NUROLF

Newcastle University

Rolling Load Facility

 

 

 

 

 

 

 

 

 

Report on:

The Performance of Pavers

for

Mechanical Installation

 

 

 

By:

John Knapton

Professor of Structural Engineering

University of Newcastle upon Tyne

Data: April 2000

 

 

 

 

EXECUTIVE SUMMARY

Four types of paver, referred to as I (rectangular), Y (Eskoo), L (Uni-coloc) and X (XeneX) have been the subject of NUROLF and other testing in order to compare their performance when subjected to heavy channelised traffic. All of the pavers can be installed mechanically. The pavers have been trafficked by 130,000 standard 8200kgf axles using the NUROLF vehicle at the Department of Civil Engineering, Newcastle University. Whilst the results show that all of the pavers are suitable for highway, industrial and other pavements, clear differences between them could prove an important factor in paver system choice. The maximum depth of rut resulting from the trial and the rate of development of rut were substantially lower for the Type X paver than for the other types. It is recognised that for most pavers such as Type I (rectangular), deformation in excess of 10mm can occur after trafficking in the order of 8 million standard axles (msa), requiring relaying. Extrapolation of rut development rates from the trial shows that a similar deformation should not occur until at least 12 msa for the Type X paver, suggesting a 50% increase in design life. For applications not subject to such high trafficking levels, less general deformation and rutting under channelised traffic should be experienced by the Type X paver.

Pull-out tests demonstrated the competence of all four types in resisting vertical point loads. However, the Type X paver developed a higher pull-out force than the others. It was also noted that the Type X paver provides a footprint area over three times that of Type I, without exhibiting any breakage problems during the NUROLF trial. Combining higher pull-out force with larger area suggests a significant advantage for the Type X paver in resisting point loads.

Although there were no discernible differences between the paver types in braking tests, differences in resistance to horizontal forces (braking and turning) would be expected for the four paver shapes and further trials on this are recommended.

Air flow tests indicated that those pavers which included right angles, either by virtue of the laying pattern or by virtue of the paver shape are more suited to those aircraft pavements subject to jet blast or propeller wash.

All of the test results indicate that the Type X (XeneX) pavers achieve significantly greater levels of stress attenuation and surface stability than the other units and remain more stable under loading.

INTRODUCTION

This Report sets out the findings of a research project sponsored by RMC Concrete Products (UK) Limited and XeneX International. The project involved full scale testing of a pavement surfaced with equal areas of the following four paver designs:

The test area was divided into four zones, each of size 2.5m x 2.5m and each differed only in respect of the type of pavers used. The four paver types were:

X - Xenex (Fig. 1)

I - 200mm x 100mm rectangular pavers (Fig. 2)

Y - Eskoo pavers (Fig. 3)

L - Uni-Coloc pavers (Fig. 4)

The purpose of the test was to compare the performance of four paver designs commonly used for mechanised installation. Mechanised installation is of growing importance and indeed is mandatory for reasons of Health and Safety in some markets. It is generally used on larger projects subjected to heavy loading and frequent trafficking. Previous research has concluded that L - shaped pavers offered enhanced structural contribution as compared with conventional pavers of similar "dentatedness" (the term "dentated" is used by some to define the degree by which the protrusions in one paver penetrate the recesses of its neighbours).

 

 

 

 

METHODOLOGY

For each of the trial sections, the pavement comprised:

80mm pavers (X, I, Y or L)

30mm laying course material

150mm lean concrete roadbase material

250mm DTp Type 1 granular sub-base material (local dolomitic limestone or "dolomite")

Each of the four paver types was subjected to NUROLF channelised full scale wheel loading up to 130,000 cumulative standard axles (Test 1). In order to simulate the most adverse combination of traffic and climate, the test area was maintained in a saturated condition throughout the whole of the testing. This was achieved by a sprinkler system which was activated before and during all of the testing. Prior to commencing the tests, the sprinkler system was activated for 30 minutes in order to ensure that the initial trafficking took place over saturated pavers. This test was conducted in the Cassie Building at the University of Newcastle upon Tyne at an ambient temperature of 20oC. The testing was carried out between 21st November and 8th February 2000 and was supervised by Prof. J. Knapton.

Additional testing included a series of dynamic tests in which the test wheel was brought to a sudden stop in order to assess the effect of severe braking on longitudinal paver displacement (Test 2). Further tests comprised the extraction of individual pavers from the test panels in order to measure the "pull-out" force (Test 3). A fourth set of tests examined the ability of the pavers to attenuate the effect of jet blast and propeller wash (Test 4).

All of these tests have allowed the suitability of the pavers tested to be assessed in relation to heavy traffic, point loading, horizontal forces, high speed traffic and channelised traffic.

SPECIFIC DETAILS OF NUROLF TESTS (Test 1)

The testing was conducted over the pavement specification as set out in the Introduction. The pavers were laid to bond patterns as shown in Figs 1 to 4 with the straight lines of rectangular units separating each panel. The laying course material comprised an artificial aggregate manufactured from steel slag complying with Category 2 of BS7533:Part 2.

The same material was introduced into the joints and a second joint filling operation was carried out to ensure that the joints were fully filled. Care was taken to ensure that the joints in all of the panels were between 2.5mm and 3mm in order to simulate well-laid units on a typical installation. The material was introduced into the joints by repeatedly brushing and vibrating. An initial wetting operation was undertaken in order to wash the jointing material fully down the paver joints. A further application of jointing material was applied. Mortar infill was used around the perimeter of the test site in order to ensure that edge effects were minimised.

The laying course material was placed over a lean concrete roadbase of thickness 150mm. The lean concrete was hand laid and compacted using a steel drum vibrating roller according to Clause 802 of DTp. "Specification for Highway Works"

Prior to commencing measurements, 20 conditioning cycles were undertaken to ensure that the pavement was correctly bedded.

 

Details of NUROLFThe NUROLF test vehicle comprises a former gully emptying vehicle that has been adapted as follows :

1) engine replaced with 60 HP 3-phase electric motor

2) guide wheels constructed on each axle to achieve constant tracking

3) rear axle weight increased to apply axle load of 14000 kgf

4) power fed to motor by computer controlled inverter

5) guidance beam fitted with limit switch to achieve

acceleration/deceleration

6) inverter set to generate 0.1g acceleration/deceleration

7) electric motor drives rear axle.

During its load cycle, NUROLF applies a vertical wheel load of 7000 kgf through its offside wheel to the centre of the test site over a test length of 9m. It commences a cycle at one end of the site - labelled 9000mm on the results charts - and accelerates linearly over the full test site so that the load wheel has attained a speed of 2.3 m/s at the centre of the test site. It then decelerates towards the end of the test site. It undertakes a complete cycle in 53 seconds and in so doing applies a horizontal force of 700 kgf always in the same direction, to the pavement.

This combination of a vertical load of 7000 kgf and a horizontal load of 700 kgf relates closely to the heaviest loading to which a pavement is likely to be subjected. By comparison, the maximum non-steering axle load normally applied by a fully laden commercial vehicle is 9500 kgf, resulting in a wheel load of 4250 kgf.

BS 7533:1992 recommends that fully channelled loading should be equivalenced to normal wandering highway loading on a 3:1 ratio. By taking account of this factor and by applying the 3.75 power law relating axle load to pavement damage sustained, the number of standard 8000 kgf axles to which one pass of NUROLF equates is 17 i.e. each full cycle of NUROLF inflicts damage onto the pavement surface that would be inflicted by 34 standard axles. When working continuously, NUROLF achieves 1550 equivalent standard axles per hour. In this test, 130,000 equivalent standard axles were applied in 84 hours running.

The NUROLF facility is shown in Figure 5.

 

Details of loading regime

Initially, a conditioning run of 20 cycles was applied and the initial survey was undertaken. Subsequently, surveys were undertaken after 1600 c.s.a., 3,200 c.s.a., 6,400 c.s.a., 10,000 c.s.a., 15,000c.s.a., 20,000 c.s.a, 32,500 c.s.a., 48,500 c.s.a., 65,000 c.s.a., 100,000 c.s.a. and 130,000 c.s.a. (c.s.a. = number of standard 8000kg axles).

 

Details of pavement measurement and assessment

NUROLF provides three-dimensional views of the deformed surface of the test using the SURFER computer program . Both before running and subsequently, a survey of the paver surface is carried out using a set of 20 Linear Voltage Displacement Transducers (LVDT's) connected to a local computer controlled Data Acquisition System. The LVDT's are mounted at 100mm centres on a stiff beam which is positioned across the 2m width of the test site. The beam is positioned initially to survey a surface profile at a distance of 100mm from end zero of the test site. The 20 relative pavement heights are stored and the beam is moved a distance of 200mm along the test site, where a further set of heights is recorded. In this way, the whole of the test pavement is surveyed and 500 sets of X, Y and Z co-ordinates are stored locally on magnetic disk. The data is transferred to the processing computer and further load tests are carried out in parallel with downstream data processing.

The survey data is fed to the SURFER program which uses a Kriging routine to generate a representative surface from the survey data. This initial surface is stored on the processing computer for subsequent subtraction from later results so that a series of cumulative surface profiles can be viewed.

ADDITIONAL TESTS

The following regime of additional tests was undertaken:

Braking Tests - Test 2

Pull out tests - Test 3

Jet blast tests - Test 4

Test 2

Ten braking events were applied to each of the four test items in order to assess the surfacing systems in relation to horizontal forces. These tests demonsrated that all of the paver types tested can withstand severe braking without displacing longitudinally. In each test, the NUROLF vehicle was decelerated at 1g so the horizontal force applied was 7000kgf at the test wheel. A unique feature of NUROLF is the opportunity to undertake this type of loading.

Test 3

Once the trafficking tests were complete, pavers were extracted from the surface using a pull-out system. Pull-out forces greater than 20kN were recorded.

Test 4

In order to gauge the suitability of the pavers tested for aircraft pavements subjected to jet blast or propeller wash, an air flow test was previously conducted as follows. Pavers of the different shapes were subjected to air blast from an air line in order to determine whether the jointing material would be easily removed

 

CONCLUSIONS

NUROLF - Test 1

The testing comprised full scale trials in which the structural performance of four paver types has been compared. The use of NUROLF ensures that the loading regime is realistic in that all of the forces applied to pavers in a true highway pavement are correctly applied during the testing. By allowing the test vehicle to track the test site, the true behaviour of a highway surfacing material in a trafficked highway is replicated. Also, the test wheel is driven through the truck's differential such that the horizontal and vertical forces normally applied to a highway surface are truly modelled.

The results are produced in terms of three dimensional graphs which show the resulting rut in magnified form in the Appendix. The graphs show that the deformation of each test item was within the deformations which would be expected in a correctly designed pavement. The maximum values are shown in Table 1.

 

Table 1 - Maximum rut depths developed in the NUROLF tests

Number of Cumulative Standard Axles

Deflexion (mm) for paver type

Type X

Type I

Type Y

Type L

1600

0.1

0.1

0.1

0.1

3200

0.1

0.2

0.3

0.2

6400

0.2

0.3

0.3

0.3

10,000

0.2

0.4

0.4

0.3

15,000

0.3

0.5

0.6

0.5

20,000

0.4

0.6

0.6

0.6

32,500

0.5

0.8

0.8

0.8

48,500

0.8

1.2

1.1

1.2

65,000

1.4

1.8

1.9

1.6

100,000

2.0

2.7

2.5

2.6

130,000

2.5

3.3

3.2

3.1

 

Table 2 shows the rate of development of the rutting - the amount of rut generated by each 30,000 standard axles. It shows how paver Type X maintains the pavement in a less rutted state than the other Types.

Table 2. - Rate of rut development in NUROLF testing

Paver type

Characteristic maximum rut depth

Approximate rate of rut development (mm per 30,000 standard axles)

I

3.3

0.9

Y

3.2

0.9

L

3.1

0.8

X

2.5

0.6

 

The Figures in the Appendix show the deformation of the test area at different levels of trafficking. The pavers were installed in almost square test items along the test zone as follows:

L - Type (Uni-coloc) zero to 2250mm

Y - Type (Eskoo) 2250mm to 4500mm

I - Type (Rectangular) 4500mm to 6750mm

X - Type (XeneX) 6750mm to 9000mm

At each survey, the figures show a rut with a corresponding heave of the bedding material to each side of the rut. This indicates that the rut has developed as a result of instability in the bedding material rather than compaction or consolidation of that material. Had compaction/consolidation occurred, there would have been no heave beside the rut. The figures show occasional peaks which then disappear at the next survey. These are grains of jointing material which were washed away from the joints during the trafficking. It is preferable to leave these grains in place rather than sweep them away prior to taking readings. Sweeping them away could have caused disturbance of the pavers which would have invalidated the results.

The results show that the four paver types have developed different levels of deformation with the Type X units having performed best and the others having developed greater deformations.

All of the deformation has occurred in the laying course material. The base has been designed to ensure that it remained undeformed, yet allowed sufficient elastic deflexion for the pavers to demonstrate their interlocking properties. The permanent deformation has occurred in the laying course material. The benefit of this approach is in taking out of the experiment the inevitable differences in strength within the base. Such differences would have swamped the differences in the paver characteristics.

The deformations as measured are all a result of the laying course material having either degraded or migrated. Such behaviour is entirely dependent upon the stresses imposed upon it. Therefore, the deformations recorded are a measure of the stress attenuation achieved by each of the four test items.

Test 2 - Braking

The sudden braking tests were undertaken to compare the four paver types in situations where severe horizontal loads are applied to the pavement, such as in braking and cornering. Ten braking events were applied on each of the four types of paver and it was observed that in each case there was no permanent movement. This finding is in line with common paver experience which suggests that well designed and installed paving systems can withstand horizontal loading.

Test 3 - Pull-out

Figure 6 shows the pull-out test equipment which allows an individual paver to be extracted from the pavement and for the force and displacement of the paver to be measured. Two XeneX pavers, two rectangular pavers and one of each of the other two paver types were extracted and the results are shown in Table 3. In order to ensure that neighbouring pavers did not rotate during the extraction process and thereby grip the paver being extracted, the test equipment applied its reaction load directly onto neighbouring pavers.

Figure 7 shows both the maximum pull-out force and the relationship between load and displacement. Although the maximum pull-out force is often referred to by others, it is of less interest than the relationship between pull-out force and displacement at low levels of displacement. Table 1 shows the pull-out force in kN at a displacement of 1.5mm. This is considered to be a more relevant figure since it is the deflexion at which initial loss of interlock occurred and is closer to the surface elastic displacements which occur in service in a highway or heavy duty pavement.

It is important to understand that the maximum pull-out force recorded for each paver is not the critical value. The maximum forces occur at deflexions which never occur in an in-service pavement. Rather, the important figures are those which occur at working deflexions. At working deflexions (typically 1mm in a heavy duty pavement), Figure 7 shows that a sudden change occurs when each test shows a reduction in pull-out force. After that, the force continues to rise. At the 1mm deflexion, the sudden reduction is a result of interlock being lost and the paver rotating. The rotated paver then regrips its neighbours and the increase in force is a result of the slipped paver raising its neighbours. For this reason, Table 1 refers to the force at initial slip.

Figure 7. Results of Pull-out testing

 

Typically, when extracted, a paver displays a linear load/displacement relationship until the load attains approximately 12kN at a deflexion of 1.5mm. At that load, a slip occurs as interlock is lost. The paver rotates then grips its neighbours so that it continues to sustain pull-out force until eventually the paver is damaged. In the case of the two Type X pavers, initial slip occurred at loads of 14kN and 13.4kN. The two Type I pavers lost interlock at 11.6kN and 12.8kN, the Type Y paver lost interlock at 9.3kN and the Type L lost interlock at 11.6kN. These figures are shown in Table 3.

 

 

 

 

 

 

Table 3. Pull-out forces achieved prior to slip.

Paver Type

Pull-out force (kN)

Type X - A

14.0

Type X - B

13.4

Type I - A

11.6

Type I - B

12.8

Type L

11.6

Type Y

9.3

 

 

From the above, it can be concluded that each XeneX is more firmly held in place than the other pavers. This is partly a result of its greater weight, partly a result of its larger perimeter and partly a result its shape. The effect of its additional weight is small in comparison with the other two effects. Interestingly, XeneX is the only "shaped" paver which displayed enhanced pull-out results as compared with rectangular pavers. Figures 8, 9, 10 and 11 show the way in which the pavers failed ultimately in the pull-out test regime.

The pull-out tests show that XeneX pavers have enhanced resistance to vertical loads than the other pavers. Typically, an additional force of several kN is required to extract XeneX as compared with the other pavers.

Test 4 - Jet blast/Propeller wash

In 1994, the Author undertook a series of trials to evaluate the performance of pavers when subjected to jet blast. Several paver types were installed in test panels and were subjected to high pressure air flow from the surface. Two factors were found to assist in retaining unstabilized sand in paver joints. The first was joint spacing: it was found that when joint width exceeded 2mm, sand could be removed from paver joints relatively easily, whereas at 2mm and narrower, joints held sand effectively. This is because jointing materials usually have a significant proportion of their grains coarser than 2mm and these grains become wedged tightly between the paver walls. The other factor which assisted in inhibiting the removal of jointing sand was the creation of 90 degree turns in the paver pattern. The two successful way of achieving this were firstly to install rectangular pavers to herringbone pattern and secondly to install Type X pavers in the pattern as tested with NUROLF. In both of these cases, the 90 degree turn effectively arrested air flow so there was little loss of jointing sand. It was concluded that all paver pavements subject to jet blast/propeller wash should be surfaced by either rectangular pavers installed to a herringbone pattern of by XeneX pavers installed as in the NUROLF tests.

All of these results indicate that the Type X pavers achieve significantly greater levels of stress attenuation and surface stability than the other units and remain more stable under loading.

ANALYSIS

Based upon the above results and discussion, the performance of the pavers tested is now evaluated. It is known that the I-shaped paver and those derived from it are suitable for highway, industrial and aircraft pavements. However, whilst a concrete block pavement construction is known to be able to withstand over 12 million standard 8200kg axles, there is some concern that the pavers and their bedding material might need to be reset at least once during the design life of the pavement. BS7533:1992. "Guide for Structural Design of Pavements Constructed with Clay or Concrete Block Pavers", Clause 4.2 states: "It may be necessary to reset the pavers during the life of a pavement if the rut depth exceeds 10mm." Experience shows that for rectangular block paving (i.e. Type I) this generally occurs at about 8 million standard axles (msa). The rate of rut development starts to reduce at 0.5 msa and continues to reduce with trafficking - in contrast with asphalt, which increases. Rut development can be interpolated against that experienced at approximately 8 msa in the case of rectangle (Type I pavers) to demonstrate that the Type X paver would not reach a 10mm rut depth until at least 12 msa.

From the NUROLF test results, it can be seen that the rut developed in the Type X pavers is shallower than that developed by the other pavers. In the test, Type X pavers developed ruts at an approximate rate of 0.5mm - 0.6mm per 30,000 standard 8,200kgf axles whilst the others developed ruts at 0.8 - 0.9mm per 30,000 standard axles. Experience indicates that pavements surfaced with pavers develop their ruts early in their life, then remain at their rutted shape. In the case of Type X, it would be expected that they would achieve the 10mm deflexion at up to 12 million standard axles, whereas, the figure is frequently achieved in 8 million standard axles with conventional pavers.

Figure 2 of BS7533:1992 is the new pavement design chart and is reproduced here.

From this, it can be concluded that when XeneX pavers are used, a pavement should be able to achieve the full design life of 12 million standard axles, but when other types are used, it may be necessary to reset them after 8 million standard axles.

As a result of these trials, it is concluded that all of the pavers tested are suitable for pavements where machine laying is typically used. However, the Type X (XeneX) paver can be seen to offer a significant increase in design life over the other three paver types (50% more than rectangular).

In addition to pavement design criteria, characteristics of the Type X paver revealed by the tests suggest that it can be considered as a preferred option in particular applications when taking a total quality approach to pavement specification, as proposed by the author in a recent paper. Applications and conditions identified from the author's Total Quality Design Chart where Type X could be considered the preferred option include:

Traffic:

Container handling port

Heavy duty industrial

1,000,000 - 10,000,000 csa (heavier trafficking)

Aircraft pavements

Environmental:

Slopes>10% (subject to further trials on resistance to horizontal forces)

Downdraft exhausts

Bus stops

Tight turning (subject to further trials on resistance to horizontal forces)

Maintenance access difficult

More channelised than normal highway

Finally, characteristics revealed of the Type X paver, and to a lesser extent of the other three pavers suggest that the stability of a well designed, accurately installed block paved pavement could allow use of the technology for new applications such as for higher speed traffic.

Click here to see NUROLF results

Click here to see photographs