3.1 Load carrying capacity
The applied load and the corresponding deflections are measured with the help of load cell and dial gauges. The load deformation behaviour is compared for both CRC and KGPC specimens by using the envelope curves as shown in Fig 5.
Envelope curves are formed by connecting all the peak values of each cycle. The ultimate and the crack load for both the specimen from the experimental and numerical results are shown in Table 4.
Table 4 Load carrying capacity
S.No
|
Specimen
|
First crack load (kN)
|
Ultimate load (kN)
|
Experimental
|
Numerical
|
Forward cycle
|
Reverse cycle
|
Forward cycle
|
Reverse cycle
|
1.
|
CRC
|
50
|
103.4
|
101.5
|
108.9
|
107.92
|
2.
|
KGPC
|
50
|
104.5
|
102.3
|
109.12
|
110.73
|
From the results it was noted that the CRC and KGPC specimens provide greater resistance against applied load slightly higher in forward loading cases. In the load displacement envelope curve, the point at which the curve changes from linearity is known as first crack load and it is observed that both the specimens have the first cracks at the same point as 50kN. With the help of potassium activators, the fly ash compounds generates more bond with the embedded reinforcement which attributed the KGPC specimen to perform equally to CRC specimens and showed similar first crack load (Sayan Kumar Shaw, 2020). The ultimate load carrying capacity of the KGPC specimens is proved to be higher than 1.08% than the CRC specimens. The reaction of potassium ions with the poly carboxylic based super plasticizer caused the fly ash mineral to gain strength in the ambient curing which remove the fluctuation or drop in the hysteresis loop and leads to the enhanced load carrying capacity (Tomas Kovarik, 2021). From the load and displacement relationship obtained from the envelope curve the parametric studies are carried out further by considering the aspects of ductility, energy dissipation capacity, stiffness degradation, drift ratio and cracking behaviour.
3.2 Ductility characteristics
Ductility is the measure on the ability of the specimen to undergo maximum deformation without failure. In order to assess the ductility characteristics maximum displacement carried out by the specimen and the ductility factors are taken into account. The maximum displacement measured on the KGPC specimens are 11.26% higher than the CRC specimens as shown in Table 5.
Table 5 Ductility factor
S.No
|
Specimen
|
Average maximum load
|
Average deflection
|
Failure deflection
|
Yield deflection
|
Ductility factor
|
Exp
|
FEM
|
Exp
|
FEM
|
1.
|
CRC
|
103.4
|
107.5
|
52.8
|
59.93
|
42.24
|
17.25
|
2.448
|
2.
|
KGPC
|
104.5
|
109.93
|
59.5
|
71.05
|
47.6
|
19.39
|
2.455
|
Presence of potassium activators enhanced the resultant sialate polymers to withstand more deformation due to increased strain rate development (Patcharanat Kaewmee, 2020). Another measure is the ductility factor which is the ratio between failure and yield deflection. The failure deflection can be found out as 80% of the ultimate deflectionin the ascending path of the load deformation curve (Ahmed Sayed Tawfik, 2014). Ductility factor of the KGPC specimens are more than equivalent to CRC specimens. Hence it is evidently proved that the introduction of potassium ions activated aluminium silicate minerals in the beam column joints exhibited greater ductility characteristics than the conventional reinforced cement concrete joints.
3.3 Stiffness degradation
When a structural member is subjected to repetitive loading the stability of the element against the applied load can be ensured by its own relative stiffness. The external load causes the decrease in energy limit which lead to the increased deformation behaviour. The increased deformation of the joints attributes to the stiffness degradation in the member followed by the development of cracks.
Due to the application of cyclic loading the beam column joints exhibits stiffness degradation followed by propagation of cracks. In the experimental results, it is noted that the stiffness of the member is highly reduced at 5th cycle of load application. The initial stiffness is calculated by the ratio of average maximum load to the deflection as shown in Fig 6.
Table 6Stiffness Degradation
S.No
|
Specimen
|
Numerical results
|
Experimental results
|
Initial stiffness (kN/mm)
|
After cyclic loading
stiffness (kN/mm)
|
Initial stiffness (kN/mm)
|
After cyclic loading
stiffness (kN/mm)
|
-
|
CRC
|
3.09
|
1.567
|
3.496
|
1.68
|
-
|
KGPC
|
3.10
|
1.796
|
3.448
|
1.958
|
When the structure is subjected to cyclic loading the beam column joint develops inner micro cracks which resulted in reduction of energy limit. The decreased energy limit of the specimen leads to the increased deformation behaviour. Due to this effect, the degradation of stiffness is induced. The application of cyclic loading consists of loading, unloading and reloading effects. The comparison between the initial and final stiffness are given in Table 6. The degradation of stiffness of the KGPC specimens are observed to be gradual and the final stiffness is 12.75% lesser than CRC specimens. The stiffness degradation results confirms to the conclusion drawn by the research work done by S.K. Shaw on flyash (S.K. Shaw, 2020). Due to the initiation of wider open cracks at the joints of CRC specimen the dilapidation of core concrete and the reinforcement grown up fast and led to the rapid stiffness degradation. Whereas, the fly ash particles activated with potassium based alkaline activators only generates minor non structural cracks at the joints and hence the stiffness degrades in the KGPC specimen more gradually.
3.4Drift ratio
The responsible structural parameter for the stability against the ground motion is member drift ratio. The measured deflection along the total length of the member is known as drift ratio. As the load cycles increased, the corresponding increment is observed on the drift ratio. According to the research work carried out by Y.Wang, it was confirmed that, the cyclic load application results in the buckling effect on the beam element. But the existing elastic nature of column still controls the joint from the drift effect (Yandong Wang, 2019). The calculated drift ratio of the CRC and KGPC specimens are shown in Table 7.
Table 7 Comparison of drift ratio from numerical and experimental results
S.No
|
Specimen
|
Drift ratio (%) from numerical investigation
|
Drift ratio (%) from experimental study
|
Forward cycle
|
Reverse cycle
|
Forward cycle
|
Reverse cycle
|
-
|
CRC
|
3.684
|
-4.43
|
3.52
|
-3.25
|
-
|
KGPC
|
4.179
|
-5.294
|
3.566667
|
-2.9
|
The drift ratio variation of CRC and KGPC specimens are compared in Fig 7.It is observed that, the KGPC specimens withstand 11.84% to 16.3% higher drift ratio than the CRC specimens at forward and reversal loading respectively without exhibiting any structural damage. Hence it is evidently proved that the employment of KGPC structural joints in the seismic prone areas will provide safely against the violent storey drift over the specially confined reinforcement in the reinforced concrete beam column joints suggested by IS 13920.
3.5Energy Dissipation Capacity:
During the strong ground motion, the building components also forced to be vibrated along the earth motion. The capacity of the elements to dissipate the absorbed energy by allowing elastic and inelastic deformation without failure decide the stability of the building. Hence the assessment of energy dissipation capacity plays a vital role in the seismic resistant structures. The energy dissipation capacity can be measured as the area under the load deflection curve which is shown in Fig 8.
The displacement variation for each cyclic load application has been showed for both CRC and KGPC specimens separately. The maximum energy dissipated by the joints are calculated from the graph and shown in Table 8. In both the experimental and numerical results, the energy dissipated by the KGPC specimen is higher than the CRC joints by 2.78% and 3.6% respectively.
Table 8 Energy Dissipation Capacity
S.No
|
Specimen
|
Energy Dissipation Capacity, kN-mm
|
Experimental
|
Numerical
|
1.
|
CRC
|
6254.25
|
7313.50
|
2.
|
KGPC
|
6485.86
|
7515.45
|
The bridging action developed by the potassium activated ions on the sialate networks attributes the excellent post cracking behaviour (Saranya, 2020) which leads to the increased energy dissipation capacity than the CRC specimens. The application of cyclic load makes the beam component to slightly undergo plastic deformation which is the reason for the minor variation between the numerical and experimental results. Whereas, the column member remains at the elastic stage which is ensured by the large energy dissipation capacity obtained from the hysteresis curve of geopolymer beam column joints (Yandong Wang, 2019).
3.6 Cracking mechanism
The repetitive loading, unloading and reloading applications of the beam column joint specimen causes alternative tensile and compressive stress development. When the generated stresses reached the ultimate strength of the material, the flexural cracks are developed from the highly stressed portion (Sujith Mangalathu, 2018). As the lateral loads are applied and transferred from the vertically placed beam element to the horizontally positioned column members through joints, the flexural cracks are initiated at this junction. The transferred lateral forces are acted as axial forces to the column member. The increase in the number of load cycles increased the rate of axial deformation which resulted in the initiation of shear cracks in the column (Gunasekara, 2016). The developed shear causes diagonal tension and compression along the joints. As the joints are provided with 90o bent anchorage bars to arrest the tension failures, compressive stresses are initiated at the joints.
When the anchorage bars subjected to compressive stresses, the further resistance to applied load is only provided by the bonding between concrete and steel which attributes development of contact pressure under the bend. Induced contact pressure generates diagonal form of compressive shear cracks (Leonardo M.Massone, 2018). To attain the equilibrium in shear the consequent diagonal tension cracks are developed on the joints. The tension ties placed perpendicular to the joints increase the ability to resist the diagonal tension caused by the lateral loads. This tension tie acted as shear panel area of the joint which safely transfers the applied lateral force to the foundation of the building through base plates (Hong Yanga, 2018). The resistance to the diagonal compression and diagonal tension developed on the joints is purely depending upon the strength of the resisting medium. When the CRC specimens are used as core concrete material in the beam column joints, wider open cracks are observed at the joints as shown in Fig 9 a. But in case of KGPC specimens the potassium activated geopolymer column acted as compressive strut which resists the diagonal compression and also serves to resist the diagonal tension by making improved compatible action with the bent up anchorage reinforcement. The flexible mode of shear resistance mechanism offered by the KGPC specimen showed non-structural micro cracks at the joints as shown in Fig 9b. Also the spalling of column cover concrete in these specimens ensures the serviceability of KGPC activated beam column joints under the seismic loading cases.