“The demand for industrial buildings and, specifically, a sub-set of that class of buildings – logistical warehousing – has followed the rise of online shopping“
The demand for industrial buildings and, specifically, a sub-set of that class of buildings – logistical warehousing – has followed the rise of online shopping. When the internet became a channel to consumers it required stock to be held not only in retail stores but in distribution warehouses also. Indeed, the advent of companies like Amazon done away with the need for a physical shop to sell products to consumers. Established retailers have had to embrace this transition and cope with a new type of competitor in recent years with the obvious upshot being an increased demand for warehousing, distribution, and delivery services.
Ownership models
Retailers choose to own or rent the property from which they conduct their business. There is no general rule but smaller warehousing (circa 2000 m2 floor area or less) is pretty much all sold by developers to small and medium sized businesses. Many businesses, especially those requiring more expansive facilities, choose to leverage the advantages of renting premises; this is typified by large warehousing and distribution centres. Some developers have responded to this demand with a build-retain-lease model, Goodman being a leading example. Build-retain-lease obviously requires the developer to have a ready source of capital and a prerequisite for this is that end users be considered ‘blue chip’ in their ability to pay rent on substantial lettable areas for long lease periods.
Design & Specification
Design life of the floor slab
Developers look to create buildings that are well suited to endure long lease periods that limit exposure to landlord maintenance costs. The slab is one element of the building that is hugely disruptive and expensive to replace or repair if it substantially fails. Thus, a warehouse’s concrete slab is carefully considered in its specification and quality of construction to ensure it is fit-for-purpose.
Specialist design and quality control
Developers have the option of retaining a specialist concrete consultant to be involved in the specification for the slab and its joints. Normally, the specialist would also be retained to monitor the method of construction and the quality of workmanship during the initial slab pours and at regular times during the slab construction. Note though, that the structural composition of the slab is designed by a suitable engineer.
Load evaluation
The slab will be designed for its intended end use, but typical loads adopted for logistics warehousing are:
7 tonnes maximum leg load (imposed by storage racking),
6 tonnes maximum forklift front axle load with pneumatic/80 mm solid nylon wheel tyres, and
40kPa live load (UDL).
Slab thickness & make-up
To accommodate the above loads, a typical specification is:
150 mm to 160 mm thick slab,
40MPa concrete at 80 mm slump,
3D fibre reinforcement at circa 30kg per m3 of concrete,
maximum shrinkage of 650 and 750 microstrain at 56 and 90 days respectively, and
a subgrade with a minimum CBR of 3%.
Floor Slab Sequencing & Installation
Sequencing: roof on, walls up
The pouring and curing of concrete, and its ultimate quality, are affected by climatic conditions during and after construction. Specifically, the slab must not be exposed rain during installation nor drying conditions (wind and sun) during installation and during curing.
For this reason, the installation of the warehouse slab is normally delayed until the building is predominantly watertight and wind-proof. So, slab follows installation of the roof and exterior walls. It is acceptable to start pouring the slab at the far end of an enclosed warehouse when, say, the roof and walls are complete in the vicinity of the pour, whilts the opposite end of the warehouse is not quite fully enclosed.
Maximum pour size
Typically, the size of each slab pour is restricted to a maximum of 900 to 1,000 m2. A pour is bordered by either exterior walls, formwork, or a completed adjacent pour. This limit is imposed to control quality whilst finishing the surface and, potentially, to reduce the impact of cracking in the finished slab.
The pour sequence of adjacent pours will be scheduled ‘hit-and-miss’ working away from an agreed start point.
Slab joints
Joints between slab pours
Joints between adjacent slab pours are formed using permanent steel formwork which incorporates twin steel top lips, circa 12 mm wide on plan, with lugs that project diagonally downwards into the slab both sides of the joint. The lips tightly abut at slab pour. The permanent formwork thus creates an ‘armoured joint’ which protects the arris of the slab at the joint interface. As the concrete cures and shrinks, a gap opens between adjacent slabs (typically with a designed restriction of 12 mm maximum after shrinkage). This allows for expansion and contraction of the finished slab during is lifespan. The steel lips afford protection against impact damage in use and, specifically, from the impact of warehouse forklift wheels. The cast-in lugs divert shock into the slab body, lessening surface shock and cracking.
Permanent formwork to create armoured slab joints
Regardless of the armour, the surface and sub-surface of the slab adjacent to the slab joint is vulnerable to cracking and damage from forklifts if the gap between the armoured joints is not filled. Wheels passing over a gap will strike the steel lip and, over time, the slab will crack and fail. For this reason, slab joints that will be exposed to forklifts are also filled with a specialist compound that is hard enough to withstand constant forklift traffic yet does not impair the ability of the joint to accommodate movement in the slab. The filler basically bridges and reduces the gap and the potential for the forklift wheels to strike the steel lips.
Some systems include a ledge at the base of the permanent formwork. The ledge supports the liquid filler upon installation and also when the joint opens due to shrinkage, ideally retaining the filler adhered to one side of the joint.
A common quality issue is the failure of the filler due to it not adhering to the face of the joint and falling through it onto the sub-base.
Joints that are not exposed to forklift traffic (those beneath warehouse racks, for example) are usually filled with a regular flexible compound.
Joints around columns
Concrete columns pass through the slab onto their individual pad footings below. To allow differential movement, the slab is isolated from columns using a formed joint that is filled with a proprietary foam expansion joint product (typically two layers of ‘Ableflex’).
Nevertheless, cracking can be a problem if the joint between the column and the slab has re-entrant corners (i.e., on plan, the joint is square or oblong). This issue can be mitigated by adopting circular joints around columns on plan (or half circles at perimeter columns).
Circular-formed joint around a structural column [plan view]
Laser-guided screed machines
The construction of slabs in larger warehouses is suited to the use of modern laser-guided finishing machines. The leading manufacturer of these pieces of equipment is Somero. The machine is operated by a single person – with operatives in attendance feeding concrete to the slab – and delivers accuracy and quality in the level and finish of the concrete’s surface that cannot be easily achieved by manual labour alone.
Surface finishing
A burnished finish to warehouse slabs is the default; it provides a smooth, durable, trafficable surface that does not trap dirt, dust, and grime. Often, the surface can be almost glass-like in the best quality work. Concrete contractors must wait for the optimum time to begin the burnishing process – a window of opportunity between the concrete surface being too wet and the surface being too dry. The wait time will depend on local climatic conditions and the amount of water introduced to the concrete. In simple terms, the surface needs to be set enough to permit operatives with power-floating machines to be on it without the concrete surface defecting. Power floats use an inverted ‘helicopter blade’ mechanism to buff the surface of the concrete.
Overhead lights reflecting in a glass-like surface finish
Curing
Concrete must be cured to safeguard its ultimate strength. The objective of curing is to retain moisture within the concrete until the setting process has progressed sufficiently. Often, the preferred curing method is a blanket system comprising a temporary membrane applied to the surface of the concrete after burnishing for 7-14 days. A typical blanket system is UltraCure.
Liquid compounds that meet the requirements of AS 3799 are more commonly used by concreting contractors although each has its own draw backs. For a start, liquid curing compounds are not removed, and the residue can detrimentally impact the visual appearance of the slab. A good example of this is the common use by concreters of chlorinated rubber compounds due to their low cost and ease of application. However, the use of chlorinated rubber is prohibited by some developers due to its tendency to retain ugly forklift tyre marks when the warehouse is operational.
An interesting product called for by some end users is the application of Ashford Formula. A north American product, it is marketed as a curing compound (although, at the time of writing, Ashford Formula does not meet the requirements of AS 3799). Ashford Formula’s big advantage is that it provides superior surface hardness by internally sealing the concrete with a process of crystalline growth. This renders the surface more dust-proof, due to wear and tear, than other curing systems. It has been commonly used in places like Bunnings.
Benchmarks
Surface level tolerances
The degree to which a surface must be accurately level becomes critical when you build a tower off it. In warehousing ‘the tower’ is the warehouse racking system having a height that is many times its base width. A few millimeters out of level at the base can result in centimeters out of plumb at the top of the rack. Racking that is not erected plum presents an unstable risk especially when typical stored product loads are considered.
Major Australian warehouse developers tend to utilize the American Concrete Institute’s F-Number System to set specifications for the specification and measurement of concrete floor flatness and levelness.
The system adopts two F-Numbers: FF for flatness, and FL for levelness. F-Numbers are linear, so, for example, FF 20 is twice as flat as FF 10.
F-Numbers are commonly stated with a Specified Overall Value (SOV) and a Minimum Local Value (MLV).
Post-pour tolerances of set concrete are usually conducted within 72 hours of a pour using a Dipstick floor profiler measurement device.
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