VOID FORMERS

Void former also called ‘anti-heave’ board is often a lightweight structural material made of continuous paper honeycomb or EPS polystyrene core developed to separate concrete slabs from reactive soil which can potentially exert damaging pressure on the structural foundations of a dwelling and to replace some of the volume of concrete (screed) in applications such as floors.

Void former can be supplied as a block, slab or bespoke shaped pieces to suit the application or project. Standard board sizes are 2400mm x 1200mm up to a depth of 1200mm. Depending of brand, void formers come in multiple name options, including:

• Cordek Cellcore HX S,
• Cellcore HX B,
• Cellcore HX Plus,
• Cellcore HG,
• Filcor 20/45/70/90/100/120/140/160/190,
• Dufaylite Residential & Commercial Clayboard,
• Whiterock Heave Stopper,
• Jablite Fillmaster,
• Kore Void Former,
• Max-frank Pecavoid® Plus,
• Max-frank Pecavoid® RD,
• Max-frank Pecavoid® C,
• Max-frank Egcovoid® 

TYPES OF VOID FORMERS

Main types of cast in void formers can be categorised as collapsible or compressible. 

COLLAPSIBLE VOID FORMERS

Collapsible void formers such as Dufalite Clayboard, Whiterock Heave Stopper or Max-frank Egcovoid® Former are a foundation stabilisers made using recyclable honeycomb cardboard core set between lightweight twin fluted polypropylene facings designed to collapse, creating a void for the ground to heave into. Unlike compressible void formers, collapsible boards are filled with water after the design compressive strength of the concrete has been achieved. By doing so, the load-bearing capacity of the board is eliminated. When dry, the board is rigid and strong enough to support the weight of wet concrete and steel reinforcement without deflection or creep compression.

Water is introduced by 36mm hole diameter void pack pipes or via the connecting hoses of the board to the panel core once the concrete is set. The amount of water is approximately 15 to 20 liter/m² depending on panel thickness. The water has to be injected with low pressure and begin with about 20% of the amount of water and after 30 minutes continue with about 30%. This degrades the honeycomb centre leading to the creation of a void. Hence the name of the collapsing board comes from. The board naturally disappears to leave a permanent 100% void with no residual density to pass on heave stresses. It’s 30% slimmer than polystyrene alternatives.

For more information on collapsible void formers click here.

For information on how to install collapsible void formers click here.

Void pack pipe simply “switches off" its compressive strength, taking it from 30kN/m2 down to a nominal 3KN/m2. This ensures that, when the ground heaves, the board yields with no resistance. The space accommodates clay expansion without exerting undue pressure on the structure above.

The resulting void continues to accommodate natural soil shrinkage and expansion to prevent structural damage. This makes the board an unique product, with its central core designed to collapse under just 3KN/m² once water has been introduced. The resulting void continues to accommodate natural soil shrinkage and expansion.

Picture credit: max-frank.co.uk

COLLAPSIBLE VOID FORMER COLOUR CODE

Each company has its own colour code. They are labeled to easily find on site which grade goes in which area. While Dufaylite Clayboard top face is usually black and the bottom face white, Whiterock Heave Stopper comes colour coded: red (60mm), green (110mm) and white (160mm). Max-frank Egcovoid® Former has off-white colour and its edges must be sealed off using sealing and repair red tape to achieve water tightness.

COLLAPSIBLE VOID FORMER RESIDENTIAL AND COMMERCIAL GRADES

While residential grade is calculated and calibrated for concrete thickness up to 1000mm, commercial grade is used on concrete thicknesses from 1000mm up to 1940mm. Collapsible void formers such as Dufaylite are available in two standard strengths, KN30 (Residential Grade) and KN90 (Commercial Grade).

On the other hand, Whiterock Heave Stopper being an alternative void former to Dufaylite’s Clayboard, is available in just one maximum strength of 120kn/m2 (good to be used as a residential or commercial applications).

Max-frank void former has maximum permissible load for dry panels of 100 kN/m2.

COLLAPSIBLE VOID FORMER SIZES

Dufaylite Clayboard® is supplied in panels 2440mm long by 1000mm wide offering coverage of 2.44m2, Whiterock Heave Stopper and Max-frank Egcovoid® former are manufactured in size 2400mm x 1200mm and standard coverage 2.88m2. 

To find out more on the differences between Cordek Cellore Hx s and Whiterock Heave Stopper click here.

To find out how to prevent heave when removing a tree click here.

Picture credit: directpiling.co.uk

COLLAPSIBLE VOID FORMER RESIDENTIAL GRADE PRODUCTS (up to 1000mm of concrete)

• 160mm DUFAYLITE RESIDENCIAL CLAYBOARD - 30kN/m² - concrete thickness up to 900mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 7/10 - 10kN/m² concrete thickness up to 220mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 9/13 - 13kN/m² concrete thickness up to 300mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 13/18 - 18kN/m² concrete thickness up to 460mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 18/24 - 24kN/m² concrete thickness up to 660mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 24/32 - 32kN/m² concrete thickness up to 900mm 
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 32/42 - 42kN/m² concrete thickness up to 1220mm 
• 100mm MAX-FRANK EGCOVOID® FORMER - 100kN/m2
• MAX-FRANK PECAVOID® PLUS GRADE 13/19 -19kN/m2 concrete thickness up to 460mm
• 160mm WHITEROCK HEAVE STOPPER - 120kN/m2- concrete thickness up to 1940mm 

COLLAPSIBLE VOID FORMER COMMERCIAL GRADE PRODUCTS (above 1000mm up to 1940mm of concrete)

• CORDEK CELLCORE HG - 65 kN/m2 - concrete thickness up to 1940mm
• 90mm/160mm/225mm CORDEK HX S UNDER SLAB  GRADE 32/42 - 42kN/m² concrete thickness up to 1220mm 
• DUFAYLITE COMMERCIAL CLAYBOARD - 90kn/m2 - concrete thickness up to 1940mm
• WHITEROCK HEAVE STOPPER - 120kN/m2 - concrete thickness up to 1940mm
• MAX-FRANK PECAVOID® RD RANGE GRADE 30/40 - concrete thickness up to 1140mm 
• MAX-FRANK PECAVOID® C RANGE GRADE 50/65 - concrete thickness up to 1940mm 
• JABLITE GROUND HEAVE PROTECTION BOARD GRADE 50/65 - concrete thickness up to 1940mm 

COLLAPSIBLE VOID FORMERS THICKNESSES

The thickness of product required will be determined by the plasticity index, which is specified by the soil engineer. Varying thicknesses are available and we’ve shown estimated thicknesses, strengths and requirements below. Generally speaking there are available in 7 thicknesses:

• DUFAYLITE® CLAYBOARD - 60mm | 85mm | 110mm | 160mm
• WHITEROCK HEAVE STOPPER® - 60mm | 110mm | 160mm
• MAX-FRANK EGCOVOID® FORMER - 35mm | 50mm | 100mm 

For more information on collapsible void former thicknesses click here.

COMPRESSIBLE VOID FORMERS

Compressible void former unlike collapsible board is a combination product made of moulded EPS collection cellular legs with twin fluted polypropylene top made to protect foundations from the effect of ground heave. Due to its unique make up and excellent compression properties, this board is able to alleviate heavy pressure on concrete slabs and it stays underneath the poured concrete. Compressible void formers are often associated with brands like Cordek®, Jablite®, Max-frank® and Kore®.

When the ground heaves or expands it is this resistance from the concrete above that counteracts the upward force from below causing the product to progressively compress under a given load. Users of products which combat the effects of ground heave are already familiar with the benefits of using the compressible type, such as Cordek® Cellcore HX S or HX B, over the collapsible fill alternatives, made from paper honeycomb. 

CELLCORE COMPRESSIBLE VOID FORMERS

Cellcore does not rely on material degradation for the system to become effective. Once the Cellcore panels are laid and the joints sealed, the surface is ready to receive the reinforcement and the concrete. Cellcore is unaffected by water so, in wet weather or when foundations are below the water table, work can continue without reducing its effectiveness.

To find out more about Cordek Cellcore Hx S and Hx Plus click here.

It is normal for this type of compressible void former to experience some compression during concrete pouring. This will normally be less then 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.

Cellcore compressible void former is more commonly used as a void former on construction sites because it is a low cost product and does not require the use of water and it is strong enough to actually walk on during placement of base spaces and reinforcement.

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. Those drainage slots are incorporated in the cellular base of each panel, to allow water trapped within the cells to escape as the product compresses.

The boards are manufactured in a range of thicknesses to meet the application – Beam, Slab or Insulated Slab.

Cordek® void former called Cellcore HX S Under Slab is the most known and used lightweight expanded polystyrene (EPS) block that can be used for concrete floor slabs to protect against the potential effects of ground heave by reducing the upward force transmitted to the structure from ground heave. Variations of the product are also available for use under floor slabs or alternatively where permanent formwork is required. 
 

Cellular polystyrene sheets are available in a variety of depths 90mm | 160mm | 225mm and grades 7/10 | 9/13 | 13/18 | 18/24 | 24/32 | 32/42 to suit most commonly encountered combinations of soil heave potential and concrete depth. 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.

Picture credit: directpiling.co.uk

JABLITE | MAX FRANK COMPRESSIBLE VOID FORMERS

Jablite® Ground Heave Protection is the second type of compressible void former that provides a stable platform to support a specified depth of wet concrete during construction.

Jablite® is followed by MAX FRANK and its Pecavoid®. This void former is manufactured from interlocking sections of expanded polystyrene. 

Pecavoid® acts as a protective layer, between the soffit of the suspended foundation and the expanding clay soil and is designed to compress when ground heave takes place. MAX FRANK’s Pecavoid® RD Range (Reduced Depth) reduces depth grades for minimal excavation depths and is available in various grades for concrete depths from 100mm up to 1140mm. The C Range is used for deep concrete suspended foundations and can be manufactured to offer ground heave resistance for up to and beyond 2m head of concrete.

For information on how to cut polystyrene void formers clik here.

To find out what are the ground heave solutions click here. 

To find out what are the heave symptoms click here.

HOW COMPRESSIBLE VOID FORMER WORK?

Each board is engineered to protect the foundations of buildings against the damage caused by the upward pressure of clay or ground heave in a given site and application situation. Compressible void formers work on principle grades that are specially calculated and calibrated for deformation of EPS under loading.

• 7/10 grade | 7 means 7 kN/m2 (safe load) | 10 means 10 kN/m2 (failure load)
• 9/13 grade | 9 means 9 kN/m2 (safe load) | 13 means 13 kN/m2 (failure load)
• 13/18 grade  | 13 means 13 kN/m2 (safe load) | 18 means 18 kN/m2 (failure load)
• 18/24 grade  | 18means 18 kN/m2 (safe load) | 24 means 24 kN/m2 (failure load)
• 24/32 grade  | 24 means 24 kN/m2 (safe load) | 32 means 32 kN/m2 (failure load)
• 32/42 grade  | 32 means 42 kN/m2 (safe load) | 42 means 42 kN/m2 (failure load)

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

Failure load - load at which the under slab will fail (permanent deformation).
Firstly the depth of the Cellcore HX S panel should be determined by the heave potential of the soil, as detailed in Table 1 below:

Table 1

Secondly, the grade of the product is determined by the depth of the concrete to be cast on the Cellcore HX S, as detailed in Table 2 below:

Table 2

COMPRESSIBLE VOID FORMER PRODUCTS

• CORDEK CELLCORE HX S
• CORDEK CELLCORE HX B
• CORDEK CELLCORE HX PLUS
• CORDEK CELLCORE HG
• JABLITE GROUND HEAVE PROTECTION BOARD
• MAX-FRANK PECAVOID® PLUS
• MAX-FRANK PECAVOID® RD Range
• MAX-FRANK PECAVOID® C Range

COLLAPSIBLE VS COMPRESSIBLE VOID FORMERS

Compressible polystyrene void formers, have a much narrower safe load-fail load window and require the designer to significantly thicken the slab in order to resist the uplift, therefore, it has to be considerably thicker than paper honeycomb, so it’s automatically bulkier than collapsible void formers.

For instance, if you are looking to buy 225mm thick Cellcore under slab, its equivalent collapsible void former thickness would be 160mm Dufalite Clayboard or 160mm Heave Stopper made by Whiterock.

Collapsible void former is 30% smaller than the compressible polystyrene alternatives and requires significantly less ground work to prepare for foundations than the traditional beam and block method.
Polystyrene doesn’t biodegrade, so it’s like a landfill under foundations. As a result, it stays underneath the poured concrete and has a much greater chance of exerting force on foundations in the event of ground movement. However, Void Pack Pipe is required every 24m2 or for every isolated area. Cordek Cellcore HX Plus compressible slab can add an insulation benefit of up to R1.0 (W/mk thermal resistance) as standard, which helps to reduce energy consumption in residential buildings.

On the other hand, a collapsible void former can not be used with integral insulation and may introduce methane as the cardboard rots down.
While paper honeycomb is delivered direct to site in a weatherproof seal, eps compressible void former can be left uncovered on site. All in all, both types of void formers share one thing in common - they are manufactured from 100% recycled materials.

To find out how to insulatate under concrete floor click here

WHY DO I NEED A VOID FORMER FOR?

The structural damage caused by clay is well understood by engineers and designers here in the UK. The problem is particularly acute in the South East although climate change is having its effect on soils in other parts of the country. In the UK ground heave is primarily caused by the rehydration of clay or expansive shale soils, overburden recovery resulting from deep excavations, mining or trees and vegetation. Whilst clay is a major problem throughout the South East and in isolated pockets nationally, ground heave due to other causes may be encountered in any region.

The solution is to create a space between ground and foundation which soils can expand into. The Building Research Establishment estimates that in situations where there are trees on shrinkable clay soils, the surface movement can be up to 100mm. Clay heave can impose both lateral and vertical pressure foundations, often resulting in cracking and expensive structural damage. In latter years it has become standard practice to protect building foundations.

Shrink swell potential in UK

VOID FILLERS

Fill material such as Filcor, Jablite Fillmaster, Sundolitt or Kore are also called a void former.  They are used for forming openings and voids within concrete structures and can be used for a wide range of applications including:

• hard and soft landscaping,
• motorways,
• underpasses,
• shopping centres,
• foundations podium slabs,
• swimming pools,
• steps,
• ramps,
• road embankments,
• slope stabilization,
• temporary ramps,
• rail platforms,
• arch forming,
• spiral staircase forming,
• column forming,
• complex shapes.

The formation of openings in concrete, either to reduce the weight of the structure or to provide access for services, calls for particular attention in the selection of the former material, positioning and subsequent withdrawal if required. In fact, the use of EPS as a structural lightweight fill has been accepted practice in the UK for over 30 years and globally for 50 years. As the EPS blocks are around 1% of the weight of traditional fill materials the net weight of the new road construction can be equal to the weight of soil removed. This alleviates pressures on existing poor ground.

FILCOR

Filcor is an exceptionally lightweight fill material, being around 1% of the weight of traditional fill materials. It is also used as a void former and a structural void former when additional compressive performance is required. In fact, Filcor EPS is a type of polystyrene block produced by the well known Cordek® company. Its close cell product has good structural properties and some inherent insulation properties too.
The low cost and lightweight of Filcor EPS makes it an ideal void former. Its use reduces the volume of concrete and therefore the weight of the structure.

Filcor works on a 1% compression figure. Basically we have all different grades from 20 to 190. Filcor 20 is the lightest grade and can take up to 800mm of concrete. In fact, Filcor 20 is light, weighing as little as 15kg per cubic meter. Next grade up is the Filcor 45 that can withstand max thickness of concrete of 1800mm. The thermal conductivity of Filcor is the rate at which heat energy passes through it and therefore the lower the value the better the material will insulate against heat loss. While there are 9 Filcor grades available, only Filcor 20 stands out from other grades. Why? It requires less raw materials to make-hence its low price.

To find out more about Filcor click here.

FILCOR PRICE

The cost of manufacturing Filcor is considered linearly proportional to its density. Filcor 20 price is the lowest compared to the other Filcor grades: 40, 70, 90, 100, 120, 140, 160, 190. For this reason Filcor 20 would be the cheapest to buy. For instance Filcor 20 2400mm x 1200mm x 600mm size cost is £196.50 ex VAT. Standard concrete slab thickness in residential construction is 4 to 5 inches. This is why Filcor 20 and Filcor 45 are often considered as a standard grade.

For Filcor prices click here.

JABLITE FILLMASTER

Jablite Fillmaster is a geotechnical product used in fill applications where a lightweight material is required to reduce stresses on underlying soils or lateral pressures to retaining walls, abutments or foundations. It has been used on projects for road embankments, road widening, rail embankments, bridge abutments, retaining walls, flood and noise bunds, culverts and temporary access solutions. Where the application requires Fillmaster to withstand long term imposed loads the designer should take into account long term compressive creep. Fillmaster exhibits stable cyclic load behavior. Under moderate loads (less than 1% deformation) the material remains elastic under cyclic loading.

KORE VOID FORMER

Kore void former has been developed for use in major works such as motorways, underpasses, shopping centres, foundations and embankments. Its main task is to decrease the volume of concrete and reduce weight while increasing strength of concrete.

SUNDOLITT

Structural Fill is supplied as large blocks of EPS which are lightweight and easy to handle and install on site (2400mmx1200mm). The EPS is inert and non-toxic there is no requirement for PPE during normal handling of EPS. Structural Fill EPS is graded by its 10% compressive strength in accordance with BS EN 13163. Grades of Structural Fill EPS have been developed with a compressive strength up to 500 kPa. Available in compressive strengths from 70 to 500 kPa the product is designated by its strength – S70, S100, S150, S200, S250, S300, S400 and S500

SIDE CLAY PROTECTION BOARDS 

Side protection board is used to prevent problems in foundations where the soil has a high clay content and is liable to heave. The product is designed to reduce lateral pressure on the side of trench foundations.

It is a low density coloured EPS for the purpose of easy identification. Heaveguard made by Cordek and Claylite from Kay-Metzeler come green in colour, Claymaster from Jablite come pink and Clayfill made by Stylite come yellow.

Those side protection boards do not compress significantly until loads exceed 20kN/m2. To produce 50% compression a load of 40kN/m2 is needed. It is not recommended for use under ground floor slabs or pile caps.

For more information how to install claymaster, heaveguard click here.

Picture credit: directpiling.co.uk

POLYSTYRENE VOID FORMER PRICE

Generally speaking thermal conductivity, density and brand itself are the basic parameters that affect void formers price. The price also depends on the EPS cost on the worldwide market. The price list of expanded polystyrene is constantly changing so the void former price as well. In general, the price list of Cordek Cellcore under slab HX S grades depends on standard factors such as domestic demand, availability of raw materials and fuel prices, including oil and inflation. As you would expect, most factories producing polystyrene use a lot of gas. The more they pay for gas the more we will pay for the final product. Moreover, the cost of transport was always included in the final product price. The raw materials needed to produce HX S under slab or HX B under beam are delivered to the factory, used for production, then once again delivered to the building site. As of 2024 you should expect to pay for commonly used 225mm Cordek Cellcore under slab (7/10 grade-max concrete depth 220mm) £117.64. On the other site its highest 24/32 (max concrete depth 900mm) grade will cost £138.29.

Realated articles:

160mm DUFAYLITE CLAYBOARD vs 160mm WHITEROCK HEAVE STOPPER

How to cut polystyrene insulation

COST-EFFICIENT UK ANTI-HEAVE PROTECTION: TOP 3 NEAR YOU

HOW TO CUT POLYSTYRENE VOID FORMER

SELECTING RIGHT VOID FORMER

Collapsible void former installation guide

Claymaster installation guide

Void former thicknesses

Ground heave solutions 

Foundation heave problems. Heave Symptoms

Don't let your foundation fail

Understanding Dufaylite Honeycomb Claybord Void Former

Filcor 20 Price

Breaking down Filcor

Understanding HX Plus

Filcor lightweight structural fill

How to insulate under concrete floor. The ultimate guide

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