A. FONÓD

DEGRADATION OF THE BEARING CAPACITY OF ASPHALT PAVEMENTS

Adrián Fonód, Ing.

Research field: Traffic load, Experimental verification of pavements

VUIS-CESTY, Ltd.Lamačska cesta 8, 817 16 Bratislava

INTRODUCTION

One of the most important tasks, on which we have to focus our attention in the area of pavement design, is guaranteeing the desiderative function of a road pavement as expressed by the term serviceability of the pavement in relation to the road user as well as to the administrator of the road. The serviceability of pavement is mainly characterized by:• roughness of the pavement - affects driving comfort,• skid resistance - affects driving safely,• bearing capacity of pavement - affects its service life,• structural condition (cracking, patching, etc.) - affects driving

comfort in addition to aesthetic effects

Because of the need to have better knowledge of pavement behavior, it is necessary to know the degradation of individual pavement material as well as the entire pavement. For projection as well for the administrator of a road, it is useful to have a sufficient amount of knowledge about a pavement’s technical state, namely at

the time of its construction, at the time of its assessment and at the time after its service life.A pavement’s technical state is determined by the variable parameters of the pavement’s properties, which are ascertained during the assessment process of the pavement’s serviceability. Those parameters change depending on time or on repeated loading from traffic. We can express this by means of a degradation model.

EXPERIMENTAL VERIFICATION OF PAVEMENTS

The experimental verification was carried out at a unique facility (the CTT circular test track) with the possibility of testing the pavements on a 1:1 scale, which was loaded with an actual vehicle with an axle weighing 115 kN (the EU standard for a permissible axle load). The owner of the facility is the VUIS-CESTY, Ltd. research institute.The basis of the CTT mechanical part consists of three loading vehicles each of which has a driving axle. The vehicles are attached

ABSTRACT KEY WORDS

• bearing capacity, • design method, • degradation model, • flexible pavement, • circular test track, • structural evaluation.

The presented article focuses on the effect of traffic load on the bearing capacity of asphalt pavements. The experimental verification was carried out at a unique facility (the CTT circular test track) with the possibility of testing the pavements on a 1:1 scale, which was loaded with an actual vehicle with an axle weighing 115 kN. The pavement bearing capacity’s variable parameter was measured regularly depending on the number of loading applications.

2005/2 PAGES 35 – 39 RECEIVED 9.3.2005 ACCEPTED 18.4.2005

352005 SLOVAK UNIVERSITY OF TECHNOLOGY

fonod.indd 35 2. 5. 2006 12:19:45

36 DEGRADATION OF THE BEARING CAPACITY OF ASPHALT PAVEMENTS

to the arms of a medium anchor clapper. The CTT’s electric part serves to drive each vehicle by means of electro-motors and to control the entire CTT mechanical facility. The facility allows a transversal shift of the loading vehicles of ± 950 mm.The vehicles move on two doubled wheels equipped with 11,00R-20 tires at a 0,7 MPa inflation pressure. The speed of the vehicles is limited to 60 km/hour, but the mean operational speed (95 %) was 30 km/hour.

The test was carried out on six asphalt pavements constructed in sections, each of which 16,0 m long (No. 6 has a length of 20.5 m). This permits the survey, during the same period of time, of several pavements differing in the composition of their layers and building materials used for the creation of the individual layers. The scheme of the experimental structures is given in Figure 2. The composition of the pavement structures was designed according to the research program. The CTT pavement structures can be classified as flexible pavements (PS 3 and PS 4) and semi-rigid pavements (the others).The subgrade soil is cohesive silt, with its Standard Proctor density of ρd = 1800 kg.m-3 and optimum water contents of 11%. The subgrade bearing capacity is characterized by elasticity modulus En = 60 MPa (PS 1,2,5,6). Increased elasticity modulus En = 90 MPa was applied to pavement structure No. 3 and 4. It was realized by means of the use of a milled asphalt mixture.

The drainage-capping layer is made:• from sand-gravel, natural material continuously graded 0-32 mm

with a 6% water content (”SP”),

• 0-32 mm crushed stone fraction; the aggregate was composed of five fractions (”SD”),

The following materials were used in base course:• soil stabilization with 5% Portland cement (”SC I”) • unbound granulated aggregate, as a mixture of fractions 0-4, 4-8,

8-16, and 16-32 mm (”MSK”),• hydraulically bound granular material (”KSC II”)

The following asphalt mixes were used for surfacing (wearing and binder-courses):• stone mastix asphalt (”AKM”),• conventional asphalt concrete, with a maximum sized aggregate of

16 mm, high Marshall stability: (”ABVH I”); ABVHAPOLLOPLAST contains modified asphalt binder; ABVH70/100 contains standard asphalt binder,

• a hot asphalt mix with a lower Marshall stability (”OKH I”), OKH70/100 contains standard asphalt binder, OKH30/45 contains harder asphalt binder.

As we can see in Figure 2, the total thickness of the pavements is 440 mm: • 150 mm subbase,• 150 mm base layer,• 140 cm surfacing (wearing and binder courses.

In Figure No. 3 loading in time is shown. During the winter time, CTT was not operated.One of the main bases for evaluating pavement efficiency is measuring the bearing capacity of the pavements tested. The measurements were executed using a dynamic loading test according to STN 73 6192 [8]. The type of falling weight deflectometer used was Kuab.The aim of the measurements of pavement of variable parameters (bearing capacity), which regularly depend on the number of load applications, was to obtain its degree of development (degradation) and to derive a specific degree of the degradation relation.The measurement sets for CTT were repeated periodically after 200,000 repetitions. The first stage was before the start of axle loading, and the last stage was after 1,200,000 total passages.The measurements were carried out on the right and left wheelpaths separately. The distance between the measured points in a longitudinal direction was 2 m. Thusly we obtained a sufficient amount of the measured values for each pavement by statistical analysis.Since the temperature of the asphalt layers has a large effect on a pavement’s deflection, it was necessary to fulfill the requirement that the temperature measured 40 mm under the surface was in a range of (10 - 30) °C.

Figure 1 Circular test track facility

2005/2 PAGES 35 — 39

fonod.indd 36 2. 5. 2006 12:19:47

2005/2 PAGES 35 — 39

37DEGRADATION OF THE BEARING CAPACITY OF ASPHALT PAVEMENTS

In order to compare the pavement response, a ”normalized” deflection was used (50 kN load and + 20°C, the average asphalt layer temperature).

The final form for re-calculation is:

(1)

where: y 0,T 20, R 50 is the maximum deflection relative to force 50 kN and

a temperature 20°C (mm), y 0,T, R 50 - the maximum deflection relative to force 50 kN at

a given temperature (mm), T - pavement temperature (- 40 mm under surface) (°C) kT - corrected temperature factor kT = 0,0015.

Further analysis of the pavement’s bearing capacity was carried out on the basis of the calculated equivalent surface modulus Eekv, which was obtained from the general equation for FWD Kuab:

Figure 2 The pavement structures on the circular test track

Figure 3 Loading in time the circular test track

Figure 4 Values of the equivalent surface modulus

fonod.indd 37 2. 5. 2006 12:20:00

38 DEGRADATION OF THE BEARING CAPACITY OF ASPHALT PAVEMENTS

2005/2 PAGES 35 — 39

(2)

where: Eekv. is the equivalent surface modulus of the pavement (MPa) y0,T20,R50 - maximum deflection (m), σ - contact pressure under the loading plate (MPa), d - radius of the loading plate (m), μ - Poisson’s ratio μ= 0.35.Figure No. 4 are illustrated the values of the equivalent surface modulis, namely, for each pavement separately.

EVALUATION OF CTT PAVEMENTS IN TERMS OF THEIR BEARING CAPACITY

The equivalent surface moduli values in pavements with an unbound subbase course (pavements Nos 3 and 4) were approximately 400 MPa smaller compared to pavements with bounded base courses.The variational coefficient (Cv) characterizes the bearing capacity’s uniformity (consistent). The value of the variational coefficient is smaller than 0.25 for each pavement so that we can characterize the values of the equivalent surface moduli as highly uniformly distributed.The measured values of the moduli are relatively high, mainly due to the increased bearing capacity of the pavement’s subgrade.The attained values of the dynamic modulus in semi rigid pavements are nearly twice as high as the values measured for the flexible pavements.The changes in the moduli of structures after repeated loading axle passages are relatively small.

With respect to the number of axle passages, the stiffness (and mechanical efficiency) of the tested pavement structures lack greater.We can generally say that the bearing capacity of the pavements after repeating loading the pavements with the vehicle axle weighing 115 kN was not altered considerately applying 1,200,000 passages. No fatigue defects occurred to the tested pavements.Application of measurements of bearing capacity of pavements in terms of pavements degradation of bearing capacityOne of the most usual characteristics for determining a pavement’s serviceability condition is to exceed the tensile strength of the bounded material, which is demonstrated as a crack. In view of this damage we can generally consider four stages of a pavement’s serviceability degradation with a focus on the pavement’s bearing capacity:1st stage – terminates by the initiation of a crack in a critical layer of the pavement’s structure,2nd stage – reflection of the crack towards the surface,3rd stage – terminates by dissipation of the pavement’s serviceability for a given traffic load,4th stage – the pavement is unusable for any traffic load.The proportional duration of the stages depends on the type of pavement structure, which is different for semi-rigid and flexible asphalt pavements.Figure No. 5 shows a theoretical degradation model of the bearing capacity of semi-rigid and flexible pavements [2]. The critical value of the parameter warns about the functional break up for a given parameter. In this case immediate renovation is necessary. The warning value of the bearing capacity’s parameter warns about an

Figure 5 Theoretical degradation model of an asphalt pavement’s bearing capacity

fonod.indd 38 2. 5. 2006 12:20:17

2005/2 PAGES 35 — 39

39DEGRADATION OF THE BEARING CAPACITY OF ASPHALT PAVEMENTS

assumed moment in time, when the parameter reaches a degree of damage requiring repair. It can be stated that the degraded condition after carrying out 1,200,000 load applications is in the first stage before initiating of first failure crack.

CONCLUSION

In general it can be stated that designing the pavements in accordance to contemporary (present) design method while fulfilling construction conditions (which means material characteristics that are in line with given standards) is still safe.The design method and criteria for a pavement provides a reliable pavement structure for a given traffic load during a design period under given climatic conditions. The evidence for this is, for example, Pavement No. 2. According to the theoretical calculations, it had to carry 400,000 passages with the identical weight as the CTT vehicle, but it actually carried 1,200,000 load applications at CTT, which represents nearly 5,000,000 standard axle loads (100 kN).This means that a pavement structure with an entire thickness of 440 mm, built on a subgrade with a design elastic modulus of En = 60 MPa in accordance with applicable technical standards and evaluated in accordance with the present design method for asphalt pavements will be ranked as a first class road with a load traffic of III (according to STN 73 6114 [7]) and a service life of at least 20 years.

We have to take into consideration that the individual materials used were supervised and the structural layers were faultlessly built in accordance with the applicable technical specifications and technical standards.It can be stated that the pavements tested at CTT even after loading simulated by 1,200,000 passages of vehicles weighing 115 kN did not indicate either fatigue defects or other failures in terms of bearing capacity. This fact is also observable in flexible pavements with an unbound base course, where the overall pavement thickness is 440 mm.The changes in the surface moduli of the structures after repeated loading of 115 kN axle passages are relatively small.A degraded condition after carrying out 1,200,000 load application is in its first stage before initiating the first failure crack.The experimental pavement testing by means of accelerated loading of a pavement section on a 1:1 scale proved that the degradation stage is in the first stage before the initiating of the first failure crack. This fact was also demonstrated during other accelerated tests (CTT) in the past [3], [4].In the case of the requirement to determine the time for the second and third stages, it is necessary to carry out a long-term test with an assumed duration of several years.The processes in the second and mainly the third stage can be varied, because the pavement damaged by cracks is no more under standard principles of development (degradation).

REFERENCES

[1] Loveček, Z., Řikovský, V., (2000) Zefektívnenie výstavby diaľničnej siete v SR a zohľadnenie zvýšeného osového zaťaženia na úroveň EU (Economic effectiveness of SR highway network pavement structures and incorporation of increased axial load in compliance with EU standards), VUIS-CESTY Ltd., research report (in Slovak)

[2] Mikolaj, J., Čelko, J., Řikovský, V., Loveček, Z., Valuch, M. (1996) Systém hospodárenia s vozovkou (Pavement management system), Technical University in Žilina, Faculty of Civil Engineering (in Slovak)

[3] Řikovský, V. (1985) Overenie životnosti netuhých vozoviek s tenkým bitumenovým krytom na Kruhovej skutočnej dráhe (Service life evaluation of flexible pavements with a thin wearing course on CTT), VUIS-CESTY Ltd., research report (in Slovak)

[4] Řikovský, V. (1989) Preukázanie životnosti vozoviek s polotuhými a tuhými podkladovými vrstvami na Kruhovej skúšobnej dráhe (Service life of flexible pavements with rigid

and semi-rigid base layers), VUIS-CESTY Ltd., research report (in Slovak)

[5] Fonód, A. (2004) Vplyv dopravného zaťaženia na funkčnú spôsobilosť asfaltových vozoviek (The effect of traffic load on the serviceability of asphalt pavements), PhD thesis, Faculty of Civil Engineering, STU Bratislava (in Slovak)

[6] Gschwendt, I., Řikovský, V. (1997) A pavement performance test for an innovative design method, VIII. International Asphalt Pavement Conference in Seattle, pp. 1467 – 1479.

[7] STN 73 6114 (1997) Vozovky pozemných komunikácií. Základné ustanovenia pre navrhovanie. (Pavements of roads: basic provisions for structural design), Úrad pre normalizáciu, metrológiu a skúšobníctvo SR (in Slovak).

[8] STN 73 6192 (1996) Rázová zaťažovacia skúška netuhých vozoviek a podložia. (Impact load test for road surfaces and subsurfaces), Úrad pre normalizáciu, metrológiu a skúšobníctvo SR (in Slovak).

fonod.indd 39 2. 5. 2006 12:20:22