Q. Can I achieve a ‘maintenance free’ floor slab?
A: No. A floor slab provides an operational platform for equipment and this very operation creates wear and tear that must be addressed. It is of paramount importance that the floor slab is subjected to an adequate housekeeping and maintenance regime in order to provide the on-going serviceability for which the floor was originally designed. This is the responsibility of the user, although maintenance contracts can be placed with nationwide concrete flooring.
To accomplish this, an industrial concrete floor slab should be subject to regular maintenance inspections by the floor user (or appointed representative) and any identified issues dealt with promptly in order to avoid unnecessary, exponential deterioration of the concrete surface and particularly of joints. Continued trafficking by mechanical handling equipment will inevitably take its toll on the concrete floor. For further details please see our concrete floor repairs and maintenance page.
Q. What is curling?
A: Curling is the warping of a concrete panel bounded by joints, saw cuts or a crack, where the edges are left higher than the centre. The process is complex and the subject of ongoing research both in the UK and USA but is primarily caused by the differential shrinkage of concrete, and can occur at corners and edges of panels. As the top surface dries and shrinks at a different rate to the bottom, the floor curls upwards, creating a ‘toast’ effect.
Whilst curling is quite common in concrete floors, it often does not cause any undue concern or disruption to the daily running of a warehouse. If good construction practice and joint detailing is carried out and attention paid to the mix design, then the practical significance of curling will usually require no further treatment.
Where induced contraction joints (or cracks) have curled significantly and present an issue regarding serviceability, the accepted method of treatment is to reinstate aggregate interlock by resin injection and then grout the void under the slab and grind smooth the surface restoring the surface regularity across the joint (or crack).
Q. What is delamination?
A: Delamination is a process whereby a thin layer (2-4mm) of a floor surface becomes detached, lifts slightly, and then breaks down rapidly when trafficked. The formation of a number of fine cracks are a tell-tale sign that delamination has occurred, and can be verified by tapping the surface with a metal object – the resulting hollow sound will confirm delamination of that particular localized patch.
The greatest cause of delamination is inadequate and inconsistent weather protection during (and for a 7 day period, after) the concreting and finishing operation. An enclosed environment is essential to ensure that cross winds do not travel across the fresh concrete at casting stage, and hence adversely affect the volume of bleed water available and required to allow the material to be worked monolithically into the base concrete.
Delamination can be repaired by squaring off the affected area, inducing shallow saw-cuts, removing the lifted surface, then filling the patch with a suitable cement or resin-based mortar system. Where small affected areas are away from frequent traffic and have yet to deteriorate, a low viscosity epoxy
resin can be injected between the lid and the base concrete to create the necessary bond.
Q. What is crazing and is it a serious problem?
A: Crazing is a pattern of fine and shallow cracks that develop over the initial few weeks and months after slab completion. These fine cracks are caused by differential contraction during drying shrinkage, giving a reduction in volume and creating tensile forces between the surface layer and the underlying concrete. This difference in the rate of contraction with depth creates the surface tension effect that leads to crazing.
Surface crazing of power trowelled concrete slabs is a very common phenomenon, affecting the majority of industrial floors, and can often be the price a client pays for wanting a “shiny” surface. Crazing is an aesthetic problem only and will usually have no negative implications on the structural integrity or abrasion characteristics of the slab, and any surface delamination that may be exhibited will be totally unrelated. Crazing is a normal occurrence, therefore no structural or serviceability issues are associated. These should be left untreated, but monitored on a regular basis for the foreseeable future in case of deterioration.
Q. Why do floor slabs crack?
A: Almost all cracks in ground floor slabs are the result of shrinkage, or more specifically, restraint to the drying shrinkage of the new floor slab. The risk of these occurring can be minimized but it is unlikely that any designer would guarantee a crack free floor slab.
Shrinkage cracks can be fully repaired by resin injection or similar methods. Resin injection will stabilise a shrinkage crack and provide a repair that is stronger than the parent material. It is a relatively simple and quick operation that results in a repair that is highly visible as a light coloured mark on the surface of the slab until such time as it ‘blends in’ to the surrounding material. It will not result in a loss of surface flatness or abrasion resistance.
Q. My floor is slippery; how can I remedy the problem?
A: This has probably been caused by inadequate cleaning of the slab; in effect the sweeping machine and foot traffic has polished the floor leading to it becoming slippery. Please see our concrete floor cleaning solutions page and also anti slip concrete flooring page.
Q. I want a coloured concrete floor slab; should I apply a coating or use a dry shake topping?
A: The Aesthetics, colour and durability of the finish to a new floor are very important and these potentially contentious issues need to be addressed thoroughly by the Client and the Specifier during the design process.
Based on experience, where the Client’s priorities are for a high quality, low maintenance, ultra hard wearing floor finish with reasonable colour consistency, then they should be guided towards a Coloured Dry-Shake Topping; but where a totally uniform, monotone coloured finish is required, they should be guided towards an applied Resin coating or screed. Please see our dry shake surface hardeners page.
Q. Why is the surface patchy in appearance?
A: Concrete is a natural material, sometimes produced with less than scientific precision, meaning that batches of concrete will have minor variations of water content and perhaps even slight aggregate variations.
A dry shake topping material goes some way to minimising this effect, but not negating it. However, the greatest cause of this patchy effect is the variable drying rate of the base slab. Moisture levels may vary across the floor when the concrete is laid. This in turn may prevent consistent timing of the floating and trowelling processes. Good concreting practice then insists that the floor be cured efficiently, which means slowing down the rate of evaporation of the water.
Any areas of the floor with appear darker will contain a slightly higher moisture level than perhaps other areas, and vice versa. These variations will gradually reduce with time as the slab dries to a consistent level. However, it is important that a good housekeeping regime is introduced, as regular cleaning will speed up the improvement.
A well laid and finished coloured dry shake floor will, if cleaned regularly, provide an excellent industrial floor finish that will outlast any form of applied finish. It must be understood though, that a coloured concrete slab will not be as uniform in appearance as a newly painted floor.
Q. How important is curing for concrete floor slabs?
A: This is one of the most crucial parts of the floor laying process. The loss of water in fresh concrete can be considerable, even when working inside of a building or warehouse. This can then lead to non-uniform shrinkage and can reduce the durability of a slab. To avoid this, all slabs should be cured immediately after the final finishing process has been completed using a suitably efficient curing compound.
Q. What are construction joints?
A: Construction joints (or day joints) are described in TR34 as ‘formed free movement joints’. They are required to separate the building into panels suitable for pouring, as rarely can an entire warehouse be cast in a single day. The construction joints are positioned at the limit of each days pour (or sometimes less when the aspect ratio needs adjusting). For ultimate performance they should be created using a proprietary steel armoured joint system which will have steel plates anchored to the concrete slab either side of the joint to prevent concrete corners (arisses) from breaking up under the action of traffic and a heavy duty dowel system.
All drying and thermal shrinkage is accommodated at the joint itself so the joint openings therefore can typically be from 10 – 25mm wide (dependant on distance between joints, shrinkage characteristics of concrete, environmental conditions etc.). A linked strategy is to try and locate the day joints away from heavily trafficked areas if possible. Please visit our steel profile floor joints page.
Q. How quickly can I load my ground floor slab?
A: We would suggest the following timescales:
1. No access is permitted onto the slab for any person other than the finishing crew until the curing membrane is thoroughly dry; this is normally 24 hours
2. No person is allowed to walk on the floor slab for a period of 72 hours after completion of finishing except for approved personnel and the floor surveyor
3. Where practical, barrier tape is to be erected to restrict access
4. At 7 days old the slab can be trafficked by light vehicles and scissor lifts
5. Heavy Goods Vehicles shall not be allowed on the slab for a period of 28 days
6. A designated single access point for HGV’s shall be identified. An unloading area adjacent to this, 15m x 36m, shall be protected with a breathable floor protection product and covered with 20mm thick plywood or chipboard
7. Lay down areas should be located in the centre of panels (minimum 5m from construction joints) and protected with 1200 gauge polythene and covered with 4mm hardboard sheeting
8. All floor protection shall be inspected at the end of each day and any damage remedied before the next day
9. Ensure that the fit out is coordinated with the casting dates (i.e. erect racking on the oldest panels first)
10. Construction joints are to be co-ordinated with racking legs
11. Avoid bolting of racking that spans construction joints until the slab is 90 days old
12. The racking should remain un-loaded for as long a period as possible (minimum 90 days)
13. All plant using the slab to be fitted with non-marking tyres or wheel covers and oil drip catchers and should be checked and maintained in good condition
14. Floor cleaning is to be carried out strictly in accordance with the recommendations to be provided by the sub-contractor or advisor
15. Maintain temporary weather protection at doors in place for a minimum of 7days after casting of slab, unless being replaced by permanent works. Keep all dock-leveller, pedestrian access and other doors closed, except when in use
Q. What is Steel Fibre Reinforced Concrete (SFRC)?
A: SFRC is a composite material; concrete with steel fibres mixed in prior to discharge. To design and build a good floor slab using SFRC, it is necessary to understand the properties of the constituent materials both individually and in relation to each other, in order to arrive at the optimum combination of concrete mix, fibre type and fibre dosage. Please see our steel fibre reinforced concrete floor slab page.
Q. If I use Steel Fibre Reinforced Concrete, will I see lots of fibres at the surface?
A: This is usually a client’s greatest concern. However, experienced contractors have perfected the choice of tools, equipment and materials to consistently achieve what appears to be virtually a ‘fibre free’ surface. This is in line with TR34, (1994) 3.2.6 Section B/vi (p30), and quote; “under normal finishing processes very few fibres will be left exposed at the surface of the slab” However, should there be a problem with a few fibres visible at the surface, TR34 also says that “any such fibres exposed at the surface will corrode and will be lost under trafficking; …. This does not lead to disruption of the concrete surface”
We strongly recommend that a client visits a number of projects where you can assess for yourself.
Q. How do I ensure the sub base is laid and compacted correctly?
A: There are many specifications regarding the laying and compacting of the final layer of sub base material and dependent upon whether the concrete slabs are external or internal or perhaps improved in some way with concrete piles are stone columns, the answer can differ substantially. We at Nationwide Concrete Flooring understand thoroughly the benefit of a solid working platform beneath the concrete slabs. For more details on how we as a reputable concrete flooring contractor lay and compact the sub grade material, please see our Laser Controlled Sub base laying page within the Services section of the site.
Q. Is it necessary to seal the joints within a concrete floor slab?
A: Joints within warehouse concrete floor slabs commonly breakdown under heavy trafficking. The joints also quickly fill up with debris and unwanted materials if not sealed. There is also the “trade off” when sealing joints as soon as the slabs have been cast and the concrete has not been allowed sufficient time to shrink fully. Due to client handover periods and potential access problems, concrete floor slab joints are often sealed too quickly. It is recommended that wherever possible joints should be sealed to minimise “arris” breakdown and debris ingress. An allowance should be made in cost sings for return visits to site once the floor has completed most of the drying shrinkage, which is usually after approximately 12 months. Nationwide Concrete Flooring use their own proprietary joint sealant known as Instant Seal 101 which instantaneously seals the sawn induced joints, immediately after the saw cutting process. Slow curing, time consuming and sometimes messy tube sealants are only required at the main construction joints and sometimes at the perimeter and steel columns. For further details on Instant Seal 101 joint sealant, the only joint sealant that requires no curing time, please see the Joint Sealants page within the Services section on the website.
Q. When are my concrete floor slabs beyond repair?
A: There is no simple answer to this question as the variables are too infinite. A common comparison can be used with “pot holes” in the main road scenario, you can only patch up for so long before it becomes more costs effective to replace the full thing!
Both client and flooring repair contractors need to be comfortable that the repair agreed is going to be suitable and will last for the length of time required before further monies are needed. After reviewing the quantity of repairs and the size of the concrete floor slab and the consideration of what the client intends to use the slabs for a decision can usually be reached quite quickly. Nationwide Concrete flooring part of The Nationwide Diamond Group currently has 4 full time concrete floor surveyors capable of looking at concrete floors anywhere in the country. For further details and options on concrete floor repairs, please click the Concrete Floor repairs & Maintenance section on the website
Floor slab overlays are another alternative to carrying out numerous concrete repairs to your old and tired floor slab. The concrete overlay can provide an excellent brand new working platform for warehouse operators without creating any further risk and costing hundreds of thousands of pounds. The only negative side to the concrete overlay is the requirement for the occupier to be out of the building for a set amount of time and the increase in existing floor levels. Nationwide Concrete Flooring has carried out numerous floor slab overlays and has a page dedicated to this element of works on their website titled Concrete floor slab overlay
Should the floor slab level become a problem for future or existing warehouse operators the full floor slab removal and reinstatement solution may become the only option. If the floor joints have settled beyond repair and or the slabs are too uneven for fork lifts to traffic, then removing and replacing the floor cannot be as expensive as you think in the long term. Rather than using expensive resin injection solutions or pumping grouts underneath the existing floors Nationwide Diamond Group can remove and replace your concrete floor extremely cost effectively and quickly to provide with the best long term solution to your long term needs. For further details please see the Concrete Floor slab removal and replacement page on the website