PERSONAL DEL INSTITUTO

Tolerances in surface levels of pavement courses are specified in the Highways Agency Specification for Highway Works (SHW), Series 700. The standard Clause 702 contains a number of tolerances which allow the contractor to complete the construction of the pavement within a variation in thickness. The section 700 tolerances can be interpreted to suggest that the following maximum and minimum layer thicknesses could remain within a contract compliant pavement, see Table 5.4.

Table 5.4 Tolerances in surface levels of pavement courses (SHW, Series 700)

Layer Tolerances

Surface course ± 6mm

Binder Course ± 6mm

Base ± 15mm

Total asphalt pavement -15mm

Sub-base +10mm

-30mm

Capping +20mm

-30mm

Construction tolerances are already taken into account in the standards’ design charts relative to the pavement thickness. Construction tolerances are particularly important when a pavement analytical design is carried out. Although design thickness proposals derived by analytical design should make allowance for construction tolerances, experience suggests that sometimes this is not the case. The analytical design method, if not conducted correctly, could lead to the design of pavement structures that are not sufficiently thick to withstand the design traffic loading. For example, if the result of an analytical design is a binder course 100mm thick, the same design should be checked for a 94mm binder course thickness to account for the construction tolerance expected for that layer (±6mm).

This study aims to see how variable the layer thickness is when a pavement is built. A review of the tolerances aims to show how well/badly built a pavement is. Sections of roads that are built thinner than the design thickness are more

likely to be weak and to deteriorate quicker when compared to pavements built within the correct construction tolerances.

The following section explains the relationship between the standard deviation of the asphalt and subbase layer thickness and the construction tolerance, for a 3-layer pavement structure (i.e., asphalt on top of subbase on top of subgrade). The calculations assumed that the layer thickness could be approximated by the normal distribution.

The Lower Specification Limit for the total asphalt layer thickness is the designed minimum thickness allowed, below which only a small percentage of the pavement is acceptable. The upper tolerance for the total asphalt thickness is not subject to any restriction although an excessive thickness is not expected.

The percentage of the constructed asphalt layer above the designed minimum thickness is , where  is determined from the standard normal distribution tables as follows (see Figure 5.5):

(5.1)

Where:

is the mean thickness of the constructed asphalt layer

is the designed minimum thickness, i.e. the design thickness minus 15mm

is the standard deviation of the asphalt layer.

Assuming independence between the asphalt and subbase surface profiles, the standard deviation of the asphalt thickness is estimated as follows:

(5.2)

where and are the standard deviations of the asphalt surface level departures and subbase surface level departures respectively. Surface level departures measure the difference between the constructed height of the

pavement and the design height of the pavement, in other words measure the error of the construction process. According to Bissell (as cited in Ollis 1997), when a process is under control, statistical theory suggests that the errors are randomly and normally distributed about a mean value. Therefore, the surface level departures are assumed to be error measurements and normally distributed (Ollis 1997).

Figure 5.5 Lower tolerance range for total asphalt layer thickness

If the asphalt mean was exactly on target (i.e. the design value), the maximum value of the standard deviation, for which the probability of a thickness measurement – for the total asphalt thickness layer – being above the standards’ requirement is, for example, 95%, can be calculated with eq. (5.1) as follows: P (x ≥ LSLa) = 0.95 Z = -1.65; and = 9.11mm -8 -6 -4 -2 0 2 4 6 8 0 5 1 5 2 _ca _ta LSLa  

Figure 5.6 Upper and lower tolerance range for sub-base layer thickness

Similarly, the percentage of the constructed subbase layer between the upper and lower specification limits is %, where  is determined from the standard normal distribution tables as follows (see Figure 5.6):

(5.3)

Where:

is the mean thickness of the constructed subbase layer is the lower specification limit for the subbase layer. is the standard deviation of the subbase layer.

Assuming independence between the subgrade and subbase surface profiles, the standard deviation of the subbase thickness is estimated as follows:

(5.4)

where and are the standard deviations of the subgrade surface level departures and subbase surface level departures respectively. If the subbase mean was exactly on target (i.e. the design value), the maximum value of the standard deviation, for which the probability of a thickness measurement – for the subbase thickness layer – being between the standards’ requirement is, for example, 95%, was calculated with eq. (5.3) as follows:

-8 -6 -4 -2 0 2 4 6 8 0 5 .1 5 .2 _csb USLsb LSLsb _tsb

P (LSLsb<x<USLsb) = 0.95

After a few iterations, by selecting = 6.08mm, then: P (-4.94<Z<1.65) = 0.4999+0.4505 ~ 0.95

Appendix A3 provides an example of construction tolerances applied to the available sites. Only for motorway M01 (the only new build site available) the design thickness is known and, therefore, only for motorway M01 is it possible to see how well or badly the construction tolerances were applied. For M01 site the design thickness is 0.260m for the asphalt layer and 0.150m for the sub-base layer. Table 5.5 shows the results of applying eq. (5.1) and (5.3) to the sections of the M01 site. Graphs in Appendix A3 show the mean, lower and upper limits for both the design and as-built asphalt and sub-base layer thickness. It can be noted that the asphalt layer thickness is generally more controlled compared to the subbase layer thickness. All sections appear to show a large number of points relative to the subbase thickness which are well outside the permitted construction tolerances for that layer (+10mm -30mm). Graphs relative to the other sites, which have all been overlaid in the past, are shown to illustrate the spread of the data when compared to new build sites. Table 5.5 Review of construction tolerances for site M01, assuming 95% confidence

Lane Asphalt tolerances Subbase tolerances

m ean (m ) stdev (m ) COV (%) m ean (m ) stdev (m ) COV (%) P(Ta>0.245m ) P(0.120m <Tsb<0.160m )

L1 CW A 0.279 0.010 3.4 0.171 0.036 20.8 100% 31% L1 CW B 0.292 0.015 5.0 0.112 0.038 33.8 100% 31% L1 CW D 0.268 0.010 3.5 0.133 0.015 11.6 99% 76% L1 CW E 0.274 0.009 3.1 0.123 0.016 12.6 100% 58% L1 CW F 0.274 0.008 3.0 0.130 0.016 12.0 100% 72% L1 CW G 0.276 0.012 4.2 0.122 0.029 24.0 100% 43% L1 CW H 0.266 0.016 6.1 0.133 0.020 14.9 90% 66% L1 ACW A 0.278 0.008 2.9 0.164 0.026 16.0 100% 39% L1 ACW B 0.276 0.008 2.9 0.129 0.014 10.5 100% 73% L1 ACW D 0.280 0.012 4.3 0.157 0.025 16.0 100% 48% L1 ACW E 0.274 0.009 3.2 0.133 0.018 13.3 100% 70% L1 ACW F 0.269 0.011 4.1 0.129 0.018 13.6 98% 65% L1 ACW G 0.271 0.011 4.1 0.146 0.017 11.8 99% 72% L1 ACW H 0.268 0.011 3.9 0.132 0.016 12.2 99% 73% C/w ay Direction