GB2376462A

GB2376462A - A low cement concrete composition

Info

Publication number: GB2376462A
Application number: GB0114012A
Authority: GB
Inventors: Anthony M Hartley
Original assignee: RMC READYMIX Ltd
Current assignee: RMC READYMIX Ltd
Priority date: 2001-06-08
Filing date: 2001-06-08
Publication date: 2002-12-18
Also published as: GB0213230D0; GB2377930B; GB0114012D0; GB2377930A

Abstract

A low cement content concrete composition for mixing with water and for foundation applications, the composition comprising cement, aggregate, a plasticiser effective to achieve a flow of at least 500 mm when the concrete composition is mixed with water and an air-entrainer. The plasticiser may be a lignosulphonate. Preferably the aggregate : cement ratio by weight is from 5:1 to 12:1, most preferably from 7:1 to 9:1. The water : cement ratio is from 0.6:1 to 1:1, most preferably from 0.75:1 to 0.85:1. A process for delivering a concrete composition to a site is also disclosed, the process comprising: <SL> <LI>(i) providing a first mixture comprising cement, aggregate and water; <LI>(ii) providing a second mixture comprising one or more admixtures; <LI>(iii) adding the second mixture to the first mixture; <LI>(iv) conveying the first and second mixtures to a site, wherein there is no mixing of the first and second mixtures during transit; and <LI>(v) mixing the first and second mixtures together at site prior to application. </SL>

Description

A Concrete Composition The present invention relates to concrete materials and, in particular, to a free-flowing concrete for foundation applications such as for houses and other buildings. The present invention also provides a method for adding one or more admixtures to the concrete composition.

Concrete is comprised principally of cement, coarse aggregate, fine aggregate and water. The coarse and fine aggregates provide strength and are classified according to their type, shape and particle size.

They are embedded in a matrix of cement paste.

Examples of naturally occurring aggregates are, processed land based and marine gravels and crushed rocks such as limestones and granites. Aggregates can be sub-divided into coarse and fine fractions and are combined proportionally to a desired overall particle distribution for a concrete mixture.

In the production of concrete using"hydraulic" cement, the addition of water initiates the chemical process associated with the setting and hardening of the cement paste. This crystallisation reaction is known as hydration. Ideally the aggregates are evenly coated with the cement paste thus binding the whole mass together.

The most commonly used cement, especially in the UK, is Portland cement; this is often blended or mixed with other hydraulic cement materials such as ground

granulated blast furnace slag and pulverised fuel ash (fly-ash) to obtain specific concrete characteristics.

Portland cement is made from a mixture of approximately 75% of a calcareous material such as limestone or chalk and 25% of a siliceous material such as clay or shale. Both materials are intimately mixed and together with some minor components are heated (burned) to a high temperature at which they fuse together. The resultant clinker is finely ground to form Portland Cement.

Portland cement is a complex mixture of multicomponent, mineral solid-solutions. Among these, are compounds of calcium silicate and calcium aluminate that hydrate in the presence of water. The essential binding component has been identified as calcium silicate hydrate (CSH).

Whilst the basic formulation of a concrete mix approximates to 80% aggregate, 15% cement and 5% water, many commercially produced concretes are specially designed mixes. These mixes are produced for specific applications, e. g. for placing underwater and for placing by concrete pumping and for other purposes. These concrete types may incorporate small amounts of chemicals known as admixtures. Admixtures modify a specific property of the fresh or hardened concrete.

For example, a retarding admixture will defer the setting of the concrete. This is useful when slow rates of placement are envisaged or for placing under high summer temperatures.

Other admixtures are used to control properties such as water permeability, concrete consistence, water demand and resistance to freeze/thaw damage.

The present invention incorporates the use of admixtures, specifically those directed towards improving the cohesiveness and workability of a freeflowing concrete for foundation applications.

The workability or the consistence of concrete is related to its ease-of-placing and is commonly quantified by the "slump" test, outlined in BS 1881, Part 102: 1983. However, the measurement of"slump" tends to be less appropriate for higher workability concrete, particularly the workability of highly fluid concrete. At very high levels of workability a different test is applied. Known as the"flow table test", as outlined in BS 1881, Part 105: 1984, this method is more sensitive to the behaviour of rheological highly mobile or thixotropic concrete mixtures.

An accepted, industry-wide standard is that when the diameter of spread of concrete, measured by the flow table test, exceeds a value of approximately 510 mm, concrete is classified as having a flowing consistency.

The workability of concrete is also related to the water: cement ratio and the mix constituents including the particle size distribution of aggregate and cement particles. For a given aggregate: cement ratio, the higher the water: cement ratio, generally the more workable the concrete is.

Despite the advantages of high workability, a high water: cement ratio can, however, adversely affect the strength of the concrete. For example, if excess free water is present, which has not been absorbed into the aggregate and cement or is not adsorbed and used in the hydration reaction, this leaves voids in the hardened concrete as it dries out. The result of this is lower strength and increased porosity leading to a potential reduction in the durability of the concrete in the structure. Additionally, excess water can result in segregation of the mix during placement.

Accordingly, it is desirable to produce highly workable concrete without significantly increasing the water: cement ratio.

This has lead to the widespread use of admixtures known as plasticisers and more recently, to the use of superplasticisers. Plasticisers and superplasticisers impart increased workability when added to concrete, without the necessity of the addition of water to help achieve the effect.

Examples of commercially available high performance plasticisers are: Plastiment FN, manufactured by Sika Pozzalith LD34, manufactured by Feb MBT Conplast P165, manufactured by Fosroc Concrete Plasticiser DA4/36/00, manufactured by RMC Admixtures Plasticisers tend to be formulated from modified lignosulphonates and polycarboxylic acids. The

lignosulphonates may be derived from fermented wood, for example, spruce.

Examples of commercially available superplasticisers are: Sikament N, manufactured by Sika Rheobuild SP3, manufactured by Feb MBT Conplast SP430, manufactured by Fosroc Superplasticisers fall generally into the categories of sulphonated melamine-formaldehyde condensates; sulphonated napthalene-formaldehyde condensates; modified lignosulphonates; acid amide/polysaccharide mixtures and other high molecular weight hydroxylated polymers and copolymers.

Superplasticisers mainly produce their effect by electrostatic repulsion, which results in rapid dispersion of cement particles. This increases the mobility of the cement paste requiring no additional water for a significant increase in concrete workability.

As previously detailed, durability of concrete may be enhanced by the use of appropriate admixtures. The durability of concrete is dependent upon a number of factors, particularly important of which is its permeability. Since aggregates such as sand and gravel are durable materials, the packing density of the aggregate has the most significant influence on porosity.

Careful grading of aggregate material ensuring good packing to a maximum density helps to reduce interparticle porosity. Ideally, the paste of cement and water then fills the spaces among the graded aggregate particles. However, in practice, some voids still remain.

The voids may be classified as either closed pores (which are impermeable) or open pores, which are permeable voids, and these are in addition to the entrapped air voids present in the concrete paste.

Open pores may be further divided into capillaries, fine striations and re-entrant shape irregularities of the aggregate particles. The capillary pores make concrete particularly vulnerable to frost attack. For example, after concrete is exposed to a critical limit of water saturation followed by freezing, there is a build-up of pressure as water freezes and expands in the capillaries.

The use of an air-entrainer as a class of admixture has been widely demonstrated for many years to improve the durability of concrete, particularly with regard to increasing resistance to damage due to cycles of freezing and thawing in exposed conditions.

The process of air-entrainment results in the presence of millions of evenly spaced air bubbles (typically of diameters between 0.025 and 1 mm) throughout the cement paste. This has the effect of relieving the expansion pressures that build up in the capillary pores as freezing occurs by blocking the capillary structure to prevent water ingress.

The air bubbles introduced through air-entrainment should be differentiated from the randomly sized air voids already entrapped in the concrete.

The most commonly used classes of material for the purpose of air-entrainment are alkali salts of wood resins; fatty acid soaps; alkyl aryl sulphonates ; alkyl sulphates and salts of fatty acids derived from animal and vegetable fats and oils. A feature which tends to be common to many air-entrainers is that they are surface active agents, or surfactants.

Examples of commercially available air-entrainers are: FRO-BE, manufactured by Sika Conplast AE383, manufactured by Fosroc Micro-air 100, manufactured by Feb MBT Concrete air-entrainer DA9/16/00, manufactured by RMC Admixtures In addition to their role in increasing the durability of hardened concrete, air-entrainers also improve workability and reduce bleeding and segregation of concrete in its newly mixed state.

For example, the multitude of microscopic air bubbles may be regarded as increasing workability by having a lubricating effect on the relative motion of aggregate particles.

In the case of the bleeding of fresh concrete, the air bubbles are understood to block the spaces between cement particles, thereby minimising the flow of water

and the sedimentation of solids throughout the cementaggregate mix.

The compatibility of admixtures with each other may depend upon whether they are premixed into a combined product, or added separately to the concrete mix during the batching process.

The present invention aims to address at least some of the problems associated with the prior art.

Accordingly, in a first aspect, the present invention provides a low cement content concrete composition for mixing with water and for foundation applications, the composition comprising: (i) cement ; (ii) aggregate; (iii) a plasticiser effective to achieve a flow of at least 500 mm, when the concrete composition is mixed with water; and (iv) an air-entrainer.

The term plasticiser as used herein is meant to encompass both conventional plasticisers and superplasticisers. The plasticiser is selected and provided in an amount sufficient to achieve a flow of at least 500 mm, when the concrete composition is mixed with water.

In the low cement content concrete composition

according to the present invention the aggregate: cement ratio by weight is typically from 5: 1 to 12: 1, preferably from 6: 1 to 10: 1, more preferably from 7: 1 to 9: 1.

The air-entrainer is provided in an amount sufficient to cause a substantially controlled and stable quantity of air to be incorporated during the mixing of the concrete without significantly affecting the strength of the hardened concrete. The resulting air content is preferably in the range of from 3 to 10%, typically from 4 to 9%, more typically from 6 to 9%.

The concrete composition as herein described will be mixed with water prior to delivery. The flow, as herein defined, is preferably in the range of from 550 mm to 650 mm.

The plasticiser and air-entrainer may be added as a single admixture or separately as two distinct admixtures.

In addition to the benefit of ease of placement, the free-flowing concrete produced from the present invention is highly cohesive and exhibits minimal surface bleeding.

Further, the free-flowing concrete is self-compacting and self-levelling and shows sufficient strength in as little as 24 hours after installation in order to allow bricklaying to proceed.

The use of a plasticiser to maintain workability reduces the required water content, which minimises

any increase in water: cement ratio. This effect partially compensates for any reduction in strength associated with air-entrainment.

Typically, the nominal cement content is from 190 kg to 250 kg.

The weight of coarse aggregate as a percentage of the weight of the total aggregate is typically from 40 to 70% and more preferably from 49 to 60%.

The weight of fine aggregate as a percentage of the weight of the total aggregate is typically from 30 to 60% and more preferably from 40 to 51%.

The aggregate: cement ratio by weight is typically from 5: 1 to 12: 1, more preferably from 6: 1 to 10: 1 and more preferably still from 7: 1 to 9: 1 ; Plasticiser is typically added in a dosage of from 0.01 to 5% by weight of cement, preferably from 0.05 to 2% by weight of cement, more preferably from 0.2 to 1.5% by weight of cement, still more preferably from 0.3 to 1% by weight of cement.

Air-entrainer is typically added in a dosage of from 0.01 to 5% by weight of cement, preferably from 0.02 to 1% by weight of cement, more preferably from 0.04 to 0.5% by weight of cement, still more preferably from 0.08 to 0.2% by weight of cement.

The concrete compositions according to the above preferred proportioning ranges may be mixed with water such that, preferably, a water: cement ratio of from

0.6 : 1 to 1: 1, more preferably from 0.70 : 1 to 0.95 : 1 and more preferably still from 0.75 : 1 to 0.85 : 1 is obtained.

The self-levelling property of the concrete compositions resulting from the above preferred proportioning ranges is most effective where the depth of concrete in the foundation exceeds 400 mm.

The typical workability of the special design concrete prior to the addition of one or more admixtures formulated in the present invention is in the range of 80 mm to 110 mm when measured by the slump test outlined in BS 1881: part 102: 1983.

The increase in the workability to the preferred range of flow of 550 mm to 650 mm following addition of one or more admixtures enables an entire house foundation to be poured from a single discharge point.

As well as significantly increasing the speed of construction, site labour and plant requirements are reduced. Further, the fluidity of the concrete reduces the temptation for site operatives to modify the mix by introducing further water, which would reduce the foundation integrity by reducing concrete strength.

The free-flowing nature of the concrete described in the present invention provides a challenge regarding how fully mixed concrete can be transported to site in full loads, without spillage.

The benefits of ready mixed concrete are well known in the industry. Of particular advantages are the consistency in quality and content of concrete mix provided, the reduced labour and supervisory costs, and the prevention of wastage of cement, aggregate and mixture on site.

In a second aspect of the present invention, there is provided a process for delivering a concrete composition to a site, the process comprising: i) providing a first mixture comprising cement, aggregate and water ; ii) providing a second mixture comprising one or more admixtures; iii) adding the second mixture to the first mixture; iv) conveying the first and second mixtures to a site, whereby there is no mixing or substantially no mixing of the first and second mixtures during transit; and v) mixing the first and second mixtures together at site prior to application.

The second mixture preferably comprises a plasticiser and an air-entrainer in an amount sufficient to achieve a flow of at least 500 mm.

Preferably, the first mixture is substantially fully mixed to the required base-mix workability, preferably in the range of from 80 mm to 110mm slump.

In the process according to the present invention, the concrete composition will preferably be conveyed from a plant to site in a truck-mixer drum. In this case, the mixer drum is held stationary while the second mixture defined in step (ii) of the above process is added to the rear of the load, but the second mixture is not mixed into the concrete. In other words, the second mixture is in contact with the first mixture but is not homogeneously mixed therewith. The load is then dispatched to site without rotation of the truckmixer drum during transit.

A substantially homogeneous mixture is only formed when the delivery vehicle arrives at its destination.

This is achieved by mixing the first and second mixtures within the drum of the truck-mixer, typically by rotation of the drum.

As well as enabling full loads of ready mixed concrete to be transported to site, the method described above has been shown to have no adverse effects on workability retention or any set-retardation.

Preferably, prior to discharge, the concrete is remixed at normal mixing speed for a minimum of approximately three minutes. No additional water is added to the concrete mix, as this is detrimental to the flow characteristics of the mix.

During transportation, some initial bleeding occurs during transit due to vehicle vibration. This is essential to the process as it provides free water to dilute the second mixture defined in step (ii) of the

above process in order that it can be rapidly dispersed into the concrete mixture during the remix cycle prior to discharge into a foundation.

The concrete compositions described in the present invention will now be exemplified, with reference to the following mix designs.

Example 1 Concrete was batched in a pan mixer to a nominal slump of 80 mm with the mix proportions as shown in Table 1.

Table 1

Material Proportion 20mm Thames Valley gravel 44% by weight total aggregate 10mm Thames Valley gravel 15% by weight total aggregate Thames Valley sand 41% by weight total aggregate Nominal cement content 220 kg/m3 Presoaked Thames Valley flint gravel aggregates and Seament Portland cement were used. Admixtures were added in the proportions given in Table 2.

Table 2

Material Proportion Plasticiser 0.40% by weight of cement Air-entrainer 0. 12% by weight of cement The plasticiser used was Concrete plasticiser

DA4/36/00, manufactured by RMC Admixtures and based on lignosulphonates derived from fermented spruce wood.

The air-entrainer was Concrete air-entrainer DA9/16/00, manufactured by RMC Admixtures and based on a blend of synthetic surfactants and fatty acid soaps.

After a period of further mixing the flow (mm) was determined together with the plastic density (kg/m3) and air content (%). Table 3 summarises the results of tests for fresh concrete.

Table 3

Result Fresh Concrete Flow (mm) 650 Air Content (%) 8. 7 Plastic Density (kg/m3) 2105 Example 2 Concrete was batched in a pan mixer to a nominal slump of 80 mm with the mix proportions as shown in Table 4.

Table 4

Material Proportion Z-UT, Thames Valley gravel 44% by weight total aggregate 10mm Thames Valley gravel 15% by weight total aggregate Thames Valley sand 41% by weight total aggregate Nominal cement content 220 kg/rn3 Presoaked Thames Valley flint gravel aggregates and Seament Portland cement were used. Admixtures were added in the proportions given in Table 5.

Table 5

Material Proportion Plasticiser 0.40% by weight of cement Air-entrainer 0. 08% by weight of cement The plasticiser was Concrete plasticiser DA4/36/00, manufactured by RMC Admixtures and based on lignosulphonates derived from fermented spruce wood.

After a period of further mixing the flow (mm) was determined together with the plastic density (kg/m3) and air content (%). In addition, 100 mm cubes were cast for testing compressive strength (N/mm2). Table 6 summarises the results of tests for fresh and hardened concrete.

Table 6

Result Fresh Concrete Hardened Concrete Flow (mm) 650 Air Content (%) 6. 5 Plastic Density (kg/m3) 2155 Compressive Strength-3. 9 (N/mm2) after 1 day

Claims

CLAIMS: 1. A low cement content concrete composition for mixing with water and for foundation applications, the composition comprising: (i) cement ; (ii) aggregate; (iii) a plasticiser effective to achieve a flow of at least 500 mm when the concrete composition is mixed with water; and (iv) an air-entrainer.
2. A low cement content concrete composition as claimed in claim 1, wherein the aggregate: cement ratio by weight is from 5: 1 to 12: 1, preferably from 6: 1 to 10: 1, more preferably from 7: 1 to 9: 1.
3. A low cement content concrete composition as claimed in claim 1 or claim 2, wherein the aggregate comprises coarse and fine aggregate.
4. A low cement content concrete composition as claimed in any one of claims 1 to 3, wherein the weight of coarse aggregate as a percentage of the weight of the total aggregate is from 40 to 70%, more preferably from 49 to 60%.
5. A low cement content concrete composition as claimed in any one of claims 1 to 4, wherein the weight of fine aggregate as a percentage of the weight

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of the total aggregate is from 30 to 60%, preferably from 40 to 51%.
6. A low cement content concrete composition as claimed in any one of claims 1 to 5, wherein the coarse aggregate is or comprises crushed rock, preferably limestone.
7. A low cement content concrete composition as claimed in any one of claims 1 to 6, wherein the coarse aggregate is or comprises land based and/or marine gravels.
8. A low cement content concrete composition as claimed in any one of claims 1 to 7, wherein the fine aggregate is or comprises sand.
9. A low cement content concrete composition as claimed in any one of claims 1 to 8, wherein the composition comprises the plasticiser in a dosage of from 0.01 to 5% by weight of cement, preferably from 0.05 to 2% by weight of cement, more preferably from 0.2 to 1.5% by weight of cement, still more preferably from 0.3 to 1% by weight of cement.
10. A low cement content concrete composition as claimed in any one of claims 1 to 9, wherein the plasticiser comprises one or more lignosulphonates, preferably being derived from fermented wood.
11. A low cement content concrete composition as claimed in any one of claims 1 to 10, wherein the composition comprises the air-entrainer in a dosage of from 0.01 to 5% by weight of cement, preferably from

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0.02 to 1% by weight of cement, more preferably from 0.04 to 0.5% by weight of cement, still more preferably from 0.08 to 0. 2% by weight of cement.
12. A low cement content concrete composition as claimed in any one of claims 1 to 11, wherein the airentrainer comprises one or more surfactants.
13. A low cement content concrete composition as claimed in any one of claims 1 to 12, wherein the airentrainer comprises one or more fatty acid soaps.
14. A low cement content concrete composition as claimed in any one of claims 1 to 13, wherein the cement is or comprises Portland cement.
15. A free-flowing concrete formed by adding water to the concrete composition as defined in any one of claims 1 to 14.
16. A free-flowing concrete as claimed in claim 15, wherein the water: cement ratio by weight is from 0.6 : 1 to 1: 1, preferably from 0.70 : 1 to 0.95 : 1, more preferably from 0.75 : 1 to 0.85 : 1.
17. A free-flowing concrete as claimed in claim 15 or 16 having a flow of at least 500 mm, more preferably from 550 to 650 mm.
18. A free-flowing concrete as claimed in any one of claims 15 to 17 having an air content of from 3 to 10%.
19. Use of the free-flowing concrete as defined in

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any one of claims 15 to 18 in a foundation.
20. Use as claimed in claim 19, wherein the foundation is for a house or building.
21. A process for delivering a concrete composition to a site, the process comprising: (i) providing a first mixture comprising cement, aggregate and water; (ii) providing a second mixture comprising one or more admixtures; (iii) adding the second mixture to the first mixture; (iv) conveying the first and second mixtures to a site, wherein there is no mixing or substantially no mixing of the first and second mixtures during transit; and (v) mixing the first and second mixtures together at site prior to application.
22. A process as claimed in claim 21, wherein the second mixture comprises a plasticiser and/or an airentrainer.
23. A process as claimed in claim 21 or claim 22, wherein the concrete composition is or comprises a free-flowing concrete as defined in any one of claims 15 to 18.
24. A process as claimed in any one of claims 21 to

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23, wherein the mixed concrete composition is deposited in a foundation.
25. A process as claimed in claim 24, wherein the mixed concrete composition is deposited in a foundation for a house or building.
26. A process as claimed in any one of claims 21 to 25, wherein in step (iii) the second mixture is added to the first mixture in a truck-mixer drum.
27. A process as claimed in any one of claims 21 to 26, wherein in step (iv) the first and second mixtures are conveyed to a site in a truck-mixer drum.
28. A process as claimed in any one of claims 21 to 27, wherein in step (v) the first and second mixtures are mixed together at site, prior to application, in a truck-mixer drum.
29. A free-flowing concrete whenever delivered to a site by the process as defined in any one of claims 21 to 28.
30. A free-flowing concrete composition substantially as herein described with reference to any one of the Examples.