Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements

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1 Journal of Civil Engineering and Architecture 12 (2018) doi: / / D DAVID PUBLISHING Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements Song Bo 1, 2, Zhang Jin-xi 1, Xue Zhong-jun 2, Zhou Xu-li 2 and Zhang Tao 2 1. School of Metropolitan Transportation, Beijing University of Technology, Beijing , China 2. Beijing Board Engineering Quality Supervision Station, Beijing , China Abstract: Compressive strength and compressive resilience modulus are two important parameters to measure the mechanical properties of semi-rigid base. The test methods of semi-rigid base cores are different from those of the laboratory samples in terms of sample acquisition, sample selection and humidity requirements. Core-drilling location, size of core sample, smoothness and humidity conditions were analyzed. The test methods of compressive strength and compressive resilience modulus were proposed. The research results show that compressive strength of lime fly-ash stabilized gravels base has a tendency of increasing during a long period. The compressive resilience modulus increases significantly with compressive strength of semi-rigid base. The compressive resilience modulus generally is 3-4 times than the recommended range of asphalt pavement design specifications. The fluctuation range of compressive resilience modulus is obviously higher than the compressive strength. The compressive resilience modulus is more sensitive to the construction variability. The overall trend between the compressive resilience modulus and the back-calculation modulus is consistent. FWD (falling weight deflectometer) back-calculation modulus can reflect the stiffness and bearing capacity of asphalt pavement. Ker words: Road engineering, semi-rigid base cores, compressive strength, compressive resilience modulus, test methods. 1. Introduction Asphalt pavement is one of the main forms of high-grade pavement. Until 2015, the mileage of asphalt pavement was more than 790,000 km in China [1]. The vast majority of asphalt pavement is composed of semi-rigid base, which materials are cement stabilized macadam or lime-fly ash stabilized macadam. With the rapid development of the highway transport and the increasing traffic load, some semi-rigid asphalt pavements had appeared different degrees of damages, most of which are due to the lack of strength and inconsistent construction of the semi-rigid base [2]. Compressive strength and compressive resilience modulus are important parameters to measure the mechanical properties of the semi-rigid base, which directly affect the service quality and life of asphalt pavements [3-5]. Corresponding author: Song Bo, doctoral candidate, research fields: materials and structures of road engineering. South Africa adopted the concept of effective modulus for semi-rigid base and applied the effective modulus to the two-stage pavement design method [6]. In the first stage, the effective modulus of the semi-rigid base is 500-3,000 MPa. In the second stage, the compressive resilience modulus of semi-rigid base decays to MPa after fatigue cracking. Shrinkage performance can be improved by modifying the graduations of base materials rather than reducing the cement content which will lead to strength loss [7]. The semi-rigid base modulus in asphalt pavement with no serious disease is greater than 2,000 MPa, The semi-rigid base modulus in asphalt pavement with serious disease is less than 2,000 MPa [8]. A test method of semi-rigid materials based on measurement of strain in the middle section of the specimen can improve the reliability of uniaxial compressive resilience modulus and reduce the influence of friction constraints at both ends [9]. The existing specifications provide test methods of compressive

2 98 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements strength and compressive resilience modulus of indoor test samples [10]. Test methods of the semi-rigid core samples are different from those of the laboratory samples in terms of sample acquisition, sample selection and humidity requirements. Therefore, it is necessary to carry out study on test methods of mechanical properties of semi-rigid base core samples to facilitate the scientific evaluation and judgment of the utilization value of semi-rigid base. In order to study the mechanical properties development of semi-rigid base, the core samples of semi-rigid base with different ages from three highways were chosen to carry out the compressive strength and compressive resilience modulus tests. The core-drilling location, size of core sample, smoothness and humidity conditions were analyzed. The test methods of compressive strength and compressive resilience modulus were proposed, and the mechanical property with age has been studied. 2. Test Materials and Methods Test materials were from core samples of S318, S50 and S203 highways in Beijing. S318 highway was completed in The core samples of semi-rigid base were drilled in S50 highway was completed in The core samples were drilled in S203 highway was completed in The core samples were drilled in Semi-rigid base materials are lime-fly ash stabilized gravels. Design strength standard of the semi-rigid base materials is that the unconfined compressive strength of 7 days is more than 0.8 MPa. The mix proportion design of the semi-rigid base materials is that, lime:fly-ash:gravels = 5:15:80. Semi-rigid base material is a kind of composite material, which is influenced by various factors during mixing and paving process. It is impossible for the coarse and fine aggregate to be completely mixed uniformly. Such variability leads to different interface shapes of the base core samples from different places. At the same time, coring in site is affected by many factors, including the location of coring, road gradient, operator proficiency, and the number of detection. Coupled with the number of coring limits, there is a certain randomness and deviation in the measurement results. According to the test standard of pavement base from EN [11, 12], the mechanical properties of the core samples are also affected by factors such as size, flatness, temperature and humidity conditions. Therefore, it is necessary to study the influence factors of the mechanical properties of the semi-rigid base core samples to improve the representative, reproducibility of the mechanics test. 2.1 Location of Core-Drilling The core sample is drilled by the testing organization before pavement rehabilitation. The continuous running age is 13 years when coring. The coring location is related to the representation of pavement core samples. If the disease-free site is completely selected, the core samples obtained will have no guiding function. If the disease location is completely selected, the integrated core may not be obtained because of the serious disease, and lack of an effective judge to the mechanical properties of semi-rigid base. According to above analysis, the choice of pavement coring location can be determined considering time, cost and traffic organization and other factors. A core sample is drilled every 100 m distance at a single lane of the highway direction, or a core every 5 FWD measuring points. The selected core location includes a wheel track, the middle of the wheel track, rutting, cracks and other representative parts of the disease. 2.2 Size of Core Sample The internal diameter of coring drill is Φ = 150 mm. The maximum particle size of the semi-rigid base in S50 highway is mm, the diameter of the core sample is generally considered to be 4 times the nominal maximum particle size of the aggregates, and the ratio of the larger core sample diameter to the maximum particle size of the aggregate is advantageous for obtaining stable values. Practical

3 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements 99 experience shows that it is likely more successfully to obtain coring samples in those sites of cracks and other disease by the Φ = 150 mm drill. The coring samples should be checked and screened. Visual inspection usually includes the cylindrical samples with obvious cracks, damages and other defects. Intact core samples are selected to mechanical properties tests. The core sample of different height-diameter ratio may get different test values. The test results of indoor forming specimens show that the higher the height-diameter ratio is, the less the fluctuation of the compressive strength values. When the height-diameter ratio is 2.0, the variability of the strength of the same mixture samples tends to zero. Due to the limited compaction of roller, the thickness of single semi-rigid base is generally less than or equal to 20 cm. In order to obtain reliable test values, the European standard BS EN recommended a core sample with a height-diameter ratio is 1.0 [13]. The British BS 6089 requires that the core-sample height-diameter ratio is not less than 0.95 and not more than 1.3 before the ending face treatment [14]. China does not yet have a regulation about the size of semi-rigid core samples. The height- diameter ratio of cement stabilized gravels or lime-lime stabilized gravel specimen from indoor forming is 1.0 [10]. Maintaining a 1.0 height-diameter ratio is advantageous for comparing the strength of cylindrical specimens during pavement construction. The technical specifications of coring samples for mechanical tests also include: the diameter error is ± 2 mm, the axis perpendicularity is less than 2, and the smoothness of two ends is ± 0.1 mm [15]. Referring to the above specifications, the technical requirements of the semi-rigid core samples are: (1) the diameter is 150 ± 2 mm; (2) the height is greater than 140 mm; (3) nonperpendicularity between end and axis is less than 2 ; (4) the smoothness of two ends is ± 0.1 mm; (5) core samples without cracks and other major defects. 2.3 Smoothness In order to meet the smoothness, height-diameter ratio of the test requirements, the core sample should be cut or capped. If the height of an intact core sample is higher than 150 mm, two ends can be cut to achieve the required smoothness. If a core sample is less than 150 mm, and greater than 140 mm, it should be capped. The loose parts of two ends are firstly removed, and then put the sample into a split round mold which is shown in Fig. 1. A split round mold consists of two semi-cylindrical steel molds, which inner diameter is 150 ± 1 mm and height is 150 mm. Both sides of a semi-cylindrical steel mold welded two steel wing plates and a handlebar. There are three symmetrical round holes in the upper, middle and lower parts respectively in the wing plate so as to use nuts to link into a split round mold. The base is a rectangular steel plate which length and width are 200 mm. Two irons mounted on one side of the base in order to connect with the handlebars of a split round mold. After wetting the end face with water, some cement paste fills into a split round mold until it is fully. Excess cement paste is scraped off by a trowel along the edges of the split round mold. The surface should be covered with a damp cloth until cement paste hardens. After hardening for 2 to 4 hours, flip the split round mold so that the uncapped end is upward, remove the cushion block and repair another end with the same method. After cement paste is hardened (see Fig. 2), move it to standard curing room and do curing 6 days. 2.4 Humidity Condition Humidity conditions significantly affect mechanical properties of the core samples. The strength of a wet sample is usually less than the dry one, but how much is uncertain. Different researches have different perspectives about the test of the core s mechanical properties under which wet conditions. ASTM C42 states

4 100 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements Fig. 1 Split round mold. Fig. 2 Examples of the capped cores. that core samples should be consistent with the wet state of the concrete in service [16]. If no conditions have been established, ASTM C42 provides two conditions. One is a receiving condition that is drying for 12 to 24 hours at a relative humidity of less than 50% and 16~17 C. The second condition is dry state that is drying for 7 days at a relative humidity of less than 60% and the same temperature. Bartlett and Macgregor s study found that the strength of core samples dried in air for 7 days was 14% higher than soaked in water for at least 40 hours [17]. They think the actual situation is more complicated than dry and wet : the humidity gradient between the outside and the inside of the core sample affects the strength in the wet or dry condition. Based on the above analysis, it is impractical to simulate the humidity conditions in service pavement base. In any case, the strength obtained from the

5 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements 101 compressive strength test of the core sample does not represent the actual strength of the pavement base. Core samples are soaked in the water for enough time to reduce the effect of humidity gradient, making the repeatability and reproducibility of the mechanical properties test better. Core samples can achieve a saturation condition that core samples are soaked in the water with 20 ± 2 C for at least 40 hours before testing. In this paper, the semi-rigid base core samples were soaked in the water with 20 ± 2 C for 48 hours before testing. 2.5 Laboratory Experiment The test method on the compressive strength and compressive resilience modulus of the in-service semi-rigid base has not yet been established in China. The design values and the required values during construction are derived from the methods provided by test methods of materials stabilized with inorganic binders for highway engineering. In order to maintain good comparability with the strength and modulus during design and construction, the test method of compressive strength of core samples is adopted for the T0815 method (see Fig. 3a), and the test method of compressive resilience modulus is for the T0808 method (see Fig. 3b). Test equipment is a computer-controlled automatic pressure testing machine made in Shanghai Hualong Test Instruments (a) Compressive strength test Fig. 3 Examples of testing process. (b) Compressive resilience modulus test

6 102 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements Co., Ltd. The deformation measuring device is two dial indicators (0.001 mm), and the test loading rate is maintained 1 mm/min. The compressive resilience modulus test is firstly completed, then the compressive strength test is carried out. After the compressive strength test is completed, the failure shape of the core sample is recorded. 3. Results In the 1980s and 1990s, there are many coal-fired power plants in the west region of Beijing. These power plants generated a large amount of industrial waste including fly-ash. The development and application of lime fly-ash stabilized gravels promote the effective recycling of wastes. The high strength and good stability of this base material make the lime fly-ash stabilized gravels gradually become the main materials of semi-rigid pavement in Beijing. Strength development process of lime fly-ash stabilized gravels shows that lime and fly-ash do not fully respond in the initial stage of mixtures and they are still in a more loose structure. With the progress of lime fly-ash reaction, the spatial network structure in the binder is gradually formed that makes the latter mixtures have a higher strength and stiffness growth. Therefore, the fly-ash stabilized gravels base has a tendency of increasing during a long period. After 6 years, the average compressive strength of semi-rigid base reaches 6.72 MPa; after 12 years, the average compressive strength reaches MPa; after 16 years, the average compressive strength reaches MPa. This is because the strength growth of lime fly-ash stabilized gravels is a long-term process. The structure strength process includes four reactions: (1) recrystallization of lime; (2) ion adsorption and exchange; (3) carbonation; (4) volcanic ash effect. The first and second reactions play an important role in the early strength of lime fly-ash stabilized gravels. The third and fourth reactions determine the late development of material s strength. Carbonation generates CaCO 3 that has a certain degree of cementing. When CaCO 3 constantly increases, it in turn hinders air from contacting the internal materials. Therefore, the carbonation of lime fly-ash stabilized gravels is a long-term and slow process. Volcanic ash effect from fly-ash generates CaO SiO 2 nh 2 O and CaO Al 2 O 3 nh 2 O. Early stage of above resultants is gelatinous matter, then they continue to transit to crystal. This reaction is also a long-term process. The variability of core compressive strength from compressive strength and compressive resilience three highways was analyzed. The variation modulus also become important indexes to evaluate the structure status of existing semi-rigid base. coefficient of compressive strength with 5 years σ is 8.33%, the variation coefficient of compressive strength with 12 years σ is 6.59%, and that with 3.1 Analysis of Compressive Strength of Core Samples 16 years σ is 5.08%. The variation coefficient of From Table 1, the compressive strength of lime compressive strength of core samples decreases with Table 1 Compressive strength of cylinder specimens with different ages. Compressive strength of lime fly-ash stabilized gravels base with 6 years Sample number Compressive strength R /MPa Compressive strength of lime fly-ash stabilized gravels base with 12 years Sample number Compressive strength R /MPa Compressive strength of lime fly-ash stabilized gravels base with 16 years Sample number Compressive strength R /MPa

7 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements 103 service ages. The variation coefficient with different ages is less than 10%. Comparing with the compressive strength of semi-rigid base during construction period, uniformity and stability of semi-rigid base have been improved with the increase of compressive strength of lime fly-ash stabilized gravels. 3.2 Analysis of Compressive Resilience Modulus of Core Samples From Table 2, compressive resilience modulus E increases significantly with compressive strength of semi-rigid base. The compressive resilience modulus generally exceeds 3-4 times than the recommended range of asphalt pavement design specifications [18]. The average compressive resilience modulus with five-year E is 3,625 MPa, the minimum compressive resilience modulus is 2,922 MPa; the average compressive resilience modulus with 10 years reaches 4,132 MPa, the minimum compressive resilience modulus is 2,629 MPa. The stiffness of semi-rigid base gradually increases with service life, which growth is conducive to the durability and stability of pavement structure. Without a significant decrease in modulus of semi-rigid base, it will make the semi-rigid base in a favorable stress state to make full use of existing semi-rigid base and thicken asphalt layers. It may become a long-life pavement. The variability of the compressive resilience modulus of the core samples from two highways was analyzed. The variation coefficient of the compressive resilience modulus of semi-rigid base with 5 years was 16.0%. The variation coefficient semi-rigid base with 10 years is 21.7%. The fluctuation range of compressive resilience modulus is obviously higher than the compressive strength, which indicates that compressive resilience modulus is more sensitive to the construction variability. The construction quality of semi-rigid basement is still a primary factor to long-term performance of asphalt pavement. 3.1 Comparing Analysis of Compressive Resilience Modulus and FWD Back-Calculation Modulus FWD (falling weight deflectometer) surveys were carried out on the asphalt pavement of S50 highway (see Fig. 4). FWD tests a point every 20 meters. A core sample was drilled every 5 points. In the FWD setup, a target stress of 700 kpa was applied through a 300 mm diameter plate. The geophones, numbered D1 to D9, were positioned as Table 3. Back-calculation was carried out using the ROSY DESIGN computer program to calculate the modulus of the pavement. A three-layer model was used consisting of subgrade, semi-rigid base and asphalt surface. The pavement layer thickness was determined from complete cores of the asphalt pavement. In the ROSY DESIGN program, a Poisson s ratio value of 0.4 was used for the subgrade, a value of 0.25 was used for the semi-rigid base and 0.35 was used for the asphalt pavement. The results of the analysis, i.e. back-calculated modulus values, are shown in Fig. 5. Fig. 5 also shows compressive resilience modulus of the corresponding core sample. Fig. 5 is comparing the compressive resilience Table 2 Compressive resilience modulus of cores with different ages. Compressive resilience modulus of lime fly-ash stabilized gravels base with 6 years E /MPa 3,351 2,922 3,396 4,242 4,215 Compressive resilience modulus of lime fly-ash stabilized gravels base with 12 years E /MPa 3,687 4,457 4,437 5,902 4,083 3,638 5,350 5,456 3,206 4,153 2,874 4,136 2,908 3,905 4,117 2,629 6,120 3,155 3,836 3,665 4,023 5,315 4,457 3,413 4,064 3,834 4,746

8 104 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements Fig. 4 FWD survey. Table 3 Geophone position. Relative distance to the plate (cm) Geophone number (inner >>>>>>>> outer) D1 D2 D3 D4 D5 D6 D7 D8 D Modulus value/mpa Sample number compressive resilience modulus FWD back-calculation modulus Fig. 5 Comparison of compressive resilience modulus and FWD back-calculation modulus. modulus of the core samples with the back-calculation values via the FWD field test data at the corresponding points. The overall trend between the compressive resilience modulus and the back-calculation modulus is consistent. The average value of compressive resilience modulus is 4,132 MPa and the average value of back-calculation modulus is 4,583 MPa. It shows that FWD back-calculation modulus can reflect the stiffness and bearing capacity of asphalt pavement. Different back-calculation modulus could reflect different pavement structure damage. 4. Conclusions (1) The compressive strength test and compressive resilience modulus test of the semi-rigid core samples are different from those of the laboratory samples in terms of sample acquisition, sample selection and humidity requirements. In order to obtain the mechanical properties of the semi-rigid base with good representative and stable test values, the core-drilling

9 Study on Test Methods for Mechanical Properties of Semi-rigid Base Cores of In-service Pavements 105 location, size of core sample, smoothness and humidity conditions were analyzed. The test methods of compressive strength and compressive resilience modulus were proposed. (2) The compressive strength of lime fly-ash stabilized gravels base has a tendency of increasing during a long period. The compressive resilience modulus increases significantly with compressive strength of semi-rigid base. The compressive resilience modulus generally is 3-4 times than the recommended range of asphalt pavement design specifications. (3) Uniformity and stability of semi-rigid base have been improved with the increase of compressive strength of lime fly-ash stabilized gravels. The fluctuation range of compressive resilience modulus is obviously higher than the compressive strength. The compressive resilience modulus is more sensitive to the construction variability. (4) The overall trend between the compressive resilience modulus and the back-calculation modulus is consistent. FWD back-calculation modulus can reflect the stiffness and bearing capacity of asphalt pavement. Acknowledgment This work was supported by Beijing Municipal Science & Technology Commission Center-to-Local for Scientific Research (Z ). This work has been also funded by Beijing Municipal Commission of Transport Research Project KJ References [1] Ministry of Transport of People s Republic of China. Road mileage in 2015 (by pavement type) [EB/OL] _ html. [2] Guo, Y. X Disease Analysis and Structural Design Parameters Studies of Semi-rigid Base Asphalt Concrete Pavement. Doctoral thesis, Central South University. [3] Sha, A. M Material Characteristics of Semi-rigid Base. China Journal of Highway and Transport 21 (1): 1-5. [4] Jia, K., Sha, A. M., and Lu, J. Q Effective Modulus Value of Semi-rigid Base Course Materials. Journal of Chang an University 29 (1): [5] Sha, A. M., Jia, K., and Lu, J. Q Deterioration Laws of Dynamic Modulus of Semi-rigid Base Course Materials. China Journal of Highway and Transport 22 (3): 1-6. [6] Theyse, H. L., De Beer, M., and Rust, F. C Overview of South African Mechanistic Pavement Design Method. Transportation Research Record: Journal of the Transportation Research Board 1539 (1): [7] Liu, Z Study on the Initial Durability of Semi-rigid Base Material. Doctoral thesis, Chongqing Jiaotong University. [8] Sun, S. W Study on the Modulus of Semi-rigid Base Structure of End-Servicing Pavement. Doctoral thesis, Chongqing Jiaotong University. [9] Wei, J. C Research on Fatigue Damage of Semi-rigid Material and Structure of Asphalt Pavement. Doctoral thesis, Chang an University. [10] JTG E Test Methods of Materials Stabilized with Inorganic Binders for Highway Engineering. Beijing: China Communications Press. [11] BS EN Unbound and Hydraulically Bound Mixtures. Test Method for Determination of the Compressive Strength of Hydraulically Bound Mixtures. London: British Standards Institution. [12] BS EN Unbound and Hydraulically Bound Mixtures. Test Method for the Determination of the Modulus of Elasticity of Hydraulically Bound Mixtures. London: British Standards Institution. [13] BS EN Testing Concrete in Structures. Cored Specimens-Taking, Examining and Testing in Compression. London: British Standards Institution. [14] BS Guide to Assessment of Concrete Strength in Existing Structures. London: British Standards Institution. [15] CECS Technical Specification for Testing Concrete Strength with Drilled Core. Beijing: China Committee for Engineering Construction Standardization. [16] ASTM C42/C42M Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. West Conshohocken: ASTM International. [17] Bartlett, F. M., and Macgregor, J. G Effect of Core Length-to-Diameter Ratio on Concrete Core Strengths. ACI Materials Journal 91 (4): [18] JTGD Specifications for Design of Highway Asphalt Pavement. Beijing: China Communications Press.