As the main production/operation/ movements happen on the shopfloor, the performance of the floor is vital and a key component for factory operations. This Paper is about The Revolution in Concrete Floors in achieving superior finishing with accuracy and flatness for Free Movement (Level FM3, FM2, FM1) without any topping or leveling course. To create such an efficient design, planning and executions play the most important role, this Paper elaborates on the same.
This Paper takes the reader through a case study of Industrial Flooring which achieved FM2 certified floor. This was meant for the Reach Truck (RT) Operating for a height of up to 13 meters (42 feet), without a side shift. There was no expansion joint in the floor where the building was 330m long and 50m wide. Moreover, there was no PCC (Plain Cement Concrete), as the base for the floor was achieved after precise pre and post planning, and execution of the steps which are highlighted below:
- Selection of joints such as Free Movement Joints, Restrained Movement Joints, Tied Joints and Isolation Joints.
- Selection of a mixed-design, based on the load/ sqm of the floor, including reinforcement considering the thermal movement and selection of shrinkage admixture to avoid any dry shrinkage.
- Selection of the right kind of floor hardener which gives better abrasion resistance and slip resistance.
- Selection of Construction and Contraction Joints based on the layout of the floor and the sequence of the concrete
- Selection of Armour Joints based on the concreting sequence.
- Adaption of Methodology for Concreting, including proper level check, proper protection of floor and making the area airtight during concreting.
- Adaption of the right kind of equipment for concreting such as Laser Screed, Truss Based Screed vibrator, VDF.
- Post concreting protection of floor and proper curing.
- Surveying of the floor to check the level and flatness of the floor.
Industrial floors play an important role when it comes to operation, as all processes are operated on the floor. The strength and durability, among other factors, are important. The flatness and the level are also crucial when it comes to MHE (Material Handling Equipment). To achieve these, a precise specification has to be derived and executed. These require attention from the design stage to post concreting stage, and various processes and measures have been discussed in this Paper.
Importance of Flatness and Levelness
The selection of the appropriate specification is crucial, especially because it is common to find over specified floors that are not suited for the clients' specific performance requirements, which require more time, resources and money to accomplish.
Surface regularity specifications are divided into two main categories, according to the type of movement the Material Handling Equipment (MHE) are able to do.
- Free Movement (FM) specifications correspond to MHE that may operate in all directions and undefined paths within the facility.
- Defined Movement (DM) specifications correspond to MHE that may operate within a defined path, area and direction within the facility. These facilities implement a Very Narrow Aisle (VNA) system.
Ground floor slabs in Warehouses and Distribution Centres are integral to the efficient operation of the facility. They are the table top on which an operator runs his business. On the surface, they appear to be one of the simplest parts of a structure to construct. However, this simplicity often leads to an underestimation of the design and construction requirements. A well designed and constructed floor will increase productivity, reduce maintenance of the building and increase the life of the equipment using the floor.
A floor design should be performance-based, and this starts with understanding the requirements of how the floor will be used. Most floor designers will be given a brief from the warehouse owner to ensure the floor will take the imposed loads, be flat, long-lasting and aesthetically pleasing, and it will be able to fulfil their responsibility to draw up the performance specification. The specialist flooring contractor should consider the construction methods, programme, cost and any value engineering they can bring to the design. Compromises may have to be made in developing the final design of the floor, but the result should be one that is economically designed and built to meet the performance requirements.
|Table 1: Floor classification as TR 34|
|Floor Class||Typical Floor Use||Property|
|FM 1||Where very high standards of flatness and levelness are required.
Reach trucks operating at above 13m without side shifts
|FM 2||Reach trucks operating at 8 – 13m without side shifts.||6.5||2.0|
|FM 3||Retail floors to take directly applied flooring.
Reach trucks operating at up to 8m without side shift.
Reach trucks operating up to 13m with side shift.
|FM 4||Retail floors to take applied screeds.
Workshops and manufacturing facilities where MHE lift heights are restricted to 4m.
|Note: Side shift is the ability of a truck to adjust the pallet transversely to the fork direction. Values mentioned in the table are in mm. Here E notifies the Levelness and F notifies the Flatness.|
This stage requires designing the total load on the floor, base, and sub-base, grade of concrete, floor hardener, construction joint, contraction joint, expansion joint, and type of sealant.
Flatness and Levelness requirements (Surface Regularity): There are essentially two methods for defining floor flatness requirements. Floors are divided into Free Movement Areas (FM) and Defined Movement Areas (DM). The FM criteria are where trucks operate at a low level when moving, such as marshaling areas, block stacking areas, and aisles greater than 2.8m wide.
Note 1: The above values are permissible 95 percentile values for Properties F and E.
Note 2: For all classifications, all points surveyed should be within 15mm from the datum.
The ASTM is a comprehensive method of measuring the overall performance of the floor based on statistical analysis known as F numbers. The standard is based on two numbers: FF for flatness (short wavelength) and FL for levelness (long wavelength). However, there is no control to the datum and the sample rate is comparatively small.
There are two parts to a floor design:
A. Structural Design: the ability to withstand the applied loads
B. Detailed Design: joint design and other performance requirements.
The structural design of a floor will follow engineering principles and calculations, and requires knowledge of the following in order to determine the slab thickness and reinforcement:
- Ground conditions
- Strength of the concrete
- Thickness of the concrete
- Method of reinforcement
- Load transfer capability of the joints
- Loading specification and loading pattern
A detailed design covers key performance requirements, including-
- Joint layout and joint design
- Construction methods
- Method of reinforcement
- Surface regularity (flatness and levelness)
- Abrasion resistance
Suitable ground conditions are essential for ground supported slabs, and it is therefore important that a ground investigation is carried out and interpreted by a competent geotechnical engineer, who must advise on the suitability of the ground along with any recommendations for ground improvements.
The primary purpose of the sub-base is to provide a level base for the floor slab. It should be capable of carrying construction traffic without rutting. The sub-base should be of a minimum 200-250mm thick well-graded material. The level tolerance of the sub-base is of great importance. A high sub-base generally means a thinner concrete floor (+ Zero to – 25mm is recommended).
Slip membranes are used to reduce friction between the slab and the sub-base so as to reduce the restraint to drying shrinkage, thus reducing the risk of unplanned cracking. A 300-micron plastic sheeting is recommended.
Concrete not only plays an important role for durability and strength, but also for levelling course. This shows the extent to which the concrete shows flexibility, if designed and executed properly.
There are comprehensive standards for the specification and testing of concrete. High strength concrete is not needed and should be avoided because it tends to have higher cement content and is more likely to shrink than lower strength concrete. It can also be difficult to finish.
Concrete shrinks as it dries out over the first 12-18 months of its life. Fresh concrete has more water than is needed for the chemical process of hardening, and some of this excess water leaves the concrete, causing it to shrink. Therefore, the amount of cement and water in the concrete should not be excessive.
The consistency of the material properties and delivery to site is the key to a well-finished floor slab. Utmost care has to be taken while executing as the "final finish is all about the concrete".
Grade of concrete for the floor was M30 Grade, with cement OPC 53 along with 20% fly ash. All concrete was mixed in batches in the fully automated Batching Plant. In no case was the temperature of concrete allowed to be more than 30 degrees, and during summer, cold water was used to keep concrete temperature within permissible limits.
Although reinforcement will give some enhancement to the structural load capacity of the floor slab, its primary function is to restrain the opening of sawn-induced joints and to maintain good load transfer properties.
The traditional method of reinforcement in jointed ground supported slabs is with steel mesh. The position of the mesh is usually specified to be 50mm from the bottom of the slab. Alternatively, floors can be reinforced with steel fibre also. The joints can become wider than predicted and load transfer between sections of the floor can be lost. This can result in floor movement at the joints and a breakdown of the joint.
Importance of Joints
Joints are the most critical element in a floor. Most maintenance requirements are related to the breakdown of joints in some form. Joints create unavoidable discontinuities in a floor which can be damaged by mechanical provided for two purposes:
- To form the boundaries of each day's concrete pour (Construction Joints)
- To reduce the risk of cracking as the floor shrinks (Contraction Joints and isolation details) Construction joints must incorporate dowels to provide a load transfer mechanism between the sections of floor.
Contraction joints are most commonly created by saw cutting. Sections of cast floor are usually cut into panels of 6m x 6m. The intention is that a crack will then form beneath the saw cut relieving drying shrinkage stresses. The reinforcement must be continuous across the sawn joint to restrain the opening of the engineered crack and to maintain adequate load transfer.
Floors must be isolated from fixed elements of the building to allow the floor to contract without cracking. This is achieved by surrounding columns with compressible materials. These are known as isolation details.
Ground supported slabs can be jointed or jointless.
- Jointed floors have construction joints at the edges of each day's concrete pour, and they typically have sawn joints at 6m intervals.
- The term jointless is something of a misnomer as all slabs have construction joints at the edges of areas of the floor that are poured in any one day. On a jointless floor, these are the only joints. Joints are provided to permit breaks in construction and to allow for shrinkage as the concrete dries out over a period of up to 2 years.
Some floors are built without sawn joints. Care must be taken to ensure that all possible steps are taken to minimize the restraint to shrinkage, including mix design, correct curing, and limiting pour sizes.
Abrasion Resistance / Durability: For most warehouse applications, a self-finished, power-trowelled slab will provide a durable working surface, provided that concrete selection is correct and good finishing techniques are applied. Surface durability is primarily a function of the densification of the surface by power-trowelling, and, most importantly, by proper curing.
Construction Methods: The floor construction method is critical in determining the overall detailed design, construction programme, and the cost. The construction method will determine:
- Daily output - programme
- Joint layout Surface regularity
- The concrete mix design
- How the floor is reinforced
- Long Strip
- Large Area Pour – Laser Screed/Truss Screed Vibrator – Jointed
- 3. Large Area Pour – Laser Screed/Truss Screed Vibrator – Jointless
Floor Flatness (FF) controls the local bumpiness of the floor surface and is primarily affected by the finishing operations after screeding, including re-straightening, and power floating. Floor Levelness (FL) controls the departure of the floor surface from the specified slope or plane of the surface. FL numbers evaluate the elevation differences along a sample line at 3m intervals. The higher the FL number, the more level the floor. Levelness of the edge forms and the accuracy of the concrete screeding operation controls the overall levelness of the floor.
For large concrete placements using truss base screeds, these tools are primarily used after screeding and before excess bleed water accumulates on the surface. As shown in Figure 5, always restraighten the "mend" and "grout" lines, otherwise, there will be high spots along these lines that will reduce the flatness of the floor. The modified highway straightedge is the tool of choice because it can also be used as a reference line to identify high and low spots on the floor surface. A bull float can smooth the surface, but it cannot perform the cut-and-fill process; and due to its short length, its use will reduce the floor flatness.
If you have trouble using the bull floater after screeding because it digs into the surface, then use an 8 to 10-foot-wide channel float or check rod. Do not use a 4 to 5-foot bull float. A bull float can smooth the surface but it cannot perform the cut-and-fill process; and due to its short length, its use will reduce the floor flatness. Also, make sure your straightedge, whether it is modified highway straightedge or not, is straight and true. If warped or damaged, you will make the floor less flat.
If needed, you can angle a re-straightening tool about 45 degrees to the axis of screeding and overlap the previous pass by about 50 percent of the straightedge width. However, be careful not to undo the flatness produced by the laser-guided screed.
A Surface Hardener is applied to the concrete surface after it is smoothed and left to dry. This helps to improve the concrete's abrasion and chemical resistance, thus significantly improving the durability of the concrete surface. It is non-metallic in nature. Non-Metallic Floor Hardener is designed for use as a surface hardener for concrete floors, which are subject to wear due to abrasion or impact from light to medium traffic.
An uneven surface will make it more challenging for material handling vehicles to operate, potentially increasing the wear to the vehicle itself. In cases of serious damage, the area may have to be isolated while repairs are undertaken. This can be costly in terms of disruption to operations as well as the actual cost of the repair itself. Abrasion can be caused by the constant movement of material handling vehicles over the surface; or by dragging items across the floor; or by impact damage caused by dropping objects on to the floor. Certain substances can also cause damage, often caused by the chemicals in the substance slowly disintegrating the concrete.
A surface hardener adds a protective layer to the concrete floor, reducing the amount of surface wear and tear caused by abrasion. This helps to extend the life of the concrete floor, reduce repair costs, minimize disruption, and keep the floor's smart appearance for years. We used "Rockland Qualidur" as dry shake floor hardener, the spreading quantity was 5 kg/sqm to achieve better abrasion and slip resistance.
Floaters are circular metal disks that clip on to the blades of either a walk-behind or a ride-on power trowel. Floaters make floors flatter by generating and moving "cream" or surface mortar. Pans easily remove surface imperfections, cut humps and fill in holes and low spots — that's why power floating with pans is so important. Expect a 10- to 25-point increase in F numbers when floating with pans instead of float blades. Pan floats are wider and flatter than float blades: pans cause less dishing and troughing due to the reduced contact pressure compared to float blades, and they minimize surface variations due to bent arms and float blades.
Pan floats are circular metal disks that clip on to the blades of either a walk-behind or a ride-on power trowel. Pan floats make floors flatter by generating and moving "cream" or surface mortar. Pans easily remove surface imperfections, cut humps and fill in holes and low spots — that's why power floating with pans is so important. Expect a 10- to 25-point increase in F numbers when floating with pans instead of float blades. Pan floats are wider and flatter than float blades, pans cause less dishing and troughing due to the reduced contact pressure compared to float blades, and pans minimize surface variations due to bent arms and float blades.
It is important to start power floating early while the concrete is still plastic. However, starting too soon can create too much surface mortar reducing the floor flatness, especially when using a truss-guided screed. Delay power floating until you leave no more than a 1/8-inch footprint in the concrete. Otherwise, you increase the risk of cutting troughs and creating surface waves that will reduce the floor flatness.
The following day, the floor is sawn into panels on a 6m grid to relieve drying shrinkage stresses. This type of floor construction is typically reinforced with a single layer of mesh 50mm from the bottom of the slab. As per the standard and codes, the joint has to be cut within 24-36 hrs of pour to achieve a crack-free floor.
Large Area Pour - Laser Screed/Truss Screed Vibrator – Jointless: This is constructed in the same manner as the jointed floor described above, but no joints are sawn. Jointless floors require good control of sub-base flatness and levelness to reduce the restraint on the underside of the slab. The large distances between construction joints can often result in wider openings, and joints can open up as much as 35mm. Careful consideration should be given to the location of these joints and the impact they have on trucks.
Curing and Protection
The floor should be cured properly and submerged in water for at least 7 days from the date of pouring, and no vehicle be allowed to move on it before 21 days of the pour date. There are different ways of curing: by water flooding the panel or by a curing membrane. At our site, we used to cover the panel by LDPE sheets and above it we poured the water, and in this way, we ensured that the floor was properly cured and taken care of.
It is worth noting that there is no purpose in selecting and implementing an appropriate specification if there is no way to check if this specification is achieved. The results of this process can be used to check if the contractor has delivered what was promised. Also, to access the need and the cost involved if the correction or an upgrade is required. The role of surveying is to get an accurate reading of the surface regularity of the floor.
All specifications measure flatness (the variability over a short distance i.e.300mm) and levelness (the variability over longer distances i.e. 1,200mm).
Flatness could be considered a measure of smoothness for the truck drivers. Levelness has more effect on the stability of the trucks, particularly those in aisles. Variability across the width of the aisle and/or between the front and the rear wheels of a truck can have a significant impact on truck swaying and nodding effects, resulting in poor efficiency and unsafe operation.
The lack of surveying often leads to endless debates about the quality of the floor and its suitability for the Material Handling Equipment (MHE) to operate on.
Face Digital Profileograph within a narrow aisle
The equipment measures the elevation across the load wheels (transverse) and the elevation between the front and rear axles of the truck (longitudinal). In addition to the elevation controls, the rate of change in elevation for both transverse and longitudinal directions is measured for every 300mm of travel down the defined path.
Although the three standards described above measure the floor properties in the same method, the property limits vary. TR34 and CEN 15620 have the same limits.
Recommendations on Surface Regularity TR34 Edition 3 2003 provide the most comprehensive guidance on the subject. For most installations, TR34 Classification FM2 is recommended when specifying a new floor, as this represents a good quality floor that can be achieved without excessive cost. For VNA applications a Defined Movement (DM) Specification should be used along with the category defined by the racking height.
The project was Certified FM2 Flooring, achieving a 97% accuracy level and flatness. It is among the first of its kind of flooring of this magnitude. Looking forward with Automatic Retrieval system (ARS) Warehouse coming in the future, the demand in FM flooring will increase with higher standards and specifications.
Through this Paper I have highlighted the way concrete has become the face of the projects and its requirement is not restricted only for the strength but to achieve levelled and flat floors that act as operation floor.
|Table 2: Comparison with Normal Floor|
|Description||Normal Floor||FM2 Floor||Advantage|
|Sub-base top layer||M10 grade PCC layer||GSB layer||
Yes. Time and cost reduction (approximately 65% reduction when compared to the cost of the PCC layer) in achieving a compacted sub-base layer. Better levels and ease of fixing formwork.
|Dry-shake hardener||Normal Non-Metallic floor hardener||Rockland Qualidur||
No. Increased cost. Yes. The target was to achieve an abrasion resistance value of 1 instead of 8 on Mohs scale which could not have been achieved as per original design.
Approx @ 5.9m
Approx @ 11.3m
Yes. Cost reduction in dowels as per original design and less number of joints. Joints designed as tied joints that will open only <1mm wide. The number of pours halved!
Equal angle armoring, 25 dia smooth rod dowels 500mm long @ 250mm c/c and sealant
|Yes. Zero cost of materials and labor. Not recommended as indoor slabs at existing temperature variance will not expand beyond poured dimensions!|
Level / Flatness
|At a nominal cost. The constructed floor with Proper Planning and Execution for Smooth Operation|
In an environment where the Material Handling Equipment (MHE) operates without suspension systems, the warehouse floor is the all-important surface that affects the speed, efficiency, maintenance needs, and often the safety and movement of the distribution center operatives and their manually or automatically-operated hardware. For this type of requirement, in today's time, the FM level floor has become mandatory.
This Paper presents that concrete has moved way ahead in digital India now for levelness and flatness. Concrete floor plays a crucial role in giving strength and durability, which is the most important part of Warehouse and Manufacturing Industries, and also for aesthetics.
This has been possible by micro-level detailing during the designing and planning stage, and most importantly during execution. Hereby, it proves that concrete, since many centuries, has served in different capacities and requirements and, going forward, there's going to be more invention, whereby concrete will evolve as the main ingredient for handling equipment when trafficked.
Guidance on the design of a floor is in detailed specified in the UK Concrete Society's Technical Report 34 (TR34)
ACI 224-Joints in Concrete Construction
About the author: Manoj Didwania is a Professional Technocrat with more than 13 years of experience in various fields of Construction including Tendering, Estimation, Procurement, Execution, Value Engineering, Planning, and Monitoring, and has executed projects in residential, industrial and logistics segments. He has worked extensively on supplementary cementitious material and has published a paper on sustainability and reckli concrete.
Acknowledgement: The article has been reprinted/reproduced from the Proceeding of UKIERI Conference with the due permission of organizer and author.