Repairing of RC Beams using Geopolymer Mortar
5.1 Introduction
5.2.2 Method of beam preparation and repairing with mortar
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
Table 5.9: Fresh and hardened state properties of CC1.
Test Result
Workability (Slump) 90 mm Compressive strength (3 days) 15.14 N/mm2 Compressive strength (28 days) 28.04 N/mm2
Tensile strength (3 days) 0.90 N/mm2 Tensile strength (28 days) 2.17 N/mm2 Flexural strength (3 days) 1.12 N/mm2 Flexural strength (28 days) 2.56 N/mm2 5.2.1.6 Reinforcement
RC beams were cast with 2 numbers of 12 mm diameter tor steel bars IS 432 (Part 1) 1982 [95] at top and bottom as main reinforcement. Stirrups were provided using 2 legged 8 mm diameter tor steel bars. Sub-section, 2.2.7 Steel reinforcement bars of Chapter 2 presents the details of the rebars used.
Figure 5.2: Details of controlled and repaired RC beams (All dimensions are in mm).
Figure 5.3: Reinforcement provided in the RC beams.
The parameters required for the design were collected from the mechanical tests which were carried out on the PCC (CM1). For evaluation of the mechanical properties, laboratory tests such as compressive, tensile and flexural strengths tests were conducted on cube specimens of size 150 mm, cylinder specimens of diameter 150 mm and height 300 mm;
and prisms of size 150 mm × 150 mm × 700 mm respectively. Using the design results, the RC beams were cast to size 150 mm × 200 mm × 2000 mm.
The casting of the RC beams was carried out as per guidelines laid in IS 516 1959 [122]. The formworks for RC beams were prepared and reinforcements as mentioned in the preceding paragraph were placed inside it (Fig. 5.4). The PCC (CM1) was prepared and placed inside the formworks in total three layers. After placing the concrete in each layer, it
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
was vibrated using a needle vibrator as shown in Fig. 5.5 to release the air bubbles and fill up the voids in the concrete. This helped the concrete to get placed homogenously inside the formworks. On completion of placing of concrete, the formworks were left undisturbed for 24 hours in the laboratory. After 24 hours from the time of casting, the formworks were removed from the RC beams. The curing of the beams were carried out in the laboratory at ambient temperature of 20 ± 2 ºC for 3 and 28 days by wrapping them with wet rags.
Figure 5.4: Formwork used for casting RC beams.
Figure 5.5: Needle vibrator used for compaction of concrete in RC beams.
On completion of the curing period, the RC beams were uncovered from the wet rags and allowed to surface dry. A fine layer of white wash was applied over the surface of the beams so that the cracks that were supposed to develop due to the loading in the test are clearly visible. The RC beams were later subjected to static flexural load using MTS actuator
Displacement based loading at the rate of 0.01 mm/s was monotonically applied. The RC beams were divided into 2 groups based on the damage level due to the loads subjected from the actuator. The groups are as follows:-
i. Fully damaged beams - Four numbers of RC beams were loaded till failure. In these RC beams, the monotonic static load was applied to achieve the ultimate load. The cracks in the beams were induced in the region of pure tension. Most of the cracks were wide with minimum width of 5 mm which propagated towards the core of the beam. This can be observed from Fig. 5.6.
Figure 5.6: Cracks in fully damaged RC beam.
ii. Partially damaged beams - Three numbers of RC beams were loaded upto 60 % of their ultimate load carrying capacity as found by the procedure followed in beams in group (i). The ultimate load in this case was considered as the ultimate load of controlled beam, B3. At such load range, the beams remained in their elastic stage.
This was observed from the load- deflection curve of the controlled beam. The cracks in the beams were induced in the region of pure tension however the enlargement was restricted. Most of the cracks were of maximum width of 1 mm. Further, the cracks did not propagate towards the core of the beam as evident from Fig 5.7.
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
Figure 5.7: Cracks in partially damaged RC beam.
The cracks that were developed due to the application of load on the RC beams were identified and marked. These cracks were then enlarged upto a depth of maximum 50 mm and width of maximum 40 mm using hammer and concrete-breaking bit to facilitate easy and effective application and penetration of repairing material (paste and mortar) as shown in Fig.
5.8. Beyond 50 mm depth, the cracks are presumed to be very fine in nature. The broken concrete bits in the cracks were removed carefully with the help of brush such that no free particles remained attached to the surface of the cracks. The beams were subjected to air blow with the help of air blower as shown in Fig. 5.9 to remove the dust and minute free particles from the surface and ready it for further treatment. Minute inspection was done to ensure that no free particles remain inside the cracks. This method of crack cleaning and surface preparation is similar to that used by the field engineers and technical persons while repairing concrete structures with epoxy resin, cement grout, etc.
Figure 5.8: Crack width in the damaged RC beam after enlargement for repairing.
Figure 5.9: Use of air blower for removal of dust particles from inside the cracks.
The repairing paste (GPP or PCP) was first applied to fill the cracks followed by the application of repairing mortar. Due to high flowabilty of the paste, it could penetrate inside the fine cracks which were not wide enough to allow the accommodation of mortar. But those fine cracks were significant to act as the source of crack development in the repaired RC beam. Hence, the fine cracks were arrested by repairing paste. The paste was applied using a syringe used in treatment related to female hygiene in hospitals, Fig. 5.10. The use of such syringe facilitated better penetration of the paste into the fine cracks.
Figure 5.10: Application of repairing paste inside the cracks using syringe.
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
The large cracks were filled up by the repairing mortar (GPM or PCM). The repairing mortar was placed inside the cracks with the help of trowel, Fig. 5.11. It was compacted with the tamping bar for better placement and removal of voids within. The mortar was continued to be placed inside the cracks till the original cross-sectional size of the beam was restored as shown in Fig. 5.12. On completion of the cracks filling process, the beams were wrapped with wet rags and cured at ambient temperature of 20 ± 2 ºC till arrival of test day. On the test day, the wet rags were removed and the beams were exposed to air blower for drying the surface.
Figure 5.11: Application of repairing mortar inside the cracks using trowel.
Figure 5.12: Mortar repaired damaged RC beam.
The experimental setup for testing the controlled and repaired RC beams is presented in Fig.
5.13. The experimental setup and testing method was same for controlled and repaired RC beams. The beams were subjected to static flexural load using MTS actuator (Maximum load:
250 kN; Maximum displacement: 250 mm). The tests were performed by applying four point loading. Displacement based loading was monotonically applied at the rate of 0.01 mm/s. The load in controlled beams was continued to be applied till the targeted load was reached.
However, in case of repaired beams the load was applied till failure of the beams. The load application was halted when the load started to fall and reached 75 % of the ultimate load achieved. The midspan displacement was recorded using spring mounted linear variable differential transformer (LVDT), L2 of capacity 100 mm. Two additional LVDTs, L1 and L3 were used to monitor the loading uniformity. The additional LVDTs were placed below the point of contact of rollers of spreader beam with RC beam as shown in Fig. 5.13 by L1 and L3. However, results from these LVDTs were not considered to prepare the load-deflection curves. Load-deflection curves are drawn according to the record of the central LVDT, L2.
Figure 5.13: Setup for 4 point loading bending test of RC beam.
The PCM repaired fully damaged beams were tested at 3rd and 28th day of casting.
PCM repaired partially damaged beams were tested only at 28th day of casting. The 3 days test for PCM repaired beam was not included since at 3 days, the strength gain by PCM is very low as being observed from the results of compressive strength test presented in sub- section, 3.3.3 Compressive strength of Chapter 4. The GPM repaired fully and partially
Roller support
Chapter 5 Repairing of RC Beams using Geopolymer Mortar
damaged beams were tested at 3rd and 28th day of casting to observe the behaviour of GPM repaired RC beam at both early and later ages.