# A Review on Design Considerations and Connection Techniques in Modular Composite Construction
## 1 Introduction  
Concrete modular construction involves manufacturing 3D modules in an off-site factory, transferring them to the site, and assembling them on-site using different modular connections. These modules are prefabricated as three-dimensional volumetric units prefinished with fixtures and fittings in a factory. These modules are individual room elements assembled and connected to form a complete building through inter-module connections. Modular construction has become increasingly popular recently, offering numerous advantages to the construction industry. These advantages include a speedconstruction process, high-quality control, reduced on-site workload, minimal shuttering and scaffolding requirements, optimal use of materials, lower labour costs, sustainability, and a lower environmental impact [1; 2]. Owing to the advantages of modular construction and its rapid development in recent times, there has been a significant shift in research focus. Modular technology is widely used globally, with high prefabrication adoption rates in China, the U.S., Germany, and the U.K. [3; 4; 5; 6; 7; 8]. In China, improvements in off-site construction supply chain efficiency and Construction 4.0 (C4.0) technologies, such as building information modelling (BIM), have accelerated modular construction adoption, enhancing project efficiency and digital adaptability [3; 4]. In the U.S., modular construction is expanding in multifamily housing to address affordability and sustainability, driven by lean construction principles and digital transformation [5]. Germany's modular construction focuses on industrialised housekeeping (IHB), utilising advanced production planning, like multi-mixed-model assembly lines, to boost prefabrication efficiency and labour optimisation [6]. In the U.K., modern methods of construction (MMC) reduce site labour and expand off-site prefabrication. Adapted contractual frameworks, such as joint contracts tribunal (JCT) and design-build procurement, support modular adoption, with early collaboration being key [7; 8]. There has been a significant shift in the construction industry, moving towards more off-site prefabrication and reducing on-site construction to accommodate sustainability and labour demands [9; 10; 11]. Although precast technology is widespread globally, modular construction has only gained attention in recent years, as evidenced by the increasing number of research articles published in this field, as shown in Figure 1. China, the U.S., and Germany have published over 70 articles indicating their advancement in precast modular construction. The U.K., Hong Kong, South Korea, and Australia have published over 30 articles on modular construction, showing the emerging trend in prefabricated construction in those countries [12; 13; 14]. Following this descending trend, other countries have shifted their focus towards modular construction in recent years, which is reflected in their publications [15; 16]. Figure 2 shows the top 20 publication sources based on the number of articles on concrete modular construction published from 2000 onwards. It can be seen that the  
Figure 1: Country/region-wise publications on concrete modular construction in the 21 st century. leading publications on modular construction are from Engineering Structures, Beton- und Stahlbetonbau, and Journal of Building Engineering and Buildings. A significant upward trend since 2020 shows the rapid development of modular construction in recent years. Connections influence precast and modular constructions' structural performance, stability, and integrity. In contrast to monolithic construction, which inherently possesses continuity, modular and precast construction systems depend on connections to facilitate force transfer among discrete components.