Flyer 2 granulate bedding for industrial floor

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Granulate bedding for industrial floors

ABT wants open formwork

As bedding for industrial floors, ABT practically always uses a well compacted coarse grain mixing granulate without intermediate foil layer.

The reasons for using rubble as bedding instead of the traditional self-levelling screed floor or a sand layercovered with foil are the following:• the load bearing capacity of the bedding is very high with the following advantages: • convenient for (steel) superstructure work. Dry bedding. • reach trucks can drive reinforcement meshes over the bedding so less lifting is required. This is highly favourable from a health and safety perspective. • elevated platforms can be used for technical installation work before the floor is poured (sprinkler, lighting, etc.). • stable position of the bottom reinforcement mesh so the bottom bedding is constant. higher load bearing capacity than a floor on steel. • lower chance of settlement of the floor shortly after pouring, whereby with a floor on piles the risk of cracks in piles marking the surface is reduced.• high horizontal resistance to shear with the ground surface so the extent of shrinkage of the floor strongly decreases and floors can be made jointless.• only water absorption from the concrete through the ground surface, so the (drying) extent of shrinkage of the concrete decreases and the concrete quality increases; the risk of cracking in the floor then diminishes.• lowered chance of bleeding and segregation of the concrete, so the top layer of the floor is more resistant to wear.• finishing the floor can start earlier.• efficient processing of residual products.• lower hydration heat generation because of high heat capacity and conductivity of the ground surface.

Layer thicknessThe thickness of a foundation layer is determined in the design. The minimum thickness is 150 mm for a floor on piles and 200 mm for a floor on shallow foundations. A somewhat greater thickness is usually used for reasons of the load bearing capacity depending on the solidity of the ground surface. Compression strengthThe compression strength can be indicatively assessed on the site by pushing the bucket of a shovel down on around


four slabs until the front wheels up come. If the slabs pe-netrate no deeper than 1.0 cm into the ground surface sufficient compression strength is ensured. There is no longer any rutting by vehicles or footsteps left behind.

If required the compression strength of the foundation layer can be more exactly established by plate loading tests with a diameter of 760 mm. Over three tests the compression strength must then on average be at least 0.06 N/mm3 and individually at least 0.04 N/mm3 . Higher or lower values can be agreed for specific projects.The compaction of the surface can also be nuclear checked with road building equipment.

Figure: compacting of the ground surface with a heavy vibrating roller

Floor designThe high horizontal resistance to shear between the floor and the foundation layer does mean, however, that the shrinkage shortening of the concrete is prevented and the stresses in the floor can be higher. The shrinkage stresses can indeed be greater, but with the uniform force, displacement peak stresses near obstacles are reduced. In the design of the floor this is taken into account by the application of low-shrinkage floor concrete and heavy fine-meshed upper reinforcement so cracking in the width remains strongly limited.

Water absorptionStrictly speaking according to the VBU the bedding is formwork and may draw practically no moisture from the concrete. In National Assessment Guideline BRL 2362, article 4.11.3/2, which describes the construction of impervious provisions, the bedding of granulated debris is already included with the following description: ‘An unbonded, flat granular mix bedding with a minimum thickness of 150 mm’. This guide is much more recent than the VBU, and specifically covers the method for (impervious) concrete floors.



Soil moistnessThe ground surface must be dry to stiff during pouring to allow appropriate absorption of moisture from the concre-te by the ground surface. With a too wet ground surface softening can occur so the load bearing capacity decrea-ses; the foundation layer then absorbs no moisture from the concrete. If puddles occur water can even mix with the concrete so the concrete quality decreases.

A too dry ground surface can also have the consequen-ce of little water being absorbed from the concrete. The moisture level of the industrial granulate with processing must be between 8 and 10%, for pouring between 8 and 15%. During the work the moisture level of the ground surface is normally visually assessed. Samples can be tested by drying.

Construction The construction of a foundation layer as bedding starts with bringing the existing ground surface to the appropriate height. Weak, humus soil stratums at the surface are removed if necessary. A solid, dry top soil layer containing many plant roots usually does not require digging up. Then the granulated debris is brought in by lorry, spread with shovels and brought to the appropriate height with a laser-controlled “kilfer board”. A vibrating roller (approx. 2 to 3 tons) compacts the surface, with a plate compactor (approx. 200 to 300 kg) being used along edges if necessary. In wet conditions it can be advisable to not vibrate too strongly because then the water is vibrated upward and the granulate is not compacted.

If with a floor on piles the ground surface on which the foundation layer applied is very weak, it may be necessary to use a root foil as separating layer so the granulate does not mix with the ground surface.

The foundation layer must be applied before the driving of the piles because the piles could subsequently be damaged by lorries driving in between. If required the layer can initially be applied less thick, then shortly before the application of the reinforcement a last layer can be applied if necessary in finer grading (0-20 mm).

Floor insulationIf an industrial floor is located in work accommodation the “building decree” may specify an insulation value. From an energy perspective, in most cases only edging strip insulation would seem desirable. The same insulation is also much more effective when this is also included in walls or in the roof. Floor insulation can also sometimes be unnecessary when underfloor heating is applied.If insulation is applied this must be fully covered with building foil to prevent PS beads from mixing with the concrete. Piles must be kept free of insulation and foil.

Based on frequently drilled cores from floors it appears that the absorption of too much water or cement from the concrete does not occur. From a test for a job in Almere it was established that the water absorption through the ground surface is very limited and can have no unfavourable influence on the quality of the concrete.

From measurements of drilled concrete cores it appeared that the strength and density of the concrete on the underside of the floor is even a little greater than on the top side. Later the water will slowly be absorbed in the concrete again so the concrete floor shrinks less quickly. Across the whole of the Netherlands and further afield, subject to clear requirements being met the rubble is allowed as equivalent bedding, and many hundreds of thousands of metres of industrial floor have already been produced with it.

RecyclingWaste products are used as the foundation layer practical-ly without exception. In most cases “clean” construction and demolition waste is applied, being industrial mixed aggregate.

If the bedding is much higher than the groundwater, slightly contaminated granulate can also be used such as blast furnace and combustion slag or asphalt granulate. Permission must be granted for this by the competent authority.

SmoothnessThe height of the ground surface is of the utmost impor-tance. With a variance of height this has direct consequen-ces on the floor thickness and coverage on the upper rein-forcement, and accordingly the load bearing capacity and cracking susceptibility of the floor. The variance compared to the laying level of the floor may be a maximum of +10/-15 mm to be able to construct the floor with top coverage of an average of 30 mm.The height of the rubble is controlled during the work using a laser tool and level staff with which height mea-surements can be carried out quickly and over a large surface area.

Figure: levelling rubble with laser-controlled equipment



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Verdere informatie is te verkrijgen bij ing. Ab van den Bos Raadgevend ingenieur

ABT bvArnhemsestraatweg 358 Velp Delftechpark 12 Delft Kammenstraat 18 Postbus 82 6800 AB Arnhem Postbus 458 2600 AL Delft 2000 Antwerpen T +31 (0)26 368 31 11 T +31 (0)15 270 36 11 T +32 (0)3 205 37 11

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If edge strip insulation is used, this is preferably placed under the rubble because this increases the load bearing capacity of the floor and reduces cracking susceptibility. Fitting the insulation vertically against the perimeter beams is an even better option. In practice dry debris and a sand layer also have an insulating effect, as included in the German VDI. Construction supervisionAs part of the industrial floor design ABT also frequently assesses the lay of the ground surface. The quality is established with a visual appraisal of the com-pression strength, the measurement of the height and the checking of the moisture level. If required pressure plate tests are conducted to check the compression strength. The assessment of the ground surface is a good starting point for the construction of the industrial floor as a whole.

SummaryGranulated debris bedding is the basis for a good load bearing jointless industrial floor with durable performance. Establishing clear criteria in the design for construction and checking them during construction greatly improves the general quality of the floor.

Flyer 2 granulate bedding for industrial floor

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