Review of current and future bio-based stabilisation products (enzymatic and polymeric) for road construction materials

Highlights

• Investigation of novel biological components for road construction application.

• Large scale demonstration of functional components of best bio-based additives.

• Macro-structural criterion includes resistance to: abrasion, erosion, water absorption & compression stress.

• Evaluation of the mode of action of bio-materials using micro-structural techniques.

• Industrial wastes as auxiliary additives for in situ soil stabilisation.

Abstract

In situ soil modification is required in order to improve the primary engineering properties of the material to meet a road construction standard. Bio-stabilised soil is an environmentally friendly, cost-effective alternative to imported granular fills, concrete, costly hauling of materials or export to a landfill. In-service soil performance and required maintenance is highly dependent on methods of stabilisation, ranging from expensive mechanical stabilisation to chemical processes. As such, many alternative materials originating from bio-based sources are being explored as potential stabilising additives to improve weak subgrade soils (i.e., dispersive, erodible and collapsible soil, and soft or expansive clays). Some key solutions include the use of bio-derived enzymes, microbes, and polymeric additives to avert road failure caused by water penetration and/or erosion. The role of microbial substrate specialisation has been largely unexplored, since the level of research done on alternative stabilisers consists mostly of small ad hoc studies. In addition, research has focused on a reduction in permeability and an increase in compressive strength using enzymes and polymers, however, the complexity of these products and their implementation for a wide range of soil types and structural applications remain limited. Currently there is a need for more supporting research methodologies and systematic approaches on the implementation of bio-based materials for infrastructure development. This also includes the simplification of bio-based products for potential construction applications. This review provides (a) an overview of soil stabilisation techniques, (b) the primary challenges that lay ahead for future research in bio-based stabilisation products application in the road sector and (c) innovations to address the challenges of using modernised techniques in the road construction industry (i.e., weak subgrade and the required maintenance thereof, as well as the development of potential bio-based additives for unpaved road construction application).

Introduction

The growing urban populations and infrastructural expansion of developing countries are understandably creating an impetus for road development. Given the unprecedented pace and scope of these initiatives, it is vital to assess the potential consequences of large-scale road and transportation projects. New roads are vital pathways in the socioeconomic growth of developing countries. Thus poorly designed or implemented road projects can result in serious cost overruns, scarcity of natural resources, and negative environmental impacts (such as high global CO₂ emissions) (Alamgir et al., 2017). In practice, concrete construction of roads will always require a certain amount of maintenance and repair, albeit ironically still at a lower maintenance frequency requirement compared to asphalt roads. This is due to unexpected wear or weathering, micro crack formations, accidental overloading, increased porosity, or other phenomena related to specific changing environmental conditions resulting in early loss of functional properties (Jonkers et al., 2016). Ordinary Portland Cement (OPC), a typical civil engineering construction material is the most used material worldwide. However, production of cement is both highly energy consuming and environmentally unfriendly (Ivanov et al., 2015), with production reaching 10 000 million tonnes/year in 2013 and will increase by 100% within the next 40 years (Pacheco-Torgal and Labrincha, 2013). Typically road projects generation of greenhouse gas (GHG) emissions occurs throughout its lifecycle phases (i.e., construction and operation, albeit to lesser extent maintenance and rehabilitation) (Alzard et al., 2019). As expected, the construction industry will continue to grow at a fast pace just to accommodate urban population that will increase to 6.4 billion by 2050 (Pacheco-Torgal, 2016). Recent estimates on urban expansion suggest that by 2030 urban land cover will increase by 1.2 million km² (Seto et al., 2012). Therefore, the demand for alternative construction materials will also rise.

In developing countries, roads are expensive to develop and maintain. There has been a concerted effort in industry to reduce construction costs particularly in challenging ground conditions. However sustainable and environmentally friendly road development is a challenge to implement. According to the South African National Roads Agency Limited (SANRAL), the total asset value of South African roads is estimated at $200 billion (2014) with the value of the paved road network making up 80% of this (approximately $160 billion). New roads and vital repairs of existing roads typically cost $300–500 thousand per kilometre for a low volume paved rural road, while constructing and maintaining a heavy freeway structure can cost millions per kilometre (United States Dollar (USD)) (National Treasury, 2015, Siyepu, 2016). In developing countries, soil roads are constructed for low traffic volume areas such as access roads in rural areas or as estate roads. In South Africa (SA) there is ample evidence pointing to lack of adequate maintenance of current road infrastructure, as well as halted development of new roads. This is worsened by the persistent increase in traffic volumes as well as moisture damage caused by heavy rains. A lack of reliable road infrastructure also undermines prospects for future development. Unproclaimed roads, which are at best gravel roads that do not offer all-weather access is a prime example of sub-par infrastructure. According to Paige-Green (2008) these types of roads can have a significant negative impact on the economic well-being of the affected communities and also result in negative environmental impacts (i.e., soil lost due to erosion, dust clouds).

The construction industry is investigating new technologies such as, deep mixing, electro kinetics, nano-materials for sensors and sensing, as well as the use of micro-organisms for groundwater remediation (Wijeyesekera, 2014). These have effectively advanced the road construction practice, resulting in a ‘new’ interdisciplinary approach to civil and geotechnical engineering asset management by integrating concepts and techniques from other disciplines. The development of microbiological processes for improvement of the soil properties is one of the more recent manifestations of this trend.

Bio-based exploration for use in construction is a potentially cost-effective, and an environmentally sustainable engineering approach to recondition and strengthen the existing road base, sub-base, or subgrade materials for an extended lifespan (Pei et al., 2015). Biological stabilisers such as Bacillus spp. produce a wide range of metabolites and eco-friendly additives that offer cumulative nature of benefits for stabilisation and compaction in structural applications (Samang et al., 2018), such as lowered cost, environmental and performance-based benefits. Several Bacillus spp. and their derivatives have been considered as potential alternatives to conventional chemical soil stabilisers, particularly in challenging ground conditions. This is clearly an excellent example of the application of multi-faceted green technologies promoting multidisciplinary research and collaboration between road/pavement engineering, green chemistry, biotechnology and geomicrobiology. It is important to pursue these greener technologies, which necessitates research and development for new eco-friendly materials as an alternative to conventional cement. The significance of harnessing geomicrobiological research (using novel bioenzymes, biopolymers or waste, such as fly ash, to catalyse various reactions that stabilise in situ soil for road applications. This includes their mode/s of action) makes it possible to devise engineering solutions for temporary and/or permanent geotechnical engineering problems.

A significant economic incentive exists for developing new and innovative, yet environmentally safe bio-based product applications. The challenge is to find low-cost, high-volume applications of bio-based products for in-place soils, and to convert waste into value-added products. This approach could be used for numerous geotechnical applications such as dust suppression, dam control, wind soil-erosion control, earthquake liquefaction mitigation, construction of reactive barriers, and long-term stabilisation of contaminated soils. With respect to their mode of action, this presents an opportunity to discover novel micro-organisms through suitable assessment of their properties such as, secretion of one or more metabolic products, enzymes of interest, or biopolymers and investigate it as soil additives. It becomes clear that due to a lack of knowledge on bio-based soil stabilisation several areas of applied research need to be addressed in the biotechnology sector. Such as, appropriate upstream processes for biomass production, downstream processes to yield stable products (including the necessary in-vitro mechanisms and field testing), effectively formulated products and the appropriate application of each soil stabiliser under various environmental conditions. This is discussed in more detail in the paper.

Soil stabilisation is defined as a process of improving the physical and engineering properties of a soil to achieve a predetermined target level of performance (Latifi et al., 2016). Stabilisation of in situ material is considered to be an important method for improving the strength and performance of the treated soil material, which contributes to prolonging the service life of the subsurface and results in a more economical design (Mgangira, 2009a).

Bearing capacity of subsoil is one of the major site selection components in geotechnical design criteria. Once the bearing capacity of soil is deemed poor, the following options are considered (a) change the design to suit the site condition, (b) remove and replace the in situ soil; or (c) abandon the site. Abandoned sites linked to undesirable bearing capacities have dramatically increased, and the outcome of this is the scarcity of land and an increased demand for natural resources. The affected areas include those susceptible to liquefaction, those covered with high level clay material and organic soils which can be problematic because of low shear strength and high compressibility, contaminated land, as well as areas in a landslide (Makusa, 2013). Problematic soil is recognised by its sudden volumetric reduction (i.e. collapsible soils), after exposure by various environmental conditions such as, increasing humidity, wind deposited sand or silt, or under water immersing conditions, hence altering cementation bonds between soil particles (Ayeldeen et al., 2017).

Drainage problems are caused by a lack of diversion of rainwater, which can lead to deterioration of the road structure or pavement surface. This is mostly due to moisture retention within the road prism and in particular within the subgrade soils relating to typical subgrade problems, as described in Table 1 (Paige-Green, 2008). The adverse effects of increase in moisture content on the soil behaviour have also been a major concern among geotechnical and pavement engineers. Soil possesses excellent performance at the optimum moisture content or below the optimum moisture content (dry-side of optimum) however, the strength of soils reduce significantly as the moisture content increases beyond the optimum (wet-side of optimum). Sensitive soils tend to swell with moisture content increase, particularly in areas of higher rainfall, as a result subsurface soil becomes too weak to support the pavement or road loads resulting in collapsed road surfaces and potholes (Stabnikov et al., 2011). The replacement of weak in situ soil is not always a good option, especially in pavement or road developments due to the ongoing construction cost conundrum in cash strapped countries, which is the typical situation in most developing countries.

An opportunity exists to improve unpaved roads in both urban and rural areas. Unpaved roads are defined as those with a surface coarse of unbound aggregate (gravel), where no binder or chemical additive is used (Netterberg and Paige-Green, 1988). Good wearing coarse on unpaved or earth roads should have the ability to resist abrasive action of traffic, erosion by water and wind, freedom from excessive dust during dry weather and freedom from excessive slippage during wet weather. Over time of service, unpaved roads experience loss of the wearing coarse material, due to traffic as well as through surface material erosion caused by heavy rainfall. The fines content diminishes over time of service, resulting in loose coarse material which tends to accumulate. In traffic, loose coarse material induces fugitive dust, which affects air, soil and water quality, roadside flora and fauna, agricultural productivity, and road safety (Greening, 2011). Additionally, it is well established that long-term exposure to traffic generated dust can be attributed to serious health problems from the effects of exposure to high concentration of airborne particulates. This loss of material causes road surface deterioration that requires re-gravelling to recover the serviceability of the road. Re-gravelling involves re-grading the road, transporting and spreading new aggregates on the affected road. Re-gravelling is environmentally unsustainable and financially not feasible as it increases maintenance costs. Therefore, the strategy by road engineers is to improve the engineering properties of the in situ materials with chemical treatment. The benefit of treated surface material (using alternative non-traditional stabilisers) is the reduction in maintenance costs. This is due to the reduction in gravel loss, and therefore reduction in frequency of grading and re-gravelling. Controlled gravel loss reduces dust loading and air-borne emissions which are hazardous to human health.

The cost savings and benefits, over the initial and long-term life cycle of the road, are apparent (Andrews, 1999). Ongoing initiatives are therefore important for developing solutions to current road construction problems as they will contribute towards improved quality of life and safety, as a result of the reduction in traffic generated dust from the treated roads.

For improvement of soil structure and engineering properties conventional approaches include dynamic compaction, soil–cement stabilisation, soil–lime stabilisation, drainage of vegetation, geotextiles, as well as injecting synthetic materials such as micro-fine cement, epoxy, acrylamide, phenoplasts, silicates, and polyurethane into the pore space to bind soil particles together (Saleh et al., 2019). This is accomplished using a variety of chemicals, jetting, and permeation grouting techniques. It must be noted that these approaches create serious environmental concerns as all chemical grouts apart from sodium silicate may be toxic and/or hazardous (Bryson and El Naggar, 2013).

In most civil engineering projects, it is difficult to obtain a construction site that will meet all design requirements without introducing ground modification. Therefore, the current practice is to modify the engineering properties of the native problematic soils to meet the design specifications (Ikeagwuani and Nwon 2019). This leads to using alternative (non-traditional/biochemical) stabilisers which are one of several methods of soil improvement. In terms of viability, this may create a viable substitute to cement but it is all dependent on cost. Non-traditional additives are diverse in their composition and in the chemical and physical manner in which they interact with soil (Rajoria and Kaur, 2014). Unfortunately, little is known about their interaction with different geotechnical materials and their fundamental stabilisation mechanisms. Despite the lack of research, an environmentally friendly solution is still advantageous, as the larger construction companies are very conscious of their environmental footprint, and thus eager to invest in smart ‘green’ construction materials (Shehata and Poulos, 2018).