# Experimental Analysis of a Novel GFRP Stiffened Pipe
## 1 Introduction  
Fiber reinforced polymers (FRPs) have been extensively used as reinforcing and structural materials in industrial, chemical and civil infrastructure, due to their advantages of light weight, high strength, corrosion resistance, easy processing, design flexibility, etc. [1; 2; 3; 4]. Nowadays, glass-fiber reinforced polymer (GFRP) pipelines buried underground are growing sharply in many industries (e.g., oil and gas, nuclear power, etc.) due to their outstanding properties, such as corrosion resistance, structural flexibility, lower weight to higher specific strength, higher moduli, excellent fatigue properties, and low installation costs [5; 6; 7; 8]. GFRP pipe is a kind of thin-walled pipe. As mentioned in [9], the typical GFRP pipe has three layers, with GFRP composites forming the middle layer, namely the structural layer, to bear load. When the buried depth of a GFRP pipe exceeds 3.5 m, the pipe should be designed adequately rigid to withstand internal and external loads with minimum support from the side soil column [10]. The buried depth of the circulating water channel in a nuclear power plant is about 15 m. If the GFRP pipe is directly buried, the pipe stiffness (PS) needs to be exceptionally large, leading to a dramatic increase in the thickness of the pipeline and higher manufacturing costs. It is because thin GFRP layers can satisfy the required levels of pipe strength, but PS requirements necessitate employing thicker glass-fiber reinforced polyester (GRP) pipes [11]. Therefore, this paper aims to propose a light-weight design and low-cost manufacturing technique for buried composite pipelines, to enhance the PS to a level acceptable to GFRP pipe producers, with neither surplus strength nor a dramatic increase in project investment. For buried or underground pipes, thick GRP mortar pipes are frequently utilized as an economical alternative, where a sand layer impregnated with resin is incorporated into thestructural layers of GFRP pipes [11]. For instance, [PERSON] and [PERSON] [12] inserted an additional layer, made of sand with unsaturated polyester resin, into the structural layer of a GFRP mortar pipe. [PERSON] et al. [13] performed analysis on the plastic behavior and strain-rate sensitivity of cylindrical polymer mortar specimens. Another typical method has been to pour the reinforced concrete round the GFRP pipe withstanding the external dynamic and static loads, where the GFRP pipe is mainly used as the internal mold to bear the internal pressure of the system and the negative pressure of the pipeline is jointly borne by the GFRP pipe and the concrete trench. The above-mentioned combination modes are appropriate where the project investment is controlled within a reasonable range, but they bring several challenges. One major concern is delamination inclined to be induced into the composite-to-sand or composite-to-concrete interface [12; 14; 15], because of the different material properties. Thus, the pipe structure fails prematurely and finally loses load capability. Other concerns are the difficulties of expensive transportation and labor intensive installation, caused by the significant weight increase in the sand layer [16; 17; 18] or the reinforced concrete layer. To tackle these problems, pipe structures with light weight and improved concrete-GFRP interfacial bonding have captured increasing attention. For instance, composite sandwiched structures consisting of stiff composite face sheets combined with core layers of light-weight materials have been proposed [19]. To further improve the stiffness of the core, extensive research has been made regarding the design, manufacturing, and mechanical evaluation of composite sandwiched structures with stiffened cores (e.g., lattice or corrugated structures) [20]. To name a few; [PERSON] et al. [21] compared different design configurations (e.g., CFRP and GFRP) used for the sandwich structure's skin, and three different types of wood as core for composite bridges. [PERSON] et al.