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Revista ingeniería de construcción

versão On-line ISSN 0718-5073

Rev. ing. constr. v.24 n.2 Santiago ago. 2009

http://dx.doi.org/10.4067/S0718-50732009000200006 

Revista Ingeniería de Construcción Vol.24 N°2, Agosto de 2009 www.ing.puc.cl/ric PAG. 195-207

 

Influence of the Bogotá environmental conditions on the mechanical behavior of an asphalt mixture

 

Hugo Alexander Rondón Quintana **, Fredy Alberto Reyes Lizcano**

* Universidad Católica de Colombia. COLOMBIA

** Pontificia Universidad Javeriana. COLOMBIA

Corresponding author:


ABSTRACT

The paper presents the evolution of the resilient modulus, permanent deformation and strength under monotonic loading of a dense hot mixture due to long term exposure to the environment in the city of Bogota D.C. (Colombia). The mixtures were made using two asphalt cements (AC) and modified asphalt cement (MAC) with ethil-vinil-acetate: AC 80-100, AC 60-70 and MAC 20-40 respectively. The mechanical properties have been evaluated every three months over a period of 20 months for the AC 80-100 and AC 60-70, and 10 months for MAC 20-40. The results show differences depending on the type of cement. All mixtures increase the modulus with time due to aging of the asphalt cement.

Keywords: Resilient modulus, permanent deformation, aging, asphalt cement, asphalt mixtures


 

1. Introduction

The main factors that affect durability of asphalt mixtures, assuming that it was properly made, are: aging and humidity damage (Airey, 2003). Then asphalt mixtures have to be designed and made not only to bear traffic loading but also environmental action. Quantifying the environmental influence over this material's behaviour is not an easy task. Nowadays, the way to quantify this considers separating every component (water, temperature, ultraviolet rays among others), and evaluates every element's influence (from a chemical and mechanic point of view) separating binder and mixtures (Kemp y Predoehl, 1981; Welborn, 1984; Kim et al., 1987; Shiau et al., 1991; Bishara et al., 2000; Bocci y Cerni, 2000; Brown y Scholz, 2000; Khalid y Walsh, 2000; Khalid, 2002; Airey, 2003; Said, 2005; Shen et al., 2006). Additionally, these researches usually use tests and equipments (e.g. thin film oven tests TFOT or rolling thin-film oven tests RTFOT, microwaves) that are not able to fully reproduce the environmental influence of each component on the mechanic and rheological mixtures and asphalt binders (Jemison et al-, 1991; Choquet y Verhasselt, 1992; Kuppenset al., 1997; Verhasselt, 1997). Most researches, even analyze these tests and equipments trying to evaluate the way how this material properties change at facing real environmental conditions. (Jemison et al., 1991; Migliori y Corté, 1998; Montepara, 1999; Montepara y Giuliani, 2000; Airey, 2003). The general conclusion shows that the main limitation of these methodologies is being unable to reproduce the actual conditions that asphalt mixture faces in situ.

One way to measure in situ the environment influence over asphalt mixtures behaviour is using test sample areas or scale test tracks. The limitation of these types of tests is that they are costly and that they do not allow to measure directly or separatly the influence of vehicle loadings and the environment. For this reason this research main objective is to make several tests to put asphalt mixture under real temperature requirements, precipitations, ultra violet rays, air, and others during five years, to evaluate the influence of these conditions over mechanical behaviour. This paper shows the evolution of the resilient modulus, resistance to permanent deformation, Marshall Stabilization and stiffness parameters of hot asphalt dense mixtures MDC-2 (according to INVIAS (2007) when this was exposed to Bogotá D.C environment. The choice was using this type of mixture that is the most commonly used in Colombia for pavement layers (which faces directly environmental conditions). Bogotá weather conditions were considered as an option due that this city is placed in an area that shows the following environmental features in one determined day: weather predominantly cold with minimum and maximum average temperatures within 50OC y 190OC, respectively, and frequent rains at any moment along the day. In the future it is considered to analyse areas with different weather conditions to better understand the environment influence over asphalt mixtures.

During the first 20 months of this project the evaluation has been focussed on asphalt cement (AC) mixtures, type AC 60-70 and AC 80-100. The evaluation for ten months of mixtures that contain modified AC (ACM) type ACM 20-40 (called Mexphalte AM), which is used as a polymer modifier called ethil-vinil-acetate, is also included. This difference in time evaluation is because the initial objective was to calculate the two types of AC produced in Colombia, and then nine months later there arises the need to additionally calculate the ACM that presented a similar formation to AC 60-70, the idea was to compare the results already obtained with this type of asphalt, which lacked a clearly defined tendency or a physical defined meaning.

2. Methodology and Materials

The stone aggregate, originally from "Subachoque" quarry (Cundinamarca, Colombia), was used to built asphalt mixtures for Marshall Test (specimens). According to the Instituto Nacional de Vias specifications (INVIAS, 2007a), this material was tested accoring: Fine and coarse aggregate sieve analysis (INV. E - 213), specific weight and fine aggregate absorption (INV. E - 222), specific weight and coarse aggregate absorption (INV. E - 223), aggregate abrasion strength (size smaller than ¾”) using Los Angeles machine (INV. E - 218), Micro Deval abrasion (NV. E-238), sulfate soundness (INV. E 220), fractured particles (INV. E - 227), test to measure clay equivalent (INV. E - 133), shape index test (INV. E 230). Table 1 shows the results reached and we can see that values accomplish the minimum quality requirements that INVIAS (2007) specification demands, to build dense MDC-2 mixtures for pavement layers.

Table 1. Aggregate Properties

The non modified asphalt cements and the modified asphalt cement were tested according to INVIAS (2007) specification requirements: penetration test, absolute viscosity test, ductility test, solubility in trichloroethylene test, water content test, softening point test and residual test after RTFOT. Additionally, modified asphalt cement was tested with twist elastic recovery. These test results are shown in Tables 2 through 4.

After testing stone aggregate and asphalt cement, five marshall specimens were made (compacted with 75 hits per face) for each asphalt percentage between 4.5 y 6.5 %, with the idea of doing Marshall design (INV. E-748, INVIAS, 2007a), in order to determine the optimum asphalt conventional mixture state. The original aggregate size distribution was modified to accomplish INVIAS (2007) specification to obtain a MDC-2 mixture based on the average values of the size range required for tha Marshall Test. The elaboration of specimens came after the MDC-2 Marshall design, (180 per type AC), mixture suitable to be exposed at Bogotá city environment. These samples were placed in the roof of the Facultad de Ingeniería de la Universidad Católica de Colombia. The specimens that received direct environmental influence were tested every three months, during the first 20 months at the beginning of this project, as the case of AC 80 100 and AC 60-70, and those using modified AC (ACM 20-40) during the 10 first months.

Resilience modulus and permanent deformation tests were performed to evaluate the mixture dynamic characteristics. The resilience modulus test, 0NV. E-749, INVIAS, 2007), was made at three temperatures (10, 20 y 30°°C), with different loading frequencies (2.5, 5.0 y 10.0 Hz) using a Nottingham Asphalt Tester (NAT) equipment, and the resistance to permanent deformation was tested under loading cycling with a stress of 100 kPa at 3600 cycles in compliance with the standard CEN (2000, EN 12697-22).

Table 2. General Asphalt Cement Characteristics AC80-100

 

Table 3. General characteristics of asphalt cement AC 60-70

Table 4. General characteristics of asphalt cement MAC 20-40

 

The mean, maximum and minimum Bogota's temperature were between 18 and 19°C, and 5-7°C respectively during these tests ( May 2007 to January 2009). The variation of mean precipitation was within 33 y 112 mm/monthly, the main values were in September, October and November these two years.

3. Analysis and Results

3.1 Marshall Test

The calculations obtained with Marshall test for specimens made with traditional asphalt AC 80-100, AC 60-70 and ACM 20-40 are registered in Tables 5 through 7, respectively.

The optimum percentages of asphalt cement, according to the records in Tables 5 through 7, are 5.3%, 5.6% and 5.6% for mixtures elaborated with AC 80-100, AC 60-70 and ACM 20-40 respectively. These percentages accomplish the minimum requirements demanded by INVIAS (2007) specification for dense MDC-2 and transit type NT1 and/or NT2.

Table 5. Marshall Test summary to asphalt mixture MDC-2 with AC 80-100

Table 6. Marshall Test summary to asphalt mixture MDC-2 with AC 60-70

Table 7. Marshall Test summary to asphalt mixture MDC-2 with ACM 20-40

 

4. Time evolution of the Marshall stiffness and stability parameters

Figure 1 shows the time evolution (t in months) of the Marshal stability parameters while Figure 1b the relationship between stability (E in Spanish) and fluid (F) (this relation is called by some researchers Marshall stiffness). This relation, in terms of physic may be understood as mechanical strength evaluated in the failure mixture state, under monotonic loading in an indirect tensile test. We can see in Figure la an increase of the stability along the exposure time for mixtures made with AC 80-100, AC 60-70 and ACM 20-40. The mixtures made with AC 60-70 with time t=17 months, increase its stability in an 81.6% comparing values reached on t=0, those made with AC 80-100 increase to a 69, 5% in t= 18 months and those with ACM 20-40 increase a 17.4%. t=8 months

Figure 1. Evolution of a) Marshall Stability and b) Relation E/F (Marshall Stiffness) with aging Time for mixtures made with AC 80-100, AC 60-70 and ACM 20-40

Figure 1b shows that the AC 60-70 mechanical srtength presents a tendency to increase upon time exposure, and in t= 17 months this raise a 41, 6% in comparison with the initial value at t=0. In case of mixtures with AC 80-100 and ACM 20-40 the increase in t=18 months and t =8 months is approximately 53% and 14.9%, respectively comparing it with the value shown in t=0. Additionally, a decay of the values related to E/F in some periods can be observed, and this can be explained by two phenomena that happen simultaneously in the mixtures:

•  Aging by oxidation experienced by asphalt cement and consequently by the asphalt mixtures, when it is required in different temperatures and exposure to ultra-violet radiation (UV).

•  Water in mixtures produces loss of bonding between aggregate and asphalt cement which gives place an increase in fluid and diminution of the fluid-stability relation.

5. Evolution of resilient modulus and resistance to permanent deformation

The time evolution of the asphalt mixture modulus is observed on Figure 2. It should be noticed that for mixtures made with CA 80-100, CA 60-70 and CAM 20-40 (Figures 2a, b and c, respectively) there is a typical modulus increase as the loading frequency raises and test temperature decreases. In the case of mixtures made with CA 80-100, the modulus tendency is to increase with the environmental exposure (Figure 3). This phenomenon may be explained mainly by the aging by oxidation that asphalt experiments due to temperature and ultraviolet ray exposure (UV).

The mixtures made with AC 60-70 and ACM 20-40 experienced a different behaviour in comparison with those with AC 80-100 (see Figures 2b-c and 3). During the first months of exposure, the resilient modulus decreased which increased permanent deformations. After that, the modulus stabilized; and finally, the tendency is to increase (specially when the test is made at 20 and 30°C), wich helps to decrease the deformation values. The modulus decrease may be explained based on the microcracks produced at low temperatures when the mixture is rigid (brittle behaviour); then, these microcracks stabilize and stiffness increases because of a similar phenomenon of aging and oxidation that happens in mixtures with AC 80-100.

Figure 2. Resilient modulus evolution with aging time for mixtures made with a) AC 80-100, b) AC 60-70 and c) ACM 20-40

 

Figure 3. Evolution of permanent vertical deformation with aging time for mixtures made with AC 80-100, AC 60-70 and MAC 20-40

There is no clear effect of water on the results because the humidity produces bonding loss between aggregate and asphalt cement, giving place to a possible module and mechanical srtenght decrease. There should be additional research in this area.

Figure 4 shows the relation (ED/EDo) between the resilient mixture modulus obtained after environmental exposure in different periods of time (ED) and the initial resilient mixture modulus at time of t=0 months (EDo). The t=20 modulus reaches, in the case of mixtures with AC 80-100, increases a 40 and 110% (depending on test and frequency temperatures) compared with the initial value. Mixtures with AC 60-70 experienced a maximum modulus decrease of 35% during the first five months of exposure and then an increase that nearly reached the initial modulus value at t=19 months. The mixtures made with ACM 60-70 showed a similar behaviour to those made with AC 20-40. It can be observed in Equation (1), for the case of mixtures made with AC80-100, and in Equation (2), for mixtures made with AC 60-70 y ACM 20-40, empirical equations that may be producing the evolution in modulus with time. There is a simulation of the results shown using equations (1) and (2) on Figure 4a and b. In the equation (1), a=1.0 and b=0.153; in the equation (2), a=0.524, b=0.102, c=0.476 and a=0.713, b=0.021 and c=0.289 for simulations of mixtures with CA 60-70 and CAM 20-40, respectively. The correlation coefficient of equations is r2=0.77.

(1)

 

(2)

The aging by oxidating have an important influence on the asphalt cement viscoelastic properties, as demonstrated by other researches (Afanasieva y Alvarez, 2004; Vargas et al., 2008). This influence is shown in the G* elastic modulus increase, produced by the increase in the asphalt molecular interaction: dispersed forces, hydrogen bridges and polar interactions, especially in asphaltenes, which give a better asphalt structuration or increase the material molecule connectivity. Low temperatures (< 40°C), facilitate asphalt rigidization which combined with the Bogota's different weather conditions, explain the observed behaviour of the asphalt mixtures used in this research. The behaviour of the ED/EDo relationship observed in AC 60-70 and ACM 20-40 mixtures may be explained based on the fact that these materials should present a lower content of lightweight fractions that are susceptible of aging, as opposed to AC 80-100. That is, its chemical composition tendency to change with aging may be lower which shown by the mechanical properties of the asphalt mixture designed with this asphalt. This fact needs an analysis beginning with the AC chemical composition under the Bogotá D.C's weather conditions.

Figure 4. Evolution of the relation of between resilient and initial modulus with aging time for mixtures made with a) AC 80-100 and AC 60-70, b) ACM 20-40

6. Conclusions

This article represents the first stage of a research project, which objective is to evaluate the mechanic property changes that a hot asphalt mixture experiences when exposed to real environmental conditions at Bogotá D.C city. In this first stage there were evaluations every three months, during the first 20 months, checking, resilient modulus, resistance to permanent deformation and Marshall test parameters on mixtures classified as MDC-2 made with AC 80-100 and AC 60-70.

The evaluation for mixtures made with ACM 20-40 was 10 months. It can be observed that mixtures made with the three AC type used, increase its mechanical strength under monotonic loading (evaluated through the relation E/F of Marshall test) with the environment exposure time.

In the case of mixtures made with 80-100 resilient modulus and resistance to permanent deformation tendency is to increase with time mainly because of asphalt oxidation phenomenon.

Mixtures made with AC 60-70 and ACM 20-40, decreased their resilient modulus during the first months of exposure increasing permanent deformation (this is may be due to low temperature microcracking); after that, an stabilization in the resilient modulus is shown within fifth and eight months of exposure; and finally, a tendency to increased resilient modulus is shown due to the toughness experienced by the asphalt oxidation phenomenon (aging).

The future project steps should measure and evaluate the evolution of mechanical parameters such the resistance of asphalt mixture to fatigue, and the chemical composition of the asphalt cements used in the mixtures experiment.

7. References

 

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