ReviewAchieving sustainability in roller compacted concrete pavement mixes using reclaimed asphalt pavement aggregates – state of the art review
Introduction
Roller compacted concrete pavement (RCCP) is a special type of concrete mixture, typically zero slump, widely popular due to its economic and fast-to-construct features (Debbarma et al., 2019c; Meddah et al., 2014; Williams, 2013). The history of RCCP applications dates back to the early 1940s, wherein it was used as an airport runway in Yakima, United States (ACI, 2001). Since then, RCCP has been extensively used for military facilities, ports, intermodal facilities, and low-to-moderate traffic streets and secondary highways in the United States (ACI, 2014). The distinct feature of RCCP is its higher fines content, typically about 16% higher than in conventional Portland cement concrete pavement (PCCP) (Harrington et al., 2010). The fines fill the voids and contribute to tight packing and consolidation of the RCCP mixture. Due to this distinct feature of better aggregate-interlocking, RCCP has a better initial load-carrying capacity than PCCP (Mardani-aghabaglou et al., 2013). However, the flow of traffic is not recommended until the pavement achieves a compressive strength of 13.8–17.2 MPa (Harrington et al., 2010). In terms of economy, the initial construction cost of RCCP is typically about 10–30% lower than that required for PCCP (Modarres and Hosseini, 2014; Mohammed and Adamu, 2018; Ramezanianpour et al., 2017). This advantage is mainly due to its lower cement content, non-requirement of formworks, dowel bars, heavy machinery, and extensive labour (Lam et al., 2017; Naik et al., 2001; Vahedifard et al., 2010; Yerramala and Ganesh Babu, 2011). As far as the construction practices are concerned, RCCP is usually placed with standard asphalt pavers and compacted using traditional vibratory drum rollers (Cao et al., 2000; Kim, 2007; Rooholamini et al., 2018; Williams, 2013). With proper compaction, it can achieve a compressive strength similar or equivalent to that of a PCCP mixture (ACI, 2014).
Over the past decades, the excessive utilization of natural aggregates for various construction activities has resulted in its shortage supply. As a result, the government authorities levied a ban on quarrying activities to preserve the natural aggregates for future demand (Singh et al., 2018a). The use of reclaimed asphalt pavement (RAP) as a replacement to natural aggregates has drastically increased in the past decades. RAP is the material obtained after the milling or removal of existing asphalt pavement for various maintenance and rehabilitation activities (Kumari et al., 2018; Monu et al., 2020; Singh et al., 2019a; Singh and Ransinchung, 2020). The generation of such wastes in huge quantities undoubtedly causes impact to landfills and increases the RAP stockpiles (Shi et al., 2019b, 2019c). These wastes if not properly managed and re-utilized, would lead to an increase in the generation of wastes at an unprecedented level causing several environmental-related concerns (Shi et al., 2019d). Although RAP is being recycled in asphalt pavements, excess RAP still remains unused, and hence, its use in concrete pavement mixes has drastically increased over the few years (Shi et al., 2018a, 2018b).
RAP has been utilized in three different fractions viz, coarse RAP, fine RAP, and combined RAP, respectively, for the productions of concrete mixes. Coarse RAP is the fraction of RAP which is larger than 4.75 mm but smaller than 25 mm, whereas, fine RAP is that fraction which passes the 4.75 mm standard sieve. Few researchers have also investigated the use of combined RAP which consists of both the coarse and fine fractions. Irrespective of the type of RAP fractions utilized, all researchers reported that the mechanical strength of the concrete could reduce by up about 10–81% (Abraham and Ransinchung, 2018a, 2018b; Al-Oraimi et al., 2009; Brand et al., 2015; Brand and Roesler, 2015; Delwar et al., 1997; El Euch Ben Said et al., 2018; Erdem and Blankson, 2014; Hassan et al., 2000; Hossiney et al., 2010; Huang et al., 2006; Okafor, 2010; Singh et al., 2017c; Thomas et al., 2018). The lower specific gravity of RAP and the formation of poor bonding between the RAP and mortar has been reported as the main reason for the reduction in the concrete strength (Ben Saïd et al., 2017; Mukhopadhyay and Shi, 2016). On the contrary, the use of RAP has a positive effect on the toughness due to its viscoelastic nature (Delwar et al., 1997; Huang et al., 2005). Although concrete is a quasi-brittle material, the failure mode in RAP concrete is more ductile as reported by Shi et al. (2020b). Most researchers recommended the use of coarse RAP up to 50% only for the production of PCCP mixes unless some specially treated RAP aggregates were utilized. Otherwise, other fractions or higher proportions of RAP should be utilized in lower layers of pavement or low-cost paving mixes (Brand and Roesler, 2015; Shi et al., 2017; Singh et al, 2017a, 2019b). Since RCCP is a low-cost paving mixture, the use of RAP in RCCP would be the next best option. Although the use of RAP in RCCP may reduce its mechanical strength, it may be advantageous in terms of the initial load-carrying capacity of the pavement wherein strength due to hydration is not a major influencing factor. Therefore, this paper aims to review the potential of RAP for the production of RCCP mixes to achieve sustainable development.
Section snippets
Research significance
This review is an effort to discuss and address several issues about the potential of RAP aggregates for utilization in RCCP mixes. Although many researchers have presented critical reviews on the suitability of RAP for conventional cement concrete mixes, the same has not been investigated for RCCP mixes. Moreover, the literature suggests the utilization of RAP aggregates for lower layers of pavements or for pavements serving low-intensity traffic only due to which RCCP best serves its purpose.
Physical properties of RAP aggregates
Although RAP aggregates are similar to natural aggregates, they are usually of inferior quality. Due to presence of low-density asphalt coating, the specific gravity of RAP usually ranges between 2.26 and 2.63 (Boussetta et al., 2018; Courard et al., 2010; Debbarma et al, 2019a, 2019c; Fakhri and Amoosoltani, 2017). Meanwhile, asphalt being hydrophobic, the water absorption of RAP is usually lower than natural aggregates (Boussetta et al., 2018; Courard et al., 2010; Debbarma et al., 2019c;
Mix design methodologies
The mix proportioning of RAP-RCCPs have been mostly designed based on the ACI 327 (ACI, 2014), PCA (PCA, 2006), and IRC: SP-68 (IRC, 2005) code of practices, respectively. All these codes of practices recommends the soil compaction method (modified proctor method) as the most easy and simple method for the formulation of the RCCP mix design. The modified proctor method as per ASTM D1557 (ASTM, 2012) involves the establishment of a relationship between the density and varying moisture contents
Optimum moisture content (OMC)
Since RCCP is a stiff mix concrete, Abram’s law does not hold valid during the initial design stage. Hence, the determination OMC is an essential design parameter for RCCP mixes (Fakhri and Saberik, 2016). The OMC value of a typical RCCP mixture made with natural aggregates usually ranges between 4 and 7% of its total volume. When RAP is utilized in place of natural aggregates, this OMC value usually decreases owing to the hydrophobic nature of asphalt (Debbarma et al., 2019a; Modarres and
Compressive strength
The type and age of RAP has a significant effect on the compressive strength of the RAP-RCCP mixes. For instance, the use of a relatively lesser-aged RAP results in a much higher compressive strength reduction than in the case of a highly-aged RAP when utilized in RCCP mixes. Debbarma et al. (2019c) reported that the compressive strength could decrease by about 26–67% when lesser-aged RAP was utilized. Whereas, a percentage compressive strength reduction of about 9–37% was observed upon the
Effect of RAP on fatigue life and fracture properties
RCCP strength parameters are considered to be the main input that controls the thickness of the pavement slab because it directly relates to the fatigue life of the material. Ferrebee et al. (2014) highlighted an important issue relative to the structural design of RCCP: does RCCP with RAP aggregates results in different fatigue life for the same stress ratio? and subsequently, is a thicker slab required due to its lower compressive strength. Modarres and Hosseini (2014) found that the fatigue
Abrasion resistance
Abrasion resistance is one functional property which needs to be vigorously addressed if an RCCP mixture is designed to act as the surface layer of the pavement. Again not much research has been executed in this context except only two research paper available (as per authors’ knowledge) wherein the abrasion resistance of RCCP mixes containing two different RAP-type was investigated (Debbarma et al, 2019a, 2019c). The abrasion resistance was measured in terms of loss in mass occurred due to
Economic and environmental benefits of RAP-RCCP
RAP being a recycled aggregate, the reduction in its cost is obvious. Singh et al. (2018c) and Singh et al. (2018b) demonstrated that the use of RAP could lower the cost of producing a cubic meter of PCCP and DLC mix by about 8% and 34%. Meanwhile Monu et al. (2019) observed 67% and 18% reduction in the cost and gas emission when RAP was used in dense-bituminous macadam mixes. Since RCCP being 10–30% times cheaper than conventional PCCP, the reduction in the cost of the mix becomes as high as
Why does RAP concrete fails?
The distinct feature of RAP is that it contains a layer of asphalt, usually ∼6–9 μm thick, around its circumference (Huang et al., 2006). Asphalt is a viscoelastic material which is mainly composed of high molecular hydrocarbons and is hydrophobic in nature (Saboo et al., 2018; Singh and Kumar, 2019). On the other hand, cement is the main binding material in a concrete matrix. In concrete made with natural aggregates, in the presence of water, cement chemically reacts due to ionization and
Implementation strategies for RAP-RCCP mixes
Literature suggests that RAP aggregates could be utilized in various paving applications such as base layer of conventional concrete pavements, low-volume roads, residential streets, or even as the surface layer depending on the type of RAP and other mineral admixtures considered. However, its utilization in field-based application is still way behind owing to hesitation amongst the government agencies and highway engineers. Some strategies to increase the utilization of RAP in RCCP mixes are
Conclusions
The following conclusions were drawn from the comprehensive review carried out on the utilization of RAP for the production of RCCP mixes:
- 1.
Asphalt being hydrophobic, using RAP could lower the OMC values of RCCP mixes but at the same time the OMC could be increased in the presence of agglomerated particles. Not only that, RAP obtained from uncontrolled milling technique could accumulate dust contaminants from the underlying layers which in turn could increase the OMC further. On the other hand,
CRediT authorship contribution statement
Solomon Debbarma: Methodology, Data curation, Formal analysis, Writing - original draft, Writing - review & editing. G.D. Ransinchung R.N.: Conceptualization, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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