**1. Introduction**

In recent years in China, the assembling of buildings with precast concrete structure has become an advanced construction technology with features of green, environmental protection and energy conservation for the building industry [1,2]. This produces an importance to the construction process of the cast-in-situ joint which relates to the entire performance and quality of monolithic precast concrete structure. In view of the monolithic precast shear-wall structures, great concerns have been made on how to safely anchorage of rebars in precast shear-walls. Methods for rebar splicing by grout-filled coupling sleeve, slurry anchor lap joints, closed-loop anchoring and their composites have been applied [3–6]. However, acting as the linking of the precast concrete walls together to subject the loads and seismic actions, the bonding performance of cast-in-situ concrete joints to precast concrete walls have not been studied sufficiently. This may affect the deformation and energy dissipation capacity of the structure. In practice, cracks along interfaces appear due to the weak bonding of cast-in-situ concrete to precast concrete. This forms a weak section of monolithic precast concrete structure. Meanwhile, the map cracking presents due to large drying shrinkage of the cast-in-situ concrete, and the cast quality problems of spongy surface and internal voids exist due to difficult compaction in narrow joint space.

**Citation:** Li, C.; Yang, Y.; Su, J.; Meng, H.; Pan, L.; Zhao, S. Experimental Research on Interfacial Bonding Strength between Vertical Cast-In-Situ Joint and Precast Concrete Walls. *Crystals* **2021**, *11*, 494. https://doi.org/10.3390/cryst11050494

Academic Editors: Antonella Sola and Amir H. Mosavi

Received: 4 March 2021 Accepted: 25 April 2021 Published: 28 April 2021

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To achieve the design criteria of equivalent cast-in-place for precast concrete shear-wall structures, the joint connection between precast concrete walls has been specified in China code JGJ 1 [3]. The strength grade of precast concrete should not be less than C30, and that of cast-in-situ concrete is better, one grade higher than precast concrete. Meanwhile, the interface of the precast concrete wall should be roughened or treated with groove keys. If a rough surface washed by pressure water is used, the rough area of the interface should be larger than 80%, and the roughness should not be less than 6 mm. However, by looking up the published literature, only a few studies were performed on the interface between cast-in-situ joint and precast concrete component [6–9].

Coming from the same bonding mechanism of concrete to concrete, studies on the bond of new to old concrete for the strengthening of existing concrete structures can be referenced to find the main influencing factors [10–13]. Firstly, the factors relate to the quality of the concrete interface. The interfacial bonding strength increases with the increasing strength of concrete, especially new concrete [10,14–16], and benefits from the spraying of cement paste on the original surface of precast concrete [10–13,17]. Secondly, the factors relate to the condition of the concrete interface. The roughening of old concrete surface is necessary to further improve the bonding strength of new to old concrete [18–20]. During the research process, several kinds of roughening methods have been applied, including indentation with steel bars, scraping with iron combs of different-shaped saw-teeth, artificial chipping, mechanical napping, sand blasting, washing to expose aggregates with pressure water, groove keys and rough formwork. This makes the interface zigzag with concrete protuberances or turns into a zone with certain thickness composited by the cohesive layer of binder paste and the permeable layer of interaction [13,17,21]. The bonding strength increases with the increase in the roughness of the old concrete. Comparatively, the best effect can be obtained with an exposed aggregate surface and mechanical napping surface [7–9,22–24]. Meanwhile, the interfacial bonding strength is also affected by the degradation of surface condition depended on the environmental actions such as carbonation, freezing and thawing, and chemical erosion. Even in a short time after casting (within 90 days), the bonding strength of new to old concretes decreases whatever the surface of old concrete is processed with different methods [24–26].

Therefore, the three kinds of factors mentioned above should be considered for the experimental study on interfacial bonding strength of cast-in-situ concrete to precast concrete. Differing from the artificial post-roughening of existing concrete surface for strengthening purpose, the interface roughening of precast concrete components should be industrialized in the precast factory. Therefore, the joint surface of precast concrete is always roughened by using the methods of mechanical napping, rough formwork, key groove or washing to expose aggregates by pressure water [7,8]. Meanwhile, a storage time after casting of precast concrete components is always created due to the out-of-sync of production and installation.

To make up for the lack of systematical evaluation of the bonding rationale for castin-situ joint to precast concrete walls, an experimental study was carried out in this paper. The precast concrete specimens were prepared in strength grade of C30 and C40, the micro-expansion self-compacting concrete was used for cast-in-situ joints in strength grade of C30, C35, C40 and C45. Three kinds of interfaces of precast concrete were made: the original, the closing net formed and the washed rough to expose aggregates. The interface of washed rough to expose aggregates was made with four levels of roughness. The storage time of precast concrete after demolding was considered at 14, 28, 56 and 90 days. The interfacial bonding strength of joint to precast concretes was experimentally studied for 32 groups of specimens by using the splitting tensile test. Results are analyzed, and the measures to satisfy the interfacial bonding strength equivalent to tensile strength of precast concrete are suggested.
