Table I.8: Classification of road pavements and traffic loading class for structural design purposes (TRH4)
Pavement Traffic Loading Class (TLC) i.t.o. number of Equivalent Standard (ES) 80 kN axle loads Bearing capacity (million 80 kN axles/lane) Typical traffic volumes and type of traffic
Approximate vehicles per day per lane (vpdpl) Description
ES0.003 < 0.003 < 3 Very lightly trafficked roads/ streets with very few heavy vehicles. Includes roads/ streets transitioning from gravel to paved roads and may incorporate semi-permanent and/or all-weather surfacing layers.
ES0.01 0.003 - 0.01 3 - 10
ES0.03 0.01 - 0.03 10 - 20
ES0.1 0.03 - 0.1 20 - 75
ES0.3 0.1 - 0.3 75 - 220
ES1 0.3 - 1 220 - 700 Lightly trafficked roads/ streets carrying mainly cars, light delivery and agricultural vehicles with very few heavy vehicles.
ES3 1 - 3 > 700 Medium volume, few heavy vehicles.
ES10 3 - 10 > 700 High volume and/or many heavy vehicles.
ES30 10 - 30 > 2 200 Very high volume of traffic and/or a high proportion of fully laden heavy vehicles
ES100 30-100 > 6 500

It is important to note that road pavement design methods are moving away from using load equivalency and standard axle loads by incorporating the axle load histogram in the design analysis, using an incremental damage approach. The simplest form of this utilises Miner’s Law with the current failure criteria. The revised South African Road Design System (SARDS)19 will incorporate the full traffic spectrum, in which case the E80 may not be used any longer for pavement design.

I.4.2.3.3 Traffic measurement and vehicle classification

(i) Traffic measurement

Traffic counts and classification are done manually or are automated using traffic counting stations installed on the road. Traffic counting is performed on a special or project basis (short-term), or on a temporary (medium-term) to permanent (long-term) basis by road authorities, as part of their road management system data collection strategy. While permanent stations provide a continuous traffic record from one year to the next, temporary stations are used on a sampling or periodic basis to collect data over a specified time period.

(ii) Vehicle classification

The appropriate vehicle classification system depends on the traffic data required and the capabilities of the traffic monitoring equipment. Heavy axle loads, associated with heavy vehicles, do most of the damage on pavements.

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For pavement design, traffic should therefore be split between light and heavy vehicles. Based on the length of the vehicle (extended vehicle classification system), heavy vehicles can be classified into three groups that are commonly used for the estimation of pavement design traffic:

  • Short heavy vehicle (S): length < 10.8 m
  • Medium heavy vehicle (M): length between 10.8 and 16.8 m
  • Long heavy vehicle (L): length > 16.8 m

 

An incorrect design traffic estimate results in the same design risk as a pavement structure of inadequate structural capacity. Traffic loading information for pavement design may be obtained from the following sources or methods: Tabulated average E80 values, published results of surveys, transportation planning models, and estimation procedures based on visual observations and project-specific traffic surveys. The level of effort and cost associated with these methods increases from the use of known results to the project specific surveys, but so does the value of the results obtained. The application of each of the above sources or methods is linked to the different categories of road/street, as recommended in Table I.9.

Table I.9: Recommended traffic investigation levels for road functional class/category
Road functional class/ category Traffic parameter      
Base year HV volume HV volume growth rate Base year E80/ HV E80/HV growth
U2-B, U3-B Traffic surveys1 Transportation
models
Published results
Traffic surveys Published results
U4-C Visual observation2 Published results Visual observation Published results
U5-D, U6-E Published results3 Published results Published results Published results

1 Project-specific traffic survey
2 Project-specific visual observation and tabulated values
3 Published results, or results from other projects with similar traffic characteristics

I.4.2.3.5 Calculation of design E80

The detailed computation of the design E80 or cumulative equivalent traffic over the structural design period involves the load equivalency of the road/street traffic, surveys of road traffic conditions, projecting the road traffic data over the structural design period and estimating the road lane distribution.

(i) Load equivalency of traffic

The Load Equivalent Factor (LEF) used to calculate the E80 relates the application of any given axle load to the equivalent damage caused relative to the standard axle, which is taken as 80 kN. The LEF is calculated using Equation I.1, as described in Section I.4.2.3.1. Table I.10 provides average equivalency factors based on Equation I.1, with n = 4.

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Table I.10: Average Single 80 kN axle equivalency factors, derived from Fave = (P/80)n, with n = 4
Single axle load, P* (kN) Average 80 kN axle equivalency factor, F** Single axle load, P (kN) Average 80 kN axle equivalency factor, Fave
< 15 0.005 115 - 124 5.021
15 -24 0.005 125 - 134 6.916
25 – 34 0.021 135 - 144 9.303
35 – 44 0.064 145 - 154 12.262
45 – 54 0.154 155 - 164 15.876
55 - 64 0.317 165 - 174 20.237
65 - 74 0.584 175 - 184 25.441
75 - 84 0.994 185 - 194 31.590
85 - 94 1.590 195 - 204 38.791
95 - 104 2.422 >205 43.118
105 - 114 3.545    

*     Single axle load with dual wheels
*     Average is based on LEF = ((Plower limit/80)n + (Pupper limit /80)n)/2

The Average Daily Equivalent (ADE) traffic can be determined by multiplying the number of axle loads (tj) in each load group in the entire load spectrum by the relevant load equivalency factor (LEFj).

By summation, the average daily equivalent traffic is calculated using Equation I.2.

 

ADE = ∑tj . Fj Eq I.2

A sensitivity analysis should be conducted on the spectrum of loads with n-values as indicated in Table I.7, especially when dealing with abnormal load spectra.

(ii) Surveys of traffic conditions

The present average daily traffic is the amount of daily traffic in a single direction, averaged over the present year. This traffic can be estimated from traffic surveys carried out at some time before the initial year. Such a survey may include static weighing of a sample of vehicles, dynamic weighing of all axles for a sample period (e.g. weigh-inmotion (WIM) survey), or estimation procedures based on visual observation (Table I.11 can be used to assist in this.)

Table I.11: Recommended E80/axle loading for visual observation technique
Description of Heavy Vehicle Loading Percentage of Vehicles Axle Load Factors
(E80/axle)
Fully Laden (%) Empty or Partially
Laden (%)
Predominantly lightly laden vehicles < 35 > 45 0.3
Fully laden, partially laden and empty vehicles 40-45 34-45 0.5
Fully and partially laden vehicles 60 - 75 < 30 0.7
Predominantly fully laden vehicles > 70   0.9

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(iii) Projection of the traffic data over the structural design period

Projection to initial design year:

The present average daily equivalent traffic (daily E80s) can be projected to the initial design year by multiplying by a growth factor determined from the growth rate:

gx = (1 = 0,01 × i)x Eq I.3
Where:  
g = growth factor
i = growth rate (%)
x = time between determination of axle load data and opening of streets in years

The traffic growth factor (g) is given in Table I.12.

Table I.12: Traffic growth factor (g) for calculation of future or initial traffic from present traffic
Time between
determination of
axle load data and
opening of road,
x (years)
*g for traffic increase, i(%per annum)
2 3 4 5 6 7 8 9 10
1 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10
2 1.04 1.06 1.08 1.10 1.12 1.14 1.17 1.19 1.21
3 1.06 1.09 1.12 1.16 1.19 1.23 1.26 1.30 1.33
4 1.08 1.13 1.17 1.22 1.26 1.31 1.36 1.41 1.46
5 1.10 1.16 1.22 1.28 1.34 1.40 1.47 1.54 1.61
6 1.13 1.19 1.27 1.34 1.42 1.50 1.59 1.68 1.77
7 1.15 1.23 1.32 1.41 1.50 1.61 1.71 1.83 1.95
8 1.17 1.27 1.37 1.48 1.59 1.72 1.85 1.99 2.14
9 1.20 1.30 1.42 1.55 1.69 1.84 2.00 2.17 2.36
10 1.22 1.34 1.48 1.63 1.79 1.97 2.16 2.37 2.59

*g = (1 + 0.01 × i)x

Computation of cumulative equivalent traffic:

The cumulative equivalent traffic (total E80s) over the structural design period may be calculated from the equivalent traffic in the initial design year and the growth rate for the design period. Where possible, the growth rate should be based on specific information. More than one growth rate may apply over the design period. There may also be a difference between the growth rates for total and equivalent traffic. These rates will normally vary between 2% and 10%, and a value of 6% is recommended.

The Annual Average Daily Equivalent (AADE) traffic in the initial year is given by

AADEinitial = ADE × gx Eq I.4

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The cumulative equivalent traffic may be calculated from

Ne = AADEinitialxfy Eq I.5
Where
fy = cumulative growth factor, based on
fy = 365 (1 + 0.01 × i)[(1 + 0.01 × i)y-1]/(0.01 × i) Eq I.6
(y= structural design prediction period)

The cumulative growth factor (fy) is given in Table I.13.

Table I.13: Traffic growth factor (fy) for calculation of cumulative traffic over prediction period from initial (daily)traffic
Prediction
period,
y (years)
Compound growth rate, i (% per annum)*
2 4 6 8 10 12 14 16 18 20
4 1 534 1 611 1 692 1 776 1 863 1 953 2 047 2 145 2 246 2 351
5 1 937 2 056 2 180 2 312 2 451 2 597 2 750 2 911 3 081 3 259
6 2 348 2 517 2 698 2 891 3 097 3 317 3 551 3 801 4 066 4 349
7 2 767 2 998 3 247 3 517 3 809 4 124 4 464 4 832 5 229 5 657
8 3 195 3 497 3 829 4 192 4 591 5 028 5 506 6 029 6 601 7 226
9 3 631 4 017 4 445 4 922 5 452 6 040 6 693 7 417 8 220 9 109
10 4 076 4 557 5 099 5 710 6 398 7 173 8 046 9 027 10 130 11 369
11 4 530 5 119 5 792 6 561 7 440 8 443 9 588 10 895 12 384 14 081
12 4 993 5 703 6 526 7 480 8 585 9 865 11 347 13 061 15 044 17 336
13 5 465 6 311 7 305 8 473 9 845 11 458 13 352 15 575 18 183 21 241
14 5 947 6 943 8 130 9 545 11 231 13 242 15 637 18 490 21 887 25 927
15 6 438 7 600 9 005 10 703 12 756 15 239 18 242 21 872 26 257 31 551
16 6 939 8 284 9 932 11 953 14 433 17 477 21 212 25 795 31 414 38 299
17 7 450 8 995 10 915 13 304 16 278 19 983 24 598 30 346 37 500 46 397
18 7 971 9 734 11 957 14 762 18 308 22 790 28 458 35 625 44 680 56 115
19 8 503 10 503 13 061 16 338 20 540 25 934 32 859 41 748 53 154 67 776
20 9 045 11 303 14 232 18 039 22 995 29 455 37 875 48 851 63 152 81 769
25 11 924 15 808 21 227 28 818 39 486 54 506 75 676 105 517 147 559 206 727
30 15 103 21 289 30 587 44 656 66 044 98 656 148 459 224 533 340 661 517 664
35 18 612 27 858 43 114 67 927 108 816 176 464 288 595 474 509 782 431 1 291 373
40 22 487 36 071 59 877 102 120 177 700 313 586 588 416 999 544 1 793 095 3 216 609

* based on fy= 365 (1 + 0.01 × i)[(1 + 0.01 × i)y- 1]/(0.01 × i)

I.4.2.3.6 Estimating the lane distribution of traffic

On multi-lane roads, the traffic will be distributed among the lanes. Note that the distribution of total traffic and equivalent traffic will not be the same. The distribution will also change along the length of street, depending on geometric factors such as climbing or turning lanes. Suggested design factors of equivalent traffic (Be) are given in Table I.14. As far as possible, these factors incorporate the change in lane distribution over the geometric life of a facility. The factors should be regarded as maxima and decreases may be justified

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The design cumulative equivalent traffic:

The design cumulative equivalent traffic per lane (Ne) may be calculated by multiplying the equivalent traffic by a lane distribution factor (Be):

Ne = E80total = (∑tj × Fj) × gx × fy× Be Eq I.7
Where:
  ∑tj × Fj = equivalent daily traffic at time of survey
  gx = growth factor to initial year (x = period from traffic survey to initial design year)
  fy = cumulative growth factor over structural design period (y - structural design period)
  Be = lane distribution factor for equivalent traffic (see Table I.14)

To check the geometric capacity of the street, the total Annual Average Daily E80 (AADE) traffic towards the end of the structural design period can be calculated from

AADE = ADE × gx Eq I.8

with gx as previously defined.

When projecting traffic over the structural design period, the possibility should be kept in mind that capacity conditions may be reached, which would result in no further growth in traffic for that particular lane.

Table I.14: Design factors for the distribution of equivalent traffic (Be) among lanes and shoulders
Total number of lanes
in both directions
E80 Lane distribution factor
Surfaced slow shoulder Lane 1 Lane 2 Lane 3
2 1.00 1.00 - -
4 0.95 0.95 0.3 -
3 0.7 0.70 0.6 0.25

Note: Lane 1 is the outer or slow lane

I.4.2.3.7 Design traffic on unpaved roads

For the design of unpaved roads, only the average daily traffic is required, as performance is mostly a function of the total traffic, with the split between light and heavy vehicles being of little importance. This is the result of the traffic-induced deformation of properly designed unpaved streets being restricted to the upper portion of the gravel surfacing. Such problems can be rectified during routine surface maintenance involving grading and spot re-gravelling, or by re-gravelling of the road.

I.4.2.4 Material and pavement selection

Pavement types and pavement material options are discussed in Section I.3.3. When selecting a pavement type,it is important to understand that the structural behaviour of pavement types differ, especially with respect to the behaviour under loading, the load sensitivity of different pavements, and the pavement behaviour over the long term. A brief description of the typical structural behaviour of different pavement types is given below to guide decision making.

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I.4.2.4.1 Flexible pavements

The typical structural behaviours of the four types of flexible pavements commonly used in South Africa are discussed separately.

(i) Unbound granular base pavements

This type of pavement comprises a thin bituminous surfacing, a base of untreated gravel or crushed stone, a granular or cemented subbase and a subgrade of various soils or gravels. The mode of distress in a pavement with an untreated subbase is usually deformation, arising from shear or densification in the untreated materials.The deformation may manifest itself as rutting or as longitudinal roughness, eventually leading to cracking. This is illustrated in Figure I.5(a).

Figure I.5: Generalised structural pavement behaviour characteristics - granular base (a); bituminous base (b)

In pavements with cemented subbases, the subbase improves the load-carrying capacity of the pavement, but at some stage the subbase will crack under traffic. The cracking may propagate until the layer eventually exhibits properties similar to those of a natural granular material. It is unlikely that cracking will reflect to the surface, and there is likely to be little rutting or longitudinal deformation until after the subbase has cracked extensively. However, if the subbase exhibits large shrinkage or thermal cracks, they may reflect to the surface.

The post-cracked phase of a cement-treated subbase under granular and bituminous bases adds substantially to the useful life of the pavement. Elastic deflection measurements at various depths within the pavement have indicated that the initial effective modulus of this material is relatively high (3 000 to 5 000 MPa) as shown in Figure I.6(a). This relatively rigid subbase generally fatigues under traffic, or in some cases even under construction traffic, and assumes a lower effective modulus (800 to 1 000 MPa). This change in modulus does not normally result in a marked increase in permanent deformation, but the resilient deflection and radius of curvature (RoC) do change, as shown in Figure I.6(b).

In the mechanistic design approach (see Section I.4.2.6.1), these phases have been termed the pre-cracked and post-cracked phases. The design accommodates the changes in modulus of the subbase and, although the safety factor in the base will be reduced, it will still be well within acceptable limits.

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The eventual modulus of the cemented subbases will depend on the quality of the material originally stabilised, the cement agent, the effectiveness of the mixing process, the absolute density achieved, the durability of the stabilisation and the degree of cracking. The ingress of moisture can affect the modulus in the post-cracked phase significantly. In some cases the layer may behave like a good-quality granular material with a modulus of 200 to 500 MPa, but in other cases the modulus may be between 50 and 200 MPa. This change is shown diagrammatically in Figure I.6(a)

The result is that the modulus of the cemented subbase assumes very low values and this causes fatigue and high shear stresses in the base. Generally, surface cracking will occur and, with the ingress of water, there may be pumping from the subbase. Therefore, regular road inspection and maintenance should be executed.

For high-quality, heavily trafficked pavements, it is necessary to avoid materials that will eventually deteriorate to a very low effective layer modulus. Many of these lower-class materials have, however, proved to be adequate for lower classes of traffic.

The surfacing may also crack owing either to hardening of the binder as it ages or to load-associated fatigue cracking. The strength of granular materials is often susceptible to water, and excessive permanent deformation may occur when water enters through surface cracks. The water susceptibility of a material depends on factors such as grading, the plastic index (PI) of the fines, and density. Waterbound macadams are less susceptible to water than engineered crushed-stone base materials and are therefore preferred to be used in the wetter regions of the country

Figure I.6: Generalised structural pavement behaviour characteristics - Cemented-subbase modular behaviour(a); Cemented-subbbase indicators (b)

(ii) Hot mix asphalt base pavements

In hot mix asphalt (HMA) base pavements, both permanent deformation (or rutting) and fatigue cracking are possible. Two types of subbase are recommended, namely either an untreated granular subbase or a weakly stabilised cemented subbase. Rutting may originate in either the bituminous or the untreated layers, or in both. This is illustrated in Figure I.5(b). If the subbase is cemented, there is a probability that shrinkage or thermal cracking will reflect through the base to the surfacing, especially if the bituminous layer is less than 150 mm thick or if the subbase is excessively stabilised (~ >5 % cement). Maintenance usually consists of a surface treatment to provide better skid resistance and to seal small cracks, an asphalt overlay in cases where riding quality needs to be restored and when it is necessary to prolong the fatigue life of the base, or recycling of the base when further overlays are no longer adequate.

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(iii) Bitumen stabilised base pavements

Although the bituminous binders in emulsion and foam-treated materials are viscous, the material is stiff and brittle, much like a cement-treated material after curing. The initial field stiffness values of these materials may vary between 800 and 2 000 MPa, depending on the bitumen binder content and parent material quality. These values will gradually decrease with increasing traffic loading in these layers to values typical for granular materials roughly between 150 and 500 MPa.

Field performance of pavements with emulsion- and foam-treated base layers indicates that they are not as sensitive to overloading as a pavement with a cement-treated base, and do not pump fines from the subbase during wet conditions under traffic, which is the first mechanism and indication of the formation of potholes in road pavements.

(iv) Cemented base pavements

In these pavements, most of the traffic stresses are absorbed by the cemented layers and a little by the subgrade. It is likely that some block cracking will be evident very early in the life of the cemented bases; this is caused by the mechanism of drying shrinkage and by thermal stresses in the cemented layers. Traffic-induced cracking will cause the blocks to break up into relatively smaller ones. These cracks may propagate through to the surfacing. The ingress of water through the surface cracks may cause the blocks to rock under traffic, resulting in the pumping of fines from the lower layers. Rutting or roughness will generally be low up to this stage, but is likely to accelerate as the extent of the cracking increases, especially in wet conditions. See Figure I.7(a).

Pavements consisting of cemented bases on granular subbases are very sensitive to overloading and to ingress of moisture through the cracks. When both the base and the subbase are cemented, the pavement will be less sensitive to traffic overloading and moisture. The latter type of pavement is generally used. The shrinkage cracks form early in the life of the pavement and should be rehabilitated by proper inspection and surface sealing. Once traffic-loadassociated cracking has become extensive, rehabilitation involves either the reprocessing (recycling) of the asphalt base, or the application of a substantial bituminous or granular overlay.

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Figure I.7: Generalised structural pavement behaviour characteristics - cemented base (a); concrete pavement (b)

I.4.2.4.2 Rigid pavements

In concrete (rigid) pavements, most of the traffic loading is carried by the rigid concrete slab and little stress is transferred through to the subgrade. The cemented subbase provides a uniform foundation and limits pumping of subbase and subgrade fine materials. Through the use of tied shoulders, most of the distress stemming from the edge of the pavement can be eliminated and slab thickness can also be reduced. Distress of the pavement usually appears first as spalling near the joints, and then may progress to cracking in the wheel paths. Once distress becomes evident, deterioration is usually rapid. See Figure I.7(b). Maintenance consists of patching, joint repair, crack repair, under-sealing, grinding, or thin concrete or bituminous overlays. In cases of severe distress, thick concrete, bituminous or granular overlays will be used, or the concrete may be recycled.

I.4.2.4.3 Semi-rigid pavements

Concrete blocks (semi-rigid pavements) spread concentrated loads over a wide area of earthworks layers. This means that blocks do not merely act as a wearing course, but also as a load-bearing course. The blocks have significant structural capacity when properly installed. The blocks themselves are generally hardly affected by high surface stresses. However, wear or abrasion of the blocks has been observed in some applications. Under traffic, concrete block pavements tend to stiffen, provided the blocks are “locked” in between kerbs or concrete beams on the edges to prevent widening of the joints between the blocks. This leads to the pavements achieving a quasiequilibrium or ‘lockup’ condition, beyond which no further deformation occurs. Many types of interlocking and noninterlocking segmental blocks are used in a wide variety of applications, which range from footpaths to driveways to heavily loaded industrial stacking and servicing yards. The popularity of paving blocks is increasing due to a number of factors: the blocks are manufactured from local materials; they can either provide a labour-intensive operation or can be manufactured and laid by machine; they are aesthetically acceptable in a wide range of applications; and they are versatile as they have some of the advantages of both flexible and concrete pavements.

A small plate vibrator is usually used to bed the blocks into a sand bedding of approximately 20 mm thick and also to compact jointing sand between individual blocks. The selection of the right type of sand for these purposes is important, since a non-plastic material serves best as bedding, while some plastic content in the sand is required to fill the joints between blocks.

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