Subbase and subgrade are the foundation of the pavement structure. It has to provide a short-term surface for traffic and a suitable platform for the placement and compaction of the high-quality asphalt layer above. In addition, it has to work as a load-bearing system for the completed pavement during the required design period (Brown & Selig, 1991).
A measure of material stiffness, which is a parameter used in the mechanistic-empirical pavement design procedure, is the resilient modulus (
The
To avoid large deformations caused by traffic load, pavement construction materials are improved by adding various additives. A lot of research was conducted on materials that might be used as additives, such as cement (Saxena et al., 2010; Ismail et al. 2014), lime (Ahmed et al., 2020; Yin et al., 2022), fly ash from different sources (Yilmaz, 2015; Mahvash et al., 2018; Noaman et al., 2022; Wang et al., 2022; Zimar et. al, 2022), and various types of fibres (Consoli et al., 2003; Yetimoğlu et al., 2003; Khattak & Alrashidi, 2006). The use of fibres as a reinforcement increases stiffness (Festugato et al. 2023), load-bearing capacity (Yetimoğlu et al., 2005; Rashid et al., 2017), the tensile strength of soils (Consoli et al., 2013; Li et al., 2014) and reduces potential liquefaction (Ibraim et al., 2010).
As the literature concluded (Consoli et al., 1998; Park, 2011; Hamidi & Hooresfand, 2013), cement addition increased the stiffness of the soil and its brittleness, while the fibres added to sandy soil changed its failure behaviour from brittle to more ductile one.
In tests conducted on compacted sand reinforced with 50 mm long polypropylene fibres, the soil monotonic mechanical parameters were improved (Festugato et al., 2023). Fibres also had an impact on the cyclic behaviour of dense sand evidenced by an increase in cycles to failure. Fibres incorporated in cemented sandy soil caused an increase in unconfined compression strength (Consoli et al., 2010, 2013; Khodabandehlou et al., 2023).
The aim of this article is to investigate the influence of the dispersed reinforcement – polypropylene fibres added in different lengths and amounts – on the resilient response of the coarse material with the slight cement addition (1.5%), used as a base or subbase in road pavement construction. Polypropylene fibres incorporated in the tested soil amended with cement had lengths of 12, 18 and 40 mm and were added in amounts of 0%, 0.2% and 0.3% by weight of dry soil.
The authors concluded from their previous research that it is not possible to provide the resilient modulus tests on non-cohesive soil without fines because the cyclic triaxial test destroys unbound non-cohesive soil (Zabielska-Adamska et al., 2021, 2023). The soil was tested as hydraulically bound with a 1.5% cement addition, chosen as the minimum amount that could improve the resilient characteristics of the tested soil. The fibre addition higher than 0.3% was not considered because it decreased the static mechanical properties of the tested soil (Zabielska-Adamska et al., 2023).
Laboratory tests were conducted on gravelly sand grSa (EN ISO 14688-1), which particle size distribution according to the EN 933-1 standard is illustrated in Figure 1. The tested non-cohesive soil is poorly graded (EN ISO 14688-2). Its coefficient of uniformity,
Grain size distribution curve of tested soil.
The gravelly sand (grSa) was tested alone, as soil stabilised with the addition of 1.5% Portland cement 42.5R, and with an addition of polypropylene fibres with variable lengths 12 mm, 18 mm and 40 mm, and diameters 0.030 mm, 0.034 mm and 0.90 mm, respectively. Fibres were added in the amount of 0.2 and 0.3% to the dry mass of the tested soil. In the case of fibres 40 mm in length, only an addition of 0.2% was used. The images of the fibres are presented in Figure 2.
Different lengths (and diameters) of fibres in the amount of 0.2% in reference to the dry mass of the sample that was added to the coarse soil: a) 12 mm (0.030 mm), b) 18 mm (0.034 mm), c) 40 mm (0,90 mm).
It can be observed (Fig. 2) that the fibres with 40 mm long have a different structure than 12 and 18 mm reinforcement. It was also visible after mixing with the soil, that the mix with 12 and 18 mm was more homogenous than the soil with the reinforcement 40 mm long, which can be observed in Figure 3. Mixtures of soil and polypropylene fibres were mixed using a laboratory mechanical stirrer for a time of 4.5 minutes, which is of great importance for the homogeneity of the tested samples.
Soil mixed mechanically with fibres: a) 40 mm long; b) 18 mm long.
The compaction curves of gravelly sand and sand stabilised with cement are presented in Figure 4. The exemplary compaction curves of soil with fibres 18 mm long are presented in Figure 5. The specific density of the tested samples and their compaction parameters, optimum moisture content (
Compaction curves of gravelly sand and gravelly sand with 1.5% of cement addition compacted by two methods: a) SP, b) MP.
Compaction curves of gravelly sand with different amounts of fibres 18 mm long compacted by two methods: a) SP, b) MP.
The specific density and compaction parameters of tested materials.
grSa | 2.65 | 9.7 | 1.974 | 9.6 | 2.022 |
grSa+1.5%C | 2.66 | 9.5 | 2.010 | 9.5 | 2.070 |
grSa+1.5%C+0.2%F_12 mm | 2.66 | 9.4 | 2.075 | 7.9 | 2.150 |
grSa+1.5%C+0.3%F_12 mm | 2.66 | 8.9 | 2.103 | 6.8 | 2.170 |
grSa+1.5%C+0.2%F_18 mm | 2.66 | 7.9 | 2.066 | 7.0 | 2.183 |
grSa+1.5%C+0.3%F_18 mm | 2.66 | 8.0 | 2.060 | 7.8 | 2.124 |
grSa+1.5%C+0.2%F_40 mm | 2.66 | 8.8 | 2.108 | 7.5 | 2.144 |
where: C – cement addition, F – fibre addition
Based on Figures 4 and 5 and Table 1, it can be concluded that the cement addition has an impact on the maximum dry density of the tested material. In both compaction methods, standard and modified, the
The main test of resilient modulus
Figure 6 is an example of the cycles to which the samples were subjected during the resilient modulus test. The maximum applied vertical stress on the soil sample is
Principle of triaxial testing with cyclic loading
Samples for resilient modulus tests were prepared in a bipartite mould of 70 mm in diameter and a height-to-diameter ratio of 2. At first, dry components: soil, cement and polypropylene fibres were mixed using the laboratory mechanical stirrer, and next the required amount of water was added to get optimum moisture content. The samples were compacted dynamically in three layers to the values of maximum dry density from the SP and MP compaction tests. The
In Figure 7 one of the initial cycles of loading/unloading is shown, where the principle of the resilient modulus test is presented. In one cycle, the soil sample is subjected to loading and unloading and the permanent and recoverable deformations are visible. After each load application, granular material undergoes some irreversible deformation (Brown, 1996). Figure 8 presents the complete 100 cycles of the 15th sequence of loading and unloading according to AASHTO T307 standard. The reduction of permanent strain with successive load/unload cycles in a given sequence is clearly visible.
Sample behaviour under cyclic loading in a triaxial cell during one of the initial cycles of the 15th sequence subjected to grSa+1.5%C+0.3%F_18 mm sample compacted with SP method.
Sample behaviour subjected to cyclic loading/unloading in the triaxial cell during 100 cycles of the 15th sequence for grSa+1.5%C+0.3%F_18 mm sample compacted SP method, tested after 7 days of curing.
The data obtained from the resilient modulus
Resilient modulus
Resilient modulus
Values of
grSa+1.5%C | 197 | 168 | 7 |
272 | 353 | 28 | |
grSa+1.5%C+0.2%F_12 mm | 73 | 165 | 7 |
162 | 177 | 28 | |
grSa+1.5%C+0.3%F_12 mm | 193 | 224 | 7 |
178 | 236 | 28 | |
grSa+1.5%C+0.2%F_18 mm | 271 | 215 | 7 |
202 | 230 | 28 | |
grSa+1.5%C+0.3%F_18 mm | 301 | 226 | 7 |
214 | 241 | 28 | |
grSa+1.5%C+0.2%F_40 mm | 51 | 140 | 7 |
145 | 153 | 28 |
It can be observed (Tab. 2) that the addition of fibres can decrease the values of resilient modulus, especially in a shorter time of curing. In the previous article of the authors, the plasticisation of samples after fibre addition was proved (Zabielska-Adamska et al., 2023). Samples compacted by the SP method with fibres 18 mm and 12 mm (only 0.3%) gained lower
Depending on their position in the pavement structure, the structure materials are subjected to different loads from the traffic. The
It can be concluded (Figs. 11–14) that the resilient modulus is affected by deviatoric and confining stress. The
Based on the test results of gravelly sand compacted with cement and dispersed reinforcement, the following conclusion can be drawn:
Addition of fibres to gravelly sand stabilised with 1.5% cement decreases the resilient modulus, independent of the percentage amount of fibres and their length. Resilient modulus for gravelly sand with 1.5% of cement addition and different amounts of variable length of the dispersed reinforcement showed the lowest value in the case of fibres 40 mm long, regardless of the compaction method and curing time. It might be connected to the structure of the fibres and the low homogeneity of this soil–fibres mixture. The highest In the case of the SP compaction method, commonly the resilient modulus was lower for the samples cured for 7 days, than for the samples cured for 28 days. In the case of the MP compaction method, the differences between values obtained for various times of curing were slightly smaller. During the resilient modulus test of the gravelly sand samples with cement and fibre addition, the hardening behaviour might be observed, as the