| Author: | MALIK, Numan |
| Title: | Performance of fiber reinforced polymer composite concrete piles in marine soils ended in rock sockets under axial cyclic and static loadings |
| Advisors: | Yin, Jian-hua (CEE) Yin, Zhen-yu (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Piling (Civil engineering) -- Testing Fiber-reinforced concrete Fibrous composites Offshore structures Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxviii, 268 pages : color illustrations |
| Language: | English |
| Abstract: | Traditional pile foundations in harsh marine environment experience steel corrosion, concrete deterioration, and timber degradation, resulting in various issues such as spalling, structural failure, and huge maintenance costs. Besides, consumption of large quantities of river sand and fresh water in the construction industry posing threat to river ecosystems, increased flooding events, and depletion of natural resources. Although seawater and sea-sand which are easily available at offshore construction projects provides an alternative solution, however traditional piling materials may experience corrosion and degradation issues with it due to salinity. The fact that fiber reinforced polymer (FRP) composites do not exhibit significant long-term degradation in typical marine environment makes it a promising alternative to steel as a reinforcing material in seawater sea-sand concrete (SSC). Therefore, replacing steel with FRP composites will be an advantageous approach. Pile foundations are often subjected to cyclic loadings caused by wind, water currents, waves, earthquakes, traffic loads and ice sheets. For instance, offshore wind turbines generate millions of rotating blade cycles on supporting piles, and pile foundations supporting the transport system always experience significant axial cyclic loads. The cyclic behavior of piles is influenced by major cyclic loading parameters such as frequency, cyclic amplitude, mean load values, the number of cycles and loading history. Although numerous research works have been carried out on the cyclic response of piles in sand, silt and soft soils, the behavior of piles ended in rock-socket under cyclic loading, with emphasis on the shaft friction behavior, is rarely reported. Apparently, it is necessary to deeply study cyclic loading influence on rock-socketed piles based on systematic tests to provide guidance and potential predictive measures. Conventional measuring devices like strain gauges and vibrating wire extensometers provide discrete strain data at certain points leading to inadequate information of the entire pile’s response. Fiber optic sensing techniques have overcome the limitations by providing distributed strain profiles, long sensing range choices, anti-corrosive, high spatial resolution, easy operation and installation, presenting a better pile monitoring solution. Given the research limitations, this thesis reports a systematic study to investigate the performance of innovative and sustainable pile foundation design namely fiber reinforced polymer (FRP) composite seawater sea-sand concrete (SSC) piles in marine soils ended in rock sockets, through a series of physical model tests under axial cyclic and static loadings. A total of six model piles with three different structural configurations (FRP tube confined, FRP rebar cage reinforced, and centered FRP rebar reinforced) were tested. A novel hybrid optic sensing technique in which discrete (multiplexed FBGs) and distributed sensors based on optical frequency domain reflectometry (OFDR) with higher spatial resolution of 1mm and high sensing accuracy of ±1με were used to measure strain profiles along the entire length of model piles. The OFDR sensors monitor the distributed strain profiles providing load distribution of the entire pile, identifying any localized regions of weakness, strain concentrations, or pile shaft non-homogeneity with higher accuracy and hence overcoming the limitations of traditional monitoring sensors. The main findings are listed as follows: (1) The pile head load-displacement response of different model piles presented a similar trend within a loading threshold and was different (hardening or softening) beyond a transition point. The FRP tube confined pile showed higher ductility and capacity comparatively, suggesting the best solution for field applications in terms of mechanical performance. (2) The strain distribution along the depth of piles showed a similar trend for the model piles with higher strains recorded in the region (0 - la/4) from the pile head. The failure of FRP tube confined and centered FRP rebar reinforced SSC piles happened within this region near the pile head. However, the FRP rebar cage reinforced SSC pile showed the maximum deformation in the same region near the pile head and rock surface. (3) The pile body gained cyclic stiffness when the maximum cyclic load level (Qmean+Qcyc) was below 30% of Qus, and degradation was observed under higher load conditions. (4) The axial strain profiles within rock-socket were utilized to develop load transfer curves to calculate reliable shaft friction values that may be used in future pile design of similar conditions. (5) The maximum shaft resistance was mobilized up to 4.5 MPa under cyclic loading in the rock socket with higher mobilization observed in the upper one-third region of the socket. The conventional design underestimates the shaft friction along the interface between the rock and the model pile. (6) The distributed circumferential strain profiles provided reliable information of the localized strain concentrations around the pile circumference, showing early detection of pile shaft cracks, lateral deformation, bending direction, and position accurately. (7) The predicted buckling load based on analytical solutions and actual buckling load from tests were in fair agreement with a minor discrepancy due to localized strains near the pile head. |
| Rights: | All rights reserved |
| Access: | open access |
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