GB2512606A

GB2512606A - A lightweight aggregate and a concrete comprising the lightweight aggregate

Info

Publication number: GB2512606A
Application number: GB1305990.2A
Authority: GB
Inventors: Philip Leonard Owens
Original assignee: NUSTONE Ltd
Current assignee: NUSTONE Ltd
Priority date: 2013-04-03
Filing date: 2013-04-03
Publication date: 2014-10-08
Also published as: GB201305990D0

Abstract

A lightweight coarse aggregate for concrete comprises a mixture of normal and lightweight coarse aggregate. The lightweight coarse aggregate is preferably expanded clay. A lightweight concrete comprises cement, fine aggregate (e.g. sand) and the coarse aggregate.

Description

A Lightweight Aggregate and a Concrete ComDrising the Lightweight Aggregate The present disclosure relates to lightweight concrete and to an aggregate suitable for the production of such a lightweight concrete.

Reinforced concrete, as a typical (normal weight) concrete is a heterogeneous material usually being a mixture of water (density 1.0 Mg/rn3); cement (density 2.75 to 3.2 Mg/rn3 depending on type) and fine and coarse aggregates. Normal weight coarse aggregates have a density 2.2 to 2.9 Mg/m3 depending on geological source. Normal weight coarse aggregates can include natural stone, crushed stone, crushed concrete, crushed brick or other selected demolition waste. The fine aggregates are normally sand. The quantity of each constituent varies, but typically for reinforced concrete the amount of water used is l/m3 (18% by volume), cement about 360 kg/m3 (12.0% by volume), sand about 590 kg/rn3 (22% by volume), and coarse aggregate about 1220 kg/m3 (47% by volume), with 1.0% entrapped air. The density of the concrete is the surn of the constituents, typically 2350 kg/m3 or 2.35 tonnes / cubic metre.

BS EN 1992 (Eurocode 2) Design of concrete structures permits the density of concrete to be varied. The density of concrete is generally fixed by the choice of coarse aggregate used. Low density concretes (also known as lightweight concretes (LWC)) are made by using coarse lightweight aggregates (LWA), which coarse lightweight aggregates are invariably manufactured light weight aggregates.

Lightweight aggregates are commonly manufactured using argillaceous minerals by expanding at high temperatures. These aggregates typically have particle densities of 0.4 to 0.7 Mg/m3 and have high absorption (15 to 30%).

Use of such manufactured lightweight aggregates, whilst allowing for lightweight concretes that have a low density, results in concretes that only have a relatively low strength. By virtue of their lower strength these concretes have limited application. They may, for example, be used in the manufacture of concrete masonry (blocks).

Past attempts have been made to provide lightweight concretes that have greater strength and thereby broader application. These past attempts have focused on the provision of modified lightweight coa rse aggregates.

UK Patent No 2218412 describes a method for manufacture of a lightweight coarse aggregate as a composite of clay and other minerals fired under controlled conditions to a prescribed density that could produce an aggregate of 1.30 Mg/rn3, which when used in concrete could result in a concrete of 1750kg/rn3 or a reduction in mass of 600kg or 24.4%.

Whilst these aggregates are clearly useful in providing lightweight concretes that have a higher strength whilst retaining a lower density, the properties of the lightweight coarse aggregates, and thereby the properties of the resultant lightweight concrete formed using such aggregates, is fixed during manufacture of the aggregates. This provides a lack of flexibility.

The present invention arose in a bid to provide an improved lightweight concrete that allows for the flexible specification of density along with strength and other requirements.

This invention relates to a concrete that is made using a coarse aggregate that comprises normal weight and lightweight coarse aggregates, as separate constituent elements, in order to significantly reduce the density or unit weight of the concrete. By using a combination of normal weight and lightweight coarse aggregates both the density and the strength can be specified as the concrete is mixed.

According to the present invention in a first aspect, there is provided a lightweight concrete comprising cement, fine aggregate and coarse aggregate, wherein the coarse aggregate comprise a mixture of normal weight and lightweight coarse aggregates.

According to the present invention in a further aspect, there is provided a coarse aggregate for a concrete comprising a mixture of normal weight and lightweight coarse aggregates.

The normal weight coarse aggregates and lightweight coarse aggregates are preferably separate from one another in the coarse aggregate mixture, that is the coarse aggregate is formed by mixing granules of one or more normal weight coarse aggregates with granules of the lightweight coarse aggregate.

BS EN 206 -1:2000 Concrete -Part 1: Specification, production and conformity, provides the following definitions for the following terms as used in the present

specification:

Concrete Material formed by mixing cement, coarse and fine aggregate and water with or without the incorporation of admixtures and additions, which develops its properties by the hydration of the cement.

Normal weight concrete Concrete having an oven-dry density greater than 2000 kg/m3 but not exceeding 2600kg/m3.

Lightweight concrete Concrete having an oven-dry density ranging from 800 kg/m3 to 2000 kg/m3.

Aggregate Granular material suitable for use in concrete. Aggregates may be natural, artificial or recycled from materials previously used in construction.

Normal weight aggregate An aggregate with an oven dry particle density greater than 2 000 kg/m3 and less than 3000 kg/m3.

Lightweight aggregate An aggregate of mineral origin having an oven-dry particle density less than 2000 kg/rn3 when determined according to EN 1097-6.

The present invention is to be construed in accordance with the above definitions.

Cements in accordance with the present invention may comprise combination cements, i.e. cements comprising a mixture of one or more CEM1 cements (i.e. cements complying with BS EN 197-1: 2000) with one or more other cementitious materials such as PEA (pulverized fly ash) or GGBS (ground granulated blast furnace slag).

According to the present invention in a yet further aspect, there is provided a rnethod of forming a coarse aggregate for a concrete comprising forming granules of a lightweight coarse aggregate and subsequently mixing the granules of lightweight coarse aggregate with granules of one or more normal weight coarse aggregates.

Most preferably, the coarse lightweight aggregate comprises lightweight expanded clay aggregate. The expanded clay aggregate may be provided in the form of pellets. Each of the pellets may be considered to comprise a granule. The pellets may comprise the expanded clay only, i.e. no other minerals or elements are added to the clay prior to pelletizing, or the pellets may comprise an inert filler, such as fines from rock crushing, PFA (pulverized fly ash) from power stations, crushed furnace bottom ash from incinerators, fine sand, argillaceous non expanding clays or other selected waste minerals in addition to the clay. The pellets are most preferably substantially spherical. The pellets preferably have a size (diameter) of 4 to 20mm, more preferably a size of 8 to 16mm with a target mean diameter of 14mm.

The lightweight coarse aggregate preferably has a particle density in the range 0.6 to 1.3 kg/m3. The lightweight coarse aggregate preferably has a 30 minute water absorbency of 7% or less. The manufactured coarse aggregate most preferably has a 30 minute water absorbency of 5% or less. The 30 mm water absorbency may be tested by following the methodology laid out in IS: 2386 (Part III) -1963 but with the time of immersion in water reduced from the specified 24 hours to 30 minutes. The reduction in immersion time replicates the time available during the mixing and hardening of the concrete when there is free water available to be absorbed by the aggregate.

By the provision of low absorbency light weight aggregate, water from the mix is effectively prevented from being absorbed into the aggregate and thereby does not play a part in the cement hydration process. Moreover, because of the low water absorption, and the low thermal conductivity of the low density concrete, the fire resistance of the concrete is dramatically improved. Note that any water locked into aggregate by age hardened cement paste, as may occur in prior art concretes, becomes a serious problem in the event of a fire, the water boils and the steam pressure causes the concrete to explode or seriously crack. Such effects may be effectively mitigated by the present invention.

The coarse aggregate mixture preferably comprises a greater amount of lightweight aggregate than natural aggregate by volume. The coarse aggregate mixture may be varied depending on the desired properties of the concrete.

In accordance with the definitions provided above, the lightweight concrete may have a density of 800 kg/m3 to 2000 kg/rn3, more preferably 1350 kg/m3 to 2000 kgjm3. The lightweight concrete may have a density of substantially 1750 kg/rn3, which may provide an optimum strength.

Non-limiting embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows the expansion of natural clay (extracted during excavations for the Crossrail project in London, UK) with temperature; Figure 2 shows the water absorption of the natural clay with temperature.

A concrete in accordance with the present invention comprises water, cement, fine aggregate and coarse aggregate, wherein the coarse aggregate comprises a mixture of normal weight and lightweight coarse aggregates.

The proportions of the constituents of the concrete may be varied, as will be readily appreciated by those skilled in the art. In particular, different specific mixtures of aggregates can be used to optimise the density as well as strength and durability of the concrete.

In contrast to prior art arrangements, where properties were varied by varying the density of the lightweight aggregate, which density was set at the manufacture of the lightweight aggregate, the properties of the concrete in accordance with the present invention may be varied by altering the mix/combinations of the coarse lightweight aggregate and coarse normal weight aggregate(s). A single specification of coarse lightweight aggregate may be used to provide a range of concretes each having different properties.

A series of exemplary concrete mixes, in accordance with the present invention, which offer a better versatility and choice to structural design and provide strengths suitable for structural concrete of +30 mN/mm2 and a fresh density in the range of 1550 to 1950 kg/m3 (thus providing density reductions of 20% to 35% when compared with concretes made with normal course aggregates only), are presented in the table below. Note the mixes are for one cubic metre of concrete in the fresh state.

Lght Weight Concrete Normal Concrete Component sutabforstrcuctur.a work iprarart) Exampe I Exampe 2 Eampe 3 Water thtres) 120 180 180 180 Cement and flyash kg) 320 320 320 320 Hne aggregate sand) kg) 606 506 506 506 Course agregate kg) LIghtweght (manufactured) 239 1.9.2 135 -Normaweght (natura} 205 452 719 1273 Tot& 1550 1750 1550 2372 % Reducton n ders'ty 35 26 18 To provide suitable mixes with different structural properties for different end uses, a concrete sample may be prepared with known constituents and known weights and/or volumes of those constituents. The sample may be tested for strength/density/consistency/workability and then changes can be made to that original mix to achieve the target strength/density/consistency/workability, as will be readily appreciated by those skilled in the art.

The normal weight coarse aggregate may comprise any known coarse aggregate used in the production of reinforced concrete, which meets the definition of normal weight course aggregate provided above, and includes known natural aggregates, screened natural aggregates, crushed rock or waste derived aggregates, including natural stone, crushed stone, crushed concrete, crushed brick or other selected demolition waste.

Whilst it will be readily appreciated by those skilled in the art that various coarse lightweight aggregates will be suitable for use, argillaceous materials are preferred, with a particularly preferred coarse lightweight aggregate being made by expanding natural clay and forming substantially spherical pellets. These pellets preferably have a diameter of 15 to 18mm with a particle density in the range 0.6 to 1.3 kg/m3 and a 30 minute water absorbency of 8% or less, more preferably 7% or less and most preferably 5% or less.

The substantially spherical pellets may be formed, in a known manner, by firing in a rotary kiln to temperatures between 1100 and 1200°C. The pellets of natural clay, which may sometimes be mixed with inert filler, are preferably formed in a rotary drum or pan pelletiser. However, alternative methods of pellet formation may be utilised. The use of the drum or pan pelletiser is preferred, since more uniform aggregate sizes are produced using the drum or pan pelletiser. The kiln will normally be in three parts: a preheating part where the pellets are dried and heated to about 650°C, the main kiln section in which the pellets are fired to 1100 to 1200°C and then the cooling section in which the pellets are cooled in a rotary or planetary cooler system where the heat is preferably recuperated by preheating the combustion air used for firing the kiln.

The temperature of firing is dependent on the clay or clay mixture being used, as will be readily appreciated by those skilled in the art. The physical changes that occur are first drying as the pellet heats up then at a particular temperature the clay becomes plastic and various gasses are released which cause the expansion and finally as the surface is vitrified forming a glassy substance on the surface which seals the surface reducing the absorbency of the thus formed manufactured lightweight coarse aggregate.

Figures 1 and 2 represent graphs that show the results of testing London clay (as recently extracted from the tunnelling operations of the Crossrail project in London). These tests show the expansion and surface vitrification being nearly completed at just below 1200°C.

To form the coarse aggregate mixture, the pellets obtained from the above described process are mixed with particles of a coarse normal weight aggregate (or a mixture of particles of different normal weight coarse aggregates).

By using a combination of normal weight and lightweight coarse aggregates, optimised strength density relationships can be obtained. A concrete may be produced with the cost advantages of lower specific weight but moreover which allows for reduced micro-cracking, increased rate of strength development, improved maturity, improved modulus of elasticity, improved tensile strain capacity and a lower coefficient of thermal expansion together with lower transport costs.

This invention applies to all types of concrete but is most useful for reinforced and/or structural concrete. By reduction of concrete density a reduction in the total weight of structures, buildings, basements, platforms, runways etc. can be used to advantage, either by, for example, going higher with the structure for the same foundations or by reducing the size of the elements of the structure and the foundations of the structure, without reducing its strength or durability and with significant cost savings when compared with conventional reinforced concrete.

Without this invention structures will be over-weight and be more expensive than they need to be.

Claims

Claims 1. A lightweight concrete comprising cement, fine aggregate and coarse aggregate, wherein the coarse aggregate comprises a mixture of normal weight and lightweight coarse aggregates.
2. A coarse aggregate for concrete comprising a mixture of normal weight and lightweight coarse aggregates.
3. A lightweight concrete or an aggregate as claimed in Claim 1 or 2, wherein the coarse aggregate mixture is formed by mixing granules of the normal weight coarse aggregate(s) with granules of the lightweight coarse aggregate.
4. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein C') the lightweight coarse aggregate comprises an argillaceous material.o
5. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate comprises expanded clay.
6. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate comprises pellets having a size of 8 to 20mm.
7. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate comprises substantially spherical pellets having a diameter of 15 to 18mm.
8. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate has a particle density in the range 0.6 to 1.3 kg/m3.
9. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate has a 30 minute water absorbency of 8% or less.
10. A lightweight concrete or an aggregate as claimed in any preceding claim, wherein the lightweight coarse aggregate has a 30 minute water absorbency of 5% or less.
11. A lightweight concrete or an aggregate as claimed in any preceding claim comprising a coarse aggregate mix, which comprises a greater amount of lightweight aggregate than normal weight aggregate by volume.
12. A lightweight concrete or an aggregate as claimed in any preceding claim, which has a density of 1300 kg/m3 to 2000 kg/m3.
13. A lightweight concrete or an aggregate as claimed in any preceding claim, which has a density of 1750 kg/m3.
14. A method of forming a coarse aggregate for a concrete, as claimed in any of Claims 2 C') to 13, comprising forming granules of a lightweight coarse aggregate and subsequently mixing the particles of lightweight coarse aggregate with particles of one or more normal o weight coarse aggregates. (115. A lightweight concrete as hereinbefore described.16. An aggregate for a concrete as hereinbefore described.