CHAPTER 6 SMALL ELEMENT PAVEMENTS

184

6.1

Introduction:

The toplayer of small element pavements consists of small precast elements that can be lifted and re-installed rather easily. The very flexible small element pavement allows easy access to the substructure and underground infrastructure, in contrast to the continuous asphalt and concrete pavements. Small element pavements are applied in The Netherlands during many centuries. In first instance burnt clay bricks have been used, but since World War II mainly concrete blocks and concrete tiles are applied. The traditional field of application of small element pavements is within built-up areas, however during the last decades also many (very heavily loaded) industrial yards have been paved with concrete blocks. In the Netherlands small element pavements are applied on a very wide scale: around 40% of the paved area has a small element pavement. The production of concrete paving blocks in The Netherlands is about 15 million m2 per year, i.e. about 1 m2/inhabitant/year. The majority of the blocks are rectangular blocks; the share of shaped (dentated) concrete paving blocks, introduced in 1952, never has been more than 10%. Burnt clay bricks are especially applied for (reconstruction of) streets and squares in historical city centres. The annual production of burnt clay bricks is 1 to 1.5 million m2. The production of concrete tiles is of the same size as that of concrete paving blocks, namely around 10 million m2 per year. Besides the mentioned paving elements every year also some 10,000 km concrete edge restraints are produced in The Netherlands. More extensive information about the design (and construction) of small element pavements can be found in (1,2,3).

6.2

General aspects:

6.2.1

Laying patterns:

Burnt clay bricks as well as rectangular and some shaped concrete paving blocks can be installed in several laying patterns (bonds). The most important laying patterns are shown in figure 6.1. However, most of the shaped concrete paving blocks (figure 6.9) fit together in only one way, which means that these blocks have to be laid either in herringbone bond or in stretcher bond. The laying pattern affects both the structural behaviour (strength) of a small element pavement and the ‘creep’ (horizontal displacements in the direction of traffic) of the elements due to the traffic loadings. In this respect herringbone bond is more favourable than stretcher bond. Furthermore concrete paving blocks are more favourable than burnt clay bricks because of the narrow joints that can be realised due to the smaller shape and dimensional tolerances of concrete blocks. By applying different laying patterns, the bond can also be used for functional reasons, for instance for traffic guidance, on traffic junctions, parking lots and squares. When applying concrete paving blocks, the different functions of paved areas can be pointed out by applying blocks with a different colour.

185

Figure 6.1: Types of laying patterns for burnt clay bricks and concrete blocks (1,2). In the past in The Netherlands burnt clay bricks and concrete paving blocks were usually installed in stretcher bond A. Nowadays on traffic lanes mostly herringbone bond B is applied, and on parking lots and industrial yards usually herringbone bond A. For temporal small element pavements normally a basket weave bond is applied because of the high production rates during construction (which limits the labour costs). Concrete tiles can be installed in basket weave bond, stretcher bond and diagonal bond (figure 6.2). Generally the stretcher bond is applied on footways and bicycle tracks. With this type of bond there are no continuous longitudinal joints, resulting in a better interlock between the concrete tiles. The relatively weak basket weave bond is only applied on very lightly loaded pavements (footways and –paths), sometimes for esthetical reasons. The diagonal bond is hardly applied in The Netherlands. There are, for esthetical reasons, some applications on squares. Besides the common uncoloured (grey) concrete tiles also coloured tiles are applied sometimes for esthetical reasons but also for functional reasons. The best example of this is the wide-scale use of red-coloured concrete tiles for bicycle tracks. The herringbone bond B (concrete blocks) as well as the diagonal bond (concrete tiles) requires especially shaped elements (so called bishop’s mitre, element A in figure 6.2) to realise a straight pavement edge along the edge restraint.

186

Figure 6.2: Types of laying patterns for concrete tiles (1). 6.2.2

Properties, advantages and disadvantages:

The most important advantages of small element pavements are: - the pavement is able to undergo unequal subgrade settlements without further structural damage (cracks) - the accessibility of the subgrade for installing or repairing underground services is very easy, except in the case of a cement-bound (sub-)base - repaving the toplayer (because of local failure, excessive rutting or unequal settlements, etc.) can be done easy, fast and without loss of expensive pavement material - the pavement is not affected by mineral oil and fuel - the pavement can be trafficked immediately after (re)laying (and compacting) the small elements - also small and irregular surfaces can be paved easily by means of small elements. Other advantages specifically for concrete block pavements and concrete tile pavements are: - skidding resistance remains sufficient, also because the rainwater is removed fast through the joints in the toplayer - the light reflection is good (also because there is no water at the pavement’s surface), especially when the concrete blocks or tiles have their natural grey colour or a light colour - application of different coloured concrete blocks or tiles, especially in combination with different laying patterns, offers possibilities to emphasize the different functions of the paved area. 187

The most important disadvantages of small element pavements are: - considering the serviceability/unevenness (and for burnt clay bricks, when wet, also the skidding resistance) a small element pavement is not very suited for roads where the traffic speed exceeds 50 to 60 km/h - special attention has to be paid to the rainwater that penetrates into the pavement structure through the sand-filled joints in the small element toplayer - the toplayer elements can creep (in the longitudinal direction) or ‘float away’ (in the transverse direction) due to traffic loadings; to prevent these deficiencies the following requirements should be met: • use elements with small tolerances with respect to shape and dimensions • apply the correct laying pattern (see 6.2.1) • realise narrow joints between the elements that are fully filled with jointing sand • apply an adequate (i.e. heavy) edge restraint (see 6.3.6) - the manual construction of a small element toplayer according to the traditional Dutch manual paving method (figure 6.3) or according to the lay-down method (figure 6.4) is very labour-intensive, while a paviour has a very hard job; however, nowadays there exists a variety of equipment for mechanised laying-down concrete blocks (figure 6.5) and tiles and even concrete edge restraints. -

Figure 6.3: Traditional Dutch manual paving method (4).

Figure 6.4: Manual laying method (4).

188

Figure 6.5: Mechanical laying method (4). 6.2.3

Applications:

The main application of small element pavements is within built-up areas (figure 6.6), especially when the subgrade is weak, when the traffic speed is low and when there are underground services. Examples of applications are: pedestrian areas, footways, bicycle tracks, parking lots, residential streets and fuel stations. The application of burnt clay bricks is mainly restricted to historical city centres, in other cases usually the cheaper concrete blocks are used.

Figure 6.6: Division of pavement types in the built-up area of Rotterdam in 1994 (total paved area 25.4 * 106 m2). Concrete paving blocks are also widely used on industrial yards such as container terminals. This kind of yards generally is located in river delta areas having a weak subgrade with high (unequal) settlements, and is subjected to very heavy (traffic) loadings with sometimes extremely high contact pressures (stacked containers!). Because of the heavy loadings a thick base of high quality (cement bound) material is absolutely necessary. On a smaller scale small element pavements are also applied for rural roads, farm yards, temporal pavements, small and irregular surfaces, etc. 189

6.3

Small element pavement structure:

6.3.1

Introduction:

Figure 6.7 shows the various components of a small element pavement structure that will be discussed in more detail in the next paragraphs. Especially the toplayer (consisting of small elements and sand-filled joints), the bedding layer and last but not least the necessary edge restraint are different from asphalt and concrete pavement structures.

Figure 6.7: Small element pavement structure. 6.3.2

Toplayer:

6.3.2.1

General:

The toplayer of a small element pavement structure consists of small precast units that can be lifted and re-installed again. To achieve a maximum load spreading (by means of shear forces, see figure 6.8) in the toplayer, the joints between the small elements should be narrow (2 to 3 mm) and continuously filled with jointing sand. In this way also the permanent deformations of a small element, relative to adjacent elements, are limited.

Figure 6.8: Load spreading in the small element toplayer through shear forces in the joints (3). 6.3.2.2

Burnt clay bricks:

Burnt clay bricks are manufactured from river-clay. In The Netherlands suitable riverclay is mainly found along the rivers Waal, Rijn en IJssel. The clay should contain about 50% (by mass) particles smaller than 50 µm and 5% (by mass) fine sand. In The Netherlands three sizes (‘formats’) of burnt clay bricks are distinguished, i.e. the Waal format, the thick format and the cobble format (table 6.1); the cobble format is further subdivided in normal and flat format. The cobble format is applied most. Cobble 190

format burnt clay bricks are manufactured with a chamfer (reduced top surface) to limit the risk of spalling of the edges of the bricks. dimension length width thickness

Waal format

thick format

195 48 85

195 64 85

cobble format normal flat 195 195 92 92 85 70

Table 6.1: Minimal dimensions (mm) of burnt clay bricks in The Netherlands (5). In the Dutch specifications (5) the required strength for burnt clay bricks is a flexural tensile strength of at least 6.0 N/mm2. Furthermore in the specifications several qualities and sorts are distinguished depending on the hardness, the deviation of the rectangular shape, the dimensional tolerances, the surface texture, etc. Burnt clay bricks have to be delivered with a KIWA-certificate that also serves as certificate of origin. Burnt clay bricks are very durable and resistant against frost/thaw-action, de-icing salt and mineral oil and fuel. Their skidding resistance is rather low, certainly under wet conditions. The serviceability (ride-ability) of burnt clay brick pavement structures is not very good, which in itself is favourable from a point of view of traffic safety in builtup areas. Burnt clay brick pavements have an appearance that is appreciated in historical city centers. The manufacturing of burnt clay bricks (burning the clay in ovens at high temperature) requires a lot of energy which makes them, at least in The Netherlands, 2 to 3 times more expensive than concrete paving blocks. 6.3.2.3

Concrete paving blocks:

Concrete paving blocks are made from cement concrete in specialised plants in a very high production rate. The manufacturing equipment requires a very dry, low-slump concrete mix with a water/cement-ratio typically within the range 0.34 to 0.38. The cement content normally is between 250 and 350 kg/m3 when type A cement (usually blast furnace cement) is applied. The aggregate mix contains about 60% (by mass) coarse sand and 40% (by mass) fine gravel, with a maximum grain size of 12 to 16 mm. Additives may be added, in small quantities, to the concrete mix. They include: - water-repellant admixtures to reduce the water absorption - superplasticizers to obtain a high early strength - pigments to colour the concrete paving blocks. In principle two types of concrete paving blocks can be distinguished, i.e. ‘rectangular’ blocks (with a rectangular or hexagonal horizontal cross section) and shaped (nonrectangular or dentated) blocks (with profiles in the horizontal and/or vertical direction). Figure 6.9 gives an overview of the most widely used block shapes (2); the main features of the distinguished categories A, B and C are: - category A: shaped blocks with geometrical interlock at all sides - category B: shaped blocks with geometrical interlock at two sides - category C: rectangular blocks.

191

Figure 6.9: Categories of block shapes. In The Netherlands for historical reasons nearly exclusively rectangular paving blocks (shape U in figure 6.9) are applied. In the Dutch specifications NEN 7000 (6) three standard formats are distinguished, i.e. the cobble format (211 x 105 mm2), the thick format (211 x 69 mm2) and the Waal format (200 x 50 mm2); the standard thicknesses are 60, 70, 80, 90, 100 and 120 mm. Most of the concrete paving blocks are manufactured with a chamfer (reduced top surface). Furthermore coloured blocks are applied more and more, not only for esthetical reasons but also for functional reasons (traffic guiding). Depending on the laying pattern (see 6.2.1) specific auxiliary blocks are necessary, for example half blocks for herringbone bond A and stretcher bond A and so-called bishop mitres for herringbone bond B (see figure 6.1). Shaped blocks mostly require more types of auxiliary blocks than rectangular blocks, such as blocks to realise a straight edge at the start or end of the block pavement and blocks to realise curves. A special type of concrete paving block is the ‘spalling-free’ block. These blocks not only have a chamfer but a strongly reduced horizontal area over the upper centimeters (figure 6.10). The objective of this reduced horizontal area is to strongly reduce the spalling of edges due to direct contact between adjacent blocks. Therefore these spalling-free blocks are especially suited for concrete block pavements with very narrow joints between the blocks and/or for pavements that exhibit great deflections under the heavy traffic loadings.

Figure 6.10: An example of a spalling-free concrete paving block.

192

The specifications for concrete paving blocks (6) concern the materials, the manufacturing, the dimensions and dimensional tolerances, and the quality (strength, durability, etc.). The properties of concrete paving blocks are determined on the one hand by the material concrete and on the other hand by the manufacturing process (pouring concrete in steel moulds and then heavy vibratory compaction). The most important properties are: - the small shape and dimensional tolerances, which make it possible to realise concrete block layers with narrow joints - a high strength, which enables to use them for very heavily loaded pavements (such as container terminals) - a high resistance against physical affection, although the combination of frost and de-icing salt may lead to scaling of the top surface of the blocks - a good resistance against mineral oil and fuel; however, (an)organic acids, sulphates, vegetable and animal oils and fats do affect the concrete - a good skidding resistance - a rather good abrasion resistance - a good light reflection, which is especially valid for non-coloured blocks and blocks having a light colour. 6.3.2.4

Concrete tiles:

Concrete tiles are also manufactured in specialised plants. The cement concrete mix composition and the manufacturing process of concrete tiles are similar as those for concrete paving blocks. Although ‘shaped’ concrete tiles are available, the great majority of the concrete tiles applied in The Netherlands are square or rectangular in shape. A distinction is made between: - small-size tiles with horizontal dimensions of 300 x 300 mm², the standard thicknesses are 45, 50, 60, 70 and 80 mm; these tiles have a chamfer (reduced top surface) - large-size tiles, with horizontal dimensions greater than 300 x 300 mm², for instance 400 x 400 mm², 500 x 500 mm², 400 x 600 mm² and 600 x 600 mm². Specifications are available only for small-size concrete tiles (7). The specifications deal with the materials, the dimensions, the shape and dimensional tolerances, the strength and the abrasion resistance. The properties of concrete tiles are to a large extent the same as those of concrete paving blocks (see 6.3.2.3). An important difference however is the fact that concrete tiles can resist much smaller traffic loadings than concrete blocks. This is explained by the fact that the contact area of (truck) tyres is smaller than the horizontal dimensions of the tile, which results in bending moments and thus flexural tensile stresses in the concrete tile. 6.3.3

Bedding sand layer:

On the base (or on the sub-base when no base is applied) a thin layer of fine granular material is necessary to eliminate the unevenness of the base (or sub-base) surface and the variation of the thickness of the small elements. This special layer is called the bedding sand layer. 193

The bedding sand layer is very close to the traffic loadings. Therefore the bedding sand should be highly elastic and very stable. Special types of sand are used for the bedding sand layer such as ‘bedding sand’, blast furnace slags sand and crushed sand; in very heavily loaded concrete block pavements fine crushed stone (for example grading 0/8 mm) is used. To prevent saturation and weakening of the bedding layer under the ‘open’ small element toplayer the bedding layer material should not contain any clay particles. Table 6.2 gives the Dutch specifications with respect to the grading of bedding sand. Retained on sieve C4 2 mm 1 63 µm

‘bedding sand’ ≤10 ≥95

Percentage (by mass) blast furnace crushed slags sand sand ≤10 ≤20 ≤60 ≥90 ≥90

Table 6.2: Grading requirements for bedding sand in The Netherlands (5). In the case that below the bedding sand layer there is a (base) material of higher quality, the bedding sand layer should be as thin as possible taking into account the construction method of the small element toplayer. The traditional Dutch manual paving method (figure 6.3) requires a bedding layer thickness of 50 to 70 mm (after compaction); in the manual (figure 6.4) or mechanical (figure 6.5) lay-down method the thickness of the compacted bedding layer should not be more than 50 mm. 6.3.4

Base:

For very lightly loaded small element pavements (such as pedestrian areas, footways, bicycle tracks and residential streets) a base is not necessary. However in more heavily loaded small element pavements a base should be applied to ensure a good traffic load spreading. For extremely heavily loaded small element pavements (such as container terminals) only a thick cement-bound base is appropriate, however this type of base material is not discussed here (1). For roads nearly always a granular unbound or lightly-bound base material is applied, and these materials will be briefly discussed here. Un unbound or lightly-bound base material has to fulfil the following requirements: - a good permeability to remove as soon as possible (to the sub-base) the rainwater that entered the pavement structure through the joints in the small element toplayer - a high angle of internal friction to prevent shear instability due to the traffic loadings - a good resistance to crushing to prevent frost-susceptibility and to prevent extra rutting due to the repeated traffic loadings - a high resistance to permanent deformation to limit the rutting due to the repeated traffic loadings. Table 6.3 gives the Dutch specifications for various unbound and lightly-bound base materials for the usually applied grading 0/40 mm (there also exist specifications for the grading 0/20 mm).

194

Retained On sieve

C63 C45 C31.5 C22.4 C16 C8 C4 2 mm 63 µm

Materials natural stone, blast furnace slags, phosphorus slags, masonry granulate, mix granulate, concrete granulate min. max. 0 0 10 10 40 40 70 60 85 94 100

hydraulic mix granulate

min. 0 10 40 60 -

mix of blast furnace slags and phosphorus slags

max. 0 10 40 70 85 -

min. 0 10 30 50 -

max. 0 10 40 70 80 -

Table 6.3 : Grading requirements (percentages by mass) for unbound and lightlybound base materials in The Netherlands (5). The most widely used base materials in concrete block pavements are mix granulate and hydraulic mix granulate (8). Mix granulate, a recycled material, is a mixture of crushed concrete (min. 50% and max. 80% by mass) and crushed masonry (max. 50% by mass). The CBR-value of the ‘fresh’ material (0 days), CBR0, should be at least 50%, while after 28 days the CBRvalue, CBR28, should be at least 1.25 * CBR0. (5). Hydraulic mix granulate is a mixture of mix granulate and hydraulic slags (between 5% and 20% by mass). The requirements for the CBR-value are on one hand CBR0 at least 50% and on the other hand CBR28 at least 1.5 * CBR0 (5). In the case of small projects (less than 3000 m2) the degree of compaction should be at least 98%. For larger projects (more than 3000 m2) the degree of compaction should be at least 97% while the average degree of compaction should be at least 101%. The thickness of the unbound or lightly-bound base is dependent on the traffic loading, the bearing capacity of the underlying layers (including the subgrade) and the properties of the base material itself. In practice the thickness of the base is between 200 and 400 mm. In the case of an unbound or lightly-bound base the subgrade remains rather easy accessible to install, expand or repair underground services like cables and piping. 6.3.5

Sub-base:

The sub-base of a pavement structure may have the following functions: - raise the road surface above the ground surface - protect the pavement structure against frost/thaw-damage - soil improvement, i.e. replacement of unsuitable subsoil - platform for construction of the overlying pavement layers 195

further spreading of the traffic loadings, in such a way that neither in the sub-base nor in the subgrade unacceptable permanent deformations occur. Because of these functions the thickness of the sub-base is dependent on the design road surface level, the frost penetration depth, the traffic loadings, the bearing capacity of the subgrade and the properties of the sub-base material.

-

In The Netherlands (nearly) always sand is used for the sub-base (because of its availability and the lack of other natural road building materials). A distinction is made between ‘sand for sub-base’ and ‘sand for fill’ (5). ‘Sand for fill’ is used at a depth of more than 1 m below the road surface, so below the depth of frost penetration. It must consist of material in which particles smaller than 2 µm are present up to a maximum of 8% by mass. The amount of particles passing the 63 µm sieve must be not more than 50% by mass. The degree of compaction should be at least 93% MPD (Maximum Proctor Density) and the average degree of compaction should be at least 98% MPD. ‘Sand for sub-base’ is applied at a depth of less than 1 m below the road surface which means that it may be within the depth of frost penetration and that it clearly is subjected to traffic load stresses. Of the particles passing the 2 mm sieve, the amount of particles passing the 63 µm sieve must be not more than 15% by mass. If this amount is between 10 and 15% by mass, then, of the particles passing the 2 mm sieve, the amount of particles passing the 20 µm sieve must be not more than 3% by mass. The degree of compaction should be at least 95% MPD and the average degree of compaction should be at least 100% MPD. 6.3.6

Edge restraints:

An adequate edge restraint (figure 6.7) is very essential for every small element pavement. The edge restraint has to lock up the small element toplayer and the bedding sand layer in the horizontal direction. The edge restraint has to be such heavy that is can resist the horizontal stresses introduced by traffic loadings in the vicinity of the edge restraint. The required mass of the edge restraints therefore is dependent on the magnitude of the axle loadings on the adjacent small element pavement. The edge restraint needs to be supported at the backside by a well compacted verge or a footway. For extremely heavily loaded concrete block pavements (such as on container terminals) even a concrete backfilling of the heavy edge restraint usually is applied. For lightly loaded small element pavements (footways, bicycle tracks, etc.) rather light ‘confining edge restraints’ are adequate; however, small element pavement structures subjected to normal road traffic require quite heavy ‘footway edge restraints’ (figure 6.11). In the Dutch specifications for precast concrete edge restraints (9) the following profiles are distinguished (figure 6.11): footway edge restraints: 130/150 * 160 mm2, 130/150 * 200 mm2, 130/150 * 250 mm2 180/200 * 160 mm2, 180/200 * 200 mm2, 180/200 * 250 mm2 confining edge restraints: 50 * 150 mm2, 60 * 200 mm2, 80 * 200 mm2, 100 * 200 mm2, 120 * 250 mm2. Straight precast concrete edge restraints have a length of 1 m. Furthermore curved edge restraints are available.

196

The specifications for precast concrete edge restraints concern the materials, the dimensions, the shape and dimensional tolerances, the strength and the abrasion resistance.

Figure 6.11: Dutch precast concrete edge restraints for small element road pavements (above) and for lightly loaded small element pavements (below); dimensions in mm (9).

6.4

The Dutch Design Method for Concrete Block Pavements:

6.4.1

Introduction:

The Dutch design method for concrete block pavements has been developed in the eighties by the CROW Working Group ‘Design of small element pavements’. The task of the Working Group was to develop a (simple) method for the structural design of small element pavements, taking into account the specific Dutch conditions. The Working Group’s research therefore was concentrated on current Dutch small element pavement structures subjected to normal road traffic, thus: - toplayer of small elements with horizontal dimensions of about 200 mm * 100 mm, especially the usual Dutch cobble format rectangular concrete paving blocks - substructure consisting of a bedding sand layer and a sand sub-base or a bedding sand layer, an unbound base and a sand sub-base. For such a pavement structure, containing only unbound materials, rutting is the most important design criterion. The Working Group has defined rutting as the characteristic rut depth RDc (30% probability of exceeding) under a 1.2 m long straightedge, while the rut depth standard RDc (the allowable rut depth) was taken as 15 mm.

197

Figure 6.12 gives a flow diagram of the research of the CROW Working Group ‘Design of small element pavements’ (1,10,11). test pavements rutting measurements

deflection measurements

progressive stiffening rutting calculations

finite element calculations rut depth standard

design charts

Figure 6.12: Flow diagram of the research of the CROW Working Group ‘Design of small element pavements’. 6.4.2 6.4.2.1

Test Pavements: Alphen 1 and 2:

The CROW Working Group has realised 2 test pavements on the exit way of the yard of a precast concrete products manufacturer in the city of Alphen a/d Rijn (peat subgrade). The test pavements were subjected to the heavy outgoing traffic, mainly truck-trailers carrying concrete paving blocks, concrete tiles, concrete sewer-pipes etc. The first test pavement (Alphen 1) was constructed in October 1982. This test pavement was divided in 2 adjacent sections, each 15 m long and 4 m wide. The pavement structure of this test pavement consisted of rectangular (cobble format) concrete paving blocks (thickness 0.08 and 0.12 m respectively) in herringbone bond B, a bedding sand layer of 0.05 m crushed sand and a sand sub-base of 0.70 m. This test pavement completely failed due to ongoing shear failure in the sand sub-base due to the heavy traffic loadings, nevertheless it remained in service until the middle of 1983. After excavation of the test pavement Alphen 1 mid July 1983 a second test pavement was constructed at the same location. This test pavement had an unbound base of 0.25 m concrete granulate on 0.45 m sand sub-base, all other features were identical to the first test pavement. It appeared from registrations at the yard’s exit gate that the test pavements Alphen 1 and 2 were subjected to about 5850 axle loadings (> 5 kN) per year. The axle load frequency distribution is given in table 6.4. 6.4.2.2

Rotterdam 1 to 6:

In October 1984 the CROW Working Group ‘Design of Small Element Pavements’, has realised 6 test pavements on the heavily loaded Albert Plesmanroad in the Rotterdam port area (clay subgrade). Each test pavement had a length of 25 m and a width of 6.8 m. Besides the test pavements there were either parking lots or an edge restraint and a footway. 198

After excavating the existing pavement structure to a depth of 1.05 m below the pavement’s surface, the following concrete block pavement structures have been constructed for the test pavements Rotterdam 1 to 6: - rectangular (cobble format) concrete paving blocks, thickness 0.09 m (test pavements 1 and 3) and 0.12 m (test pavements 2, 4, 5 and 6) respectively, in herringbone bond B (test pavements 1, 2, 3, 4 and 6) and herringbone bond A (test pavement 5) respectively - a bedding layer of 0.05 m crushed sand - an unbound base of 0.30 m concrete granulate (test pavements 3 and 4) or 0.30 m mix granulate (test pavements 5 and 6) - a sand sub-base, thickness varying between 0.91 m (test pavement 1) and 0.58 m (test pavements 4, 5 and 6). During some weeks in 1984 and 1985 axle load measurements were done with a weight-in-motion system. It appeared form these measurements that the test pavements Rotterdam 1 to 6 were subjected to about 1,250,000 axle loadings (> 5 kN) per lane per year. The measured axle load frequency distribution is given in table 6.4. The test pavements Rotterdam 1 to 6 actually have been in service until the middle of 2000 without any significant maintenance. axle load (kN) range average 5 – 20 10 20 – 40 30 40 – 60 50 60 – 80 70 80 – 100 90 100 – 120 110 120 – 140 130 140 – 160 150 > 160 170

frequency distribution (%) Alphen Rotterdam 0 60.8 1.5 12.75 18.4 15.75 39.7 6.40 35.0 2.75 4.1 1.20 1.0 0.30 0.3 0.045 0 0.01

Table 6.4: Axle load frequency distributions for the test pavements Alphen and Rotterdam. 6.4.3

Measurements:

Falling Weight Deflection measurements and rutting measurements have been done regularly for determination of the development of the resilient deformations (deflections) and the permanent deformations (rutting) on the test pavements. 6.4.3.1

Falling Weight Deflection measurements:

In a Falling Weight Deflection measurement a dynamic (pulse) load of 50 kN is applied on the pavement’s surface through a circular plate with a diameter of 300 mm (figure 6.13); the contact stress thus is 0.707 N/mm2. The pulse loading time is about 0.025 s. The deflections (d) were measured at distances of 0, 0.3, 0.5, 1.0, 1.5 and 2.0 m from the load centre.

199

Figure 6.13: Principle of Falling Weight Deflection measurement. Figure 6.14 shows the development of the average maximum deflection d0 (in the load centre) of the test pavements Alphen and Rotterdam. In this figure corresponding test pavements and test sections (that only are different with respect to the thickness of the concrete paving blocks) have been combined because the block thickness had no significant effect on the measured deflections.

S = sand sub-base only CBS = mix granulate base and sand sub-base CCS = concrete granulate base and sand sub-base Figure 6.14: Development of average maximum deflection d0 (due to 50 kN load) on the test pavements Alphen and Rotterdam. Figure 6.14 shows a substantial decrease of the maximum deflection (i.e. an increase of the stiffness) of the pavement structure as a function of time (the number of load repetitions) for the test pavements Alphen 2 and Rotterdam 1 to 6. These test pavements exhibit the characteristic ‘progressive stiffening’ behaviour of stable concrete block pavements. 6.4.3.2

Rutting measurements:

The development of the rutting on the test pavements Alphen and Rotterdam is shown in figure 6.15. Considering the rut depth standard (see 6.4.1), figure 6.15 shows the 200

development of the characteristic rut depth RDc (30% probability of exceeding) under a 1.2 m long straightedge. Similar to figure 6.14 also in figure 6.15 corresponding test pavements or test sections have been combined, because the thickness of the concrete paving blocks also had no significant influence on the development of the rutting on the test pavements.

S = sand sub-base only CBS = mix granulate base and sand sub-base CCS = concrete granulate base and sand sub-base Figure 6.15: Development of the characteristic rut depth RDc (30% probability of exceeding) on the test pavements Alphen and Rotterdam. A comparison of the rutting behaviour of the test pavements Alphen 1 and 2 clearly demonstrates the very beneficial effect of the unbound base of 0.25 m concrete granulate in the case of a very weak peat subgrade, The base not only prevented the test pavement Alphen 2 from instability (shear failure), but also limited the rutting to an acceptable level considering the heavy traffic loadings and very weak subgrade. The rutting behaviour of the test pavements Rotterdam 3 + 4 (base of 0.30 m concrete granulate) and Rotterdam 5 + 6 (base of 0.30 m mix granulate) is about equal. The rutting behaviour of the test pavements Rotterdam 1 + 2 (with a sand sub-base only) is clearly worse although shear failure did not occur. With the exception of the completely failed test pavement Alphen 1 the development of the rutting on the remaining stable test pavements is described by the equation: RDc = a p . N

bp

(6.1)

where: RDc = N

=

ap, bp =

characteristic rut depth (mm), 30% probability of exceeding, under a 1.2 m long straightedge cumulative number of equivalent 80 kN standard axle load repetitions per lane, taking into account the lateral wander of the traffic (reduction factor 1.0 and 0.5 respectively for the test pavements Alphen 2 and Rotterdam respectively). rutting coefficient with respect to the total pavement structure

201

The calculations (table 6.5) clearly show that the rutting behaviour (i.e. the rutting coefficient ap and bp) of concrete block pavements is dependent on the total pavement structure including the subgrade. Test pavement Alphen 2: 0.45 m sand sub-base + 0.25 m concrete granulate base + 0.05 m crushed sand bedding layer Rotterdam: 1+2 0.90 m sand sub-base + 0.05 crushed sand bedding layer 3+4 0.60 m sand sub-base + 0.30 m concrete granulate base + 0.05 m crushed sand bedding layer 5+6 0.58 m sand sub-base + 0.05 m mix granulate base + 0.05 m crushed sand bedding layer

ap

bp

1.379

0.211

0.133

0.381

0.930

0.162

0.666

0.190

Table 6.5: Rutting coefficients ap and bp of the test pavements Alphen 2 and Rotterdam 1 to 6. 6.4.4

Analysis of measurements results:

6.4.4.1

Resilient deformation behaviour (deflections):

The measured resilient deformation behaviour (deflection curves) of the test pavements Alphen 1 and 2 and Rotterdam 1 to 6 was analysed by means of a twodimensional finite element model (11). In this model the concrete block toplayer is represented as a pure shear layer: the concrete paving blocks, represented as ‘rigid bodies’, are interconnected by means of linear-elastic vertical springs and they only can translate in the vertical direction. The layers below the concrete block layer (bedding sand layer, unbound base (if any), sand sub-base and subgrade) are modelled as continuous elements, characterised by their dynamic modulus of elasticity and Poisson’s ratio. The finite element calculations confirmed the ‘progressive stiffening’ behaviour of the test pavements, with the exception of the completely failed test pavement Alphen 1. 6.4.4.2

Permanent deformation behaviour (rutting):

As the test pavements Alphen 2 and Rotterdam 1 to 6 clearly showed the ‘progressive stiffening’ behaviour of stable concrete block pavements, the permanent deformation behaviour (rutting) of these test pavements was analysed by means of the ‘progressive stiffening theory’ (11). The ‘progressive stiffening’ coefficients of the various granular layers of these test pavements were calculated, assuming that no permanent deformation occurs in the subgrade because of the low traffic load stresses in that layer.

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6.4.5

Design method for concrete block road pavements:

6.4.5.1

Rutting calculations:

To enable the development of design graphs for concrete block pavements, subjected to normal road traffic, rutting calculations by means of the ‘progressive stiffening theory’ have been performed for 84 different concrete block pavements. The ‘progressive stiffening’ coefficients, used in these calculations, were based on the analyses of the rutting behaviour of the test pavements Alphen 1 and Rotterdam 1 to 6. Furthermore a ‘post-compaction factor’ was introduced in the calculations as it is impossible to optimally compact during construction a thin sand sub-base, and to a smaller extent the granular base (if any), on a very weak subgrade. For each of the 84 concrete block pavements the calculated development of rutting was described by means of equation 6.1. From this equation it follow for the life N of the concrete block pavements: 1

 RDc  b p  N =  a   p 

where: N =

(6.2)

RDc = ap, bp =

allowable number of equivalent 80 kN standard axle load repetitions per lane in the wheel track (channelised traffic) rut depth standard (taken as 15 mm, see 6.4.1) rutting coefficient with respect to the total pavement structure

6.4.5.2

Design graphs:

Figure 6.16 shows the design graph that was developed by the CROW Working Group D3 for concrete block pavements with a sand sub-base only. The dynamic elastic modulus of the subgrade (E0) ranges from 30 N/mm2 (peat) to 140 N/mm2 (sand), so the graph covers (nearly) all the Dutch subgrades. It appears from figure 6.16 that the life of a concrete block pavement with only a sand sub-base is mainly determined by the subgrade modulus E0 and, at low E0-values, also by the thickness of the sand sub-base. Figure 6.16 also contains a failure curve. This curve gives the minimum sand sub-base thickness to prevent failure (i.e. shear failure in the sand sub-base) of the concrete block pavement under normal road traffic. Five design graphs were developed for concrete block pavements with an unbound (granular) base of concrete or mix granulate and a sand sub-base on a subgrade with a modulus E0 of 30, 40, 60, 100 and 140 N/mm2, again to cover (almost) all Dutch subgrades. Figure 6.17 gives two examples of these design graphs. Figure 6.17 demonstrates that the life of a concrete block pavement in the case of a high E0-value (for instance 140 N/mm2) is dependent on the thickness and the material of the unbound base. In the case of a low E0-value (for instance 40 N/mm2) the pavement life is also dependent on the thickness of the sand sub-base. The design graphs for concrete block pavements with an unbound base do not contain a failure curve because shear failure in such pavement structures under normal traffic loadings is very unlikely. 203

Figure 6.16: Design graph for concrete block road pavements consisting of rectangular concrete paving blocks (cobble format, thickness ≥ 80 mm) in herringbone bond, 50 mm crushed sand bedding layer and a sand sub-base (10,11).

Figure 6.17: Design graph for concrete block road pavements consisting of rectangular concrete paving blocks (cobble format, thickness ≥ 80 mm) in herringbone bond, 50 mm crushed sand bedding layer, an unbound base and a sand sub-base (10,11).

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It should be realised that the design graphs, shown in figure 6.16 and figure 6.17, do not take into account the frost penetration depth criterion. In the case of a frostsusceptible subgrade the total thickness of the pavement structure (concrete block toplayer + bedding sand layer + unbound base (if any) + sand sub-base) should be at least about 0.7 m (western parts of The Netherlands) and about 1.0 m (northern and eastern parts of The Netherlands) respectively to prevent frost- and thaw-damage. 6.4.5.3

Design examples:

To illustrate the use of the design graphs, presented in the proceeding paragraph, in table 6.6 some design examples are given, i.e. for N=4*104 (lightly loaded pavement, for instance a residential street) and for N= 2.5*106 (heavily loaded pavement, for instance road in industrial area). In the examples it is assumed that the total thickness of the unbound base and the sand sub-base should be minimal (to limit the construction costs). Subgrade modulus E0 (N/mm2) Design traffic loading N Pavement structure Concrete paving blocks (m) Bedding layer, crushed sand (m) Base, concrete granulate (m) Base, mix granulate (m) Sand sub-base Subgrade modulus E0 (N/mm2) Design traffic loading N Pavement structure Concrete paving blocks (m) Bedding layer, crushed sand (m) Base, concrete granulate (m) Base, mix granulate (m) Sand sub-base

1)

2) 3) 4)

5)

S

4*104 CCS

0.08 0.05 0.90

0.08 0.05 0.104) 0.405)

S

4*104 CCS

0.08 0.05 0

0.08 0.05 0.104) 0

40

2.5*106 CCS CBS2)

CBS

S2)

0.08

-

0.08 0.05 0.30 1.10

-

CBS

S3)

2.5*106 CCS

CBS

0.08 0.05 0.10 0

0

0.08 0.05 0.13 0

0.08 0.05 0.154) 0

140

S = sand sub-base only CCS = concrete granulate base and sand sub-base CBS = mix granulate base and sand sub-base This pavement structure is not possible for the combination of E0 = 40 N/mm2 and N = 2.5*106 This pavement structure is not possible for the combination of E0 = 140 N/mm2 and N = 2.5*106 This base thickness is a theoretical thickness; in practice the thickness of the unbound base material, mostly having a grading 0/40 mm, normally is 250 or 300 mm, which results in an overdesigned pavement structure The sand sub-base thickness should be greater than 0.40 m tot prevent frost- and thaw-damage

Table 6.6: Design examples for concrete block road pavements on a subgrade with a dynamic modulus of elasticity E0 = 40 N/mm2 and E0 = 140 N/mm2 respectivily.

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6.5

References:

1.

Houben, L.J.M. Structural Design of Pavements – Part V: Design of Small Element Pavements Lecture Notes CT4860, Faculty of Civil Engineering and Geosciences, TU Delft, Delft - 2003

2.

Shackel, B. Design and Construction of Interlocking Concrete Block Pavements Elsevier Applied Science, London - 1990

3.

Concrete Block Pavements (Concrete Paving Blocks) (in Dutch) Publication 44; S.C.W.; Arnhem – 1978 (since 1985 S.C.W. is part of CROW, Ede)

4.

Comparison of construction methods for concrete block paving (in Dutch) Publication 78; CROW; Ede - 1993

5.

Specifications for Road Construction 2000 (‘Standaard RAW Bepalingen 2000’) (in Dutch) CROW; Ede - 2000

6.

Specifications for Concrete Paving Blocks (in Dutch) NEN 7000; Nederlands Normalisatie-instituut; Delft - 1985

7.

Specifications for Concrete Tiles (in Dutch) NEN 7014; Nederlands Normalisatie-instituut; Delft - 1974

8.

Recycled Granular Materials (in Dutch) Publication 12; CROW; Ede - 1988

9.

Specifications for Concrete Edge Restraints (in Dutch) NEN 7015; Nederlands Normalisatie-instituut; Delft - 1972

10.

Manual for Design of Concrete Block Road Pavements (in Dutch) Publication 25; CROW; Ede - 1988

11.

Design of Concrete Block Pavements for Roads (in Dutch) Publication 42; CROW; Ede - 1991

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