coatings Article Effect of Rejuvenating Agent on the Pavement Properties of Cold Recycled Mixture with Bitmen Emulsion Chun Li 1, Jian Ouyang 2,*, Peng Cao 3, Jingtao Shi 1, Wenting Yang 4 and Yuqi Sha 5 1 PetroChina Fuel Oil Limited Company Research Institute, Beijing 100195, China; [email protected] (C.L.); [email protected] (J.S.) 2 School of Transportation and Logistics, Dalian University of Technology, Dalian 116024, China 3 College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China; [email protected] 4 School of Civil Engineering, Dalian University of Technology, Dalian 116024, China; [email protected] 5 School of Energy and Powder Engineering, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] * Correspondence: [email protected] or [email protected] Abstract: A traditional cold recycled mixture with bitumen emulsion (CRMB) has a relatively low cracking resistance and moisture susceptibility, which greatly limits its application scope. A rejuvenating agent was employed to improve the pavement properties of CRMB. To avoid the rejuvenating agent having an adverse effect on the new bitumen, reclaimed asphalt pavement (RAP) was firstly treated by the rejuvenating agent, and the effect of rejuvenating time on the pavement properties of CRMB was investigated. Results indicate that the addition of a rejuvenating agent can greatly improve the ductility and moisture susceptibility of CRMB. Meanwhile, although the rejuvenating agent can soften aged bitumen, the addition of a rejuvenating agent can still increase the indirect tensile strength of CRMB and does not greatly reduce the rutting resistance of CRMB. This phenomenon exists because the rejuvenating agent can be both beneficial to the membrane Citation: Li, C.; Ouyang, J.; Cao, P.; structure of the bitumen emulsion mastic and aged bitumen. It can also greatly improve the bonding Shi, J.; Yang, W.; Sha, Y. Effect of interface between RAP and the bitumen emulsion mastic. The rejuvenating time between RAP and Rejuvenating Agent on the Pavement the rejuvenating agent can affect the pavement properties of CRMB. To obtain better pavement Properties of Cold Recycled Mixture properties, the optimum recommended rejuvenating time is between 5 and 7 days. Overall, the with Bitmen Emulsion. Coatings 2021, addition of rejuvenating agent can be a good choice to improve the pavement properties of CRMB. 11, 520. https://doi.org/10.3390/ coatings11050520 Keywords: cold recycled mixture; bitumen emulsion; rejuvenating agent; pavement properties; rejuvenating time Received: 27 February 2021 Accepted: 26 April 2021 Published: 29 April 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Environmental sustainability and cost-effectiveness are promoting the worldwide published maps and institutional affil- diffusion of low-energy and low-emission technologies for pavement construction and iations. rehabilitation. In this context, the recycling of reclaimed asphalt pavement (RAP) based on a cold recycled mixture with bitumen emulsion (CRMB) is one of the most effective and low environmental impact technologies. CRMB consists of bitumen emulsion, additional water, filler, unheated virgin aggregates, and RAP. Compared to conventional hot-recycled bitumen mixture, the maximum amount of RAP, generally between 70% and 100% of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the total aggregate blend, can be recycled in CRMB [1]. The effective utilization of RAP This article is an open access article can greatly preserve natural resources and reduce the disposal costs. No extra heating is distributed under the terms and required in the production of CRMB, so it can minimize energy consumption, dust, and conditions of the Creative Commons gaseous emissions [2]. As a result of these advantages, CRMB is becoming more preferred Attribution (CC BY) license (https:// in pavement construction and rehabilitation. creativecommons.org/licenses/by/ Although CRMB has been widely applied in the pavement construction and rehabili- 4.0/). tation, its application is mainly restricted to the base and subbase layers of motorways [3]. Coatings 2021, 11, 520. https://doi.org/10.3390/coatings11050520 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 520 2 of 16 Therefore, the surplus value of RAP cannot be maximized by this technology. To better reuse RAP, road engineers in China have been trying to use CRMB as a surface layer material (mainly intermediate and bottom courses of asphalt layer) in recent years. How- ever, there are many challenges to achieve this ambitious goal. The primary challenge is the mechanical properties of CRMB. CRMB has a high modulus and good rutting resis- tance [4–6] because cement hydrates can greatly increase the stiffness of bitumen binder [7]. However, because of the high void content and the addition of cement, CRMB features low tensile strength and low ductility [8,9], so that its cracking resistance is weak compared to conventional hot mix asphalt and hot recycling asphalt [10]. CRMB also has a high moisture susceptibility [11,12]. As a result, CRMB has high risks of cracking and moisture damage when it is employed in the surface layer. To reduce these risks, the pavement properties of CRMB should be greatly improved, especially for tensile strength, ductility, and moisture susceptibility. To reduce the risks of cracking and moisture damage, an indirect tensile test is sug- gested to be better than the Marshall test in the mix design of CRMB [3,9]. Meanwhile, fac- tors influencing the indirect tensile strength (ITS) and pavement properties of CRMB were intensively studied by scholars recently, such as cement content and cement types [13–15], other cementitious fillers and content [11,16], water content [9], bitumen content [15], ag- gregate gradation [17,18], curing condition [19–21], and compaction method [22]. These studies are all very meaningful to understand the effect of raw components and fabrication conditions on the ITS and pavement performances of CRMB and obtain the optimal mix design of CRMB. Comparing the results, the most efficient way to improve the tensile strength and moisture damage of CRMB is adding more cement into mixture. However, the deformability of CRMB can be greatly degraded if more cement is added [9], thus increasing the risk of cracking. The cement content of CRMB should be between 1% and 2% to prevent cracking in the application. Overall, the pavement properties of CRMB under the optimal mix proportion should be further improved to promote the use of RAP. To further improve the pavement properties of CRMB under the optimal mix pro- portion, the deficiencies of CRMB in the mechanical properties should be firstly analyzed. There are significant differences between CRMB and hot recycling asphalt in the materials composition and RAP treatment method. Firstly, the hot bitumen with a very low viscosity can easily wet and coat aggregate well. However, bitumen droplets are difficult to penetrate to the surface texture of aggregates. Its coating ability on aggregate is highly related to the wetting ability of bitumen emulsion. In this context, the addition of surfactants (i.e., superplasticizer and wetting agent), which can greatly reduce the viscosity of fresh cement bitumen emulsion mastic and the contact angle between asphalt emulsion and aggregate, can greatly increase the ITS of CRMB and have no adverse effects on its ductility [8,23]. Secondly, although the RAP content is small in the hot recycling asphalt, a rejuvenating agent is employed to restore the technical properties of the aged bitumen of RAP in the technology [24,25]. However, no treatment method is employed for RAP, and no extra heating is used to melt the aged bitumen in the application of CRMB; thus, the RAP is normally as black aggregates in the traditional CRMB. The high amount of RAP is only adhered by a small amount of cement bitumen emulsion mastic. As a result of the poor ductility and adhesive ability of aged bitumen, CRMB has relative low pavement properties. To further improve the pavement properties of CRMB, adding a rejuvenating agent to restore the technical properties of aged bitumen may be a good choice according to the experience of hot recycling asphalt [26]. With the above regards, a rejuvenating agent is tried to improve the mechanical properties of CRMB in this study. Since the reactivity between the rejuvenating agent and aged bitumen at ambient temperature is much gentler than that at high temperature, more concerns should be focused on the treatment process of RAP by rejuvenating agent in CRMB. To efficiently restore the aged bitumen in RAP and avoid softening the new bitumen in cement bitumen emulsion mastic, a better treatment method may be to firstly Coatings 2021, 11, 520 3 of 16 blend RAP and the rejuvenating agent and then cure this blending for several days. In consideration of this, the objectives of this paper are as follows: • To study the effect of a rejuvenating agent on the pavement properties of CRBM; • To find a suitable treatment process of RAP by rejuvenating agent; • To discuss the possible mechanisms of the effect of the rejuvenating agent on the pavement properties of CRBM. 2. Materials and Specimens Preparation 2.1. Materials Slow setting cationic bitumen emulsion, Portland ordinary cement P.O42.5, recycled asphalt pavement (RAP), crushed sand, and limestone powder were employed to fabricate CRMB. The Portland ordinary cement P.O42.5 is from Dalian Xiaoyetian Cement Co., Ltd. in Dalian, China. The properties of cationic bitumen emulsion are listed in Table1. The employed RAP was obtained from the surface layer of one motorway in Liaoning Province, China, whose gradation is shown in Figure1. The bitumen content in RAP was determined as 4.8% by combustion method. According to the gradation of RAP, virgin crushed sand, limestone powder, and cement were introduced to get a suitable aggregate gradation of CRMB, which is shown in Figure1.