PSI - Issue 81

Yuriy Panchuk et al. / Procedia Structural Integrity 81 (2026) 509–513

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non-prestressed reinforcement. Beam deflections at midspan were measured using 6PAO deflection gauges. Additionally, dial indicators were installed to record beam displacements at the supports. To measure slippage of the prestressed reinforcement at the beam ends, dial indicators were also used. The initiation and development of cracks were recorded visually, and crack widths were measured using a microscope. At each loading stage, the load was maintained for 5 minutes to record instrument readings. The main strength and deformation properties of concrete are presented in Table 1.

Table 1. Strength and deformation properties of concrete f b,prism , MPa E b0 , MPa (η=0)

f bt,prism , MPa

E b , MPa (η=0.3)

λ R

32.1

28830

25340

0.353 2.4

3. Results and discussion For all tested beam specimens subjected to a single short-term load, failure occurred in the normal cross-section when the stresses in the tensile reinforcement reached the yield strength, accompanied by crushing of the compressed concrete. The failure mode was characterised by an uncontrolled increase in deformations (beam deflections) with spalling of the most highly compressed concrete face between the concentrated loads. The destroyed part of the compressed concrete exhibited a brittle structure intersected by longitudinal and transverse cracks. Analysis of the stress-strain state of the beams showed that the tensile reinforcement, as well as the concrete in both the tensile and compressed zones, behaved jointly within the stress range from η = 0 to η = 0.3, with elastic deformation of the materia ls. At η = 0.3, the reinforcement strains were ε sp = 0.00319 and ε s = - 0.00252, while the concrete strains were ε b = 0.0000118. The midspan deflection of the beams decreased from f = - 2.25 mm (η = 0) to f= - 0.54 mm. The height of the compressed concrete zone was 10.73 cm. With an increase in the stress level to η = 0.5, the first cracks appeared in the tensile concrete zone beneath the points of application of the concentrated forces. Due to the reduction in stiffness of the element, a redistribution of stresses occurred in the beam cross-section. As the cracked portion of the tensile concrete was excluded from load transfer, the tensile forces were mainly carried by the working reinforcement. The relative strains of the reinforcement increased to ε sp = 0.0 0396 and ε s = 0.000233, indicating an inflection point in the reinforcement strain diagrams. The height of the compressed concrete zone decreased to 8.03 cm, while the deflection increased to f = 1.77 mm. Within the stress range of 0.5 ≤ η ≤ 0.7, a slight slowdown in the growth of compressed concrete strains and a minor reduction in the height of the compressed zone were observed. At stress levels exceeding η = 0.7, an accelerated increase in tensile reinforcement strains, a further reduction in the heig ht of the compressed concrete zone, and a rapid growth of beam deflections occurred. Immediately prior to failure, the relative strains in the working reinforcement and in the extreme compressed concrete fibre were ε sp = 0.00588, ε s = 0.00249, and ε b = 0.00205, respectively. The height of the compressed concrete zone decreased to 6.04 cm, and the beam deflection reached f= 11.9 mm. The stress in the most highly compressed concrete fibre exceeded the prism compressive strength by a factor of 1.72. Beam failure occurred in the tensile zone when the stresses in the tensile reinforcement reached the yield limit and was characterised by a gradual failure process. Upon the onset of reinforcement yielding, a sharp increase in beam deflections was observed, followed by crushing of the compressed concrete zone, which is typical for beams with a slenderness ratio h/l < 0.5. The average ultimate bending moment of the flexural elements was Mu = 19.0 kNm, while the crack width reached a crc = 0.5 mm. During testing of the reinforced concrete beams with mixed reinforcement, no slippage of the prestressed reinforcement within the concrete was recorded throughout the entire loading process. The results of the experimental studies of reinforced concrete beams with mixed reinforcement subjected to a single short-term load are presented in Table 2.

Table 2. Results of experimental studies η M u , kNm ε b , mm/mm

ε sp , mm/mm

ε s , mm/mm x, cm f, mm a crc , mm

0 0

-0.000450 0.00291 -0.000322 0.00297 -0.000168 0.00303 0.0000118 0.00319 0.000314 0.00350 0.000662 0.00396 0.000903 0.00442 0.001097 0.00481 0.001376 0.00556 0.001721 0.00588

-0.000412 -0.000396 -0.000363 -0.000252 -0.000096 0.000233 0.000554 0.000819

-

-2.25 -1.72 -1.11 -0.54 0.39 1.77 3.42 4.76 7.53 9.17 11.91

- - - - -

0.1 1.9 0.2 3.8 0.3 5.7 0.4 7.6 0.5 9.5 0.6 11.4 0.7 13.3 0.8 15.2 0.9 17.1 1.0 19.0

11.84 11.56 10.73

9.19 8.03 7.38 7.15 7.05 6.71 6.04

0.05 0.15

0.2

0.00121 0.00170 0.00249

0.35 0.45

0.00205

-

0.5