In warehouses and logistics buildings the concrete slab and flooring are critical to the effective functioning of the operations. However, it is often the perception that the concrete floor is one of the most straight forward elements of the project, and many times the overall attention paid to design and construction detail is less than proportional to its ultimate importance in the efficient operation of the facility. The expectation is that these large area floors must be constructed with lowest possible cost and provide problem free service year after year.
Function of the Floor Slab
The floor slab is constructed to provide a suitable wearing surface on which the operations in the facility may be carried out efficiently and safely. In the case of ground-bearing floor slab, the concrete slab distributes the applied loads without deformation or cracking to the weaker subgrade below. Piles supporting slabs are designed as a suspended ground slabs.
These requirements may also apply to other commercial floors whether they are trafficked concrete or are finished with high performance flooring systems. The following checklist discusses some of the principle issues for consideration when specifying and designing concrete floor slabs for logistics facilities. Specific slab construction properties may differ between industries or even within the same industry.
Typical Floor Slab Requirements for Warehouses and Logistic Facilities:
- Support operational and stationary loads without cracking and deforming
- Minimize the number of exposed joints
- Utilize maintenance isolation joints that do not impede vehicle operating speed
- Provide a durable abrasion resistant and dust-free surface
- Appropriate levelness and flatness tolerances to support material handling systems (“MHE”)
- Balance surface texture traction with cleanability
- Flexibility to accommodate possible future changes in operations
- Provide a safe and pleasant working environment
Floor Loadings
Load bearing concrete slabs-on-ground face two types of loadings: static and dynamic loadings. Static loads include for example, block stacking, equipment and machinery and storage racking systems. Dynamic loadings include material handling equipment (“MHE”), and other traffic including: forklifts, pallet stackers and other vehicles.
Static floor loads can be divided into three different types:
Uniformly acting loads
Uniformly distributed loads are generally larger foot print distributed load, for example timber pallets or paper rolls stacked on one another. In most other commercial buildings floors are designed for nominal loading, which are substantially lower than the distributed loads in industrial areas. When machines and production equipment are installed directly on the floors, their foundation can be regarded as a uniform load. In this kind of situation, it is important to consider and dampen potential vibration.
Examples:
- Block stacked pallet loads and paper reels (unit loads)
- Loads from fixed machinery and equipment
- Nominal loadings for light commercial and recreational use
Point loads
Point loads arise from any equipment or structure mounted on legs with baseplates. Fixed conveyor systems deliver a variable point loading and require vibration consideration. The most common static point loads are from storage racking. In a conventional static racking system, the loading is transmitted to the slab through the baseplates. Baseplates have a relative small effective contact area with floor. Most baseplates are fixed into the floor with bolts distributing the load.
Examples:
- Arena seating
- Clad rack buildings
- Mezzanine legs
- Point loads from fixed machinery
- Stacker crane mountings
- Storage racking legs
- Wheel loads from materials handling equipment
Line loads
Line loads, as the name suggests, are loads that act along a line, for example the weight of an internal partition wall resting on a floor, calculated in units of force per unit length. Some storage systems or equipment mounted on rails are linear loads that can be placed anywhere on a floor and can be of uniform, stepped, or varying magnitude.
Examples:
- Mobile dense racking system
- Partition walls
- Rail mounted fixed equipment
Trafficking has a great impact on the floor and its design. Material handling equipment present dynamic and point loads. Forklifts, pallets trucks and stackers move pallets and containers for bulk products or for order picking. Individual items are collected from storage, moved to packaging, and then to dispatch. Different kind of traffic can be divided by their function and type to: MHE operating in free-movement areas and wide aisles and MHE operating in very narrow aisle.
Typical vehicle operating “at floor level” is a pallet transporter, hand truck or trailer, often having maximum 3 tons capacity and small load carrying polyurethane wheels. Small and hard wheel contact surface generate high local pressure on the floor surface. Floor surfaces on which this equipment operates are typically flat and level. This light load transport equipment is commonly found in food distribution and other logistics centers. To avoid joint damage and subsequent spalling, contraction joints should be designed with narrow openings and/or filled with load bearing flexible resin to support the traffic.
Very narrow aisle (VNA) lift trucks require high flatness and levelness floor tolerances. This equipment operates in a narrow and fixed aisle between the high racking, picking or placing pallets. The wheels of this equipment are typically hard neoprene rubber. The vehicle has a fixed path and does not usually cause extreme and aggressive abrasion to the floor surface. This truck typically has three wheels and is guided by rails at the sides of the aisle or by inductive guide wires. Floor surfaces in VNA areas should be flat and level with no wide, stepped or uneven joints. In semiautomated facilities, consideration must be given to areas were the vehicle conducts frequent turns, especially when the third wheel rotates in place.
In free-movement areas and wide aisles, counterbalance lift trucks fitted with telescopic masts (forklifts) are frequently used for material handling. The load carrying capacity can be 10 tons or more, however in industrial buildings they normally do not exceed 4 tons, depending upon the load distribution. Lift heights are limited and do not normally exceed 7 meters. The tires are either solid rubber or pneumatic, generating less surface pressure than small hard wheels. These vehicles tolerate uneven surfaces and accommodate wider joint openings than hard wheel MVE. The softer tires, however, tend to pick up debris and scrap which results in excessive floor wear due to the high abrasion.
Structural Design and Slab Types
To ensure that the concrete floor will continue to carry its design loading successfully, it is vital to design and construct the subgrade as carefully as the floor itself. Pressures exerted on the subgrade due to loading are usually low because of the rigidity of concrete floor slabs and loads from forklifts wheels or high rack legs are spread over large areas. Thus, concrete floors do not necessarily require strong support from the subgrade. However, subgrade support must be reasonable uniform without voids or abrupt changes soften support.
Subgrade soils are considered problem soils when they are highly expansive or highly compressible such as silts and clays that do not provide reasonable uniform support. Proper classification of the subgrade soil must be conducted to avoid problem subgrades. The classification report provides information for needed subgrade improvement measures and design parameters for the concrete slab specification.
The structural design of the concrete floor slab on-ground is dominated by the sub-grade conditions and the floor loadings. The two design options are a ground-bearing slab, or a pile supported suspended slab. If consolidation of plastic soils is determined to be a potential problem a suspended slab may be the only effective solution, in which the floor slab is built on piles or between ground beams.
Both design types can be reinforced with steel mesh or fibers, or can be post-tensioned. Polypropylene macro-fiber technology is becoming more popular for ground bearing slabs.
Joints Design Requirements
Warehouses and logistic centers have a high volume of vehicular traffic. In order to maintain the long-term functionality and safe operations of these facilities, unplanned concrete cracks must be minimized and repaired, while planned expansion and contraction joints must be detailed to support the traffic. Proper design of the concrete mix, use of concrete reinforcement, satisfactory curing, and appropriate joint spacing all contribute to crack prevention. Cracking occurs when the tensile stress in a section of slab exceeds the tensile strength of the concrete. Unplanned cracks in a warehouse or logistic facility floor will quickly lead to deterioration causing safety issues and potential product damage. When cracks do occur, they must be cleaned and filled with traffic supporting semi-flexible resin.
Isolation joints design to accommodate normal structural movement are generally sealed with a highly flexible sealant. This practice will not work in warehouses and logistic facilities when the isolation joint is in a traffic pattern. A specialized joint system must be specified that will accommodate the movement and support the traffic without creating a discontinuity in the level surface.
Contraction joints, in theory, accommodate the movement created by the shrinking of the concrete slab as it cures. In practice, these joints continue to see movement due to temperature and humidity changes. These sawcut joints must be filled in areas expecting vehicular traffic. Left untreated, hard wheels will impact the joint edge leading to spalls. Similar to the treatment of cracks, traffic supporting a semi-flexible resin is used to fill these joints.
Abrasion or wear resistance is the ability of a surface to withstand deterioration caused by rubbing, rolling, sliding, cutting and impact forces. The abrasion mechanisms will vary greatly in different applications. Complex combinations of different actions can occur, for example, truck traffic, foot traffic and scraping. Excessive and early wear can result from under specified or under strength concrete or weak surface strength related to construction conditions.
Dry Shake surface hardeners, chemical hardeners, and high performance coatings provide cost-effective solutions to achieving a high abrasion resistance. Each of these enhances the performance of the concrete floor and can meet the specifications required for specific applications.
Abrasion resistance of the floor depends strongly on the composition the concrete and the hardness and toughness of the topping material, including finish coatings. There are number of tests available to measure the wear and impact resistance. Some measure the hardness of the material itself, some the surface wear resistance capacity. Standards EN BS 8204-2:2002 and ASTM C779 and ASTM C944 give guidance on abrasion resistance, performance classes, service conditions and typical applications.
Concrete is a porous material with limited chemical resistance. Organic and mineral acids react with the alkaline cementitious material eroding the surface. Many other agents, including most foods, oils, and some chemicals, attack concrete over time. Where chemical attack is likely, the floor should be protected with chemically resistant material and coating that resists the aggressive substance.
The final appearance of a concrete floor will never be as uniform as a coated surface finish. Concrete floors are constructed from naturally occurring materials, finished by techniques that cannot be controlled as precisely as in a factory process, and the conditions during installation will vary.
A typical concrete floor has a grey color. However, there ways to produce concrete floors with colors and provide different kind of appearances. Dry shakes hardeners containing pigment, providing a colored finish to the floor. The concrete floor can be colored by adding colorant in the concrete mix or by using acid staining or water-based dyes to provide surface color. A recent innovation uses a colored hardener in which fine pigments suspended in water are blended on-site with liquid floor hardeners. Light color shades, like yellow, beige, light grey or even white, provide higher reflectivity and brightness in the room. This may reduce illumination requirements and save energy costs. In large warehouses this can have a big impact on the sustainability rating.
Trowel marks and discoloration from burnishing are often a consequence of the normal variations in setting of the concrete or from poor finishing, such as over-troweling. Excess curing compound can cause darker areas. These wear and disappear with the time and use of the floor without having an effect on the surface.
Final Remarks
Only by providing the right combination of load carrying ability, controlling cracks, treating joints, appropriate tolerances and wearing surface performance will a warehouse floor allow the operations to be carried out as expected, with maximum efficiency and cost-effectiveness. Any defect in specification or workmanship will be exposed by the constant, demanding traffic found in these environments. Thus, the most important requirement for the floor in warehouse and logistics facilities is to provide a problem-free platform for the operations relating to functionality, durability and economy.