GB2495591A - Stone Paving construction having a hydraulically bound mixture in the base layer - Google Patents

Abstract

The construction 10 comprises a stone upper layer 12, a stiff bound lower layer 14 and a bonding layer 16 between the upper layer and lower layer. The stiff bound lower layer 14 comprises a compacted hydraulic bound mixture of aggregate or soil bound using a low energy hydraulic binder. The stone upper layer comprises plurality of stone elements 20 arranged with joints 26 therebetween and a joint filling material arranged in the joints which acts compositely with the stone elements to form a continuous surface layer. The low energy hydraulic binder may comprise an alkali-activated pozzolan, for example a blend of pulverised coal fly ash and Portland cement. The pulverised coal fly ash may be high alumina siliceous coal fly ash. The joint filling material may be mortar and the bonding layer may comprise a fine concrete. A method of constructing a stone paving is also claimed.

Description

NATURAL STONE PAVING CONSTRUCTION

This invention relates to a natural stone paving construction for use in the construction of paved areas for general urban vehicle loading. This invention also relates to a hydraulic bound mixture for use in natural stone paving construction.

Natural stone can be a durable building material. The compression strength of natural stone can be high and the material is commonly used in structures such as bridges and buildings. Natural stone elements have been used in roadway and footway construction for many centuries. Typically the stability of a roadway having stone elements forming the surface relied on the stone elements having sufficient size and mass. Stone surface elements were laid on a foundation which comprised the natural ground or a granular layer which enhanced the bearing capacity of the natural ground. Stone surface elements were bedded onto a sand or fine quarried aggregate layer to achieve an even surface profile. Sand or fine quarried aggregate filled the joint space between elements to lock the elements together and ensure stability of the stone surface elements under the vertical and horizontal loading generated by the movement of vehicles.

On level ground large stone elements in the form of slabs provide an even surface for the movement of vehicles and people. However, slabs are prone to fracture under heavy vehicle loading unless they are large, and large elements are difficult to place and remove when accessing services beneath roadways and footways. The use of smaller elements such as setts became more common and practical for urban roadway construction, and in Europe small cubes were used to form footways.

Construction using such small stone elements is sustainable) since the elements can be lifted and relayed when bedded and jointed using sand or fine quarried aggregate.

Roadways having stone surfaces bedded on sand or fine quarried aggregate, having joints filled with sand or fine quarried aggregate, and supported on a quarried aggregate and natural soil foundation form a flexible construction. Flexible construction is used here to mean that the surface and foundation of the construction can displace under vehicle loading with little loss of strength of the construction. Although the construction is sustainable in that repair and maintenance is achieved by removing and replacing the surface elements, such construction is not always practical in the urban environment of the 21st century.

Vehicle loading in modern urban environments is frequently from the movement of buses and delivery vehicles; the movements are often canalised as a consequence of urban design.

Rigid stone surface construction has developed into a technology for heavy vehicle loading situations. A rigid stone surface construction requires the joints to transfer shear and form a continuous surface layer capable of resisting both punching and flexing. Punching shear failure occurs when the joints around the perimeter ofa wheel contact area fail through relative vertical movement of adjacent elements; rotational shear failure occurs when the joints fail in the overlap area between elements in adjacent rows as the surface layer flexes. The behaviour applies to setts and slabs; with setts rotational shear is sustained by an overlap area that is short and deep, whilst with slabs the overlap area is shallow and long.

Typically present design of flexible stone surface and foundation construction is based on a stiff unbound aggregate foundation supporting a surface formed by elements locked together by joint friction. Such a construction is sustainable but it has limited vehicle loading capacity, particularly in conditions of canalised vehicle movement.

Typically present design of rigid stone surface construction is based on traditional road construction with a foundation, a structural layer sustaining vehicle loading called the base layer, and a surface layer to create an even profile and friction for vehicle control. Present design codes require the surface layer to be bonded to the base layer.

Present design guidance takes no account of composite performance.

JP 07113204 discloses a paving stone construction in which a second conventional ready mixed concrete layer is placed on a conventional concrete base layer. A layer of cement containing both Portland cement and blast furnace slag is placed on the second ready mixed concrete layer and paving stones are placed thereon. Such a construction suffers from cracking under heavy traffic.

It is an object of the present invention to provide a natural stone pavement construction which takes account of composite performance and which overcomes one or more of the aforementioned problems.

According to the invention the direct support layer beneath a rigid stone surface must be stiff to minimise the rotational shear stresses generated within the joints of the surface from flexing under vehicle loading. The direct support layer and the foundation must also be resistant to deformation. The surface elements may be bedded into the direct support layer or laying course to achieve an even surface profile. The laying course is typically a fine concrete with a high elastic modulus.

The upper foundation layer, beneath the laying course, must be sti if with a relatively high elastic modulus to provide adequate support to the surface and protect the natural soil foundation from stress. Bonding the direct support layer to the upper foundation layer and the upper stone elements creates a composite pavement construction.

According to a first aspect of the present invention there is provided a stone paving construction comprising a stone upper layer, a stiff bound lower layer and a bonding layer between the upper layer and lower layer, wherein the stiff hound lower layer comprises a compacted hydraulic hound mixture, the hydraulic bound mixture comprising an aggregate or soil bound using a low energy hydraulic binder, wherein the stone upper ayer comprises a plurality of stone elements arranged with joints therebetween and a joint filling materia' arranged in the joints which acts compositely with the stone &ements to form a continuous surface layer, and wherein the continuous surface layer acts compositely with the stiff bound lower layer and the bonding hyer to form a composite structural construction.

The low energy hydraulic binder may comprise an alkali-activated pozzolan.

The ow energy hydraulic binder may comprise a blend of pulverised coal fly ash and Portland cement. The pulverised coal fly ash may be high alumina siliceous coal fly ash.

The low energy hydraulic binder may comprise between 60% and 70% by weight high alumina siliceous puverised coal fly ash. The low energy hydraulic binder may comprise between 30% and 40% by weight Portland cement.

The high alumina siliceous coal fly ash may comprise between 35% and 60% by weight alumina.

The Portland cement may have a strength grade of 42.5. Grade 42.5 ordinary Portland cement develops a minimum compression strength of 42.5 MPa after twenty eight days.

The aggregate in the hydraulic bound mixture may be a well graded 0/10 crushed rock aggregate The hydraulic hound mixture maybe a T2 mixture as defined by BS EN 14227-3, Fly Ash Bound Mixtures. The hydraulic bound mixture maybe a T2 mixture as defined by BS EN 14227-5, Hydraulic Road Binder Bound Mixtures.

The hydraulic bound mixture may have a one year strength measured as a direct tensile strength of between 0.6SMPa and 0.8MPa, in accordance with EN 13286-40, or as an indirect tensile strength of between 0.8MPa and 1.OMPa, in accordance with EN 13286-42. The hydraulic bound mixture may have an elastic modulus at one year of between 25GPa and 35GPa.

The stone upper layer may comprise a plurality of natural stone elements laid side by side. The term "side by side" includes arrangements in which stone elements are laid in a grid pattern, in rows in which the elements of one row lap with the elements of an adjacent row, or in any other arrangement in which the lateral faces of adjacent stone elements are adjacent to each other or separated by a gap. The natural stone elements maybe slabs or setts.

The natural stone elements may be laid with a gap between adjacent elements, the gap being filled with a joint filling material. The joint filling material maybe mortar.

The mortar maybe a fine concrete. The mortar may comprise a mixture of sand, cement and water. The ratio of water to cement by weight is preferably greater than 0.28. The mortar may be manually packed into the gap, typically with a water to cement ratio of between 0.28 and 0.50. The mortar maybe placed into the gap by applying to one or both adjacent edges of adjacent stone elements and then butting the adjacent edges towards each other, typically with a water to cement ratio of between 0.50 and 1.00. The mortar maybe applied to the gap as a grout or slurry, for example by squeegeeing, typically with a water to cement ratio of 1.00 or more.

The natural stone elements may be between 40 and 250mm thick.

The natural stone elements may have a length of between 70 and 2500 mm. The natural stone elements may have a width of between 70 and 1500 mm. The natural stone elements may be cuboid and have a length substantially equal to the width, typically between 70 and 150 mm. The natural stone elements may be setts and have a length between 150 and 300mm and a width between 70 and 150 mm. The natural stone elements may be slabs and have a length between 300 and 2500 mm and a width between 200 and 1500 mm.

The bonding layer may comprise a fine concrete with an elastic modulus greater than 25GPa at 28 days. The fine concrete may comprise a mixture of aggregate, cement and water. The aggregate may have a maximum aggregate size of 6 mm.

The bonding layer may typically have a compressive strength of between 3OMPa and SOMPa.

The bonding layer may have a thickness of between 10 and 45 mm.

The stone paving construction may comprise a soil foundation beneath the stiff bound lower layer. The soil foundation may have an enhanced bearing capacity, for example by modification using lime, or by reinforcing with hydraulic binder. The lime modifies the chemistry and then the physical nature of the soil when this is formed by clay; the hydraulic binder bonds particles together and is the only option with a granular soil.

According to a second aspect of the present invention there is provided a hydraulic bound mixture for use in stone paving construction, the hydraulic bound mixture comprising an aggregate or soil bound using a low energy hydraulic binder.

The low energy hydraulic binder may comprise an alkali-activated pozzolan.

The low energy hydraulic binder may comprise a blend of pulverised coal fly ash and Portland cement The pulverised coal fly ash may be high alumina siliceous coal fly ash.

The low energy hydraulic binder may comprise between 60% and 70% by weight high alumina siliceous pulverised coal fly ash. The low energy hydraulic binder may comprise between 30% and 40% by weight Portland cement.

The high alumina siliceous coal pulverised fly ash may comprise between 35% and 60% by weighi alumina.

The Portland cement may have a strength grade of 42.5.

The aggregate in the hydraulic bound mixture may be a well graded 0/10 crushed rock aggregate The hydraulic bound mixture may be a T2 mixture as defined by BS EN 14227-3, Fly Ash Bound Mixtures. The hydraulic bound mixture maybe a T2 mixture as defined by BS EN 14227-5, Hydraulic Road Binder Bound Mixtures.

The hydraulic bound mixture may have a one year strength measured as a direct tensile strength of between 0.6SMPa and 0.8MPa, in accordance with EN 13286-40, or as an indirect tensile strength of between 0.BMPa and 1.OMPa, in accordance with EN 13286-42. The hydraulic bound mixture may have an elastic modulus at one year of between 2SGPa and 35GPa, measured in accordance with EN 13286-50, EN 13286-51, EN 13286-52 orEN 13286-53.

According to a third aspect of the present invention there is provided a method of constructing stone paving comprising the steps of: preparing a formation;

B

laying and compacting a hydraulic hound mixture comprising an aggregate or soil bound using a low energy hydraulic binder to form a stiff bound lower layer on the formation; laying a bonding layer on the lower layer; and forming a stone upper layer on the bonding layer by arranging a plurality of stone elements with joints therebetween on the bonding layer and arranging a joint filling material in the joints to act compositely with the stone elements to form a continuous surface layer; and curing the hydraulic bound mixture, the bonding layer and the joint filling material so that the continuous surface layer acts compositely with the stiff hound lower layer and the bonding layer to form a composite structural construction.

The low energy hydraulic binder may comprise an alkali-activated pozzolan.

The low energy hydraulic binder may comprise a blend of pulverised coal fly ash and Portland cement The pu]verised coal fly ash maybe high alumina siliceous pulverised coal fly ash.

The low energy hydraulic binder may comprise between 60% and 70% by weight high alumina siliceous pulverised coal fly ash. The low energy hydraulic binder may comprise between 30% and 40% by weight Portland cement The high alumina siliceous pulverised coal fly ash may comprise between 35% and 60% by weight alumina.

The Portland cement may have a strength grade of 42.5.

The aggregate in the hydraulic bound mixture may be a well graded 0/10 crushed rock aggregate The hydraulic hound mixture maybe a T2 mixture as defined by BS EN 14227-3, Fly Ash Bound Mixtures. The hydraulic bound mixture maybe a T2 mixture as defined by BS EN 14227-5, Hydraulic Road Binder Bound Mixtures.

The hydraulic bound mixture may have a one year strength measured as a direct tensile strength of between 0.6SMPa and 0.8MPa, in accordance with EN 13286-40, or as an indirect tensile strength of between 0.8MPa and 1.OMPa, in accordance with EN 13286-42. The hydraulic bound mixture may have an elastic modulus at one year of between 25GPa and 35GPa.

The step of laying a stone upper layer on the bonding layer may comprise laying a plurality of natural stone &ements side by side. The natural stone elements may be slabs or setts.

The natural stone elements may be laid with a gap between adjacent elements. The gaps may be filled with a joint filling material. The joint filling material in the gap may be mortar. The mortar may be a fine concrete. The mortar may comprise a mixture of sand, cement and water. The ratio of water to cement by weight is preferably greater than 0.28. The mortar maybe manuafly packed into the gap, typically with a water to cement ratio of between 0.28 and 0.50. The mortar may be placed into he gap by applying to one or both adjacent edges of adjacent stone elements and then butting the adjacent edges to wards each other, typically with a water to cement ratio of between 0.50 and 1.00. The mortar may be applied to the gap as a grout or slurry, for example by squeegeeing, typically with a water to cement ratio of 1.00 or more.

The natural stone elements may be between 40 and 250 mm thick The natural stone elements may have a length of between 70 and 2500 mm. The natural stone elements may have a width of between 70 and 1500 mm. The natural stone elements may be cuboid and have a length substantially equal to the width, typically between 70 and 150 mm. The natural stone elements maybe setts and have a length between 150 and 300 mm and a width between 70 and 150 mm. The natural stone elements may be slabs and have a length between 300 and 2500 mm and a width between 200 and 1500 mm.

The bonding layer may comprise a fine concrete with an elastic modulus greater than 2SGPa at 28 days. The fine concrete may comprise a mixture of aggregate, cement and water. The aggregate may have a maximum aggregate size of 6 mm.

The bonding layer may have a thickness of between 10 and 45 mm.

The step of preparing a formation may include preparing a soil foundation. The bearing capacity of the soil foundation may be enhanced, for example by modification using lime, or by reinforcing with hydraulic binder.

The method may include the step of selecting the length, width and depth of the stone elements according to design tables and/or equations based on the traffic load to be applied to the stone paving and the properties of one or more of the stone elements, the mortar used to fill the joints, bonding layer and the hydraulic bound mixture.

The properties may include one or more of tensile strength, compressive strength, shear strength, bond strength and stiffness.

The method may include the step of selecting the depth of the lower layer according to design tables and/or equations based on composite structural behaviour of the lower layer, the bonding layer and the upper stone layer. The design tables and/or equations may use one or more parameters selected from the following: the traffic load to be applied to the stone paving, the physical properties of the stone elements, the physical properties of the mortar used to fill the joints, the physical properties of the bonding layer and the physical properties of the hydraulic bound mixture.

The physical properties may include one or more of tensile strength, compressive strength, shear strength, bond strength and stiffness.

The invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 shows a section through a stone paving construction according to the present invention.

With reference to Fig. 1, the present invention provides stone surfaces which can sustain commercial vehicle and public service vehicle movement through rigid composite construction 10 by bonding a continuous rigid stone surface upper layer 112 to a stiff lower layer 14 of bound material. The upper 12 and lower 14 layers, which are bonded together by a bonding layer 16, form a composite construction 10, which is in turn supported by a foundation of soil 18 that may have an enhanced bearing capacity upper layer, or layers.

The lower layer 14 of the composite construction is continuous and bound. A hying course or bonding layer 16 is laid on the lower layer 14, and the stone dements 20 of the upper layer 12 are laid on the laying course 16. The laying course 16 is of sufficient depth to accommodate the thickness tolerance of the stone surface ekments 20 of the upper layer 12 and the construction tolerance of the top surface 22 of the lower layer 14. The laying course 16 becomes a bonding layer, which bonds the upper layer 12 to the Thwer layer 14.

The stone surface elements 20 are laid with gaps or joints 26 between them. The joint space is filled with mortar to stabilise the elements 20. Mortar, sometimes called fine concrete, is a mixture of fine aggregate, for exampk sand, and cement.

The consistency of the mortar is varied by the addition of water; required to hydrate the cement. Adding more water will reduce the strength of a sand-cement blend.

The blend percentage of sand and cement can be varied to create mortars of different compressive and tensile strengths. The water and cement contents are increased to increase fluidity and strength of the mortar. Typically the blend of water to cement by weight must be greater than 0.28. With a weight ratio of about 0.5 or more, the mortar will be plastic or mouldable in consistency. With a weight ratio of about 1.0 or more, the mortar will be pourable in consistency, and is often termed a grout Joints 26 maybe manually packed using a plastic or drier consistency of mortar. A mortar having a plastic consistency may be spread onto the edges of the elements before butting the edges together, to form a joint 26 filled with mortar. A mortar having a pourable consistency may form a slurry which can be spread into the joints 26, for example using a "squeegee" method.

The composite construction 10 acts to assure the ability of the surface stone elements 20 to act as a continuous layer 12. The lower layer 14 is designed as a hydraulic bound mixture (HBM). The HBM comprises natural or secondary aggregates or soils bound using a low energy hydraulic binder and roller compacted; a low energy hydraulic binder is formed by an alkali-activated pozzolan, the pozzolan being an industrial by-product in the form of pulverised coal fly ash.

The low energy hydraulic binder is of limited strength in order to achieve an I-IBM that can be laid as a continuous layer which will not be subject to unrestrained shrinkage cracking.

Portland cement is a traditional hydraulic binder used for general building construction. It achieves relatively rapid strength gain and relatively high compressive strength. Portland cement is used with secondary aggregates and soils for road construction but it has limitations, the most significant being unrestrained cracking. The solution when used with road construction using Portland cement is to pre-cracka HBM layer.

The low energy hydraulic binder of the present invention is a blend of high alumina siliceous pulverised coal fly ash and Portland cement in the proportions 60% to 70% by weight high alumina siliceous pulverised coal fly ash, such as from Longannet power station in Scotland, and 30% to 40% by weight Porfiand cement of strength grade 42.5. High alumina siliceous pulverised coal fly ash is defined as having a percentage by weight of alumina in the siliceous fly ash of more than 30%, typically between 35% and 60% by weight.

The hydraulic binder is mixed with a well shaped and well graded 0/10 crushed rock granite aggregate, such as G2 defined in the European Standard series BS EN 14227. The resulting mixture should achieve the characteristics of a T2 mixture as defined by BS EN 14227-3, Fly Ash Bound Mixtures) or BS EN 14227-5 Hydraulic Road Binder Bound Mixtures. The 1-year strength characteristics of the HBM should be between 0.6SMPa and O.8MPa direct tensile strength (0.BMPa and 1.OMPa indirect tensile strength) and between 2SGPa and 35GPa elastic modulus. The rate of strength gain measured as indirect tensile strength should be between 0.3SMPa and 0.4MPa at 3 days and between 0.4SMPa and 0.SOMPa at 7 days. The characteristic values should be measured according to European Standard guidance.

When a lower layer having these properties is used, no pre-cracking is required.

The binder layer or laying course l6is a fine concrete with an elastic modulus greater than 2SGPa at 28 days. The laying course 16 should be fully shrinkage compensated, for exampk by introducing into the concrete an additive, such as aluminium powder, which expands in contact with lime, from the hydration of the cement in the concrete, and water. The resulting expansion compensates for the shrinkage of the cement paste.

Shear resistance is provided by the composite action of the mortar or grout in the joints 26 with the joint faces of the stone elements 20. Typically the mortar or grout in the joints 26 has a shear strength of at least 1.6 MPa. Shear forces occur in the joints 26 and may be direct, meaning that two adjacent stone faces move vertically relative to each other, or maybe rotational, meaning two adjacent faces rotate in counter direction, for exampk when stone elements are laid in overlapping rows.

Any loading gives rise to shear; the value of the shear generated depends on the area of joint in shear and the vehide oading. The combination of area in shear and loading should not generate a shear stress beyond a factor of the limiting or test strength. The factor is selected according to the loading frequency.

Typically the mortar or grout in the joints 26 generates a bond adhesion strength of at least 1.5MPa in composite action with the joint faces of the stone elements. Bond adhesion strength sustains tensile stress generated in joints when the surface deflects under the action of vehicle wheel loading.

To achieve composite action of the pavement 10 the horizontal adhesion between the stone upper layer 12 and the bonding layer 16, and the horizontal adhesion between the bonding layer 16 and the lower layer 14, must be sufficiently large.

The bond adhesion strength should exceed the lower tensile strength of the two adjacent materials. Since the HBM in the tower layer 14 has a lower tensile strength than the stone elements 20 of the upper layer, the bond adhesion strength shoukl exceed 0.8 MPa. In practice, to provide a factor of safety, it has been found that the bond adhesion strength should be at least 1.0 MPa.

The design of a stone paving construction 10 of the present invention to withstand a particular design loading involves two steps. First the dimensions and properties of the surface layer 12 are selected to achieve a structurally continuous upper layer 12.

Secondly the dimensions and properties of the base layer 14 are selected, being dependent on the thickness of the surface layer 12.

The surface layer 12 is formed from stone elements 20. The plan dimensions of each stone ekment, namely width and length, may depend in part on aesthetic considerations, but they will also, with the depth, be a factor of the ability of the surface layer 12 to sustain generated joint shear, both direct and rotational. When the stone elements are slabs the flexural strength, or modulus of rupture, controls the depth of units of preferred plan dimensions for anticipated wheel loading. The required depth is the greater of the depth calculated from unit analysis and the depth calculated from joint ana'ysis, based on the anticipated oading of the surface layer.

Example 1

Loading condition: Moderate loading frequency or Site Category I with default loading of 80kM axle weight and 4Okn wheel load.

Stone characteristics: Class 1 igneous or Class 2 sedimentary stone. Surface specification: Joint width 6-Bnim, joint mortar and laying course mortar compliant with requirements of BS7533-12 Solution approach: For a 40kN wheel load and a stone slab width between 300 mm and 900 mm, the inventor has found that a suitable slab thickness is 75mm.

Taking a trial width of 700mm, and using conventional stress analysis, the ength of rectangular slab maybe in the range 900mm (i.e. k=1.29, where k = the ratio of length to width of the slab) to 1400mm (i.e. k=2.00). Taking k=2.00 as the worst case for load across the width of slab, the load distribution across the slab is 36kN, and the slab is almost one-way spanning. The effective contact stress is 36/0.0707=500kPa, with a circukr contact diameter of 300mm.

Using a depth of slab of 75mm, and effective joint depth of 65mm, the horizontal tensile stress across the joint between rows of slabs is 0.55MPa, which is ow loading frequency.

The vertical strain in the laying course or bonding layer 16 is given by: E=o/E=500x103/25x109 = 20x10-6, where E for the laying course is taken as 2SGPa The vertical deflection of the laying course 16 is given by: 6=txE =30x20x10-6 = 600x10-6mm, assuming laying course is 30mm in depth The horizontal deflection across the joint 26 is given by: 6=600x10-6 x 65/350 = lllxlO-6mm The horizontal strain across the joint 26 is given by: E=111/7x106 = 16x1ft6, where 7 is the width of the joint in mm The horizontal stress across the joint 26 is given by: = 16x106x30x109 = 4BOxlO3Pa = 0.4BMPa, where the modulus of the joint mortar in 30x109 Pa The loading frequency permitted with this stress is Moderate/Low; the slab dimensions should be adjusted, to reduce the horizontal stress.

One option is to reduce the maximum length of slab thus reducing the k factor.

Taking the slab length as 900mm and the Ic factor of 1.29, then the effective contact stress is reduced to 38SkPa (27.3/0.0707) and the horizontal stress in row joints is reduced to 0.37MPa, which is Uigh loading frequency. Using a k factor of 1.5 creates an effective contact whee' load of 3lkN and effective contact stress of 436kPa; this translates to a horizontal stress of 0.42 MPa which corresponds to Moderate loading frequency. This ana'ysis suggests that the k factor range for the target loading frequency is between 1.29 and 1.5, which gives a length of slab between 900mm and 1050mm.

Tables of generated moments on a slab surface course corresponding to various wheel loads and corresponding resistance moment can be prepared by analysis and testing. Referring to such a table (not shown), and taking the minimum length as 900mm, and adjusting for a fatigue factor, where necessary, the cumulative resisting moment adjusting for an effective joint depth of 50mm is 10.64kNm, which is greater than the applied moment of 9.O0kNm. Referring to the same table [not shown], data for a slab 1050mm long shows that the minimum effective joint depth reduces to 30mm. For a minimum effective joint depth of 50mm translates to a slab depth of a maximum value of 60mm with plastic consistency laying course mortar.

The punching shear for a slab 900x700x75mm thick subject to a 4OkN wheel load is l92kPa and even with a full fatigue factor applied the limiting shear stress becomes 769kPa, which can be achieved with a sand-cement mortar.

Hence the design solution is to use a slab having the following dimensions: YOOx700x7Smm thick Other design solutions are possible. For example the length of the slab may increase to 1050mm; the width of the slab may increase to 900mm, which will allow an increase in length of slab. Moving to a Class 2 igneous stone would reduce the minimum depth of slab to 60mm using a plastic consistency laying course mortar and the slab size can reduce to 900x600x6Omm thick.

The initial design depth of the slab maybe limited by the joint shear capacity. If a fine concrete joint grout giving higher joint shear performance is used, then the slab may be thinner.

When using the design tables referred to above, a factor is used in design that accounts for the frequency of loading; the factor is used with the joint shear value used in design. The factor, Kjj1, is taken as 0.25 with high frequency loading and the ability to create an indefinite life of surface, the value of Kj0111 increases to 0.4 with occasional loading frequency.

Once the dimensions of the slabs have been selected, the thickness and properties of the lower composite layer are calculated. The design of the lower composite layer is based on the value of Strength Ratio (SR) modified by factors relating to the performance characteristics of the material (Kilyd.) and the risk with a design (Ksfty). SR is the ratio between flexural strength and tensile stress generated at the underside of the HBM lower composite layer. SR values vary with frequency of loading, ranging from 1.5 with occasional loading to 3.0 for high frequency loading.

The values of SR are based on experience and correlation with published data by the Transport Research Laboratory (TRL) in the UK. The value of RHyd. is taken as 0.35, a value defined from experience; the value of Ksarety is taken as 1.0 unless a project is of high risk and value when the value would be reduced.

The generated flexura] stress (direct tension) is calculated by a simple linear elastic theory) in accordance with conventional pavement design. The parameters used in the design are the design vehicle wheel contact stress) the foundation bearing capacity and the characteristics of the composite construction 10, including the upper layer 12 and the lower layer 14. The calculations use the following equation to calculate the radius of curvature of system at depth z below surface contact stress.

R = (B x a/((1-p?) x ao)/(1+(1+((1.S/(1-[i)) x (z/a)2B) x (1+(z/a)2)2.5 where: R = radius of curvature of system at depth z below surface contact stress z = depth of interface between HBM base and foundation E = elastic modulus of single material system a = radius of circular surface contact area p. = Poisson's ratio of single material system = surface contact stress The radial or tensile strain at the depth of the interface between the HBM base and the foundation is calculated using the following equations: Er = z/2/R and = EHBM X Er where: Er = radial (tensile) strain at depth of interface between HBM base and foundation = radial stress at bottom of HBM base layer The single material system is created by translating the depth of a materia' layer to one equivalent lo that of the lower layer material. All layers are first converted to that of the soil foundation. The equivalent thickness expression for a three layer system (surface, base and foundation) is calculated using the following equation: h0 = hi x (E1/E2 x ((1-R22)/(1-R12))1/3 x f where: he = equ valent thickness of upper layer = thickness of upper layer E1/E2 = elastic modulus of upper and lower layers respectively = Poisson's ratio for upper and lower layers respectively f = factor taken as 1.0 for first interface and 0.8 for second interface

Example 2

Vehicles with an 8 tonne axle are to move over a stone surface of thickness 125mm; the laying course is 15mm and the I-IBM base hyer is 400mm in thickness. The surface elastic modulus is taken as 3SGPa, the laying course elastic modulus is taken as 3OGPa, and the elastic modulus of the base is taken as 3GPa. The effective elastic modulus of the base is taken as 0.2GPa. The tensile stress at the bottom of the base HBM should not exceed O.3SMPa. Poisson's ratio for the layers is as follows: surface and hying 0.15, base 0.35 and foundation materia' 0.4. The contact area of a vehicle wheel is taken as circular with a radius of 150mm. Check the thickness of the HBM base layer for a limiting radial stress in the HBM base layer of 0.2MPa.

Solution aijoroach: As the surface and laying course have similar elastic modulus values, the same Poisson's ratio value and the laying course is thin relative to the surface and base layers then the surface and laying course will be combined in the analysis and converted to a single layer 140mm thick having an elastic modulus of 3OGPa. The system is now a three layer system.

Firstly, convert the system requires to a single layer of foundation material using the equivalent thickness equations: h01 = 140(30/3 x (10.352)/(10.152))h/3 x 1.0 = 291mm h02 = (291+400)(3/0.2 x (1-0.423/(1-0.3Yfl1/3 x 0.8 = 1343mm Secondly, calculate the tensile stress at the bottom of the HBM base layer The surface contact stress cr0, assuming a uniform stress distribution, is given by 40/22/7 x 0.152 = S66kPa The radius in flexing at the lower interface level is given by: R = ((0.2x109x0.1S)/ ((10.42) x 566 x 1O)]/(1÷((1+(1.5/0.6J) x 80.162)) x (1÷80.162)25 m R = (63.099/281.567) x 59344.688 = 13299.11 m The radial stress at the bottom of the HBM base layer is given by: = (1.343/2)/13299.11 = 50.49 x 10-6 = 3 x 109 x 50.49 x 10-6 = 0.15 MPa, the value is less than the limiting value of 0.2 MPa.

Hence the thickness of the HBM base layer is sufficient Typically the elastic modu'us of the upper layer 12 is taken to be in the range 30GPa to 35GPa, while the elastic modulus of the HBM lower layer 141s taken as 3GPa.

Although if uncracked the H BM layer would have an elastic modulus of 3OGPa, a factor of 0.1 is applied to the ebstic modulus to take account of the fact that the HBM layer will be subject to diffuse cracking, as opposed to unrestrained cracking.

This cracking greatly reduces the effective modulus and contr6ls the value of KEjyd..

The factor of 0.1 is lower than that recommended in prior art design guidance when HBM is used inroad construction, but takes account of the lower strength compared to that normally expected in road pavements.

Typically the composite construction 10 is laid directly on a soil formation 24; the formation 24 is the surface of the soil foundation 18 used to construct the pavement The bearing capacity of a foundation is conventionally defined in units of CBR and expressed as a percentage. With general roadway design the bearing capacity of a foundation is expressed as foundation surface modu'us in units of MPa. There is an empirical relationship between CBR and foundation surface modulus which is used for design purposes.

If a soil foundation has a low bearing capacity, the effect is to limit the degree of compaction possible of the HBM lower composite layer. For the purposes of the design of a composite pavement according to the present invention, an HBM layer elastic modulus of 3GPa is assumed with foundations of bearing capacity of 30 percent CBR and greater, of 1GPa with foundations of bearing capacity of 15 percent CBR and tess, and of 2GPa with foundation bearing capacities between 15 percent CBR and 30 percent CBR.

In practice the soil foundation can be an existing roadway foundation, made ground, or natural soil. An existing roadway foundation or made ground may be expected to provide a higher bearing capacity than natural ground.

The hydraulic hound mixture and stone paving construction of the invention provide an efficient method of design of stone pavements, which make best possible use of the bearing capacity of the ground) and allow efficient use of the stone materiaL which is an expensive component of stone paving construction. The invention makes use of the composite construction of the stone upper layer 12, the HBM lower layer 14 and the bonding layer 16 to provide a structurally efficient paving construction.

There are two modes of composite action in the present invention. Firstly the stone elements are lormed into a continuous layer through the composite action created by the joint mortar, and secondly the continuous surface layer then acts compositely with the HBM layer for structural efficiency. The invention makes use of the composite joint shear strength of the joints between the stone elements, and the tensile strength across the joints) and the bond strength between the upper layer and the lower layer. All these factors achieve a composite construction which distinguishes the invention from current practice.

Claims (1)

1. <claim-text>Claims 1. A stone paving construction comprising a stone upper layer, a stiff bound lower layer and a bonding layer between the upper layer and lower layer, wherein the stiff bound lower layer comprises a compacted hydraulic bound mixture, the hydraulic bound mixture comprising an aggregate or soil bound using a low energy hydraulic binder, wherein the stone upper layer comprises a plurality of stone elements arranged with joints therebetween and a joint filling material arranged in the joints which acts compositely with the stone elements to form a continuous surface layer, and wherein the continuous surface layer acts compositely with the stiff bound lower layer and the bonding layer to form a composite structural construction.</claim-text> <claim-text>2. A stone paving construction according to claim 1, wherein the low energy hydraulic binder comprises an alkali-activated pozzolan.</claim-text> <claim-text>3. A stone paving construction according to claim 1 or 2, wherein the low energy hydraulic binder comprises a blend of 60 to 70% by weight high alumina siliceous pulverised coal fly ash and 30 to 40% by weight Portland cement.</claim-text> <claim-text>4. A stone paving construction according to claim 3, wherein the high alumina siliceous pulverised coal fly ash comprises between 35% and 60% by weight alumina.</claim-text> <claim-text>5. A stone paving construction according to any previous claim, wherein the aggregate in the hydraulic bound mixture isa well graded 0/10 crushed rock aggregate 6. A stone paving construction according to any previous claim, wherein the hydraulic bound mixture has a one year strength measured as a direct tensile strength of between O.6SMPa and 0.BMPa, in accordance with EN 13286-40.7. A stone paving construction according to any previous claim, wherein the hydraulic bound mixture has an elastic modulus at one year of between 2SGPa and 3SGPa.8. A stone paving construction according to any previous daim, wherein the stone upper layer comprises a plurality of natural stone elements laid side by side.9. A stone paving construction according to claim 8, wherein the natural stone elements are laid with a gap between adjacent elements, the gap being filled with mortar.10. A stone paving construction according to claim 8 or 9, wherein the naturth stone elements are between 40 and 250 mm thick, between 70 and 2500 mm in length and between 70 and 1500mm in width.11. A stone paving construction according to any previous claim, wherein the bonding ayer comprise a fine concrete with an elastic modulus greater than 25GPa at 28 days.12. A stone paving construction according to claim 11, wherein the fine concrete comprises a mixture of aggregate, cement and water, and wherein the aggregate has a maximum aggregate size of 6 mm.13. A stone paving construction according to any previous daim, wherein the bonding layer has a thickness of between 10 and 45mm.14. A stone paving construction according to any previous claim, further comprising a soil foundation beneath the stiff bound lower layer.15. A stone paving construction according to claim 9, wherein the mortar has a water to cement ratio by weight of greater than 0.28.16. A stone paving construction according to cilaim 9 or 15, wherein the mortar in the gap between adjacent stone elements generates a bond adhesion strength of at least 1.SMpa in composite action with the joint faces of the stone elements.17. A stone paving construction according to any previous claim, wherein the bond adhesion strength between the bonding layer and the stone upper layer exceeds the tensile strength of the stiff bound lower layer.18. A stone paving construction according to any previous claim, wherein the bond adhesion strength between the bonding layer and the stiff bound lower layer exceeds the tensile strength of the stiff bound lower l.ayer.19. A stone paving construction according to any previous claim, wherein the bond adhesion strength between the bonding layer and the stone upper layer and the bond adhesion strength between the bonding layer and the stiff bound lower layer are both greater than 0.8MPa.20. A stone paving construction according to any previous claim, wherein the bond adhesion strength between the bonding layer and the stone upper layer and the bond adhesion strength between the bonding ayer and the stiff bound lower layer are both greater than 1.OMPa.21. A method of constructing stone paving comprising the steps of: preparing a formation; laying and compacting a hydraulic bound mixture comprising an aggregate or soil bound using a low energy hydraulic binder to form a stiff bound lower layer on the formation; laying a bonding layer on the lower layer; forming a stone upper layer on the bonding layer by arranging a plurality of stone elements with joints therebetween on the bonding layer and arranging a joint filling material in the joints to act compositely with the stone elements to form a continuous surface layer; and curing the hydraulic bound mixture, the bonding layer and the joint filling material so that the continuous surface layer acts compositely with the stiff bound lower layer and the bonding layer to form a composite structural construction.22. A method according to claim 21, wherein the stone paving is a stone paving construction according to any of claims 1 to 20.23. A method according to any of claims 21 to 22, wherein the step of laying a stone upper layer on the bonding layer comprises laying a plurality of natural stone elements side by side.24. A method according to any of claims 21 to 23, wherein the natural stone elements are laid with a gap between adjacent elements, and wherein the gap is filled with mortar.25. A method according to any of claims 21 to 24, wherein the natural stone elements are between 40 and 250 mm thick, between 70 and 2500 mm in length and between 70 and 1500 mm in width.26. A method according to any of claims 21 to 25, wherein the bonding layer comprise a fine concrete with an elastic modulus greater than 25GPa at 28 days.27. A method according to claim 26, wherein the fine concrete comprises a mixture of aggregate, cement and water, and wherein the aggregate has a maximum aggregate size of 6 mm.28. A method according to any of claims 21 to 27, wherein the bonding layer has a thickness of between 10 and 45mm.29. A method according to any of claims 21 to 28, wherein the step of preparing a formation includes preparing a soil foundation.30. A method according to any of claims 21 to 29, further including the step of selecting the length, width and depth of the stone elements according to design tables and/or equations based on the traffic load to be applied to the stone paving and the properties of one or more of the stone elements, the mortar used to fill the joints, bonding layer and the hydraulic bound mixture.31. A method according to claim 30, wherein the properties include one or more of the tensile strength, compressive strength, shear strength, bond strength and stiffness.32. A method according to any of claims 21 to 31, further including the step of selecting the depth of the lower layer according to design tables and/or equations based on composite structural behaviour of the lower layer, the bonding layer and the upper stone layer, wherein the design tables and/or equations use one or more parameters selected from the following: the traffic load to be applied to the stone paving, the physical properties of the stone elements, the physical properties of the mortar used to fill the joints, the physical properties of the bonding layer and the physical properties of the hydraulic bound mixture.33. A method according to claim 32, wherein the physical properties include one or more of the tensile strength, compressive strength, shear strength, bond strength and stiffness.34. A stone paving construction as hereinbefore described with reference to the accompanying drawing.35. A method of constructing stone paving as hereinbefore described with reference to the accompanying drawing.</claim-text>