The Impact of Polypropylene Fibre Addition on the CBR Value

One of the basic parameters for determining the bearing capacity of the subbase and subgrade is the California Bearing Ratio (CBR) test. The method was developed in the 1930s in the USA by the California State Highway Department and subsequently applied to the design of road pavement structures [1]. Determination of the CBR is a static test and consists in establishing the pressure needed to press a standardised piston into a sample of the soil to be tested, with reference to a standard compacted crushed rock [2]. Studies carried out by engineers indicate that the standard CBR method does not fully capture the specific conditions of shear forces acting on the subsoil. However, CBR values are greatly related to the determination of compaction, so this test can be used as a method of earthwork evaluation [3]. The CBR test is one of the generally accepted tests for determining the bearing capacity of the soil in Poland.

The CBR test is applicable to both the design of the upper road layers as well as to the aggregate base and subbase. Bases or subbases characterised by poor technical parameters and not meeting the CBR requirements can be improved by using various types of additives. The additives that have a significant effect on improving the bearing capacity are chemical additives such as: cement, lime, and calcium fly ash. A review of the literature shows that the most common chemical additive is cement. The percentage of cement additions was for sand, e.g. [4]: 2.5%, 5% and 7.5% and for clay, e.g. [5]: 5%, 7%, 10%, 12%, 15% and 20%. Cement stabilisation has shown a positive influence on soil compaction parameters, increasing the dry density of the soil skeleton while decreasing the optimum moisture content [6]. Tests of the CBR parameter conducted on clayey sand with a cement addition in the range of 5–7.5%, showed that the CBR increases with an increase in the stabiliser addition [7]. Lime is also one of the oldest and most frequently used additives. There are different types of lime used for soil stabilisation, e.g.: Calcium Oxide (CaO) or Calcium Hydroxide (Ca(OH)₂). The addition of such a stabiliser ranges from 5% to 10% relative to the dry mass of the soil [8]. The addition of lime also significantly improves the parameters of the CBR values. A review of the literature shows that the optimum value of an additive to the soil tested, which was clay, is about 10%. With a higher amount of additive, the CBR parameter started to decrease [9].

The addition of stabilisers is not the only method to improve soil strength parameters. Synthetic fibres are an increasingly popular addition. They are resistant to aggressive chemical environments and have a low self-weight and high abrasion resistance. The percentage addition of exemplary polypropylene fibres to the dry sand mass is in the range of 0.1–0.5%. An additional property of synthetic fibres affecting the strength parameters of the soil mix is their length. The most commonly used fibre lengths are 12, 18, 25 and 40 mm [10, 11]. The addition of fibres as a dispersed reinforcement to sand increases both the compactibility of the soil and its strength parameters. Sand with the addition of 0.5%, 0.75% and 1.0% fibres showed an increase in shear strength with increasing fibre content [12].

An increasingly common case is to reinforce the soil with a stabiliser and fibres at the same time. The influence of sand mixture with 1.5% cement addition and different amounts of fibres (0.1%, 0.2% and 0.3%) of 18 mm length was studied, where the positive effect of these 2 additives on the soil strength parameters was also observed [13]. In the other study of sand, the amount of cement ranged from 2% to 8%, while the amount of dispersed reinforcement in the form of polypropylene fibres (3, 6, 9 and 12 mm in length) ranged from 0.5% to 2.5%. The highest soil strength parameters were for 0.5% fibre addition. It was also observed that the greater the fibre addition, the more problematic the proper mixing of the soil [14].

A review of the literature shows that the amount of research on non-cohesive soil stabilised with fibre and cement is not numerous. In addition, high percentages of fibres to dry mixture were investigated, therefore it was decided to conduct research with small percentage additions of cement and fibres. The tests were conducted on two samples of the same natural soil taken from the same deposit. The results of the CBR values were determined for the natural soil and the soil with the different addition of cement (1.5%, 3%, 4.5%, 6%), as well as for their mixtures with 18 mm long polypropylene fibres at 0.1%, 0.2% and 0.3%. The effects of compaction and curing time of the samples stabilised with hydraulic binder were also determined. The samples were compacted using the standard and modified Proctor methods at the optimum moisture content, and then were tested immediately after compaction and after 7 and 28 days of curing.

Table 2 summarises the results obtained from the CBR test on gravelly sand and its mixtures with cement and fibres. The table includes a classification according to compaction method, time of curing and application of an additional load of 2.44 kPa. Figure 6 shows a variation of the CBR values with the percentage of polypropylene fibres, while Figure 7 shows the evolution trend of the CBR values with the percentage of cement addition and curing time for both compaction methods.

Figure 6.

Figure 7.

The gravelly sand tested without any admixtures showed relatively high values for the CBR values. The loading of the sample by 2.44 kPa had an increasing effect on the CBR. Sample I obtained higher CBR values than sample II, this is due to the fact that sample I is characterised by better graining coefficients (C_U and C_C) and has a higher density. The compaction method also has a significant effect on the CBR values. Samples compacted using the modified Proctor method achieved even more than the 2–3-fold increase in the CBR parameter, greater for sample II than for sample I. The additions of 0.1% and 2% polypropylene fibres resulted in over twice the growth of the CBR for samples compacted using the standard method. Interestingly, increasing the fibre percentage to 0.3% resulted in a reduction in the CBR values of more than 10%. For samples compacted using the modified Proctor method, the observations are different. Only the sample with 0.2% fibre addition achieved a slightly higher value than the specimen without the fibre addition. The values are as follows: 71.72% and 68.28%. It should be noted that, when we compare the compactibility of the material with the addition of fibres (Figure 4) and the dependency in Figure 6, a similar relationship can be seen. Samples with 0.2% polypropylene fibre addition obtained higher values for compaction parameters and the CBR than those with 0.3% fibre addition. Samples II compacted by the modified Proctor method with 0.1% and 0.3% fibres achieved lower CBR values than those without fibre reinforcement. Although the fibre addition had a positive effect on the compaction of the reinforced sand, the CBR values deteriorated.

A review of the literature shows that several researchers have reached similar conclusions regarding the reinforcement of non-cohesive soil with polypropylene fibres. The addition of fibres to the sand resulted in a decrease in the strength of the soil, especially at higher stresses. It was concluded that the reinforcement changed from brittle medium to a more plastic medium [13, 22]. From the further literature review, we can also find papers where the authors have a different opinion and indicate that the addition of fibres has a positive effect on the strength parameters of the soil [23].

Analysing the effect of the cement addition at different percentages (1.5%, 3%, 4.5%, 6%) two different cases can be seen. When the CBR value was tested immediately after mixing the soil with cement, it can be seen that the addition of the stabiliser reduced the CBR values. This tendency is noticeable for both compaction methods, although it is more pronounced in the case of the modified Proctor, as can be seen in Figure 7. Analysing Figure 7, we can also see that the CBR value increased with the addition of cement, but only when the sample was cured for 7 or 28 days. The curing time has a significant effect on the obtained results since the mixture of cement, soil and water needs an appropriate time to reach its maximum strength parameters. In general, the longer the treatment time, the higher the CBR index results.

The addition of 0.3% fibre to the cement-stabilised soil also increases the CBR values. The improvement is evident both depending on the curing time and the method of compaction. Increasing the duration of curing from 7 to 28 days increases the parameter slightly for the standard method and approx. 30% for the modified method. Analysing the sample compacted by the standard Proctor method and curing by 28 days, an increase of approximately 2-fold in the CBR value can be observed. In the case of modified compaction, the increase in the CBR value is only about 20%.

From the literature review, it appears that many researchers have investigated the effect of the addition of polypropylene fibres on the parameter of the stabilised soil. Their studies show that the fibres in lime-stabilised clay mainly act to increase its plasticity [24]. There are no publications in the literature that directly refer to non-cohesive soil stabilised with cement reinforced with polypropylene fibres.

Based on the research conducted and the analysis of the results obtained, the following conclusions were formulated: 1)

The fibre additions affect the compaction of gravelly sand and fibre mixture, improving it. However, the addition of 0.3% fibre worsens the compaction, compared to 0.2% fibre, which is more visible for the modified compaction.