1. Introduction
Soil stabilisation through the addition of a binder (and water, if needed) in road construction increases or maintains the strength and durability by hydraulic binding or carbonation. This method is widely used for increasing the load-bearing capacity of weak subgrades by making capping layers and for executing road-base layers. After compaction and hardening, the soil–binder mixture acquires mechanical properties unlike those of unbound soil. These properties include indirect tensile strength (Rit) and unconfined compressive strength (Rc), both due to the bonding of the grout to the soil. The mixture of the bonded soil with cement results in a significant increase in the load-bearing capacity and strength of the subgrade. In addition, by transferring tensile forces, the soil layer stabilised by binders allows for a better distribution of loads than in unbound (non-stabilised) soils, and it is more durable and resistant to weather conditions, such as moisture and frost.
The most commonly used soil stabilisers include Portland cement, lime (hydrated or quicklime), fly ash, hydraulic road binders [
1,
2,
3,
4,
5], chemical stabilisers, polymers, bitumen emulsion [
6,
7,
8], and by-products [
9,
10]. The soil forms a semi-rigid layer after it is mixed with the binder and undergoes compaction and hardening. This is because its strength parameters have smaller values (
Rc < 5 and
Rit < 0.5 MPa) compared with those of traditional concretes. However, these low strengths significantly increase the load-bearing capacity of the stabilised subgrade, as determined by deformation modulus (
Ev) tests conducted in situ. This is beneficial because the stabilised subgrade of road structures has a relatively high elasticity and low susceptibility to shrinkage cracking.
Two systems are used in the design of binder-stabilised road layers: European Standard EN 14227-1 [
11] and EN 14227-15 [
1]. One is based only on the unconfined compressive strength,
Rc, and the other on the indirect tensile strength,
Rit, and the modulus of elasticity,
E (stiffness modulus), after a contractual period of curing [
12,
13,
14] under laboratory conditions. The curing time, which ranges from 28 to 360 days, depends on the type of binder used. Conventionally, 28-day curing is applied to cement-stabilised mixtures. Binder-stabilised soil specimens are normally compacted based on the required compaction ratio. After curing, the aforementioned mechanical properties are determined. This applies to laboratory conditions in which the mixture formula is determined for implementation in the field.
In practice, low-bearing soils are often stabilised using the in situ method [
2,
5] in a layer with a thickness of 15–40 cm, which is then compacted with a roller. A subgrade of low bearing capacity may occur under the stabilised layer, making it difficult or even impossible to compact the stabilised layer to the
DPr values assumed in the mixture design. This assumption is true for silty and clay soils, which become plasticised during the compaction process.
In road engineering, compaction is determined by the compaction ratio,
DPr:
where
ρd (g/cm
3) is the dry density of the soil skeleton determined in the field by a direct test and
ρds (g/cm
3) is the laboratory maximum dry density of the soil skeleton using the Proctor test based on the European Standard EN 13286-2 [
15].
European Standard EN 13286-1 [
16] allows the vibration hammer, vibration press, static press, or the Proctor apparatus to be used to determine the maximum dry density of the skeleton of unbound and bound soils and mixtures. Each of these has a different effect on the compaction of the material, and the methods cannot be interchanged [
17,
18,
19]. The consistent use of one selected compaction method throughout the stabilisation mixture design process is required.
The Proctor test is one of the most commonly used methods to simulate real field conditions in a controlled laboratory environment [
3,
20], and the tests presented in this study were conducted using this method. The required values of the compaction ratio in traffic engineering usually range between 98% and 103%
DPr, as determined by the Proctor test. Typically,
DPr ≥ 100%, and the requirements for other material properties after compaction and hardening (e.g., compressive strength,
Rc, indirect tensile strength,
Rit, California Bearing Ratio, CBR, and deformation modulus,
Ev) are related to this value [
21,
22,
23].
The mixture of soil and binder forms a load-bearing skeleton after the hardening process. The soil particles that are not bound by the binder act as a filler of the load-bearing skeleton and a shock absorber of external forces, imparting susceptibility [
24]. The unbound soil also serves as a shock absorber of internal forces (such as those due to temperature and shrinkage), causing soil stabilised by binders to be characterised by lower shrinkage than cement concretes. It has a completely different structure (semi-rigid) than unbound soil (flexible) or bound concrete (rigid). Although most of the relationships between the mechanical properties of hydraulically bound and unbound soils [
25,
26] have been established and properly documented, the mechanics of soils stabilised with a small amount of binder have not been studied. Analysed studies have mainly examined the differences in laboratory compaction methods [
20,
21,
27,
28] or focused on the effect of binders and different stabilisation additions [
1,
11,
29], as well as the comparison of individual test methods, e.g., tests between the static and dynamic stiffness modulus [
30]. Some of the research on soil stabilisation and its compaction concerned mixtures of high strength or the effect of a delayed compaction time and its influence on the parameters of the bound mixture, which is very important in this technology [
31,
32].
The analysed studies proved the significant influence of compaction on unbound soils due to their deformation properties. Regarding stabilised soils, does this indicate that compaction can be considered less important in the case of slightly stabilised soils with binders due to their lower deformability than unbound soils?
This study examines the justifiability of the above considerations. It presents the results of laboratory tests to determine the effect of compaction conditions on the mechanical properties of low-strength cement-stabilised soils. The individual values of compressive strength and indirect tensile strength were determined according to standard procedures [
33,
34]. To determine the dynamic stiffness modulus, a standard method [
35] was used with the necessary modifications considering the specificity of these mixtures, described further in
Section 2.3.3.
A novelty is that the tests were carried out on low-strength stabilisation mixes, using three Portland cement additives, and the samples were compacted with different compaction energies corresponding to 95%, 98%, and 100% DPr. Based on the research, the percentage values of the mechanical parameters under different compaction ratios in relation to values at 100% DPr were established. Furthermore, the percentage values can be used to predict how weak the strength parameter will be in relation to the assumptions made at 100% DPr, which may have engineering utility.
4. Conclusions
The analysis of the test results of soil stabilisation with low-strength cement showed that the values of the mechanical parameters: unconfined compressive strength, , indirect tensile strength, , and dynamic stiffness modulus, depend on the state of compaction. For the compressive this dependence was exponential in form, whereas for the indirect tensile strength and dynamic stiffness modulus, it was linear. The compaction ratio in the upper range, = 98–100%, had a significant influence on the unconfined compressive strength. On the other hand, the changes in compressive strength were much smaller and not statistically significant when < 98%. Moreover, the influence of the compaction ratio on indirect tensile strength and the dynamic stiffness modulus was not as clear and statistically significant as in the case of compressive strength.
It was observed that the rates of change in compressive strength and indirect tensile strength depend on the hardness of the specimen, which in turn depends on the cement addition. The comparisons of the percentage changes in compressive strength and indirect tensile strength at different compaction ratios with these parameters at the standard compaction of = 100% showed that the strongest mixtures were the least resistant to changes in the compaction ratio. Moreover, weaker mixtures with a lower cement content were found to be less resistant to compaction ratio changes for compressive strength and indirect tensile strength. A different trend was observed when the dynamic stiffness modulus values were analysed at different compaction states: the mixture with the lowest cement content was the least susceptible to compaction ratio changes.
The values of the parameters at a compaction equal to 100%
were taken as the standards for comparison in the analyses. The percentage values of the parameters relative to these standard levels (
Figure 11,
Figure 12 and
Figure 13 and
Table 6) have great engineering utility because they can be estimated at any compaction level, even when it is not technologically possible to obtain satisfactory in situ compaction. Furthermore, they can be used to predict how weak the strength parameter will be in relation to the assumptions made at 100%
.
There were linear correlations between the tested parameters: indirect tensile strength,
Rit, unconfined compressive strength,
, and dynamic stiffness modulus,
, for each compaction condition (
Figure 14,
Figure 15 and
Figure 16). It should be stated that the relationships between these tested parameters (
Rit,
) were constant for each compaction state.