Frequently asked Questions:

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.

 

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.

Concrete can be thought of as graded aggregate bound together by mortar. The mortar content of good quality concrete should be optimised to reduce problems of porosity, shrinkage, adverse chemical reaction etc. without detriment to the placing characteristics.

Traditional ground-bearing slab construction uses saw-cut joints to induce shrinkage cracks and this can lead to user problems such as arris breakdown and curling. SFRC technology can be used to produce heavily reinforced floors that do not require sawn induced contraction joints (so called ‘jointless’ or ‘jointfree’ floors).

The purpose of steel fibres in concrete is to reinforce the mortar and bridge the aggregate. If the correct type of steel fibre is added to the concrete in sufficient quantity it will control shrinkage by preventing micro-cracks within the mortar from joining together to form larger shrinkage cracks.

 

Q. How does SFRC differ from traditional mesh reinforced ground floor slabs?

A: Concrete reduces in volume as the water needed for workability evaporates and it is the tensile stresses caused by this ‘shrinkage’ that leads to most cracks in ground floor slabs.

In a traditional slab design, the development of micro cracks as the cement paste changes state is not controlled. As a result, the micro cracks join together and unrestrained shrinkage cracks develop. These will widen to the point that aggregate interlock is lost and load transfer from one side of the crack to the other cannot take place. In this case, a slab will be designed to crack at prescribed positions (formed or saw cut induced joints at approximately 6m centres). We could describe these as “planned cracks”.

With SFRC at a sufficiently high fibre content, the jointless design is based on enhancing the flexural strength of concrete due to the uniform distribution of steel fibres throughout the matrix. As the concrete shrinks it develops micro cracks throughout the matrix. As the micro cracks develop they cause the steel to go into tension to withstand the shrinkage stress. At a suitable dosage rate of steel fibres, the shrinkage is controlled. No induced contraction joints are needed, hence the description of these types of slabs as ‘jointless’ or ‘jointfree’ even though these systems require day or construction joints in just the same way as more traditional options.

Whilst shrinkage for a jointless SFRC slab is controlled, it is not eliminated. This is evidenced at by the opening of the steel profile joint by, at times up to 15 - 20mm. The width of the joint demonstrates the total crack widths that would otherwise have accumulated across the panel for an unreinforced design (with saw cut joints).

It should also be noted that nominally reinforced floor slabs will see day joint width of between 60 – 80% of this figure. This is why very careful detailing is required for all ground floor slabs, and particular attention should be paid to panel sizes and joint positioning.

 

Q. If I use SFRC, 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 the quality of the finished floor achieved by the contractor in question.

 

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