DESIGN OF THE TEST SPECIMENS

In real engineering practice, the objective of strengthening a RC beam in shear would be to avoid the shear failure, forcing the shear strength to be higher than the flexural strength and, consequently, inducing a more ductile flexural failure. However, to be able to quantify the shear strengthening effect with the proposed technology, the objective in this experimental campaign was to raise the shear strength but without reaching the flexural strength.

Twenty small scale RC beam specimens were produced, all with the same geometry and longitudinal reinforcement. A rectangular cross-section of 80-mm wide (b), and 150-mm deep (h), is shown in figure 5.1. The total length of the beam specimens was 900 mm and the tests were carried out by loading the beams at a central point, testing both sides identically and at the same time. The longitudinal reinforcement was calculated to ensure enough bending strength to trigger shear failure and it was composed of one 16 mm steel rebar (As = 201 mm²) in beam. To achieve good anchorage conditions in such small beam specimens, both ends of the bars were welded to a plate. The beams were not internally shear reinforced. The desired concrete compressive strength was 40 MPa.

Considering a central point load configuration (figure 5.1b) for the tests and an ultimate strength for the steel longitudinal reinforcement fu = 642 MPa, as it will be shown later, the ultimate bending moment in conventional flexural analyses is Mu = 13.76 mkN and, consequently, a maximum load of Pbending = 80.94 kN, and a maximum shear force of Vbending

= 40.47 kN. Considering the shear analysis in compliance with (Comisión Permanente del Hormigón 2008) Standard, the maximum load is Pshear = 34.7 kN and shear force Vshear = 17.3 kN, without taking into account the coefficients of reduction of the material considered in the cited standard. Therefore, a shear failure is expected for the reference beams (not strengthened beams) and this failure load will be raised thanks to the addition of the external strengthening.

The 20 beam specimens, as well as the test cubes and test cylinders for the determination of concrete compressive and splitting strength, respectively, were cast from two different concrete batches, that define the two phases.

The shear span, a, was equal to 340 mm (figure 5.1c) and the effective depth, d, was calculated from Eq. 5.1:

Experimental program for active shear strengthening of RC beams using Ni-Ti-Nb wires

𝑑 = ℎ − 𝑟 − 𝜙_{𝑙𝑜𝑛𝑔}

2 = 150 − 15 − 8 = 127 mm (5.1) where,

- h is overall depth,

- r is covering of longitudinal reinforcement, and - 𝜙_{𝑙𝑜𝑛𝑔} is longitudinal reinforcement diameter.

Thus, the effective depth of the beams is d = 127 mm, with a/d approximately equal to 2.68.

The characteristics of the beam specimens of the 2 phases are summarized in table 5.1.

Figure 5.1. a) Main RC beam geometry characteristics, b) schematic drawing of central point load configuration, c) geometry and dimensions for reference beams (without external strengthening), and d)

geometry and dimensions for specimens with external strengthening (SMA wires).

Chapter 5

Table 5.1. Characteristics of the tested beam specimens

Phase 1 beams follows: the nomenclature begins with a short test code (1.1 to 10.2) for fast identification.

Experimental program for active shear strengthening of RC beams using Ni-Ti-Nb wires If this code is followed by “a” or “b”, it means that the beam specimen was tested twice, “a”

indicating the first test. Next, S3 (3-mm diameter spiral) or U3 (3-mm diameter U-shape stirrup) indicates the type of shear strengthening used, followed by “100” or “075”, indicating the pitch in mm of the Ni-Ti-Nb pseudo-rectangular spiral or U-shape stirrup. The next field consists of three letters, “UCR”, “PCR” or “COL”, indicating that the beam specimen was un-cracked when the strengthening spiral was placed and activated (UnCRacked), that it had been previously loaded until a shear crack appeared (Pre-CRacked), or that the beam specimen had already been tested until collapse, and, after the collapse, the strengthening spiral had been activated (COLlapsed). Note that the placement of the spiral or U-shape stirrup was always performed before carrying out any test or pre-cracking. The next term indicates whether the strengthening spiral had been activated before the beam test (A) or had been placed but not activated before the beam test (NA). Some of the beams have a last term indicating an additional characteristic for the specimen: if the lower part of the spiral is placed in a groove, this term is a (G) and if the spiral is wrapping only the compression chord of the beam (approximately the upper mid part of the cross section of the beam specimen), this term is a (S). To ensure the repeatability of the findings, two identical beams were tested for each set of criteria testing both sides identically and at the same time (for example, beams 2.1 and 2.2, where the second number indicates the first and the second tested beam specimens of beam type 2).

The first column of table 5.1 is the test number and its nomenclature designation, the second one is the age of concrete in days at testing, and the third and fourth columns are the concrete compressive strength, fcm (MPa) and the splitting strength, fsp (MPa), respectively. The fifth to eighth columns are referred to shear strengthening: 5^th is diameter and spacing of stirrups (or pitch for spiral strengthening) in mm, 6^th is the state of the wire (activated or not activated), 7^th and 8^th are the inclination angles in degrees of front and rear links (angle between longitudinal axis of the beam and the direction of the link). Finally, the ninth column is a comment about test performance and/or about geometry of strengthening.

Figures 5.2 and 5.3 depict beam geometry, strain gauge location and characteristics of every tested beam in phase 1 (fig. 5.2) and phase 2 (fig. 5.3). Note that the spacing of the external reinforcement (or pith for the spiral) near the point of load application is reduced so that it does not overlap with the load application plate.

Chapter 5

Figure 5.2. Beams 1 to 5 geometries and strain-gauge locations of the beam specimens for phase 1

Experimental program for active shear strengthening of RC beams using Ni-Ti-Nb wires

Figure 5.3. Beams 6 to 10 geometries and strain-gauge locations of the beam specimens for phase 2

Chapter 5

5.3 FABRICATION OF THE TEST SPECIMENS. CONCRETE AND