GB2293376A

GB2293376A - Concrete compositions

Info

Publication number: GB2293376A
Application number: GB9419283A
Authority: GB
Inventors: Rod Dengate; Alan Maries; Richard Day; Ron Franklin; John Chandler; Dirch Bager; Brian Schou Nielsen; Palle Nepper-Christensen; Hans Henrik Baache; Anders Henrichsen; Ivar Schacke; Richard Malmose; Jan Holm Petersen; Michael John Abrams
Original assignee: Trafalgar House Technology Ltd
Current assignee: Skanska Technology Ltd
Priority date: 1994-09-23
Filing date: 1994-09-23
Publication date: 1996-03-27
Anticipated expiration: 2014-09-23
Also published as: GB9419283D0; GB2293376B

Abstract

A concrete composition comprises a cementitious component of relatively fine particles and a coarse aggregate which consists of particles at least 70% by weight of which are shaped such that the ratio (Vp:Vs), between the volume (Vp) of a given particle and the volume (Vs) of the notional sphere of minimum volume capable of containing that particle wholly within it, is in the range 1:2 to 1:6. The composition has fewer than 10% by volume of particles having a size in the range 2 mm to 5 mm as determined by retention on British Standard sieves. The compositions can be laid to form a very dense concrete with high mechanical stability which is suitable for use in the construction of load-bearing surfaces e.g. roads, and runways.

Description

CONCRETE COMPOSITIONS This invention relates to concrete compositions.

In particular, but not exclusively, it relates to concrete compositions useful in the construction of roads, runways and the like.

Concrete is a well-known material which is used in an enormous variety of applications. Its strength means that it is useful for constructing roadbases.

However, when used in the construction of roadbases, because cement-bound materials tend to expand and contract in response to changes in ambient temperature and moisture, concrete is generally laid in slabs five to six metres long separated by joints. Such joints are designed to relieve thermally-induced stresses and to accommodate temperature-related movements in the concrete pavement. At least two layers of asphalt may be laid over the concrete roadbase in order to prevent cracks in the concrete pavement being propagated in the road surface. A disadvantage of such a construction is that the joints between the slabs of concrete must be properly maintained; if they are not, very high temperatures may cause the pavement to break up. The joints also increase construction costs and may have a detrimental effect on the rideability of the pavement surface.

In order to avoid the above-mentioned problems with joints, it is known to lay concrete as a continuous slab. However, the slab must be reinforced, for example with steel, so that it does not crack due to expansion and contraction. Such a construction is known as Continuously Reinforced Concrete Roadbase (CRCR) and, owing to the required reinforcement, is expensive and requires specialist plant for its construction.

It has become popular to use asphalt in the construction of roads because it is easy to handle and can be laid quickly and easily using a paving machine.

However, asphalt roads suffer from the disadvantage that heavy traffic may cause the road to become rutted, hence requiring additional expensive maintenance.

Concrete pavements are known to have an inverse relationship between ultimate strength when hardened and the amount of mixing water incorporated in the concrete mix; for example, for a modern Portland cement, Duff Abrams' law is expressed as: 14000 5X where S is the cylinder compressive strength of moistcured concrete at 28 days (in lb/sq.in.) and x is the water/cement ratio, by apparent volume, assuming that 1 cu.ft. of cement in a bagged state of compaction weighs 94 lb. It is therefore generally desirable to employ a dense mix with relatively little mixing water in order to generate a high strength pavement.

Dense mixes, however, are difficult to work and there is therefore a conflicting requirement as between maximising density for high ultimate strength and maximising workability to facilitate the fabrication of the concrete.

We have found that a dense, yet workable, mix can be formulated provided that the larger particles present in the mix (i.e. the coarse aggregate) are selected to have certain particular characteristics in relation to their size and shape.

According to a first aspect of the present invention, there is provided a concrete composition for use in the construction of load-bearing surfaces such as roads, runways and the like, which comprises a cementitious component of relatively fine particles; and a coarse aggregate, characterised in that the coarse aggregate consists of particles at least 70% by weight of which are shaped such that the ratio (Vp:Vs), between the volume (Vp) of a given particle and the volume (Vs) of the notional sphere of minimum volume capable of containing that particle wholly within it, is in the range 1:2 to 1:6, and in that the composition has fewer than 10% by volume of particles having a size in the range 2 mm to 5 mm as determined by retention on British Standard sieves.

The ratio Vp:Vs, as expressed as a fraction (Vp/Vs), represents a quantitative measure of particle shape. The closer the particle approximates to a sphere, the nearer the fractional ratio approaches unity. For particles which are needle-like, the fractional ratio becomes very small. Expressed differently, if the volume (Vp) of the particle is taken as unity, then the value of the volume (Vs) is 1 for a perfect sphere and increasingly large for particles of increased elongation. The value of V9 when Vp is taken as unity (1) will be referred to hereinafter as the "Abrams number". Elementary geometry indicates that, for a particle which is a parallelepiped of relative dimensions lxlx2, the Abrams number of that particle is 3.85.For a particle which is a parallelepiped of relative dimensions lxlx3, the Abrams number of that particle is 6.37. For a particle which is a parallelepiped of relative dimensions lxlx6, the Abrams number is 20.44.

In terms of achieving a dense-packed structure, and thus one which is of high ultimate strength, the coarse aggregate used in the concrete should have an Abrams number in the range 2 to 6, i.e. the particles should be neither spherical nor acicular, but rather should approximate to an equi-axed form. Obviously, the particles of aggregate employed will have more or less irregular shapes; nonetheless the Abrams number can easily be determined by simple means and is believed to be a guide to the ability of the mix to lock together into a dense concrete of high strength.

For particles of coarse aggregate, the Abrams number can be determined by (a) the volume of liquid displaced by a given particle (to give Vp); and by a calliper measurement to determine the maximum distance between any two points on the surface to the particle, since this value represents the diameter of the notional sphere whose volume is Vs. Simple calculation thus enables the ratio Vp:V9 to be determined.

Whilst the inventors do not wish to be bound by theory, it is believed that the ratio Vp:V9 of the coarse particles is such that, when they are compacted mechanically, they form a dense, "locked-solid" structure in which the coarse aggregate particles are locked together in contact with one another and in which the particles of the cementitious component of the composition can flow into the spaces between the coarse aggregate particles.

Such a structure is illustrated diagrammatically in Figure 1 of the accompanying drawings. Particles 1 of coarse aggregate can be seen compacted together to form a locked solid structure. The cementitious component 2 is able to flow into the spaces between the particles 1, with the result that a very dense concrete can be formed with a high mechanical stability in its fresh state.

The presence of fewer than 10%, and preferably fewer than 5%, of particles having a size in the range 2 mm to 5 mm enables the composition to be worked relatively easily.

The concrete produced by the composition of the present invention can have a high strength, high fracture energy and a good durability, with little shrinkage and creep.

Preferably, the cementitious component comprises Portland cement, microsilica, pulverised fuel ash, a plasticiser and sand.

At least 90% by volume of the cementitious component may have a particle size below 2 mm, preferably below 1 mm.

The particle sizes of the cementitious component may be such that there is a continuous grading of particle sizes below 1 mm, preferably below 2 mm. Such a continuous grading of particle sizes allows the particles to form a dense mortar which can flow into the spaces between the coarse particles, resulting in a dense, strong concrete.

In a preferred embodiment, the ratio Vp:Vs is in the range 1:2.5 to 1:5, and is preferably in the range 1:2.6 to 1:4. Particles having a ratio Vp:V5 in these ranges tend to be of an angular and cubic shape.

Accordingly, they have a high degree of internal friction, which, together with the cohesive strength of the cementitious component produces a concrete with a high mechanical stability in its fresh state.

Preferably at least 90%, more preferably 95%, by volume of the coarse aggregate has a particle size greater than 5 mm.

In one embodiment, the coarse aggregate has a maximum particle size of 50 mm. In this embodiment, the particle sizes of the coarse aggregate may be such that there is a continuous grading of particle sizes in the range 5 mm to 50 mm. The coarse aggregate of this embodiment is preferably a blend of single-sized 40 mm, 20 mm, 14 mm and 10 mm coarse aggregate.

In another embodiment, the coarse aggregate has a maximum particle size of 20 mm. The particle sizes of the coarse aggregate may be such that there is a continuous grading of particle sizes in the range 5 mm to 20 mm. Preferably, the coarse aggregate is a blend of single-sized 20 mm, 14 mm and 10 mm coarse aggregate.

The coarse aggregate may be derived from one or more of granite, igneous rock, carboniferous limestone, crushed concrete and river gravel. Preferably, the particles of coarse aggregate have a rough surface so that, when mixed to form concrete, the mortar will adhere to the particles. Furthermore, it is preferred if the aggregate is crushed and has a low flakiness index.

The continuous grading of coarse aggregate above 5 mm ensures that the coarse aggregate particles can be packed as closely as possible. This means that the resultant concrete will be as dense as possible and will have a high mechanical stability in its fresh state.

In a preferred embodiment, the particle sizes of the sand are in the range 50 pm to 3 mm. The sand may be china clay waste sand and/or washed red sandstone.

The pulverised fuel ash may have particle sizes in the range 1 pm to 250 pm. Moreover, the cement may be a low alkali, high sulphate resistant Portland cement.

Preferably, the plasticiser is one or more of a naphthalene-based plasticiser and a lignosulphonatebased plasticiser. More preferably, the plasticiser is naphthalene sulphonate as this has been found to be most effective in dispersing the cement, microsilica and pulverised fuel ash throughout the concrete composition when it is mixed.

In one embodiment, the ratio of pulverised fuel ash to cement is in the range 1:2 to 2:1 and is preferably 1:1.

The microsilica may form 6% by weight of the combination of cement, microsilica, pulverised fuel ash and plasticiser. The plasticiser may form 1% by weight of the combination of cement, microsilica, pulverised fuel ash and plasticiser.

Figure 2 of the accompanying drawings is a graph illustrating the grain size distribution of two cement compositions ("FF" and "FA") in accordance with the present invention. The percentage of the composition by volume passing is plotted against the size of the sieve. It can be seen that the percentage passing increases approximately exponentially until a sieve size of 1 mm. Then between 1 and 5 mm, there is a "gap" in which little of the composition passes through. After 5 mm, the amount passing again increases exponentially.

The graph illustrates the continuous grading of particle sizes below 1 mm and above 5 mm with very little of the composition having a particle size in the range 1 to 5 mm.

According to a second aspect of the present invention, there is provided a method of forming concrete comprising mixing the composition according to the first aspect of the present invention with water.

The ratio between the volume of water and the volume of the combination of the cement, microsilica and pulverised fuel ash is preferably in the range 1:6 to 1:1. Such a ratio ensures that the concrete has suitable flow characteristics and has a high mechanical stability in its fresh state.

Preferably, the composition and water are mixed together for a time in the range of 1 to 15 minutes.

In one embodiment, the method comprises the steps of (a) mixing at least the coarse aggregate, sand and cement together to form a premix, and (b) mixing the remaining components of the composition and the water with the premix. The microsilica and plasticiser are preferably provided pre-dispersed in water and are mixed with the premix in step (b). Preferably, step (a) is carried out for a time in the range 0.5 minutes to 3 minutes, more preferably 1 minute, and step (b) is carried out for a time in the range 1 minute to 10 minutes, more preferably 5 minutes. It was found that using such a method, the time required to produce a well-mixed concrete could be reduced.

In another embodiment, the method comprises the steps of (a) mixing the cement, microsilica, pulverised fuel ash, plasticiser and a part of the coarse aggregate together to form a premix, (b) adding the water and mixing, and (c) adding the sand and the remainder of the coarse aggregate and mixing.

Preferably, one third of the coarse aggregate is added in step (a). In this method, step (a) may be carried out for a time in the range 0.25 minutes to 3 minutes, preferably 1 minute, step (b) is carried out for a time in the range 1 minute to 5 minutes preferably 3 minutes, and step (c) is carried out for a time in the range 1 minute to 5 minutes, preferably 3 minutes.

According to a third aspect of the present invention, there is provided concrete produced by the method of the second aspect of the present invention.

According to a fourth aspect of the present invention, there is provided a method of casting the concrete of the third aspect of the present invention, comprising the step of vibrating the concrete at a frequency higher than 100 Hz.

In order to achieve the relatively high compaction required to attain the "locked-solid" structure of coarse aggregate particles, it is necessary to vibrate the concrete at a relatively high frequency.

Preferably, this frequency is higher than 150 Hz. The vibration may be applied by the vibrators and/or screeds of an asphalt paving machine such as, for example, a Vogele 1700 or Super 2000 machine.

Preferably, the method further comprises the step of applying a force to the concrete after casting thereby to generate cracks therein. To facilitate this step, the concrete is preferably prepared at the time of its casting, e.g. by providing preformed lines or zones of weakness, or by incorporating means for generating high local internal pressure at predetermined regions of the concrete. The force is preferably applied a time after casting in the range 1 to 48 hours.

Shrinkage of concrete and temperature variations will create tension in a concrete slab that is greater than the ultimate tensile strength of the concrete.

Thus naturally-generated cracks can be expected to be formed in unreinforced concrete. When used in the construction of roads, the effect of heavy traffic exacerbates the problem, causing a deterioration in the road surface. As mentioned, concrete used in roads is conventionally provided with joints at regular intervals to relieve any stress in the concrete slabs.

The generation of cracks in the fresh concrete in accordance with the invention is intended to relieve any stress in the concrete slabs. The concrete can also be laid more quickly and efficiently as a continuous, joint-free slab, rather than as a series of slabs interrupted by joints. If the cracks are introduced relatively soon after casting, the crack line is expected to occur in the paste of the concrete.

Thus the crack will not be formed in the coarse aggregate particles. As a result, the coarse aggregate particles will interlock across the crack and will ensure that the load transfer across the crack is high and that relative vertical movement of the concrete on either side of the crack is minimised.

In one embodiment, the force is applied by detonating an explosive thread located underneath the concrete, which is preferably detonated a time after casting in the range 1 to 15 hours. The force of the explosion forces the concrete upwards with the result that a crack is formed. It has been found that an explosive thread having a charge of 10 g of TNT per metre produces favourable results.

Alternatively, the force is applied by means of an expanding member placed in a recess in the concrete.

In one embodiment, the expanding member expands chemically and, in another embodiment, the expanding member expands mechanically.

The force may applied by impacting a member placed in a recess formed in the concrete. The recess may be a groove.

According to a sixth aspect of the present invention, there is provided a road comprising a layer of concrete according to the third aspect of the present invention covered by one or more layers of asphalt.

According to a seventh aspect of the present invention, there is provided a road comprising a layer of concrete cast by the method of the fourth aspect of the present invention covered by one or more layers of asphalt.

The invention will now be described with reference to the following Examples.

ExamDle 1 A concrete roadbase was constructed according to the following method.

Firstly, granite was selected which consisted substantially of particles in the range 5 to 25 mm and approximately 80 % by weight of which had an Abrams number in the range 2 to 5.

Then, concrete according to the mix design set out in Table 1 was mixed in a 7 m3 Rex Chainbelt free-fall mixer.

A two-step mixing procedure was used in which water, superplasticiser, micro-silica and coarse aggregates were mixed for 150 seconds before the cement and sand were added and mixed for a further 300 seconds. The total cycle time during production including loading and discharging, was found to be minimum of eight minutes. Due to the special mix design which produces a very stiff and sticky concrete, the capacity of the mixer was reduced by approximately 50%.

For this reason a batch volume of 2.5 m3 was chosen, resulting in an overall production rate of 20 m3 per hour.

Materials kg/m3 Portland cement 220 Fly ash 110 Microsilica 20 Filler sand 215 Superplasticiser 5 Sand 640 Granite 1305 Water, including absorbed water 92 TOTAL 2607 Water/powder ratio 0.26 Table 1 During production, samples were also taken for analysis of water content and homogeneity. At the beginning of the production rather wide variations in water content were recorded. The situation was later improved by adjustments made at the mixing plant.

The wet paste/aggregate ratio was heavily influenced by the fluctuating water content, but the dry ratio was found to be reasonably close to the design mix. Sieving analyses on particles above 0.25 mm showed rather large deviations from the designed composition.

As expected, the strength was also found to be affected by the unstable water content. The compressive 28 day strength in cylinders taken at the construction site varied between 50 MPa and 82 MPa.

Construction of the roadbase took place over two days. On the first day, paving was carried out to the full thickness of the roadbase. On the second day, two layers of 100 mm were laid one upon the other.

On the first day of production and when paving the top layer on the second day, the paving operation was carried out by a Vögele asphalt paver fitted with a modified heavy duty screed. The paving of the bottom layer on the second day was done by a traditional asphalt paver. Concrete was loaded into the pavers as in conventional asphalt paving.

Two methods were used in the establishment of cracks. In the first method, rubber hoses were placed at intervals from 0.5 to 2 m on top of the subbase layer before construction of the roadbase. Shortly after the paving process, the hoses were put under hydraulic pressure until a visible crack appeared on the surface of the pavement.

In the second method, the hoses were replaced by detonating threads with a TNT charge rate of three grams/m, and detonated approximately four hours after the paving process. The explosion resulted in fine visible cracks in more than 95% of the predetermined crack lines.

Evaluation of the completed roadbase was based on a number of cores drilled in places where: - cracks were identified on the surface - good compaction had been obtained - an open surface structure indicated insufficient compaction.

The analyses carried out on the cores included macro analysis and fluorescent analysis of cut sections as well as a visible description of induced cracks.

The analyses of cut sections from cores, drilled in concrete paved during the first day of production, indicated a well-compacted concrete. The designed slab thickness was also accomplished. Cores drilled from the second day of construction, however, showed that a somewhat lower compaction had been achieved in the base layer.

Inspection of the cores drilled over visible crack lines in the surface revealed that the cracks had only penetrated a few centimetres into the slab and therefore cannot be expected to act as crack-inducers.

During the paving process it had not been possible to produce a surface with an evenness sufficient for an asphaltic wearing course layer to be placed directly on top.

The problems with the formation of cracks were overcome by mechanical distress of the hardened slab.

The distress was accomplished by dropping a heavy steel plate on its edge over the crack lines. This operation also resulted in the development of secondary cracks.

The complete crack pattern was inspected and marked with paint before a photographic registration.

Two Falling Weight Deflectometer (FWD) surveys were carried out in order to asses the bearing capacity of the roadbase. One survey was made approximately two months after construction and the second was made approximately four months later after the mechanical distress of the concrete slab.

The deflections measured in the second FWD survey were generally found to be larger then in the first FWD-survey, indicating that the creation of the crack pattern had resulted in a lower bearing capacity.

Seasonal factors may also have been responsible for these differences. In both cases, however, the FWD surveys indicated good bearing capacities.

After some levelling operations to the concrete surface, the section was paved in seven months after construction with a traditional 40 mm (80 kg/m2) asphaltic wearing course (a stone mastic asphalt with polymer-modified bitumen).

Example 2 Over 75% of particles in a microgranite were found to have an Abrams number in the range 2 to 4.5 and therefore this granite was used in the four different mix designs shown in Table 2.

The concrete was prepared by mixing the coarse aggregate, sand and cement together for 1 minute, and then mixing the microsilica and plasticiser (which were provided pre-dispersed in water as a slurry) and the water with the premix for 5 minutes.

The test pavement was divided into five sub-sections, each 200 m long, where the four different concrete mixes were used, three with maximum aggregate size of 20 mm and one with 40 mm.

The concrete roadbase was paved over 9 working days. Concrete was batched to 3 m3 in a split-drum mixing plant of 4 m3 capacity in a two-minute mixing cycle, giving a maximum output of 90 m3/hour.

Table 2

MATERIALS Proportions for concrete mix No.

1 3 5 6 Dry weights (kg/m3) Aggregate (single size microgranite): 40 mm - - - 784 20 mm 418 420 410 461 14 mm 467 469 456 10 mm 418 420 410 Sand:: China clay waste (I) 551 - - - China clay waste (11) - - 598 553 Washed red sandstone - 626 - - China clay waste (III) - - - 184 Portland cement 186 180 197 166 Pulverised fuel ash 223 220 223 203 microsilica (stabilised 27 26 28 25 suspension in water) (litres/m3) Superplasticiser 10.3 10.5 10.5 9.5 Water 124 94 112 87 Concrete was laid by the same Vögele 1700 modified asphalt paver used for construction of the test pavement of Example 1. The roadbase was paved in widths of four and a half and five metres, in a sequence that ensured that the longitudinal joint was a construction joint formed against hardened materials.

Apart from a small part of one of the sub-sections, the concrete was laid and compacted in one layer.

Where mechanical treatment was used to induce cracks at predetermined positions, the intended line of the crack was conditioned either by drilling holes or by sawing a slot in the green concrete.

In all cases, the crack was generated by means of a Gomaco 'Whip-Hammer' fitted with a cylindrical impact head, applying 10 blows to each paved strip fully to ensure creation of the crack. The best results were obtained if the impact was applied 36 hours after casting. Figure 3 shows a schematic layout of the mechanically induced cracks.

Except for one section, the concrete contained minerals which it was hoped would encourage 'intrinsic' microcracking in absence of macro-cracks formed by mechanical treatment. If any microcracking was produced, it was insufficient to relieve the strain imposed on the slab by the first cycle of thermal contraction. This was confirmed by subsequent petrographic analysis.

Consequently, discrete transverse cracks developed naturally at wide intervals within 24 hours of construction. Figure 4 shows a schematic layout of the natural cracks recorded prior to paving of the surface layers.

The quality of the batched concrete was assessed by testing cube specimens and beams. In addition, cores in the hardened roadbase were cut from apparently good and bad areas.

With few exceptions, the relationship between compressive and flexural strength was found to be similar to that for conventional concrete.

Significantly, cube specimens from all the mixes achieved mean compressive strengths greater than 30 MPa after three days, with 28-day strength of between 62 and 87 MPa and 91-day strength between 77 and 98 MPa.

A satisfactorily normal gain of strength with age was thus exhibited.

Measurements of the saturated densities of roadbase cores showed that the majority were compacted to at least 93% of the average density of beams and cube specimens (or the theoretical maximum dry density). Density profiles down the cores were also determined by nuclear scanning which showed that, in general, the lowest third of the roadbase was noticeably less well compacted than the top part. This observation was confirmed by examination of the concrete microtexture by fluorescence petrography, where the upper zones of the cores exhibited better compaction, but poorer paste quality than the lower zones (i.e. uneven water distribution).

The concrete roadbase was surfaced with bituminous regulating and wearing courses approximately one month after construction.

Example 3 One objective of this Example was to demonstrate the possibility of producing a low-cost pavement material with sufficient strength and durability properties.

A test pavement 400 m long with a width of eight metres was constructed as follows.

Two mix designs introducing cheaper constituents were proposed for the test pavement section. The only difference between the two mixes was the type of coarse aggregate. Natural gravel pit aggregates of which over 75% of particles have an Abrams number in the range 2 to 6 and recycled cement of which over 70% of particles have an Abrams number in the range 2 to 6 were selected.

In the first 200 m of the test pavement, natural gravel pit aggregates were included in the mix, whereas recycled concrete was to be used in the remaining 200 m section. The mix designs are shown in Table 3 below:

MIX TYPE Gravel pit Recycled aggregate concrete (kg/m3) (kg/m3) Portland cement 275 275 Microsilica 20 20 Plasticiser 4 4 All in gravel pit 1170 1170 aggregate 0/35 mm All in gravel pit 970 aggregate 6/24 mm Recycled concrete 6/21 - 880 mm Water 104 104 TOTAL 2543 2453 Water/powder ratio 0.35 0.35 Table 3 The relatively long mixing time required in Example 1 was found to comprise a cost premium on the overall construction cost. For this reason, a mixing time of 120 seconds for the newly-developed concrete was decided.

Production of the concrete took place at the same production plant as used in Example 1.

During production, samples were taken for assessment of strength and strength development. In the subsequent laboratory testing, the reduced mixing time was found to have affected the compactability of the concrete, which in turn resulted in compression strength somewhat lower than expected. In the design, a compressive 28 day maturity cylinder strength of 60 MPa at 100% compaction was aimed at.

Table 4 shows the daily production, average degree of compaction in relation to theoretical maximum dry density, and the compressive strength of cylinders obtained at 28 days maturity. All cylinders showed signs of poor compaction (honeycombs). The estimated cube strength is given in parentheses.

Production Day Compaction of Compressive cylinders MPa strength MPa 1st 96% 42.2 (49) 2nd 95% 41.8 (48) 3rd 96% 39.3 (45) Table 4: Compaction and compressive 28 day strength of cylinders (dia. 150 mm, height 300 mm) In the previous testing of the mix designs in the laboratory, an almost linear relation between strength and compaction had been found. Using this relation the 'potential' strength of the material can be estimated to be between 49 MPa and 56 MPa. It seems as though roughly 10* of the estimated strength has been lost due to the reduced mixing time. This coincides, unfortunately, with a loss of compactability.

The paving was carried out in two lanes of 4.40 m and to the full slab thickness by the same modified Vögele 1700 asphalt paver as used in Examples 1 and 2.

The second lane was paved while the concrete of the first lane was still workable. Only the areas adjacent to the longitudinal joint was compacted with a vibratory plate after installation of tie bars.

Transverse cracks were created by the explosive power of detonating threads, placed on top of the subbase prior to paving of the concrete slab. The charge rate was in all cases 10 g TNT per linear metre.

This charge rate had previously been found not to cause damage to the pavement.

Each of the two sections with different concrete composition were subdivided into approximately 70 metre long stretches where cracks were formed at intervals of 1, 2 and 3 metres.

The detonating threads were detonated between six and ten hours after paving. A survey showed that cracks were created in approximately two thirds of the predetermined locations. Most of the unidentified cracks were found in the sections with one metre intervals.

Figure 5 shows a schematic layout of the identified cracks and the subdivision of the test pavement into sections with different concrete compositions and intended crack intervals.

Cores taken after hardening of the concrete verified the laboratory results where poor compaction and low strength properties had been established.

The compressive strength of cut 99 mm diameter/100 mm high samples cut from the top and bottom of cores, were determined in order to give a general picture of the slab as well as any differences with depth.

Average results of the tests are given in Table 5 below. The estimated cube strength is given in parentheses.

Average compaction of the 95% top upper part of the layer Average compaction of the 90% bottom part of the layer Average compressive strength 31 MPa of the top of the cores (29) Average compressive strength 24 MPa of the bottom of the cores (22)

Table 5 The evenness of the paved concrete slab was determined by both 'Viagraph' and 'Bumpmeter' surveys in the outer wheel path of both lanes.

The test pavement was paved with a hot rolled Stone Mastic Asphalt two months after construction.

The material properties with regards to grading of the aggregates, bitumen content and reflection fulfil the normal requirements for this type of wearing course. Voids in cores, however, are 25% above the specified maximum.

Example 4 Table 6 shows the mix design of a concrete composition in accordance with the present invention, together with the particle sizes of the components of the composition.

Table 6

Compo@ant Volume Weight % passing (D.S. sizes) (m@) (kg/m@) 37.5 2@ 20 14 10 5 2.36 1.18 600 300 150 75 3@ mm m Cement 0.0689 215 100 100 100 100 100 100 100 100 100 100 98 54 10 Flyach 0.0895 188 100 100 100 100 100 100 100 100 100 100 99 96 40 Microsilica 0.0128 26.9 100 100 100 100 100 100 100 100 100 100 100 100 99 Plasticiser 0.0000 4.9 100 100 100 100 100 100 100 100 100 100 100 100 100 Water 0.0938 94 100 100 100 100 100 100 100 100 100 100 100 100 100 Sand 0.2389 627 100 100 100 100 100 100 99 95 80 46 12 6 0 20 m@ aggregate 0.1585 421 100 100 93 16 1 0 14 mm aggregate 0.1770 470 100 100 100 93 25 2 0 10 mm aggregate 0.1585 421 100 100 100 100 94 1 0 TOTAL 0.9979 Size 100.0 100.0 98.8 84.3 67.8 47.4 46.6 45.6 41.8 33.1 24.3 18.7 9.0 distr.

sy weight Theoratical 2469 Dy volume 100.0 100.0 98.9 85.4 70.0 51.0 50.3 49.3 45.7 37.4 29.2 24.5 14.9 density

Claims

CLAIMS:

1. A concrete composition for use in the construction of load-bearing surfaces such as roads, runways and the like, which comprises a cementitious component of relatively fine particles; and a coarse aggregate, characterised in that the coarse aggregate consists of particles at least 70% by weight of which are shaped such that the ratio (Vp:Vs), between the volume (Vp) of a given particle and the volume (Vs) of the notional sphere of minimum volume capable of containing that particle wholly within it, is in the range 1:2 to 1:6, and in that the composition has fewer than 10% by volume of particles having a size in the range 2 mm to 5 mm as determined by retention on British Standard sieves.

2. A composition as claimed in claim 1, wherein said the cementitious component comprises Portland cement, microsilica, pulverised fuel ash, a plasticiser and sand.

3. A composition as claimed in claim 1, having fewer than 5% by volume of particles having a size in the range 2 mm to 5 mm.

4. A composition as claimed in claim 1, 2 or 3, wherein at least 90% by volume of said cementitious component has a particle size below 2 mm.

5. A composition as claimed in claim 4, wherein at least 90% by volume of said cementitious component has a particle size below 1 mm.

6. A composition as claimed in any preceding claim, wherein the particle sizes of said cementitious component are such that there is a continuous grading of particle sizes below 1 mm.

7. A composition as claimed claim 6, wherein the particle sizes of said cementitious component are such that there is a continuous grading of particle sizes below 2 mm.

8. A composition as claimed in any preceding claim, wherein the ratio (Vp:V5) is in the range 1:2.5 to 1:5.

9. A composition as claimed in claim 8, wherein the ratio (Vp:V5) is in the range 1:2.6 to 1:4.

10. A composition as claimed in any preceding claim, wherein at least 90% by volume of said coarse aggregate has a particle size greater than 5 mm.

11. A composition as claimed in claim 10, wherein at least 95% by volume of said coarse aggregate has a particle size greater than 5 mm.

12. A composition as claimed in any preceding claim, wherein said coarse aggregate has a maximum particle size of 50 mm.

13. A composition as claimed in claim 12, wherein the particle sizes of said coarse aggregate are such that there is a continuous grading of particle sizes in the range 5 mm to 50 mm.

14. A composition as claimed in claim 13, wherein said coarse aggregate is a blend of single-sized 40 mm, 20 mm, 14 mm and 10 mm coarse aggregate.

15. A composition as claimed in claim 12, wherein said coarse aggregate has a maximum particle size of 20 mm.

16. A composition as claimed in claim 15, wherein the particle sizes of said coarse aggregate are such that there is a continuous grading of particle sizes in the range 5 mm to 20 mm.

17. A composition as claimed in claim 16, wherein said coarse aggregate is a blend of single-sized 20 mm, 14 mm and 10 mm coarse aggregate.

18. A composition as claimed in any preceding claim, wherein said coarse aggregate is derived from one or more of granite, igneous rock, carboniferous limestone, crushed concrete and river gravel.

19. A composition as claimed in any one of claims 2 to 18, wherein the particle sizes of said sand are in the range 50 urn to 3 mm.

20. A composition as claimed in any one of claims 2 to 19, wherein said sand is china clay waste sand and/or washed red sandstone.

21. A composition as claimed in any one of claims 2 to 20, wherein said pulverised fuel ash has particle sizes in the range 1 urn to 250 Bum.

22. A composition as claimed in any one of claims 2 to 21, wherein said cement is a low alkali, high sulphate resistant Portland cement.

23. A composition as claimed in any one of claims 2 to 22, wherein said plasticiser is one or more of a naphthalene-based plasticiser and a lignosulphonatebased plasticiser.

24. A composition as claimed in claim 23, wherein said plasticiser is naphthalene sulphonate.

25. A composition as claimed in any one of claims 2 to 24, wherein the ratio of pulverised fuel ash to cement is in the range 1:2 to 2:1.

26. A composition as claimed in claim 25, wherein the ratio of pulverised fuel ash to cement is 1:1.

27. A composition as claimed in any one of claims 2 to 26, wherein said microsilica forms 6% by weight of the combination of said cement, pulverised fuel ash, microsilica and plasticiser.

28. A composition as claimed in any one of claims 2 to 27, wherein plasticiser forms 1% by weight of the combination of said cement, pulverised fuel ash, microsilica and plasticiser.

29. A method of forming concrete comprising mixing the composition of any preceding claim with water.

30. A method as claimed in claim 29, wherein the ratio between the volume of water and the volume of the cementitious component is in the range 1:6 to 1:1.

31. A method as claimed in claim 29 or claim 30, wherein said composition and said water are mixed together for a time in the range of 1 to 15 minutes.

32. A method as claimed in any one of claims 29 to 31, comprising the steps of (a) mixing at least said coarse aggregate, sand and cement together to form a premix, and (b) mixing the remaining components of said composition and said water with said premix.

33. A method as claimed in claim 32, wherein said microsilica and plasticiser are provided predispersed in water and are mixed with said premix in step (b).

34. A method as claimed in claim 32 or 33, wherein step (a) is carried out for a time in the range 0.5 minutes to 3 minutes and step (b) is carried out for a time in the range 1 minute to 10 minutes.

35. A method as claimed in claim 34, wherein step (a) is carried out for 1 minute and step (b) is carried out for 5 minutes.

36. A method as claimed in claim 29, 30 or 31, comprising the steps of (a) mixing said cement, microsilica, pulverised fuel ash, plasticiser and a part of said coarse aggregate together to form a premix, (b) adding said water and mixing, and (c) adding said sand and the remainder of said coarse aggregate and mixing.

37. A method as claimed in claim 38, wherein one third of said coarse aggregate is added in step (a).

38. A method as claimed in claim 36 or claim 37, wherein step (a) is carried out for a time in the range 0.25 minutes to 3 minutes, step (b) is carried out for a time in the range 1 minute to 5 minutes, and step (c) is carried out for a time in the range 1 minute to 5 minutes.

39. A method as claimed in claim 38, wherein step (a) is carried out for 1 minute, step (b) is carried out for 3 minutes, and step (c) is carried out for 3 minutes.

40. Concrete produced by the method of any one of claims 29 to 39.

41. A method of casting the concrete as claimed in claim 40, comprising the step of vibrating said concrete at a frequency higher than 100 Hz.

42. A method as claimed in claim 41, comprising the step of vibrating said concrete at a frequency of 150 Hz.

43. A method as claimed in claim 41 or 42, further comprising the step of applying a force to said concrete after casting thereby to generate cracks therein.

44. A method as claimed in claim 43, wherein said force is applied at a time in the range 1 to 48 hours after casting.

45. A method as claimed in claim 43 or claim 44, wherein said force is applied by detonating an explosive thread located underneath said concrete.

46. A method as claimed in claim 45, wherein said detonating thread is detonated from 1 to 15 hours after casting.

47. A method as claimed in claim 43 or claim 44, wherein said force is applied by means of an expanding member placed in a recess in said concrete.

48. A method as claimed in claim 47, wherein said expanding member expands chemically.

49. A method as claimed in claim 47, wherein said expanding member expands mechanically.

50. A method as claimed in claim 43 or claim 44, wherein said force is applied by impacting a member placed in a recess formed in the concrete.

51. A road comprising a layer of concrete cast by the method of any one of claims 38 to 49 optionally covered by one or more layers of asphalt.

51. A method as claimed in claim any one of claims 47 to 50, wherein said recess is a groove.

52. A road comprising a layer of concrete as claimed in claim 40 covered by one or more layers of asphalt.

53. A road comprising a layer of concrete cast by the method of any one of claims 41 to 51 covered by one or more layers of asphalt.

Amendments to the claims have been filed as follows CLAIMS:

1. A concrete composition for use in the construction of load-bearing surfaces such as roads, runways and the like, which comprises a mortar component of relatively fine particles; and a coarse aggregate, characterised in that the coarse aggregate consists of particles at least 70% by weight of which are shaped such that the ratio (Vp:Vs), between the volume (Vp) of a given particle and the volume (Vs) of the notional sphere of minimum volume capable of containing that particle wholly within it, is in the range 1:2 to 1:6, and in that the composition has fewer than 10% by volume of particles having a size in the range 2 mm to 5 mm as determined by retention on British Standard sieves.

2. A composition as claimed in claim 1, wherein said the mortar component comprises Portland cement, microsilica, pulverised fuel ash, a plasticiser and sand.

3. A composition as claimed in claim 1 or claim 2, having fewer than 5% by volume of particles having a size in the range 2 mm to 5 mm.

4. A composition as claimed in claim 1, 2 or 3, wherein at least 90% by volume of said mortar component has a particle size below 2 mm.

5. A composition as claimed in claim 4, wherein at least 90% by volume of said mortar component has a particle size below 1 mm.

6. A composition as claimed in any preceding claim, wherein the particle sizes of said mortar component are such that there is a continuous grading of particle sizes below 1 mm.

7. A composition as claimed in claim 6, wherein the particle sizes of said mortar component are such that there is a continuous grading of particle sizes below 2 mm.

8. A composition as claimed in any preceding claim, wherein the ratio (Vp:Vs) is in the range 1:2.5 to 1:5.

9. A composition as claimed in claim 8, wherein the ratio (Vp:Vs) is in the range 1:2.6 to 1:4.

19. A composition as claimed in any one of claims 2 to 18, wherein the particle sizes of said sand are in the range 50 Sm to 3 mm.

21. A composition as claimed in any one of claims 2 to 20, wherein said pulverised fuel ash has particle sizes in the range 1 jim to 250 jim.

23. A composition as claimed in any one of claims 2 to 22, wherein said plasticiser is one or more of a super-plasticiser and a lignosulphonate-based plasticiser.

24. A composition as claimed in claim 23, wherein said plasticiser is a naphthalene sulphonate type plasticiser.

28. A composition as claimed in any one of claims 2 to 27, wherein the plasticiser forms 1% by weight of the combination of said cement, pulverised fuel ash, microsilica and plasticiser.

29. A concrete composition substantially as hereinbefore described in Example 1, 2, 3 or 4.

30. A method of forming concrete comprising mixing the composition of any preceding claim with water.

31. A method as claimed in claim 30, wherein the ratio between the volume of water and the combined weight of the mortar component is in the range 1:6 to 1:1.

32. A method as claimed in claim 30 or claim 31, wherein said composition and said water are mixed together for a time in the range of 1 to 15 minutes.

33. A method as claimed in claim 30, 31 or 32, comprising the steps of (a) mixing said cement, microsilica, pulverised fuel ash, plasticiser and a part of said coarse aggregate together to form a premix, (b) adding said water and mixing, and (c) adding said sand and the remainder of said coarse aggregate and mixing.

34. A method as claimed in claim 33, wherein one third of said coarse aggregate is added in step (a).

35. A method as claimed in claim 33 or claim 34, wherein step (a) is carried out for a time in the range 0.25 minutes to 3 minutes, step (b) is carried out for a time in the range 1 minute to 5 minutes, and step (c) is carried out for a time in the range 1 minute to 5 minutes.

36. A method as claimed in claim 35, wherein step (a) is carried out for 1 minute, step (b) is carried out for 3 minutes, and step (c) is carried out for 3 minutes.

37. Concrete produced by the method of any one of claims 30 to 36.

38. A method of casting the concrete as claimed in claim 37, comprising the step of vibrating said concrete at a frequency higher than 100 Hz.

39. A method as claimed in claim 38, comprising the step of vibrating said concrete at a frequency of 150 Hz.

40. A method as claimed in claim 38 or 39, further comprising the step of applying a force to said concrete after casting thereby to generate cracks therein.

41. A method as claimed in claim 40, wherein said cracks are formed at intervals of 1, 2 or 3 metres.

42. A method as claimed in claim 40 or claim 41, wherein said force is applied at a time in the range 1 to 48 hours after casting.

43. A method as claimed in claim 40, 41 or 42, wherein said force is applied by detonating an explosive thread located underneath said concrete.

44. A method as claimed in claim 43, wherein said explosive thread is detonated from 1 to 15 hours after casting.

45. A method as claimed in claim 40, 41 or 42, wherein said force is applied by means of an expanding member placed in a recess in said concrete.

46. A method as claimed in claim 45, wherein said expanding member expands chemically.

47. A method as claimed in claim 45, wherein said expanding member expands mechanically.

48. A method as claimed in claim 40, 41 or 42, wherein said force is applied by impacting a member placed in a recess formed in the concrete.

49. A method as claimed in claim any one of claims 45 to 48, wherein said recess is a groove.

50. A road comprising a layer of concrete as claimed in claim 37 optionally covered by one or more layers of asphalt.