###### Abstract
The design model underestimates the bearing resistance of post-tensioning anchorage zones with rectangular cross-sections but overestimates it when the area is a square that is larger than the minimum anchorage spacing. The numerical results by [PERSON] and [PERSON] [8] showed that the ultimate bearing strength is hardly affected by anchorage spacing or specimen size if it is greater than the minimum spacing. If the wedge action is dominant and stress is solely concentrated immediately beneath the bearing plate, the result could be reasonable. However, additional confinement or strengthening by the surrounding concrete should be expected in a large specimen. Recently, some research groups [9; 10; 11; 12] investigated the use of ultra-high performance concrete (UHPC) in prestressed concrete beams where slender anchorage zones with less rebar reinforcement are required. Due to the high compressive and tensile strength of UHPC compared to conventional concrete, the use of UHPC could significantly reduce or eliminate the need for rebar reinforcement in the local anchorage zone; it can most likely be replaced by steel fiber. Based on extensive experimental and numerical research, [PERSON] et al. [11] proposed a design equation for the ultimate bearing strength in which the coefficient of strength enhancement by spiral rebar was formulated as a linear function of concrete compressive strength with positive slope. Theoretically, the greater strength of concrete should result in less strength enhancement by lateral pressure, so a more rigorous study may be required to clarify the effect of spiral rebar in the anchorage zone using UHPC. [PERSON] et al. [13] conducted a study on local anchorage zone behavior according to the relative size of the bearing plate based on numerical analyses, and they compared the results with the design equations of [PERSON] and Roberts-Wollmann [5] and those of European post-tensioning anchorage systems such as Dywidag and VSL. The study concluded that estimation of the design model is highly affected by not only the minimum anchorage spacing but also the confining reinforcement details. [PERSON] et al. [14] and [PERSON] et al. [15] carried out load-transfer tests of more than 30 specimens of post-tensioning anchorage concrete blocks. The research mainly focused on the effects of steel fiber and rebar reinforcement on the ultimate bearing strength of the local anchorage zone, but the relative sizes of the concrete cross-section and anchorage bearing plate were not considered. [PERSON] et al. [16] extended that research based on a numerical approach to identify the effect of horizontal ribs for the additional bearing area in special anchorage devices, and they proposed the modified bearing capacity equation based on the model of [PERSON] and Roberts-Wollmann [4; 5]. [PERSON] et al. [17] and [PERSON] and [PERSON] [18] focused on stress distribution, crack formation, and ultimate capacity with respect to the bursting force in the anchorage zone. [PERSON] et al. [19], [PERSON] et al. [20], [PERSON] et al. [21] performed on the influence of confining reinforcement on the bearing strength of concrete blocks. Their research took similar approaches as other research, such as the investigation of the existing and proposed design equations for better estimation. Concrete strain and crack opening on the specimen surface are very difficult to measure because the strain is highly affected by the crack that is irregularly formed. So most research investigated strength rather than strain or crack opening. In this study, 11 specimens were manufactured considering both the anchorage spacing (or concrete block size) and confinement effect and were tested to fracture in accordance with EAD 160004-00-0301 [22]. In the first group of specimens, the ratio of supporting concrete surface to bearing plate gross area (\(A/A_{g}\)) varied from 1.82 to 9.52. In the second group, the confinement effect of spiral rebar design was investigated by varying the spiral diameter and rebar size.