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Concrete for industrial floors

ABT stands for jointless industrial floors without limitations. Reducing shrinkage is one of the most important aspects under the principle of jointless design. Research has been carried out for some time to reduce shrinkage in concrete, and in the course of the years much experience has been gained in adapting concrete mixes for specific cases. The concrete mixture used for an industrial floor largely determines the ultimate quality of the floor.

ABT therefore regards floor concrete as an integral part of the floor design. During execution the most important properties are checked by measuring specific parameters. ABT constantly strives to optimise the concrete mixture on the basis of these findings.

High requirements are made of the concrete mixture to be used in monolithic industrial floors. This entails the examination of the limits of prevailing standards for floor concrete. This text describes the most important principles with the ultimate formulation of a ‘standard’ mixture.

When determining the concrete composition the following aspects must be taken into account:• desired workability of the concrete during pouring• stability of the concrete mixture• shaping and bonding behaviour in the short term for the finishing of the floor• generation of heat as a result of hydration and consequent cooling• strength development in the long term for the load bearing capacity of the floor• shrinkage susceptibility of the concrete mixture; risk of cracking

Specific requirements that can be made of floor concrete are:• liquid sealing• resistance to frost and road salt• high resistance to wear

WorkabilityIn most cases consistency range F4 or F5 is desired for workability. The desired consistency class is often indicated using a slump test. In consistency class 4 and 5 (generally the case for floor concrete), according to NEN-EN 206-1 the flow is normative for measurements. It is of great importance to establish the required consistency beforehand. A flow of 600 mm on arrival at the work is very likely not the same flow as at the end of the pipe with a pipe length of 80 meters. Aspects such as pipe length, mesh size and any gradients are decisive when determining the appropriate consistency.

Concrete for industrial floors

The upper limit of the consistency (stability) of the mixture must be monitored by the mixer.

figure: pouring concrete with a concrete pump

StabilityThe aggregate must have the right grading to ensure the stability of a concrete mixture. An unstable mixture can result in bleeding or in extreme cases segregation. When bleeding occurs this is particularly harmful to the quality of the top layer of the floor. A ‘too stable’ mixture can, however, result in a reduction of workability while the flow is still high. This also increases the likelihood of plastic shrinkage.

Strength developmentTo be able to finish the floor in good time after pouring, whereby the top layer of the floor is smoothed and sealed, bonding must already take place in the concrete after just a few hours. This finishing process must be completed approximately 12 hours after pouring. Too fast bonding means the floor cannot be finished. On the other hand with slow strength development the top layer of the floor will be insufficiently hard.

Hydration starts approximately 2 hours after pouring (depending on various factors) and continues for a long time. The 1st night after pouring hydration is fully under way and will then gradually slow. The course of hydration can be controlled by the adaptation of the cement types and quantities. This is dependent on the mortar temperature and outside temperature, floor thickness and other circumstances (open air or in a concrete hall).

After approximately 7 to 10 days the floor must be able to take loads to not impede the progress of the building work. An average compression strength of 25 N/mm2 is then desired. The ultimate strength after 90 days must at least suffice with regard to strength class C28/35 for

constructies civiele techniek bouwmanagement bouwkunde installaties

Concrete for industrial floors

Aggregate granulate gradationGrading of 0-32 mm is used in most cases. With thin (overlay) floors with a thickness of 50 – 80 mm it may be necessary to partly or wholly add 4-16 mm grit, being so-called spramex concrete. With very thin (overlay) floors with a thickness of 30 – 50 mm, 2-8 mm grit granulate gradation can even be used, the so-called microconcrete. With a high dosage of steel fibres of over 30 kg/m3 a fine grit part is added to the concrete practically as standard. The granulate gradation may be ‘optimised’ with a maximum of 20% 4-16 mm grit. The grading of the aggregate must suffice with regard to the limits of the A-B line according to the VBT (regulations for concrete technology) for reasons of stability, low water requirements and workability.

Figure: granulate gradation

The granulate gradation may be ‘optimised’ with a maximum of 20% 4-16 mm grit. The grading of the aggregate must suffice with regard to the limits of the A-B line according to the VBT (regulations for concrete technology) for reasons of stability, low water requirements and workability.The 70% sieve residue at 1 mm is favourable cost wise, but a strong ‘lounger’ with a flat section requires more water so there is more shrinkage. This problem can usually be remedied by also adding fine sand (0-2) besides coarse sand (0-4).

Fine partsTo obtain concrete that can be pumped, is workable and sufficiently stable it must have sufficient fine parts smaller than 0.250 mm. For normal floor concrete this is at least 140 litres, with steel fibre concrete it is at least 145 litres. Spramex concrete requires 5 litres extra (total 150 l). Fine parts and microconcrete 10 litres extra (total 155 l).

Figure: the flow is also measured at the work site

the load bearing capacity of the floor. A much greater strength is undesirable because then much higher percentages of reinforcement are required to limit the crack width in a floor.

Shrinkage susceptibilityConcrete shrinks as a result of a chemical reaction, drying out and cooling. As a result of the shrinkage of concrete undesired splits and cracking can occur in a floor.Shrink-free concrete is not achievable within the preconditions the concrete mixture must, however, be allowed to shrink as little as possible. Hence granulate bedding is applied under the concrete to among other things reduce shrinkage.

Liquid sealingIndustrial floors can be made ‘impermeable’. The term impermeable means liquid on the top of the floor cannot penetrate to the bottom of the floor. Then in any case the following conditions must be satisfied:• minimum strength class C20/25• maximum penetration depth of the liquid amounts to 50 mm• the concrete density must be sufficient• floor working details must be attunedYou can find more information about this subject in CUR 65 (design) and CUR 44 (verification).

Frost and road salt resistanceThere is a major risk of the weathering of floors outdoors where road salt is spread. Making concrete extra dense and strong can create high resistance to the effects of wear.

The concrete must have a water cement factor of 0.45 for reasons of technical standards. ABT deliberately departs from this because if the standard is strictly followed the cement content must be increased. Then, however, shrinkage and accordingly cracking also increase. CEM III/B used as the basis for ABT concrete mixes has very good resistance to the penetration of chlorides. Here ABT also counts fly ash from the perspective of durability for the water binding factor, with a water cement factor approaching 0.45.The application of air bubbles works against monolithic finishing. Hence ABT usually uses the basic mixture described in more detail.

Mixture compositionThe principles mentioned must be seen as the schedule of requirements for the concrete mixture composition. When determining a mixture the ingredients below can then vary:• aggregate granulate gradation• fine part content• water dosage• cement content• cement type• binding agent• fillers• type and dosage of superplasticizer• steel fibre dosage

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Concrete for industrial floors

Water additionOnly a part of the water present in concrete is required for the hydration of the cement. The other part is required for working the concrete. Because the remaining water immediately causes more shrinkage and therefore more cracking it must be limited to the extent possible. Hence only the strictly required quantity of water is added to the mixture (the minimum water requirement of 165 l with 4-32 mm grit). The increasing of the consistency is further ensured by the superplasticizer. The values con-crete mixing plants use for absorbed water differ greatly. Absorbed water is water that at the start of bonding is not involved in the hydration process. Despite it not having to be counted for reasons of technical standards, ABT is of the opinion that in the long term it will evaporate from the concrete so will cause shrinkage. As a result, the choice of cement content and type and any binding agent level is determined.

Cement contentFor reasons of technical standards and for strength development, 280 kg cement can usually suffice. Because a large amount of water is not bonded here, this is not desirable from the perspective of drying shrinkage and crack sensitivity. To bind this free water the total amount of binder must be 360 kg. Depending on the working conditions and environment class, the cement content used will vary from 280 kg cement to 340 kg, combined with 80 to 20 kg of fly ash from blast furnace slag respectively. Then with a water content of 165 litres and a binding agent level of 360 kg (ABT calculates k=1 for fly ash because the value is revised after 90 days) there is a theoretical water cement factor of 0.46. If, however, absorbed water is counted (usually 10 to 15 litres) the water cement factor amounts to approx. 0.50. The cement/added binder proportion will depend on the environment class, processing conditions and be determined in consultation with the parties. The advantage of this mixture is that adjustments can be made depending on hydration heat. In specific cases (loading pit, impermeable) the mixture with 340 kg cement and 20 kg fly ash must be applied.

Figure: concrete pouring summary

Cement typeThe basis used is blast furnace cement (CEM III/B 42,5 LHHS). This cement initially has relatively slow strength

development and gives a high density to the concrete so shrinkage susceptibility decreases. At low temperatures, with the finishing of the floor, a part of the cement can be replaced by faster reacting Portland cement (CEM I). One must exercise caution here with shrinkage in mind.

Binding agentMaterials are added to concrete that do not immediately harden like cement but cause an increase in the compression strength of the concrete. The best known is the application of fly ash, but ground blast furnace slag is often used in combination with Portland cement. At higher temperatures, up to a maximum of a third part of cement can be replaced by an alternative binder for reasons of technical standards. This also functions as an internal water binder, so drying shrinkage decreases and the greater relaxation reduces cracking susceptibility. Silica Fume is not used in floor concrete.

FillersFillers can be used to increase the content of fine parts. As distinct from binding agents these do not or barely increase strength development so they cannot also be used as a replacement for cement. The best known example of a filler is limestone powder. ABT preferably uses fly ash, however.

PlasticizerSuperplasticizer is applied to bring the workability of concrete up to the necessary level. The dosage is determined by the concrete technologist. The starting point is that a minimum of water is used with the appropriate consistency being obtained by means of superplasticizer. The so-called 3rd generation of super-plasticizer is being increasingly used. In principle ABT advises against its use because many cases of damage are known of which no clear cause is identifiable.

Figure: air content measurement

Steel fibresFor construction steel fibre floors ABT uses steel fibres with end anchoring because the effectiveness of these fibres is the highest. The fibres are preferably mixed at the plant, but mixing on site is also an option. Steel fibre concrete is preferably mechanically processed and transported on the site using dumpers.

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Concrete for industrial floors

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

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Basic mixture for floors on granulated debrisBased on the aspects above, ABT uses the following low-shrinkage floor concrete mixture as the basis:• granulate gradation 4-32 mm inside ABHA A-B lines (possibly optimised with a 4-16 mm part)• cement content default value 300 kg cement and 60 kg fly ash, depending on circumstances changes between 280 kg and 340 kg cement, supplemented by 80 to 20 kg fly ash or ground blast furnace slag respectively, to be determined in consultation with ABT• water content 165 litres of water• fine parts at least 140 litres• superplasticizer 2nd generation

With the purchase of the concrete mixture ABT can advise on the proportions of cement/fly ash and cement sorts to be used. Several days before pouring ABT receives a mixture calculation, on the basis of which any last adaptations can be made.

TemperatureProcessing concrete in industrial floors is greatly dependent on temperature, because it is a relatively thin plate with usually a very large surface exposed to the ambient temperature. The following table can be used as a guide.

Table 1: mixture depending on mortar temperature

Execution supervisionDuring execution a mobile ‘laboratory’ can be used to control the weight by volume, the setting and flow, air content, water content and accordingly the water binding factor. Samples can also be manufactured for testing the compression strength, splitting tensile strength, bending tensile strength and shrinkage of the concrete. Attendance of the start of pouring and the adaptation of the concrete mixture (if necessary) has the purpose of having the actual work take place as closely as possible in line with the design principle.

Figure: determining water content on the site

Specifications The following text can be used as specifications for an ABT concrete mixture: Concrete C28/35 composition according to ABT in accordance with the ‘Concrete for industrial floors’ flyer.• environment class XC2• fine aggregate: sand• coarse aggregate: grit• filler: coal fly ash, quantity in consultation with the management• cement content: maximum 340 kg/m3

• additive: superplasticizer, for approval by the manage- ment. Quantity: approx. 1% of the cement volume.

The following information must be provided by the supplier:• composition of the mortar• the consistency range• type and class of the cement• nature of the aggregates• the brand of the cement with proof of origin• the cement content• the sieve analysis of the aggregates• the weight proportion of the aggregates• the largest granule• water cement factor• the slump• the additives

SummaryABT strives to make matching the concrete mixture a part of the design process for an industrial floor. The general quality of a floor is greatly improved by consulting on the concrete mixture and the properties, while making adjustments as necessary when carrying out the work.

The concrete mixture is a part of ABT’s endeavours for the design of a ‘sustainable’ environment by reducing the use of primary raw materials, and the reduction of greenhouse gas emissions with the use of residual products.

Figure: rack warehouse in use