VACUUM PROCESS FOR STRENGTHENING CONCRETE STRUCTURES

Transcription

1 VACUUM PROCESS FOR STRENGTHENING CONCRETE STRUCTURES Amando Padilla R., Antonio Flores B., Guillermo Landa A. and Iván Panamá UAM Azcapotzalco ABSTRACT This research is focused to study the effectiveness of externally bonded GFRP glass fiber fabric in increasing the flexural strength of concrete beams using vacuum process. The procedure is very similar to vacuum bag process and helps to release entrapped air between layers and resin penetration and increase mechanical interlocking. Four point bending flexural tests were conducted up to failure on concrete beams strengthened with one and two GFRP glass fiber fabric (woven roving of 600 grams/m 2 )bonded manual and by vacuum process. Also beams were tested without external reinforced. Comparisons were made between test results on flexural strength and the rigid (elastic modulus) of the beams. For one glass layer fabric, flexural strength was increased up to 47% on concrete beam strengthened by hand process and up to 51% by vacuum process. In the case of rigid the effect of the process was more notorious. Rigid was increased up to 48% on concrete beam strengthened by hand process and up to119% by vacuum process. INTRODUCTION For about 20 years ago, fiber reinforced polymer (FRP) systems for strengthening concrete structures were developed as an alternative to traditional strengthening techniques, such as steel plate bonding, section enlargement, and external post tensioning. FRP strengthening systems use FRP composite materials as supplemental externally bonded reinforcement. FRP systems offer advantages over traditional strengthening techniques: they are lightweight, relatively easy to install, and are noncorrosive. One of the critical issues is to promote continuous intimate contact between the concrete surface and the FRP system. In order to increase the adhesion between both phases, a vacuum process was employed for strengthened concrete beams. EXPERIMENTAL Concrete beams were strengthened using glass woven roving and epoxy resin system. Concrete beams were prepared using Portland cement. Water cement weight relation was equal to Concrete matrix properties are reported in Table 1.

2 Table 1 Concrete properties Property Unit Value Compression strength MPa 27.0 Compression Modulus GPa 22.1 Slump cm 12 Density Kg/m 3 2,200 Each beam is reinforced with three 3/8 diameter steel rebars. Rebars arrangement is shown in Figure 1. Concrete beams reinforced with these rebars were subjected to static four point bending. Cracking, load and beam deflection were recorded along the test. Beam specimens were cast in wood molds and compacted on a vibration table. Beam dimensions were: width 15 cm, height 15 cm and total length 60 cm. The specimens were de molded after 24 hrs and stored in a curing room at 20 C and 98% relative humidity for 28 days. 15 cm Lo=15 cm 15 cm 3 cm 3 cm L= 45 cm Fig. 1 Schematic beam cross section and loading outline External reinforced method As it is well known, that behavior of concrete members strengthened with FRP systems is highly dependent on a sound concrete substrate and proper preparation and profiling of the concrete surface, it was necessary for the concrete surface be free of loose or unsound materials (See Figure 2). Also the corners should be rounded to prevent stress concentrations in the FRP system and voids between the FRP system and the concrete. Roughened corners were smoothed with epoxy putty. Finally, before applying the external reinforcing, concrete beam was drying. Fig. 2. Beam preparation. Clean surface and rounded corners

3 Figure 3 shows the schematic placing of the glass fiber fabric reinforcing. In this case the fabric is a 600 g/m 2 woven roving. The applied epoxy resin to the fabric is in a relation 1:1 in weight. Load External reinforcing Hand process Fig. 3 Schematic external reinforcing placing on the concrete beam Reinforcing fabric was applied by hand layup method using brush and roller, applying first resin over surface concrete and fabric. Then fabric is placed on the beam surface and is compacted by hand using metallic rolls and brushes as it is shown photographs in Figure 4. Fig. 4. Reinforcing application by hand process, using brush and rolls to release burbles air and joint fabric to concrete surface Vacuum process Reinforcing fabric was applied by vacuum process as similar way of hand process. Previously, it is necessary to set double face rubber compound (sealant tape) over the concrete surface around the area where the fabric is going to be place, after that, it is also fixed the polyethylene spiral wrap close to sealant tape. This is shown in photographs of Figure 5.This spiral wrap helps to get a uniform vacuum in the entire bag.

4 Fig. 5. Double face tape and polyethylene spiral wrap over surface concrete application Resin is applied to surface concrete and over fabric, then the fabric is placed over the concrete and it is covered by PVC film, which works as vacuum bag. PVC film is fixed on the sealant tape. See photographs on Figure 6. A B C Fig 6. Photograph A shown resin application over concrete surface; photograph B shown carbon fiber application and photograph C shown PVC film fixed on sealant tape. Finally vacuum is applied and atmospheric applies an uniform pressure over all the fabric area, moreover release air atrampment. In Mexico City the maximum reached vacuum was 21 inhg, which is equivalent to around 0.71 bar (7,200 kg/m 2 ). Pump used has 1/3 HP. On the other hand, photographs in Figure 7 shows the vacuum process done in laboratory over concrete beams. As you can observe the film is setting over the table work.

5 A B Rubber double tape Polyethylene spiral wrap C D Fig. 7. Beams are strengthened by vacuum process. A) Double face rubber is just placing. B) and C) PVC film is placing over beams with reinforcing fabric and D) Vacuum is applied with a small pump Crack patterns and behavior In all the beams, at the beginning a certain number of cracks appear, after, cracks start growing. Before cracking occurs, the behavior of the beam is independent of the external reinforcing and the adhesion degree between concrete an external reinforcing. However once cracking occurs in the concrete, the adhesion degree has an important effect.

6 The first cracks are flexion cracks, which when they reach the steel bar they stop. However the main cracks are diagonal tension type (shear cracks). These cracks are characterized by a high growing and they are responsible for concrete structure failure. This was observed specialty when reference beams were carried out. When diagonal tension cracks appears and growth, beam failure happened as is shown in photography in Figure 8. Fig. 8 Left side Diagonal tension cracks appear on the reference beam and cause brittle failure. Right side the same type of cracks appear but they are contra rested by the external reinforcing One layer reinforced beams. Figure 9, shows the characteristic curves of the beams witness and the beam strengthened with one glass fabric layer by hand lay up and vacuum process. As one can observe the first shear crack appears around 70 KN on the reference beam and it continues growing with the increase of the load until an average value of the order of 90 KN. 160, ,000 Reinforcing 1 layer 120,000 Load (N) 100,000 80,000 60,000 40,000 20,000 Reference Manual Process Vacumm Process Deflection (mm) Fig. 9 Characteristic curves for witness beam and reinforced beams by hand and vacuum process

7 On the other hand, in the beams with external reinforcement, the growth of the faults in tension diagonal is contra rested by the lateral fiber glass reinforcement. This leads load capacity increase of both beam reinforced by hand lay up as reinforced by vacuum process. As one can observe the beam reinforced by vacuum process shows the highest load capacity. The corresponding values of shear cracks appear area around 90 and 115 KN. This represents a load increase about 30% and 45% in beams reinforced bay hand layup and vacuum process respectively. The vertical non continues lines in Figures 9 and 11 shown the moment when shear cracks appear, and as one can observe the beam rigid decreases a little and the load continues increasing without loss the relation load/deformation until reach maximum load. Also, as we can observe in the same Figure 9 there is an increase of the slope for the curves corresponding to reinforced beams. The slope represents the rigidity of the structural element. Here the effect of the external reinforced is higher than in load capacity. In that way using hand layup process there is an increase of 48 %, instead with vacuum process the increase is 119%. These increases promote that concrete beam fails by concrete crush. The other advantage was a greater adhesion between the reinforcement and the surface of the beam reached, with vacuum process. Some beams reinforced by hand process shown a delamination of the fiber from the surface concrete. The fiber separation occurs on the zone where diagonal tension cracks appear. In the beams reinforced by vacuum process, the concrete and the reinforced layer were separated from the bulk. In this case, the failure is on the concrete substrate which shows a good adhesion. These cases are shown in photographs of Figure 10. Fig.10 Left side: Fiber delamination on beam reinforced by hand process. Right side: concrete is pullout with the fiber, the failure is on the concrete phase. Two layers reinforced beams. The behavior of beams reinforced with two glass fiber layers is similar to those observed with one glass fiber layer. Vacuum process allows higher load capacity and especially a higher rigid. In

8 Figure 11, which shows the load vs. midspan deflection, one can observe the curves corresponding to wittiness beam and reinforcing beam by both process. The application of two layers increases the beam rigid. The corresponding rigid increase, compared with control, for hand and vacuum process are 117% and 135% respectively. Obviously these increments are not always desired, due to this cause a brittle concrete beam failure. Here the research was focused to check the effect of the kind of process on concrete beams strengthened. 140,000 Reinforcing with 2 layers 120, ,000 Load (N) 80,000 60,000 40,000 20,000 Reference Manual Process Vacumm Process Deflection (mm) Fig. 11 Characteristic curves for witness beam and reinforced beams by hand and vacuum process CONCLUSIONS Process has an important effect on the strengthening concrete structures, as the reported results shown, specialty on the beam rigid. Vacuum process leads to get the same FRP system, the highest beam load capacity and rigid. Vacuum process could be an excellent process for strengthening concrete structures; due to the guaranties the adhesion between glass fiber and concrete surface, as was observed in collected data and FRP compact process is independent of labor hand. Particularly the rigid in the elastic zone is increased almost to the double value whit vacuum process.

9 From the point of view of costs, additional costs are not relevant due to auxiliary materials such as PVC film and double face rubber tape, they are cheap; and polyethylene spiral wrap is reusable. Additional labor hand is lower.