Torsional Strengthening of RC Beams with Continuous Spiral Near-Surface Mounted Steel Wire Rope

Six rectangular RC beams with 250 mm depth, 150 mm width and 2000 mm length were cast using ready-mixed concrete. The beams were designed intentionally to display torsion failure at beams’ central part. The length of the central part was set to 1.0 m to allow for the formation of single spiral crack at the angle of 45° within the full length of the test region part of the beams. Figure 1 illustrates the steel reinforcement details and the cross-section dimensions of the beams. Accordance to the (ACI 318 M-14 2014), all beams were under-reinforced to simulate torsion-deficient beams under possible loading condition. The minimum spacing of the transverse reinforcement was also intentionally exceeded allow the observation of torsional failure and prevent the stirrup from restricting the torsional cracks. The groove details for all five strengthened beams are shown in Fig. 2. Grooves with a width of 20 mm and a depth of 20 mm are cut in the concrete beam cover area of 25 mm, and steel wire rope (Ø8 mm) was embedded in these grooves. For these beams, the c/c perpendicular spacing’s of grooves were set to 113, 141, 188, 283 and 566 mm.

For all six beams, ready-mixed concrete was used. The supplied concrete’s compressive strength (f′c) was evaluated after 28 days. In calculating the compressive strength, the average results of three concrete cylinders (100 mm in diameter and 200 mm in height) on the day of the test were considered. Sikadur-30P epoxy was used as an adhesive to fill the grooves of the strengthened beams.

Three bars with the same diameters and three pieces of steel wire rope (Ø8 mm) were subjected to uniaxial tensile tests to determine their yield and ultimate strength (f_y, f_u) as recommended by (ASTM A370-10 2010).

Table 2 summarises the mechanical properties of the concrete, stirrup steel bar, steel wire rope, the average compressive strength of the concrete (f′c) and the average test results for the steel bars. Table 3 shows the epoxy’s properties depending on the specifications supplied by the manufacturer.

After 28 days of curing, grooves were cut into the concrete cover of the specimens to install the strengthening steel wire rope. These grooves were cut in the spiral direction around the beam cross section, angle of spiral inclination Ø was 45° and dimensions were greater than 1.5 db × 1.5 db (ACI 440.2R-08 2008) (where db refers to the NSM steel wire rope reinforcement diameter). For cutting, a special concrete saw (Handle Grinder) was used with a diamond cutting saw blade. A hammer drill and chisel were used to remove the remaining concrete lugs and roughen the grooves’ deeper surface. As shown in Fig. 3, these grooves were then smoothed and cleaned with a wire brush and a high-pressure air jet. As shown in Fig. 4, the strengthened steel wire rope was inserted into the spiral grooves by fixing in one end with the aid of anchor fix and stretching in the other end with the aid of small stretcher tool. These grooves were then filled around the steel wire rope with epoxy adhesive groove filler (Sikadur-30LP) and the surface was levelled as shown in Fig. 5. The beams were stored for 2 weeks to maintain the full strength of the epoxy.

The rig of the test is shown in Fig. 6. A 200-ton hydraulic jack on the active support was used to transfer the load. The load had a lever arm of 500 mm from the vertical beam axis. To measure the load applied periodically, a compression load cell with a capacity of 100 tons was used. The hydraulic jack had a moving length of 250 mm and the beams were lengthened longitudinally after cracking. To prevent any lateral constraints and avoid any resulting compression, the beams were allowed to move freely by supporting the ends of the beams on the rollers at the un-resistance support. The angle of twist of the free end (the point of application of the torque) was determined using a dial gauge at that point with the aid of the downward length of the lever arm.

The loading frame in the laboratory of the civil engineering department was used to apply the load on the beam samples. A proper support condition was arranged to allow rotation about the longitudinal beam axis and the lever arms were attached to the specimen to provide a torsional moment, as shown in Fig. 6. When the position of the lever arm happens to coincide with the support, the beam was constantly subjected to pure torsion. The lever arm was kept beyond the two supports in order to apply simultaneous bending and torsion.

The hydraulic testing machine in the civil engineering laboratory was used to test the beam samples, as shown in Fig. 7. The supports should be twisted and the applied load should be transferred to the two points that express the moment arm from the centre of the machine. Figures 6 and 7 show the loading frame used in this experiment, which was consisted of an I-section (200 mm × 80 mm × 8 mm) fastened to dual steel channels (100 mm × 50 mm × 8 mm). These steel channels were tied from the bottom by strong bolts after being placed around the beam cross-section. Concerning the longitudinal beam axis, the lever arms provided the required 500 mm eccentricity. As shown in Fig. 6, The 200 mm depth and 2 m length I-section steel spreader beam was used to transfer the loads from the centre of the machine to the two lever arms. Using the recorded videos and load increments from the data logger, increment readings from the dial gauges were recorded at each loading while the cracks were recorded when they occurred.

The continuous spiral NSM steel wire rope reinforcement ratios for the strengthened specimens are shown in Table 4. The volumetric ratios of continuous spiral NSM steel wire rope reinforcement, ρ_snsm, were calculated using Eq. 1 (Chalioris and Karayannis 2013).

The twist angle was measured using a dial gauge that was connected to the lever arms’ bottom at a point of 500 mm from the centre of the beam longitudinal axis. This dial gauge recorded the lever arm’s downward value to quantify the twist angle in radians.

The ultimate torsional moment carrying capacities of the control and strengthened beams are shown in Fig. 8. The ultimate torsional moment carrying capacity of the strengthened beams semi-linearly improved relative to the control beam. Beam SNSM113 had the maximum ultimate torsional moment (25.75 kN m), beam SNSM566 had the minimum torsional moment (12.58 kN m) and the torsional moments of all the other strengthened beams lie between the two peaks.

The spiral NSM steel wire rope reinforcement ratios for the strengthened beams and the corresponding increase in their ultimate torsional moment (T_u) with respect to the control beam are shown in Fig. 9. Beam SNSM113 had the maximum spiral NSM steel wire rope reinforcement ratio of 0.86% and the maximum increase in ultimate torsional moment of 140% relative to the control beam. By contrast, beam SNSM566 had the minimum spiral NSM steel wire rope reinforcement ratio of 0.17% and the minimum increase in ultimate torsional moment of 17%. For beams SNSM141, SNSM188 and SNSM283, the values of spiral NSM steel wire rope reinforcement ratios were 0.69%, 0.52% and 0.35%, respectively. The corresponding increases in their ultimate torsional moment (T_u) with respect to the control beam were 112%, 67% and 47%.

The torque–twist relation of all beams is illustrated in Fig. 12. The ultimate torsional moment (T_u) and the post-cracking torsional response of all the strengthened beams indicated that the continuous spiral NSM steel wire rope reinforcement strongly affected the torsional capacity and the overall behaviour. The increase in the amount of the steel wire rope provided caused significant enhancement in the torsional carrying capacity and ductility of all strengthened beams. The control beam had lower carrying capacity of torque and higher twist angle values than the beams strengthened by spiral NSM steel wire rope under the same load. Strengthened beam SNSM behaved the same as the control beam with different changes at the latest stage of the torque–twist curves. Furthermore, the torque–twist angle tendency of all beams did not show any significant changes before the cracking. Throughout the post-cracking stage, all curves tried to show a consistent slope to reach the ultimate torque of beams due to the stirrup or the external spiral NSM steel wire rope reinforcement that resists the torque loaded on the beams. Consequently, the torsional rigidity of the beams increased and the loading stopped after reaching the ultimate torque. Beam SNSM113 was the most ductile (high torque carrying capacity and less angle of twist) amongst all the tested beams and was followed by SNSM141, SNSM188 and SNSM283.

The results of ultimate bending moment and ultimate mid-span deflection for control and strengthened beams are listed in Table 5. It can be seen that the ultimate bending moment and ultimate mid-span deflection of strengthened beams increases linearly according to the continuous spiral NSM steel wire rope reinforcement ratios (ρ_SNSM). Among all, SNSM113 has maximum ρ_SNSM (0.0086), its enhancement percentage in ultimate bending moment and mid-span deflection ratio with respect to control beam is 140% and 3.17, respectively. While, SNSM566 has minimum ρ_SNSM (0.0017), its enhancement percentage in ultimate bending moment (M_u) and mid-span deflection ratio with respect to control beam is 17% and 1.03, respectively. The percentage of enhancement in ultimate bending moment (M_u) for beams SNSM141, SNSM188, and SNSM 283 with ρ_SNSM of 0.0069, 0.0052, and 0.0035 are 112%, 67%, and 47%, respectively. The mid-span deflection ratio with respect to control beam for beams SNSM141, SNSM188, and SNSM 283 with ρ_SNSM of 0.0069, 0.0052, and 0.0035 are 3.04, 2.32, and 1.90, respectively.

For a complete knowledge and evaluation of the continuous spiral NSM steel wire rope contribution on the response and for comparison reasons of test results in terms of torsional ductility indices, the following torsional ductility indices are introduced, which are used by (Chalioris and Karayannis 2009): \(\vartheta_{Tmax}\) is the rotation at the maximum torsional moment, degree/m; \(\vartheta_{85Tmax}\) is the rotation at the point of 85% of the maximum torsional moment, assumed as the end of the reliable response range, degree/m

Values of the above mentioned torsional ductility indices for the tested beams are listed in Table 6. The results showed that the increase of continuous spiral NSM steel wire rope ratio results in a significant improvement of the post-elastic behaviour of the beams.

The failure modes of the control and strengthened beams are shown in Fig. 13. The torsional moment forced all the tested beams to fail. The number of cracks in the strengthened beams was larger than that in the control beam; thus, the former beams had higher tensile stress than the latter beam. The development of flexural cracks at the mid-point length of one or both of the vertical faces of the beam was observed in the control beam. Torsional tension cracks were formed and propagated in some kind of spiral pattern. The continuous spiral NSM steel wire rope strengthened beams demonstrated different failure modes and mechanisms since steel wire rope delay crack growth and propagation in strengthened beams. For low spiral NSM steel wire rope reinforcement ratios this improvement in first crack strength is of minor important. As the load increased, the cracks expanded gradually, the two arms rotating in the opposite directions relative to each other around the longitudinal axis of the beams. Most of the concrete cracks in the strengthened beams were located between the spiral NSM wire ropes through the concrete surfaces. In addition, steel wire rope failure in specimen SNSM113 was observed from the edge of the central portion of the beam specimen, and the RC beam suddenly failed. For SNSM141, the first crack appeared at the centre of the test region and in both sides of the specimen. Such cracks propagated and distributed in a spiral pattern throughout the central part of the test area, and the widening of the major crack in the middle of the test region resulted in a failure in that region.

By increasing the spacing between spirals NSM steel wire ropes the number of cracks decreased. The failure modes of beams SNSM113, SNSM141, SNSM188 and SNSM283 were the same. In particular, they firstly experienced sudden failure due to reaching the ultimate load of steel wire rope and then crashing of concrete. However, each beam had a different number of spiral cracks and spreading through the test region.

In SNSM566, most of the cracks concentrated at the centre of the test region, and failure occurred due to crashing of the concrete at that region.