UNDER BEAM     

Under beam such as Cordek® Cellcore HX B, Max-frank Pecavoid® Plus+, RD, C or Jablite® is a lightweight expanded polystyrene (EPS) block designed for use beneath reinforced concrete ground beams and pile caps to protect against the potential effects of ground heave. It is made of moulded collection cellular legs with twin fluted polyproylene top. 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. It is available in strips 2400mm long and widths from 300 to 1200mm in increments of 25mm to suit the associated beam and in overall depths of 85, 155 and 220mm. That way you can be sure that the legs coincide with where the top of the formwork meets and we don't experience any of the legs breaking off in the product overturning in the bottom of the trench.  Anti-heave boards are available in a variety of depths Cordek®  85mm | 155mm | 220mm and grades 7/10 | 9/13 | 13/18 | 18/24 | 24/32 | 24/32. Max-frank Pecavoid® Plus+  8/12 | 9/13 | 10/15 | 13/19, Max-frank Pecavoid® RD 15/22 | 18/24 | 24/32 | 30/40, Max-frank Pecavoid® C 25/35 | 30/40 | 35/45 | 40/50 | 45/55 | 50/65, Jablite®  7/10 | 9/13 | 10/15 | 13/18 | 18/24 | 24/32 | 30/40 | 40/50 | 50/65 to suit most commonly encountered combinations of soil heave potential and concrete depth. One of great things about anti-heave Under Beam sheet is it is very simple to work with and doesn't have much dust off when cut on site. Either the 155mm or 220mm depth under beams are the most commonly used in the UK. It is closely related to the large clay content in the south of the country (high shrinkage category). Panels should be installed on a smooth, flat, well compacted surface comprising of a sand/cement or concrete blinding. The product must not be used on the vertical faces of concrete foundations. It is normal for this type of compressible under beam to experience some compression during concrete pouring. This will normally be less than 10mm, dependent on the depth of the concrete and the method and rate of placement. Once the concrete has been poured, subsequent creep deflections will be minimal. It is not anticipated for anybody walking on the product during installation. HX B Under Beam Grade 7/10 price is the lowest compared to the higher grades: 9/13, 13/18, 18/24, 24/32, 32/342. Cordek® Cellcore price list is available upon request. 

HOW TO UNDERSTAND THE DIFFERENCES BETWEEN UNDER BEAM GRADES?

The appropriate product grade is selected according to the maximum thickness of wet concrete that the product will be required to support during construction of the project. The appropriate product depth is selected such that its specified compression under fail-load is equal to, or greater than, the maximum anticipated ground movement owning to clay heave as established from site investigations. 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 B, 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®, Max-frank 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 B, 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® , Max-frank Pecavoid® RD, Jablite®
• 24/32 grade | 24 kN/m2 (safe load) | 32 kN/m2 (fail load) | Concrete Depth 601mm - 900mm | Cordek® , Max-frank 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® Cellcore HG, Pecavoid® C, Jablite®
• 32/42 grade | 32 kN/m2 (safe load) | 42kN/m2 (fail load) | Concrete Depth 941mm - 1220mm | Cordek® Cellcore HX B
• 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® Cellcore 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® Cellcore HG, Pecavoid® C, Jablite®

Safe load – sometimes stated as the Normal Working Load (NWL) is the maximum safe force that a HX B material offers without fear of breaking. Remember the safe load is the maximum load!

Failure load – load at which the under beam will fail (permanent deformation).

Cellcore under beam used to be called CC*. Its grades were also different. 

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

CC performed in grades (under beam):

100mm depth

8/12 - 270mm (max concrete thickness)
10/15 - 350mm (max concrete thickness)
15/22 - 550mm (max concrete thickness)
20/30 - 770mm (max concrete thickness)

175mm depth

8/12 - 270mm (max concrete thickness)
10/15 - 350mm (max concrete thickness)
15/22 - 550mm (max concrete thickness)
20/30 - 770mm (max concrete thickness)

250mm depth

8/12 - 270mm (max concrete thickness)
10/15 - 350mm (max concrete thickness)
15/22 - 550mm (max concrete thickness)
20/30 - 770mm (max concrete thickness)

300mm depth

8/12 - 270mm (max concrete thickness)
10/15 - 350mm (max concrete thickness)
15/22 - 550mm (max concrete thickness)
20/30 - 770mm (max concrete thickness)

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

Essentially it is a very simple product to specify. Depth of the product is in line with the amount of anticipated heave (potential for ground movement). The depth of concrete placed above panels must not exceed that stated in the relevant product literature (grade). The depth (85mm, 155mm or 220mm) of the under beam 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

The depth of the under beam sheet should be determined by the heave potential of the soil, as detailed in Table 1 below. Further guidance on predicted ground movements in shrinkable soils is given in NHBC Standards 2021 Chapter 4.2

Table 1

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 more information click here

For Plasticity Index calculator click here

UNDER BEAM INSTALLATION

Under beam 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 beam panels should be sealed with formwork tape or a DPM (Damp Proof Membrane) layer. 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 beam 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 beam should have at least three legs in each direction to remain structurally sound.

WHAT THE CONTRACTORS ON SITE WOULD BENEFIT BY USING UNDER BEAM AS A MATERIAL?

One of great things about under beam is it’s very simple to work with and doesn't have much dust off when cutted on site. Each grade has a color code. They are labeled to easily find on site which grade goes in which area.

CORDEK® SHEETS VS MAX-FRANK PECAVOID® VS JABLITE® GROUND HEAVE PROTECTION

Generally speaking there is not much difference between those 3 brands. Cellcore Cordek under beam is manufactured from EPS moulded into a hexagonal cellular compressible base for use under reinforced concrete ground beams, with a 5mm thick top of twin-wall polypropylene associated sheet. Pecavoid® range is the new and improved ground heave solution from MAX FRANK. Unlike Cordek® sheets, Pecavoid® range comes with a square shaped EPS legs covered with a yellow 4.5mm polypropylene top. Cordek® sheets grades are color coded, Max-frank are labeled - last 4 digit numbers indicating grade. All of the ground heave protection boards have a BBA certificate and meet NHBC Standards. All Jablite Ground Heave Protection boards are supplied with a black 5mm bonded polypropylene top sheet in 300, 450 and 600mm pre-cut boards as standard. Similar to Cordek® or Max-frank® products, Jablite® Expanded Polystyrene (EPS) core is a great alternative to Cordek®, designed to include drainage slots in every node (where four faces meet). The slots allow water that could be trapped in the collapsed cells to escape as the board compresses.

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