Solution Features & Benefits

• The most cost effective quality flooring solution
• Fast-track construction by Laser Screed with output up to 2000 m2
• With care, slab should be free of cracks
• Saves cost and time by eliminating reinforcement bars.
• Joints will require some maintenance
• No Dry Shake Hardener is required with use of Steel Fabric
• No concerns over Fibres at slab surface with use of Steel Fabric
• Can be constructed to high tolerances of up to TR-34 FM2 Special

Design & Specification

Modern Slabs on Grade are designed on the basis of limiting the induced tensile flexural stresses in the concrete to a safety factor below the ultimate flexural strength of the concrete, therefore slab thickness is the most important factor in the design.  Restrained Saw Cut Joints are required in the Slab to relieve tensile stresses caused by Drying Shrinkage etc and prevent random cracking.  No reinforcement is used to resist the flexural stresses, however a nominal amount of reinforcement is required to ensure that the slab has sufficient ‘ductility’ or toughness in case some cracking does occur.

The soil is assumed to be elastic and of reasonable and uniform properties.  Although it is generally recommended that the Sub-Grade should have a minimum K-Value (Modulus of Sub-Grade Reaction) of 0.035 N/mm3, the Engineer approving the design must ensure that the soil will not result in excessive long term settlement and especially differential settlement.  It is always recommended that the Client engages a specialist Soils Engineer to review the Soil Report/ Borehole logs and advise on whether the soil conditions are suitable for a Ground Supported Slab.  A Plate Bearing Test is recommended to confirm the K-Value. 

Using modern design codes, slabs have become thinner.  As a result, the floor designer must take careful consideration into load transfer across joints and serviceability issues such as deflection and crack control.  Secondary reinforcement can be provided by a light single sheet of Steel Fabric or 20-30 Kg/ m3 of Steel Fibres.  The Steel Fabric should be continuous across the saw cut joints to provide suitable load transfer. 

The area of Steel Fabric reinforcement used should be around 0.1 to 0.125% in order to prevent restraint to shrinkage.  As the design theory is based on preventing cracks from occurring at the slab surface and the amount of steel is so small, the location is not important and modern designs allow the Steel Fabric to be placed in the bottom of the slab.  Although Steel Fibres may provide better crack control, at low dosages they provide limited load transfer across saw cut joints and as such there is an higher risk of defects such as differential settlement.

Regarding Performance Specification for the slab, CoGri Korea recommends the use of the U.K Concrete Society’s Technical Report No 34 for Floor Flatness and Abrasion Resistance Specifications.  The appropriate floor flatness specification should be selected from Tables 4.2 (general warehouse floors) and Table 4.3 (VNA or defined movement floor areas), based on anticipated lift height.  Generally large pour floor construction methods can only be expected to achieve tolerances up to FM2 Special and then only be experienced concrete flooring specialists.  CoGri Korea is firm believer that the Client should get what he is paying for.  Therefore we always recommend that Compliance Testing is carried out by a third party testing company in accordance with TR-34.



Methodology

Firstly it is important that the building is completely enclosed and weather-proof prior to construction of the Sub-Base and slab.  A 200 mm layer of dry, well graded Sub-Base material is required to uniformly transfer the loads from the slab to the Sub-Grade soil below.  The Sub-Base must be well compacted and finished as flat and level as possible to prevent any restraint to shrinkage in the slab which could result in mid panel cracking.  The Sub-Base must be levelled to a tolerance of +0, -25 mm to ensure that the actual slab thickness is as per design.  Clients are well advised to check the slab designs carefully if sub-base tolerances are specified as +/- 15 mm as a 180 mm slab could soon become a 150 mm slab.

(설명입니다)

One of our competitors floors with slab thickness varying from 130 to 190 mm”

Once the Sub-Base has been approved by the CoGri Korea Project Engineer, the SOG preparation work can commence.  The Slab must be fully isolated from the Sub-Base with a 0.3 mm Slip Membrane lapped 300 mm and preferably taped.  The slab must also be fully isolated from any fixed structure, in case of settlement in the structure or slab.  Formed and Induced Joint layout must be carefully planned to avoid any restraint to Drying Shrinkage, especially around columns.  The key to success with any Concrete floor construction is attention to detail at all stages which is why CoGri Korea always ensures a high level of Site Management, supervision and Quality Control at all stages of the floor construction. 

Formed Free Movement Joints (Construction Joints) in SOG floors can be expected to open up by up to 20-25 mm depending on various factors, due to Drying Shrinkage.  Construction Joints are the weakest point in the slab, especially in heavily trafficked warehouse floors where the joints are designed to open up.  Therefore it is highly recommended that a cast in place Armoured Joint system with individual plate dowels for load transfer are installed in the floor.  This will protect the arisses of the slab from trafficking and provide the necessary long term durability, whilst preventing any restraint to shrinkage.  However the Client should be aware that cheap copied Armour Joint systems can do more damage that good to the floor. 

The concrete mix must be carefully designed in order to provide a low shrinkage concrete and good quality finish.  Comprehensive trial mixes will be carried out by the CoGri Korea Engineers for every single project.  Once the CoGri Korea Project Engineer has inspected and approved the preparation work, the concrete can be placed and finished with areas up to 2000 m2 per day. 

Strict standards of Quality control will continue throughout the concrete delivery and placement.  Concrete is discharged directly from the mixer trucks and compacted and screed using the CoGri Korea S-240 Laser Screed.  If Steel Fabric is used, a ‘just in time’ method is used with the Steel Fabric lifted up onto concrete spacer blocks in advance of the concrete screeding.  If Steel Fibres are used, it is highly recommended that a Dry Shake Hardener is applied to prevent any fibres being exposed at the surface.  As the concrete sets, various finishing equipment and procedures are utilised to provide a highly burnished and abrasion resistant direct concrete finish. 

Curing of the concrete is critical for the long term strength and abrasion resistance of the concrete.  Therefore it is important that a high quality Curing Compound is applied to the concrete immediately following completion of the Power Trowelling.  Covering the slab surface with a thin plastic sheet will not stop the concrete drying out prematurely.  CoGri Korea has achieved Abrasion Resistance of up to BS 8204 AR Special class without the use of liquid hardeners, with high standards of quality control and floor finishing.

As soon as the concrete as set enough to enable cutting without damage to the concrete, but generally within 24-48 hours of placement, the Induced Joints should be created by saw cutting to a depth of 1/3 to 1/4 of the slab depth with a saw cut width of 3 mm.  Soff-Cut Floor Saws can be used to enable early age cutting but this provides no difference in floor quality and the selected solution depends on the Flooring Contractors preference. 

CoGri Korea recommends that a high quality medium grade Joint Sealant is used to seal the saw cut joints as late as possible prior to operation of the floor.  This will allow some flexibility while the majority of the concrete shrinkage takes place during the first 12 months, but also some protection to the joint edges.  CoGri Korea recommends that the semi-permanent joint sealant is replaced with a Joint Sealant having a Shore ‘A’ Hardness of at least 65 after 1-2 years to ensure a long term low maintenance floor.

Compliance Testing should be done as soon as the floor construction is completed to ensure that the floor meets the performance specification required and the Client receives the standard of flooring it is paying for.