Strengthening Substandard Road Materials with Nanoemulsion-Based Stabilisation: An Experimental Study

Abstract

This study considered using nanotechnology-based products to stabilise and enhance the properties of a substandard quality road construction material. Two stabilisers, herein referenced as Nano-A and Nano-B, were utilised to stabilise a G6 gravel material with a California bearing ratio (CBR) of 25% and a Plasticity Index (PI) of 4%. X-ray diffraction (XRD) tests showed that the material contained 58% Quartz with 36.5% clay minerals in the fraction passing the 0.075 mm sieve. The impact of the stabilisers on the material was evaluated through CBR, unconfined compressive strength (UCS), and indirect tensile strength (ITS) tests. The results revealed that both stabilisers significantly improved the bearing strength of the material by as much as 53 and 92% for Nano-A and Nano-B, respectively. Significant improvements were also noted in UCS and ITS results. The study also revealed that both rapid and 28-day curing conditions result in similar strength effects in the samples. Nano-B also showed great potential for improving the material’s hydrophobic properties as measured by the resistance to water damage. The research findings indicate an enhanced load-bearing capacity of the stabilised material, ensuring long-lasting pavement infrastructure. These enhancements demonstrate the effectiveness of Nano-A and Nano-B in improving the properties of substandard road construction materials. Tests on more substandard materials are recommended.

1. Introduction

It is necessary to use construction materials of sufficient quality and strength when building roads. However, as such materials are not readily available, alternative strategies are considered to improve available substandard materials to ensure their utilisation in road construction through various stabilisation techniques. Road material stabilisation improves substandard material properties through increased mechanical strength and load-bearing capacity. The selection of an appropriate stabiliser depends on several factors, including the material’s properties, compatibility, and desired improvements [1].

In South Africa (SA), road materials are classified as G1 to G10, referencing the quality of the road materials based on various properties particularly, the strength [2]. G1 to G3 are composed of high-quality crushed stone road materials, while G4 to G10 refer to natural granular materials ranging from gravel to silts and clays [2, 3]. G1 to G3 are costly and not readily available for road construction compared to G4 to G10, which often requires stabilisation to enhance their performance when utilised as road construction material.

Traditional stabilisers such as cement, lime, bitumen, and fly ash are some of the most commonly used products for road material stabilisation [4, 5]. However, these products often present challenges, including developing brittle materials susceptible to cracking and compatibility with certain material types, e.g., materials containing mica and clay minerals [6, 7, 8, 9, 10]. The limitations associated with these traditional stabilisers in road construction objectify the need to seek or consider alternative products suitable for the current demand.

Over the years, several alternative (non-traditional) products have been appraised with regard to their capacity to enhance substandard road materials [11, 12], including nanotechnology-based stabilisers. These stabilisers exploit nanomaterials’ unique properties to control the materials’ properties at the nano-scale [13, 14]. Advances in nano-science and nanotechnologies have made it possible to use this technology for pavement engineering [3, 7, 15], with initial trials concentrated on asphaltic materials to improve binder performance and durability [16, 17]. However, recent studies have shown that nanotechnology-based stabilisers can also be applied to granular materials for the construction of pavement structural layers. This includes the utilisation of bitumen emulsions modified with nanotechnology to effectively stabilise substandard road materials [3, 6, 7]. The modification of bitumen emulsions with nanotechnology improves tensile strength and provides resistance to deformation in road materials compared to un-modified bitumen emulsions [18].

Numerous studies [3, 6, 7, 18, 19, 20, 21, 22, 23, 24] on bitumen emulsions modified with nanosilanes indicated that these products have a unique advantage of drawing on the properties of nanotechnology to improve road construction material properties by forming siloxane bonds, one of the strongest bonds in nature [25]. These products are hydrophobic and thus impart this property to stabilised road materials such that they become water-resistant [18, 25]. Water resistance is an invaluable property, particularly for road materials found in tropical regions, which are susceptible to secondary weathering effects, usually through exposure to moisture. In this case, the bitumen emulsion acts as a carrier fluid, aiding the transportation and penetration of the nanomaterial into the intergranular network of the road material. Additional benefits of utilising bitumen emulsions modified with organofunctional nanosilanes in road material stabilisation include providing adhesion between particles and encapsulating fine aggregates.

Similar to the road material classification presented in TRH14 [2], encompassing categories from G1 to G10, cement-stabilised materials (C1 to C4), and bituminous stabilised materials (BT1 to BT3), Jordaan et al. [6] propose a classification system for road materials incorporating nanomaterial stabilisation, termed nano-modified emulsion (NME1 to NME4 and NM-EG5). In this classification, the “1” suffix denotes the highest quality road material, with subsequent suffixes indicating diminishing material quality. These classifications hold significant importance as they facilitate a meaningful comparison between the road material categorisation outlined in TRH14 [2] and that of road materials stabilised with nanomaterials.

As mentioned above, a number of commercial solutions have been developed and launched as workable nanotechnology-based alternatives to traditional bitumen emulsion in road material stabilisation. Therefore, a thorough evaluation of these solutions is required before considering their incorporation into standard processes for building roads.

A report elsewhere [26] indicates an abundance of substandard road materials in several locations in the province of KwaZulu-Natal that are unsuitable for road construction. KwaZulu-Natal is a provincial location in South Africa. It has a varied climate ranging from coastal humidity to inland aridity and different types of soils, including sandy soils or clay-rich soils, making it appropriate for studying the effectiveness of various road stabilisation methods in these complex environments. A research gap exists to investigate the potential of improving the performance of these substandard road materials to ensure their utilisation in building quality roadways through stabilisation with nanoemulsions as an alternative to traditional stabilisers.

This research considered two commercially available products for the stabilisation of substandard road materials. The stabilisers were modified bitumen emulsions incorporating nanotechnology products, herein referred to as Nano-A and Nano-B.

Nano-A is a non-traditional water-based modified bitumen emulsion with nano-polymer, and according to the product’s datasheet, the stabilising polymer of Nano-A has elastomers that require sufficient compaction to bind them to the soil matrix, enabling it to gain strength once cured. The curing procedure in Nano-A is achieved through moisture loss in the mix, a concept similar to reverse osmosis – moisture escapes from inside the mix into the atmosphere. Still, it prevents water from penetrating, forming a water-resistant layer as curing occurs. Nano-A requires ultraviolet light to enhance its strength property.

Nano-B is a bitumen emulsion modified with organofunctional nanosilane, enabling the fusing of soil granules with the nanosilane. As the emulsion breaks and water is expelled from Nano-B stabilised road material, a hydrophobic effect will be created in the material. There is limited information regarding the effectiveness and test procedure in stabilising road materials with Nano-A, unlike Nano-B, which has gained some attention in this regard. Furthermore, the recommendation and procedure for testing Nano-B are contained in the Draft document TRH24 [27] and will be adopted for this study.

The research aims to compare the performance of these two modified bitumen emulsions for stabilising substandard road materials for the construction of road structural layers. The study also assessed the materials’ resistance to moisture damage as well as the effect of rapid and 28-day aged curing methods.

The importance of this study is to ensure the use of in situ substandard road material in road construction through stabilisation with nanoemulsions, which will be instrumental in minimising the need to haul good quality road materials to the construction site.

2. Materials and test methods

Three materials were used in this study. The first is a G6 natural gravel road material obtained from Stockville quarry in Pinetown, KwaZulu-Natal, Eastern South Africa (SA). The others are two nanotechnology-modified bitumen emulsions, Nano-A and Nano-B. The choice of these stabilisers stems from their availability in the research region as nanotechnology products for road material stabilisation. Furthermore, both Nano-A and Nano-B are modified bitumen emulsions with nanotechnology and are claimed to have the capability to provide water-proving protection for road material. It is appropriate to subject both products to similar test conditions to evaluate and provide a fair comparison of their effectiveness in stabilising road material. Therefore, as discussed below, the test recommendation and procedure for nano-modified emuslion (NME) were employed for this investigation.

Classification tests were conducted on the road materials in accordance with relevant SA road material test methods, as shown in Table 1. The mineral composition of the road material and its compatibility with Nano-A and Nano-B are evaluated in Sections 3.1 and 3.2. Therefore, X-ray diffraction (XRD) scans were conducted on a whole sample of the material and on a sample of a fraction passing the 0.075 mm sieve per the literature review [20, 21].

Name of test	Aims	Test methods
Sieve analysis	To classify the construction material	TMH1, Method A1(b) [28]
Atterberg limits		TMH1, Method A2, A3 [28]
MDD/OMC	Strength determination	TMH1, Method A7 [28]
CBR		TMH1, Method A8 [28]
Indirect tensile strength		TMH1, Method A8 [28]
Unconfined compressive strength		TMH1, Method A14 [28]

Table 1.

Test methods for the investigation.

The two stabilisers used are proprietary products bound by copyright restrictions, so no chemical analysis tests were conducted on them. However, their physical properties are provided in Table 2.

Property	Nano-A	Nano-B
Colour/appearance	Dark grey	Thick, milky brown liquid
Flammability	None	None
Odour	Slightly	Slightly
Solubility in water	Yes	Yes
Toxicity	None	None
Boiling point	Same as water	Same as water
pH	8–9	10–11

Table 2.

Properties of the research stabilisers.

The gravel material was stabilised with 0.7 to 1.5% of Nano-A and Nano-B. The trends in mechanical strength properties were evaluated and compared. The test procedures espoused in Draft TRH24 [21, 27] for NME were adopted for material specimen preparation, which rely primarily on the guidelines in TMH1 and TG2 [28, 29]. Tests conducted included CBR, ITS and UCS, as presented in Table 1. Six specimens were prepared for each test, with three specimens tested for relevant properties immediately after curing. In contrast, the other tests were conducted after 4 hours of wet conditioning (soaking in water).

Air-drying specimens in the laboratory for 28 days can be time-consuming and lead to construction delays. However, any curing period can be used for testing, with 7, 28, and 90 days being the most common [30]. This study compared the two curing methods, rapid curing and air-drying for 28 days, by testing the UCS and ITS of specimens of Nano-A. Only Nano-A specimens were tested for both curing conditions, while Nano-B specimens were tested after rapid curing. This is attributed to testing limitations. The various strength tests performed on the specimens are shown in Figure 1.

Figure 1.

(a) Specimens, (b) CBR test arrangements, (c) ITS test arrangements, and (d) UCS test arrangements.

3. Results and discussion

3.1 Material classification

The particle size distribution curve of the road material is shown in Figure 2. The maximum aggregate size is 37.5 mm, and its grading coefficient G_c is 22.42. The material has a Grading Modulus (GM) of 2.3. Table 3 provides the results of additional classification tests conducted on the road material. Based on the results and referencing SA standards [2, 31], it is determined that the road material is a G6, which is relatively substandard natural gravel for pavement construction.

Figure 2.

Particle size distribution curve for the road material.

Liquid limit (%)	30
Plastic limit (%)	26
Plasticity index (PI)	4
Linear shrinkage	—
Coarse (%)	49.7
Sand (%)	42
Clay and silt (%)	8.14
Optimum moisture content (%)	5.8
Maximum dry density (kg/m^3)	2148
CBR at 93 Mod AASHTO (%)	25

Table 3.

Characteristic properties of the road material.

3.2 XRD test

The mineralogy report for the road material from the XRD test is presented in Table 4. The test on the sample indicates that it is predominated by quartz (58%), while the fraction passing the 0.075 mm sieve contains 36.5% clay mineral (23.9% Kaolinite and 12.6% smectite) and 7.7% mica. The quartz, a silica tetrahedron, is necessary to form siloxane bonds compatible with the stabilisers [32]. Clay and mica (especially the muscovite sub-group) are among the undesirable minerals in construction materials as they tend to absorb and retain water in compacted layers [2]. The combined quantity of clay minerals and mica (44.2%) is critical in determining the quantities of stabiliser necessary to mitigate their impact.

Mineral Groups	Total Sample	0.075 mm Fraction
Primary mineral – Not subject to weathering
Quartz (%)	58	20.4
Total	58	20.4
Primary mineral – Subject to weathering
Plagioclase (%)	20.1	17.8
Microline (%)	20.6	16.9
Diopside (%)	0.1	0
Haematite (%)	0	0.7
Total	40.8	35.4
Mica
Muscovite (%)	1	7.7
Total	1	7.7
Clay Minerals
Kaolinite (%)	0.2	23.9
Smectite (%)	12.6	12.6
Total	12.8	36.5

Table 4.

Mineralogy report for the road material.

3.3 Mixing procedure for the stabilisation

The appropriate application rate for the stabiliser was selected following the recommendation of Jordaan and Steyn [21]. The authors recommend the application rates based on the CBR of the unstabilised material, i.e., 32% (at 95% of Mod AASHTO density), the quantity of the material passing the 0.075 mm sieve (8.14%), and the quantity of the secondary minerals identified through the XRD tests, 44.2%. It is shown in Figure 3, based on these results, that the recommended stabiliser by mass lies in Zone 2. Based on the preceding details, the application range for both products, Nano-A and Nano-B, was 0.7 to 1.5%, with Nano-A applied at 0.7, 1, and 1.5%, while Nano-B was applied at 0.7, 1, 1.2, and 1.5%. ITS and UCS tests were performed at the specific application ranges for Nano-A and Nano-B. The CBR test was carried out at the optimum content determined from the ITS and UCS tests for Nano-A and Nano-B.

Figure 3.

Selection of an appropriate nano-modified emulsion technology for naturally available road material. Based on Jordaan and Steyn [21].

3.4 CBR test

The results of CBR tests on specimens stabilised with 1 and 1.2% of Nano-A and Nano-B, respectively, are presented in Figure 4. Nano-B had the most significant impact on the soaked CBR value due to its hydrophobic effect, which created a firm interlocking of the particles in the specimens. This was not the case for Nano-A stabilised material. The soaked CBR test is a crucial attribute for evaluating the bearing capacity of soil material on site when it is in contact with a water source (surface or underground).

Figure 4.

The California bearing ratio for the stabilised and unstabilised specimens at 93, 95 and 98% Mod AASHTO.

The CBR of unstabilised material at 93 and 95% Mod AASHTO met the minimum CBR requirement of 25% for a G6 material. However, the CBR of Nano-A stabilised material at 98% Mod AASHTO was below the minimum CBR requirement of 80% for a base course layer. On the other hand, Nano-B stabilised material exceeded the CBR requirement even at 93% Mod AASHTO. It is important to note that CBR tests are irrelevant for material selection for modified bitumen emulsions with nanotechnology. Still, they are included to accommodate traditional methods of material selection [21].

3.5 ITS test

The results of ITS tests for specimens stabilised with Nano-A and Nano-B at 0.7 to 1.5% are shown in Figure 5. Both Nano-A and Nano-B improved the performance of ITS specimens compared to the unstabilised specimens. The ITS_dry results for unstabilised specimens cured rapidly are 2.4% lower than those cured for 28 days. Specimens stabilised with 0.7% Nano-A cured rapidly have ITS_dry results that are 2.8% lower than those cured for 28 days in the air. The maximum ITS_dry results of 152 and 168 kPa were achieved for 1% Nano-A specimens cured rapidly and for 28 days in air, respectively. ITS_dry results of 149 and 161 kPa were achieved for 1.5% Nano-A specimens cured rapidly and for 28 days in air, respectively. The optimum content for Nano-A is 1%, as the strength attained at this content is greater than at 1.5%. Specimens stabilised with 0.7% Nano-B cured rapidly have ITS_dry results of 148 kPa, while the peak ITS_dry result of 152 kPa was achieved at 1.5%. The ITS_dry results for Nano-A specimens increase with increasing content, while the strength attained at 1 and 1.2% with Nano-B specimens is slightly lower than the strength at 0.7%.

Figure 5.

ITS for the stabilised and unstabilised specimens.

Another crucial aspect of this study is determining the specimens’ capacity to resist moisture damage during the 4 hours of soaking, which was evaluated through the retained tensile strength (RTS), as expressed in Eq. (1) [21, 28]. RTS measures the effectiveness of the tensile strength of the materials, expressed as a percentage of the wet and dry ITS specimens:

$$RTS=\frac{IT{S}_{wet}}{IT{S}_{dry}}\left(\%\right)$$E1

During the 4 hours of soaking the specimens in water, it was observed that the unstabilised specimens dissolved after about 30 minutes. Samples stabilised with Nano-A at 1 and 1.5% remained stable throughout the 4 hours of soaking in water and still were not tested for ITS_wet due to water absorption in the specimens, while specimens stabilised at 0.7% dissolved after about 2 hours of soaking. The ITS_wet of samples stabilised with Nano-B increases as the content of the stabiliser increases from 0.7 to 1.5%. At 1.2 and 1.5% of Nano-B, the ITS_wet design requirement in draft TRH24 [28] (>120 kPa) for NME4 (nano-modified emulsion) stabilised material was satisfied. A retained tensile strength (RTS) of 53.4, 81.0, 97.9%, and 97.4 were obtained at 0.7, 1, 1.2, and 1.5% of Nano-B. Therefore, the optimum content for Nano-B is 1.2% since no significant increments in strength at 1.5% were recorded. The ITS results for Nano-A and Nano-B showcased how they affect the strength properties of the specimens in the dry and wet states.

3.6 UCS test

As shown in Figure 6, UCS results for specimens stabilised with Nano-A and Nano-B and unstabilised specimens that were cured rapidly or for 28 days in the air are compared. The results for the UCS_dry (dry test) show a difference of 3.8% between the two curing methods, with the air-cured specimens recording 1.83 MPa and the rapid-cured specimen 1.76 MPa. Similar results were observed with the specimen stabilised with 1% Nano-A. A UCS_dry of 2.29 MPa was obtained for the air-cured Nano-A stabilised specimens, while 2.17 MPa was obtained for the rapid-cured, resulting in a difference of 5.3%. No significant improvement was recorded as Nano-A was increased from 1 to 1.5% in the specimens. The observed difference can be attributed to moisture variation in the specimens due to the curing conditions.

Figure 6.

UCS of specimens stabilised with Nano-A and Nano-B.

The results also show that the UCS_dry (dry test) for Nano-B performs better than Nano-A specimens, although a little decline in the UCS_dry was observed at 1 and 1.2% with Nano-B. The observed variation in strength between Nano-A and Nano-B can be related to their unique chemical composition.

The resistance to moisture damage of the specimens was also evaluated through the Retained Compressive Strength (RCS), expressed in Eq. (2) [21, 28]. RCS is a measure of the effectiveness of the compressive strength of the materials, expressed as a percentage of the wet and dry UCS specimens:

$$RCS=\frac{UC{S}_{wet}}{UC{S}_{dry}}\left(\%\right)$$E2

It was observed that the unstabilised UCS specimen dissolved within 30 minutes of being soaked in water for 4 hours. At 0.7%, Nano-A specimens dissolved within 2 hours, while those of 1 and 1.5% were stable throughout the 4 hours of soaking and still not be tested due to excessive moisture in the specimens.

RCS of 34.0, 67.0, 73.0, and 78.0% were obtained at 0.7, 1, 1.2, and 1.5% for specimens stabilised with Nano-B. UCS_wet for Nano-B specimens exceeds the UCS_wet design requirement of 0.75 MPa for NME4 stabilised material. In contrast, the minimum requirements of 65% for RCS were exceeded at 1 to 1.5% for Nano-B specimens, which indicates the specimen’s resistance to deterioration in wet conditions.

Overall, Nano-A provides no significant protection to specimens during the 4 hours of soaking in water, while Nano-B creates a hydrophobic effect in the specimen’s matrix during the soaking. However, UCS_dry results for Nano-A and Nano-B exceed the requirement by TRH4 [2] for a cemented natural gravel (C4) base course layer equivalent to natural stabilised material with nano-modified emulsions of category 4 (NME4). In terms of structural capacity, the NME4 stabilised layer can withstand a traffic loading between 1.0 MESA to 3.0 MESA [28]. These results indicate that Nano-B has the potential to improve the performance of stabilised road material in wet conditions, while Nano-A provides no significant improvement in this regard. The variations in the performance of specimens stabilised with Nano-A and Nano-B in wet conditions are closely related to their chemical compositions.

3.7 Conclusion and recommendations

This study compared the effectiveness of two nanotechnology-based stabilisers, Nano-A and Nano-B, in improving the strength of substandard G6 gravel material for use in a pavement structure. The results demonstrated that both Nano-A and Nano-B, at concentrations ranging from 0.7 to 1.5%, significantly improved the UCS_dry and ITS_dry of the specimens.

Notably, Nano-A performed well in enhancing the ITS dry specimens, while Nano-B exhibited the highest effect on the UCS dry specimens. Furthermore, the minimum RTS and RCS requirements were met with Nano-B concentrations at 1, 1.2, and 1.5%. The superior performance of Nano-B in wet conditions can be attributed to the presence of nanosilane in its composition, resulting in a hydrophobic effect on the specimen. The elastomers of Nano-A did not provide significant protection against water absorption in the specimens and, hence, failed to satisfy the moisture resistance capacity as mentioned in its product’s datasheet.

Interestingly, increasing the concentration of Nano-A from 1 to 1.5% did not significantly improve ITS_dry and UCS_dry results. On the other hand, Nano-B concentrations of 1.2 and 1.5% created similar effects on ITS_dry and ITS_wet results, but a slight improvement in UCS_dry and UCS_wet was observed as the Nano-B content increased from 1.2 to 1.5%. Thus, the optimum content for Nano-A and Nano-B was determined to be 1 and 1.2%, respectively.

The study’s insights into the curing process open new avenues for optimising construction schedules. The similarity in strength outcomes between rapid and 28-day curing conditions suggests that significant time savings could be achieved without compromising material quality.

The study findings indicated that the enhancement in UCS_dry with Nano-A and Nano-B exceeded the base course requirements for cemented-stabilised natural gravel material of category 4 (C4). Based on the CBR results at 95% Mod AASHTO, the Nano-A stabilised sample is suitable for the subbase, while the Nano-B stabilised sample is adequate for the base course at 98% Mod AASHTO. Applying these nanotechnology-enhanced stabilisers to substandard paving materials would facilitate the use of in situ materials for building quality roads.

Considering the investigation and analysis performed in this study, several recommendations are proposed for future research:

1. Further research is necessary to investigate the long-term interactions of the stabilisers with the road material.

2. This research was performed on only one road material type (G6); therefore, it cannot be generalised to all substandard materials; hence, further studies on other substandard road materials are recommended.

3. Variations in temperature, humidity, and wind and their effects on air-cured specimens should be evaluated to understand better and establish a correlation between air-cured and rapid-cured samples.

4. The effect of these stabilisers on the performance of materials with higher plasticity should be investigated to explore their potential for broader applications.

5. Lastly, the recyclability of the material stabilised with these stabilisers should be evaluated within the context of the circular economy and their compatibility with existing recycling equipment.

Addressing these recommendations would contribute to a deeper understanding of the stabilisers’ capabilities and allow for more informed decision-making in implementing nanotechnology-based solutions for pavement materials. Ultimately, these advancements would promote sustainable infrastructure development and contribute to the use of in situ materials, leading to the construction of durable and high-quality roads.

Acknowledgments

The authors would like to express their sincere gratitude to the manufacturers of the stabilisers for granting permission and support to conduct this investigation. Additionally, the authors extend profound appreciation to the University of KwaZulu-Natal, where the study was conducted, for providing the necessary resources and facilities that made this research possible. Special thanks are also due to the management of Giba Gorge Business Park and Soilform for their invaluable cooperation in supplying the gravel material used in the study. Their contributions have been instrumental in the successful completion of this research project.

Conflict of interest

The authors declare no conflict of interest in this study.