined by the flatness specification. As a result the longitudinal grinding profile (along the aisle) consists of a number of very gradual slopes. This enables the grinding profile to follow the general profile of the existing floor, while removing those parts of the floor surface that are not compliant with the flatness specification.
From one side of the aisle to the other, the aim is to provide zero tolerance between the left and right load wheel tracks.
The Laser Grinder easily adapts to grind in a number of different formats to suit all types of VNA forklift trucks and MHE. It can grind individual defined wheel tracks for three or four-wheeled VNA trucks, or the full width of an aisle can be ground to accommodate any number of different potential wheel tracks, says Dare.
The joint repair division of Concrete Grinding recently repaired the two main 24-metre transverse construction joints in a 2,000 sq m concrete floor slab in a weekend and, it says, at a much lower cost than alternative methods.
Uneven joints between the three sections of the floor slab in the warehouse at Milton Keynes run by furniture fittings distributor Blum UK were causing the wire-guided man-up trucks to malfunction frequently.
Originally Blum UK had considered carrying out major structural repairs to the slabs, which would have taken at least six week-ends and been very disruptive and expensive. Concrete Grinding’s solution was to stabilise the joint edges by pumping a high-strength resin foam into the void beneath them, cutting back the joint arrises to a depth of 50mm and then re-building the joints with a high-performance epoxy mortar. Finally, the joints were re-cut in the same position as before, filled with a semi-rigid compound, ground to ensure a completely even surface and sealed.
The contract was completed in one weekend, which was quicker, cheaper and less disruptive than the alternative.
First for Stanford
Construction of a 14,000 sq m concrete floor at Witton, Birmingham, is claimed by Stanford Industrial Concrete Flooring to be the first constructed by Laser Screed techniques to the highest Free Movement demands of FM1, TR34 3rd Edition, without grinding and to be measured against UKAS accredited surveying standards by international surveyors, Face Consultants for the very first time.
The floor was constructed using the laser controlled Somero SXP Laser Screed, a precision spreading and compacting machine, newly delivered to Stanford, offering enhanced concrete surface regularity compliance.
Designed and detailed by engineers Sprigg Little of Sutton Coldfield, the floor construction is for main contractor, Ashford Construction of Coleshill, to carry 105kN racking leg loads for a speculative development. The FM1 specification was chosen to minimise any corrective grinding required to bring it up to the required Defined Movement specification, the ultimate surface tolerance for very narrow aisle (VNA) picking machinery. Initial surveys to DM2, TR34 3rd Edition reveal very minor grinding is required to convert to a dedicated VNA scheme
Technical Report TR34 produced by The Concrete Society is the essential guidance on the design and construction of ground concrete industrial flooring.
Rising above the masses
Installing a mezzanine floor in a warehouse is an obvious way of increasing the usable space and appears relatively cheap and easy to do. However, there can be pitfalls for the unwary.
Alan Mooge, head of sales and marketing at mezzanine International, points out that health and safety issues are critical in the design of mezzanines. “They are probably going to need some form of fire protection,” he says, pointing out that only those that are mainly for storage or less that 50 per cent of the floor area will not. The cost of this is about 55 per cent of the cost of the mezzanine because it involves hanging a fire-rated ceiling below the mezzanine.
Getting the specification right can be a compromise, says Mooge. People generally want the fewest supporting columns possible but this means using thicker beams which can lead to headroom problems. It can also increase the load imposed on the floor by the column so getting information on the load-rating of the floor is important and, especially for an older building, can be difficult.
Nevertheless, says Mooge, the market is reasonably buoyant at the moment – for example, Mezzanine International recently won a substantial order in France.
RDA Projects recently won a £2.8 million contract for Next Distribution, to install a 26,000 sq m two level mezzanine structure at the Next warehouse development in Wath Upon Dearne, Rotherham. Due for completion in June 2007, the mezzanine accommodates a sortation and storage system for Next’s expanding mail-order business.
RDA is an independent structural engineering company and targets professional specifiers such as construction firms, architects, surveyors, retailers, shopfitters, distribution & logistics specialists and production companies. Sales and marketing director Adele Lancaster says: “We design our solutions from scratch and back them up with £3 million Professional Indemnity Insurance, which I think is unique in our industry.”
Technical director Richard Bates says: “Around 80 per cent of our on-going business is repeat, where clients return to us again and again. We have one particular client in Leeds who now has ten of our mezzanine floors in their premises, one for every year we have been in existence.”
Health and Safety is a vitally important factor in the warehouse and the safety of pallet loading areas on mezzanine floors is increasingly coming under the spotlight. The safety of warehouse staff is paramount and too many accidents occur due to poorly guarded loading areas. But chains, sliding rails and lift out bars are open to abuse. Gardner Mezzanines has recognised this with the introduction of an up and over ‘Guard Gate’. Simple to operate and easy to install, the ‘Guard Gate’ comes in four widths – 1.6m, 2.0m, 2.4m, and 3.0m . It is powder-coated in a blue finish with high visibility yellow horizontal safety bars. To complete the safety package, Gardner Mezzanines will supply Durbar wearing plates with front edge protection. Gardner Mezzanines also install on site using their own fitters.
Sometimes the need is for speed and flexibility in constructing a floor. Ergo Floor Systems has had some notable success in this sector with its interlocking floor tile system which goes under the name of Traficline.
Organisations like Sainsbury’s and NHS Logistics are among Ergo customers. Ergo’s Neil Pimkin points out that the heavy duty PVC interlocking floor tile system is simple to install over existing sub-floors even if they are damp, cracked or contaminated. The tiles are capable of withstanding high levels of foot or forklift traffic and they highly resistant to impact and vibration. NHS Logistics, for example, used the tiles on a mezzanine floor at its Runcorn distribution centre.
Pimkin points out that one of the advantages is that the flooring can be laid with very little disruption as surface is usable as soon as it has been laid. The tiles have an exclusive stain proof surface treatment which means they are more resistant to marking and easier to clean. Traficline is recyclable available in five surface finishes.
Joinrtless floor for Morrisons
Twintec designed and installed a jointless, steel-fibre reinforced concrete ground supported floor slab at a new cold store for WM Morrison in Corby, working with main contractor, Clugston Construction and the consulting engineers AWT Partnership. The finished floor slab is virtually fibre-free and the survey results for flatness are outstanding.
The floor slab was cast on top of heavy duty insulation and within the confines of insulated wall panels which prevented direct discharge of the concrete from mixer trucks, making concrete pumping necessary.
This also ruled out the use of the Laserscreed and Topping Spreader and required high levels of skill and experience with flood pour techniques to achieve the quality demanded by the client.
A tight programme meant that Twintec had to use two production teams working side by side. A total of 34 Twintec staff, worked simultaneously on the project using 630 cubic metres of concrete a day to cast jointless panels up to 3,300 sq m a day working to FM2 Special flatness tolerance in accordance with TR34 (3rd Edition) 2003.
The quality and consistency of the concrete was of prime importance and Twintec in-house technicians together with Hanson developed the most appropriate mix design