UNDER SLAB 

Under slab void former, called an 'anti-heave material' is a lightweight expanded polystyrene (EPS) compressible block designed for use beneath reinforced concrete floor slabs to protect against the potential effects of ground heave. It is made of moulded collection cellular legs with a 10mm twin fluted polypropylene top which is strong enough to actually walk on during placement of base spaces and reinforcement.

Structural void former legs are made of EPS with drainage holes in which enables the product to be used in areas either way you have a high water table or in a coastal environment. The slots allow water that could be trapped in the collapsed cells to escape as the board compresses. None of the anti-heave slabs have insulation properties. The product is dimensionally stable under varying conditions of temperature and humidity. It is rot-proof and water-resistant and will continue to perform effectively as a void former for the life of the building.

UNDER SLAB BRANDS

Concrete under slabs are often associated with well known brands such as Cordek®, Jablite® or Max-Frank®. All brands offer similar products with small differences. Cordek® and its detail product HX S under slab is supplied with white polypropylene top, opposite to Jablite black top. On the other hand Max-frank and its Pecavoid® come yellow on one side. All polystyrene under slab void formers are extremely easy to cut with a compact Stanley knife, due to the hollow and polystyrene free nature of the construction.

Cordek’s HX S material EPS legs create octagonal shape, while Max-Frank® and Jablite® ground heave protection boards come square shaped. It doesn't make much difference for concrete, however if you consider supporting facts, Cordek® will be the absolute winner offering 6 legs support, not only 4. Moreover, Cordek® has its own color code system that allows you to easily find the right product on site which grade goes in which area. Max-Frank® are labeled with last 4 digit numbers indicating grade. All of the ground heave protection boards have a BBA certificate and meet NHBC Standards.

UNDER SLAB DEPTHS & GRADES

Under slab expanded polystyrene void former sheets are available in a variety of depths including: 90mm, 160mm, 225mm and standard grades 7/10, 8/12, 9/13, 10/15, 13/18, 13/19, 15/22, 18/24, 24/32, 32/42.

Each grade has adequate strength to support a specified depth of wet concrete during construction and will collapse under its specified fail-load, limiting the upward pressures experienced during periods of clay heave.  

If you are about to pour concrete more than 1220mm thick, you should consider buying either Cordek® Cellcore HG, Max-Frank Pecavoid® RD or Max-Frank Pecavoid® C range. Those slabs are especially made for deep concrete suspended foundations up to and beyond 1940mm head of concrete.

Below you will find all available Cordek under slab thicknesses and their corresponding grades:

Pic. Cordek products

HOW TO UNDERSTAND THE DIFFERENCES BETWEEN UNDER SLAB GRADES?

Grades are specially calculated and calibrated for deformation of EPS under loading. The grade must be adequate to take the additional 50mm concrete load.

• 7/10 grade | 7 kN/m2 (safe load) | 10 kN/m2 (fail load) | Concrete Depth 100mm - 220mm | Cordek® HX S, Jablite®
  
• 8/12 grade | 8 kN/m2 (safe load) | 12 kN/m2 (fail load) | Concrete Depth 100mm - 260mm | Max-Frank Pecavoid® Plus+
• 9/13 grade | 9 kN/m2 (safe load) | 13 kN/m2 (fail load) | Concrete Depth 261mm - 300mm | Cordek® HX S, Pecavoid® Plus+, Jablite®
• 10/15 grade | 10 kN/m2 (safe load) | 15 kN/m2 (fail load) | Concrete Depth 301mm - 340mm | Max-Frank Pecavoid® Plus+, Jablite®
• 13/18 grade | 13 kN/m2 (safe load) | 18 kN/m2 (fail load) | Concrete Depth 341mm - 460mm | Cordek® HX S, Jablite®
• 13/19 grade | 13 kN/m2 (safe load) | 19 kN/m2 (fail load) | Concrete Depth 341mm - 460mm | Max-Frank Pecavoid® Plus+
• 15/22 grade | 15 kN/m2 (safe load) | 22 kN/m2 (fail load) | Concrete Depth 461mm - 540mm | Max-Frank Pecavoid® RD
• 18/24 grade | 18 kN/m2 (safe load) | 24 kN/m2 (faill oad) | Concrete Depth 541mm - 660mm | Cordek® HX S, Pecavoid® RD, Jablite®
• 24/32 grade | 24 kN/m2 (safe load) | 32 kN/m2 (fail load) | Concrete Depth 601mm - 900mm | Cordek® HX S, Pecavoid® RD, Jablite®
• 25/35 grade | 25 kN/m2 (safe load) | 35 kN/m2 (fail load) | Concrete Depth 901mm - 940mm | Max-Frank Pecavoid® C, Pecavoid® RD
• 30/40 grade | 30 kN/m2 (safe load) | 40 kN/m2 (fail load) | Concrete Depth 941mm - 1140mm | Cordek® HG, Pecavoid® C, Jablite®
• 32/42 grade | 32 kN/m2 (safe load) | 42 kN/m2 (fail load) | Concrete Depth 941mm - 1220mm | Cordek® HX S
• 35/45 grade | 35 kN/m2 (safe load) | 45 kN/m2 (fail load) | Concrete Depth 1141mm - 1340mm | Max-Frank Pecavoid® C
• 40/50 grade | 40 kN/m2 (safe load) | 50 kN/m2 (fail load) | Concrete Depth 1341mm - 1540mm | Cordek® HG, Pecavoid® C, Jablite®
• 45/55 grade | 45 kN/m2 (safe load) | 55 kN/m2 (fail load) | Concrete Depth 1541mm - 1740mm | Max-Frank Pecavoid® C
• 50/65 grade | 50 kN/m2 (safe load) | 65 kN/m2 (fail load) | Concrete Depth 1741mm - 1940mm | Cordek® HG, Pecavoid® C, Jablite®

Safe load – The maximum load which can be applied to the product during the construction process. This is calculated as the total dead weight of the concrete and reinforcement (assumed to be 25kn/m3) plus an allowance of 1.5kN/m3 for the imposed load (foot traffic, heaping of concrete etc). Remember the safe load is the maximum load!

Failure load - (permanent deformation) the load at which the legs of the Cellcore panel will collapse if the ground heaves under the concrete structure. The slab, beam or pile cap must be designed to accommodate this upward load, taking into account the self-weight of the concrete structure.

Cellcore under slab used to be called CP*. Its grades were also different. 

*Cellcore CP has been discontinued and is no longer available. Therefore it is used for reference only. It have been upgraded to Cellcore HXS

CP performed in grades (under slab):

150mm depth

10/15 – 350mm (max concrete thickness)
15/22 – 550mm (max concrete thickness)
20/30 – 770mm (max concrete thickness)

225mm depth

10/15 – 350mm (max concrete thickness)
15/22 – 550mm (max concrete thickness)
20/30 – 770mm (max concrete thickness)

300mm depth

10/15 – 350mm (max concrete thickness)
15/22 – 550mm (max concrete thickness)
20/30 – 770mm (max concrete thickness)

350mm depth

10/15 – 350mm (max concrete thickness)
15/22 – 550mm (max concrete thickness)
20/30 – 770mm (max concrete thickness)

INSULATED UNDER SLAB

Cordek® Cellcore HX S, Max-Frank Pecavoid® Plus+, RD, C are just standard products and provide no insulation value for underneath the floor slab. If you are considering insulating your building, then you should consider looking at this product called Cellcore HX Plus or Jablite® ground heave protection insulated slab. It is a combination product, it has a range of cellular legs with different depths of insulation (30mm, 50mm, 75mm, 80mm, 100mm, 125mm, 130mm, 150mm) glued onto the top of those cellular legs dependent upon the target U-value that you need to hit.

It has been designed for use beneath reinforced concrete floor slabs to protect against the potential effects of ground heave with the additional benefit of providing insulation. Only Cordek® Cellcore HX Plus is made in 25mm increments, which means insulation material (Filcor) is added (50mm, 75mm, 100mm, 125mm, 150mm).

In order to undertake a U-value calculation to determine the appropriate thickness of insulation to be incorporated within the Cellcore HX Plus panels, we will require the following information relating to the proposed floor slab: Floor area | Exposed perimeter distance | Floor build-up including any finishes (with dimensions) | Wall build-up (with dimensions) | Target U-value.

HOW DO I KNOW WHAT DEPTH OF UNDER SLAB DO I NEED?

Depth of the product is in line with the amount of anticipated heave (potential for ground movement). The depth of concrete placed above sheets must not exceed that stated in the relevant product literature (grade). The depth (90mm, 160mm or 225mm) of the under slab panel should be determined by the heave potential of the soil (NHBC shrinkage category).

There are 3 potential volume soil plasticity indexes:

10-20 Low
20-40 Medium
40-60 High

Further guidance on predicted ground movements in shrinkable soils is given in NHBC Standards 2021 Chapter 4.2
Plasticity Index (PL) is a measure of Plasticity of the Soil. 

In order to find PL, we have to find LL & PL.

Liquid Limit (LL) - The water content at which the soil will have very less shear strength. (The water content at which the flow characteristics are introduced in the soil, almost it can flow).

Plastic Limit (PL) –The water content at which a thread of soil with 3mm diameter begins to crumble. (The water content at which the soil starts to behave as plastic).

 Plasticity Index (PL) = Liquid Limit (LL) - Plastic Limit (PL) 

For Plasticity Index calculator click here

VOID FORMER FLOOR INSTALLATION 

Concrete under slabs are designed to resist nominal foot traffic during the construction process. Heavy foot traffic directly on the panels, particularly the edges, should be avoided or alternatively the use of walking boards should be adopted. Void former sheets are suitable for horizontal (flat) installation only and should not be installed vertically or on a sloping surface.

Panels should be positioned with the white, black or yellow polypropylene board facing upward and the moulded EPS cellular structure below. Where a membrane e.g. DPM is proposed, this should be positioned above the void former panels, not beneath, to maintain its integrity should ground heave occur. Structural void former should be placed on a firm, level surface and the bottom of the excavation properly compacted and blinded with a layer of concrete or sand/cement blinding.

Panels should be covered with a 50mm thickness of concrete blinding where heavy reinforcement is proposed or where the reinforcement will be subjected to significant point loads from traffic or other imposed loading. For piled ground beams, the top of each pile should be trimmed so that it extends slightly above the proposed underside of the ground beam. Each pile should penetrate the void former to allow for an approximate 50mm thickness of concrete blinding on top where applicable and a keying depth which is in accordance with the engineer's design. When required, the product can be cut to shape with a fine-toothed saw.

Care should be taken to ensure that, after cutting, exposed ends of the cellular stiffening ribs do not exceed a maximum length of 50mm. Joints between under slab sheets should be sealed with formwork tape or a DPM (Damp Proof Membrane) layer to avoid any grout loss between the panels. Reinforcement should be fixed and adequately supported to ensure that the correct depth of concrete cover is achieved and to ensure that the maximum imposed load beneath each support is appropriate to the grade of sheet being used. Concrete should be placed with care to avoid overloading the under slab panels. If the depth of concrete proposed exceeds that stated for the panel type being used, multiple pours may be considered providing that the initial pour does not exceed the maximum permissible depth allowed and the load from subsequent pours is transferred to the foundations and not the sheets. Any necessary cutting should be kept to a minimum.

It is recommended that the under slab should have at least three legs in each direction to remain structurally sound. Sheets should not be cut to a width less than 300mm to maintain stability.

CORDEK CELLCORE HX S, HX PLUS VOID FORMER DETAIL

Table shows 90mm HX range sizes

Table shows 160mm HX range sizes

Table shows 225mm HX range sizes

Thickened slab detail may be found in our Downloads section.

Fairly easy access to all PDF files.

Datasheet ǀ Declaration of Performance - DoP ǀ BBA Certificate ǀ Installation Guide ǀ Certificates ǀ Brochure ǀ SPEC Sheet

Pictures credit:https://cordek.com/case-study/