Testing by Four-Point-Loading

All beams were tested under four-point loading using Universal Testing Machine (UTM) with a capacity of 200 kN and a load rate of 0.3 kN/s which was manually changed appropriately for testing. Beams were positioned on a support at the beam ends, which makes the beam to be simply supported. The vertical displacement at the point of the load application were measured automatically by the UTM-200kN machine. The load was gradually increased automatically with a rate of 0.3 kN/s while visually monitoring the concrete's surface for shear and flexural cracks up until failure.

With a clear span of 550 mm between the supports, all beams were 4-point monotonically loaded. As illustrated in Figure 3.12, the loading points were placed 225 mm from each support and 200 mm from the other loading point. The load at first crack, ultimate load at failure, maximum mid-span displacement values were recorded.

Failure modes of the beams were visually observed and recorded. Load strengthening and ductility improving ratio were calculated by comparing the ultimate load and maximum mid-span deflection of respective test beam with the control beam. Figure 3.13 below shows the actual testing by four-point loading using UTM-200kN machine.

FIGURE 3.12: Illustrative sketch of the test setup

FIGURE 3.13: Actual testing by four-point loading using UTM-200kN machine

CHAPTER 4

RESULT AND DISCUSSION 4.1 Cube Test Result

A concrete trial mix by using three (3) cubes of size 150 x 150 x 150 mm was done with two (2) different mix designs where they vary in slump of 30-60s and 60-180s. As shown in Table 4.1 below, the results of Design Mix 1 and Design Mix 2 indicate an average of cube compressive strength of 20.68 MPa and 18.41 MPa, respectively. The average cube compressive strength values were converted to a cylinder compressive strength, by a ratio of 0.8. This conversion is necessary due to the beam design process that utilizes a 20 MPa of concrete characteristic strength, fck

of cylinder, according to Eurocode 2. A 7-days of wet cured concrete is typically around 80% of the full characteristic strength at 28-days. 80% of 20 MPa is around 16 MPa. Therefore, the most suited design mix was Design Mix 1, with slump value of 30-60s due to the average cylinder compressive strength of 16.54 MPa.

TABLE 4.1: Cube test results for design mix 1 and 2 (wet cured for 7 days) Design mix 1 (wet cured for 7 days)

Cube Ultimate load (kN) Cube compressive strength, fck (MPa)

Cylinder compressive strength, fck (MPa)

Cube 1 211.30 21.13 16.91

Cube 2 201.00 20.10 16.08

Cube 3 208.10 20.81 16.65

Average 206.80 20.68 16.54

Design mix 2 (wet cured for 7 days) Cube Ultimate load (kN) Cube compressive

strength, fck (MPa)

Cylinder compressive strength, fck (MPa)

Cube 1 185.70 18.57 14.86

Cube 2 198.10 19.81 15.85

Cube 3 168.50 16.85 13.48

For the process of RC beam casting, it is necessary to also cast concrete cubes for the compressive strength test. This measure is done to check the concrete characteristic strength at the day of beam testing. Thus, three (3) concrete cubes of 150 x 150 x 150 mm in size were cast with the same concrete mix used to cast the six (6) numbers of RC beam specimens. As shown in Table 4.2 below, an average compressive strength of 26.81 MPa and 21.45 MPa for cube and cylinder, respectively were obtained at 28-days of wet curing. With the beam design process that utilizes 20 MPa grade of concrete characteristic strength by cylinder, therefore the concrete mix was proven to be acceptable.

TABLE 4.2: Cube test result for test specimens (wet cured for 28 days) Cube test for test specimens (wet cured for 28 days)

Cube Ultimate load (kN) Cube compressive strength, fck (MPa)

Cylinder compressive strength, fck (MPa)

Cube 1 261.30 26.13 20.90

Cube 2 273.60 27.36 21.89

Cube 3 269.50 26.95 21.56

Average 268.13 26.81 21.45

4.2 Ultimate Load and Maximum Mid-Span Deflection

The ultimate load of the tested beams and their maximum mid-span deflections are provided in Table 4.3. All the strengthened beams experienced significant increases in load-carrying capacity and deflection compared with the control beams.

The results indicate that a higher bonded length of NSM-CFRP sheets can sustain more load than the control beams. Thus, it is clear that NSM-CFRP sheets are effective in increasing the load carrying capacity of beams strengthened thereby.From Table 4.3, the increase in ultimate load is reported in terms of load strengthening ratio, while the increase in maximum mid-span deflection is reported in terms of ductility improving ratio, where both are denoted by a percentage (%). Refer to mathematical equations below for the calculation of load strengthening and ductility improving ratio.

𝐿𝑜𝑎𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ𝑒𝑛𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝑇𝐵 − 𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝐶𝐵

𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝑙𝑜𝑎𝑑 𝐶𝐵 𝑥 100 (1) 𝐷𝑢𝑐𝑡𝑖𝑙𝑖𝑡𝑦 𝑖𝑚𝑝𝑟𝑜𝑣𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 =𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑇𝐵 − 𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝐶𝐵

𝑀𝑎𝑥. 𝑑𝑒𝑓𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝐶𝐵 𝑥 100 (2) The load strengthening and ductility improving ratio of the test beams is compared with the control specimen CB due to the identical beam design. For different bonded length of NSM-CFRP sheet for 1-0, 2-10, 3-30, 4-50, and TB-5-70, the load strengthening ratio is found to be 48.93%, 47.00%, 40.36%, 21.54%, and 8.95% respectively while the ductility improving ratio is found to be 83.47%, 66.53%, 43.39%, 9.30%, and 2.48% respectively. This shows that regardless of NSM-CFRP bonded length, the strengthened beam is capable of sustaining a higher load and deflection than that of the control beam. Furthermore, it is clear that the strengthening effectiveness by NSM-CFRP sheets increases when the bonded length of NSM-CFRP increases through the depth of beams.

From the results, it is clear that the best NSM bonded length for strengthening of RC beam was with a full-depth coverage due to it having the highest load strengthening and ductility improving ratio. It can be seen that the ultimate load after strengthening for beams with a full bonded length of NSM-CFRP (TB-1-0) is higher than the ultimate load observed for the control beam and other strengthened beams.

The specimen TB-1-0 failed with an ultimate load of 64.65 kN and maximum

mid-improving ratio increased by 48.93% and 83.47% respectively over the control specimen CB. This can be attributed to the fact that the bonded length of the NSM-CFRP sheets increased the beam’s shear capacity and delayed the delamination of the sheet by increasing the bonded area hence reducing the stress concentrations.

TABLE 4.3: Ultimate loads and maximum mid-span deflections of specimens

Specimen Load at first crack (kN)

Ultimate

load (kN) Failure mode Maximum deflection (mm)

Load strengthening ratio (%)

Ductility improving ratio (%)

CB 9 43.409 Shear 4.84 - -

TB-1-0 22 64.650 Flexure and concrete

crushing 8.88 48.93 83.47

TB-2-10 18 63.813 Flexure and concrete

crushing 8.06 47.00 66.53

TB-3-30 14 60.930 Flexure, shear, concrete

crushing and peeling 6.94 40.36 43.39

TB-4-50 12 52.760 Shear and peeling 5.29 21.54 9.30

TB-5-70 12 47.296 Shear and peeling 4.96 8.95 2.48

The load-deflection curve is a crucial curve that must be created in order to assess structural performance. For all tested specimens, the relationship between ultimate load and maximum mid-span deflection response is shown in Figure 4.1.

According to the bonded length of NSM employed in strengthening various beams, load-deflection behaviour appears to vary. The following can be used to summarise the load-deflection curve: prior to the cracking stage, the relationship was linear;

however, when cracks developed, the slope of the curves deteriorated with increasing stress.

FIGURE 4.1: Load-deflection curves of all test specimens

The load-deflection curve indicated that test specimens TB-1-0, TB-2-10, and TB-3-30 had better ductility and strength, where they failed with flexural mode of failure at a higher deflection, compared to specimens TB-4-50, TB-5-70 and CB where they fail with shear mode of failure with lower ductility and strength. Specimens that fail with better ductility were indicated by a higher deflection, while specimens that fail with better strength were indicated by a higher ultimate load. Specimens TB-1-0, TB-2-10, and TB-3-30 had better ductility and strength were due to the steel reinforcements that were able to yield first and take up the loads at plastic region before they failed. While specimens TB-4-50, TB-5-70 and CB had lower ductility and strength were due to the failure happened in the elastic region where the steel

0 10 20 30 40 50 60 70

-2 0 2 4 6 8 10

Load (kN)

Deflection (mm)

Load vs. Deflection

CB TB-1-0 TB-2-10 TB-3-30 TB-4-50 TB-5-70

interpretation of typical stress-strain and load-deflection curves where elastic and plastic region were indicated, as well as the points of yielding of steel, ultimate load and fracture.

FIGURE 4.2: Typical stress-strain curve

FIGURE 4.3: Typical load-deflection curve

In summary, the findings demonstrated that as the bonded length of NSM-CFRP sheets increased, so did the ultimate loads and maximum mid-span deflections of the strengthened specimens. This could validate the idea that employing higher NSM-CFRP bonded length might lead to a stronger strengthening process.

4.3 Failure Modes

The failure modes of all tested specimens are depicted in Figures 4.4 to 4.9. The load at first crack was recorded visually, as shown in Table 4.3. The outcomes provide a generalised understanding of how NSM affects the behaviour of strengthened beams.

It was discovered that the NSM-CFRP was able to prevent shear cracking in the majority of beams strengthened with various bonded lengths of NSM-CFRP.

Regardless of the bonded lengths, it was discovered that the majority of the NSM-CFRP-enhanced beams were stronger than the control beams. Reduced NSM-CFRP length from whole beam depth had resulted in shear failure, whereas full depth length results in flexural failure. This improvement can be attributed to the NSM-effect CFRP's on the concrete's increased tensile cracking strength. As an extra interpretation, using 45º inclined NSM-CFRP had raised the shear capacity and at the same time enhanced the flexural capacity, according to the failure modes of all test specimens. A component of the inclined NSM-CFRP force that is horizontal and perpendicular to the flexural cracks may be the cause of the increased flexural capacity by avoiding the formation and spread of flexural cracks within the NSM-CFRP.

At an ultimate load of 43.409 kN, the control specimen had pure diagonal shear cracking as the damage mode (see Figure 4.4). It is evident that specimen CB failed as a result of shear cracks that caused the concrete cover to delaminate and peel off. This failure mode was developed due to the beam's inadequate shear reinforcement while having adequate flexural reinforcement. At 9 kN, shear cracks first appeared, and they kept growing until failure.

FIGURE 4.4: Failure mode of CB

Both specimens TB-1-0 and TB-2-10 demonstrated a similar failure mode, which is flexural cracks and concrete crushing, as shown in Figures 4.5 and 4.6. With an ultimate load of 63.813–64.65 kN, which is higher than that of all other test specimens, the first crack first occurred at 18–22 kN. It was observed that there were no shear cracks formed and propagated through the NSM-CFRP areas. This might be understood as the shear crack coming into contact with a high-scale NSM-CFRP strength that was able to prevent crack formation, giving the specimen additional resistance to shear crack propagation and so increasing the specimen's shear capacity.

In contrast to all other test specimens, a delayed failure at a higher stress was brought on by the avoidance of shear crack growth. This could be due to the transition from shear to flexural failure mode that took longer time and resulted in a higher load-carrying capacity. Flexural cracks started to form and spread towards the specimen's mid-span, in which the largest moment occurs.

FIGURE 4.5: Failure mode of TB-1-0

FIGURE 4.6: Failure mode of TB-2-10

In the case of specimen TB-3-30, as depicted in Figure 4.7, it was seen that both flexure and shear cracks were developed, followed by concrete crushing and concrete cover peeling off, which had caused a failure in both flexure and shear. The specimen may be considered in the mid-range of strengthening effectiveness among all strengthened test specimens due to its’ ability in slightly delaying crack formation compared to the control specimen CB where the first crack appeared at 14 kN, with an ultimate load of 60.93 kN. The sides' concrete cover peeled off as a result of shear cracks that developed and spread at the ends of NSM-CFRP. While flexural cracks formed at the mid-span of the specimen was smaller than those that had appeared in specimen TB-1-0 and TB-2-10. With the formation and propagation of both shear and flexural cracks up until failure, it may be interpreted that the specimen was under the transition of flexure-to-shear strength capacity deficient.

FIGURE 4.7: Failure mode of TB-3-30

Specimen TB-4-50 showed a similar failure mode to specimen TB-5-70, as shown in Figure 4.8 and 4.9, with the same load at the first crack of 12 kN but with different ultimate load, that is 52.76 kN and 47.296 kN. The specimens demonstrated a failure mode of shear and peeling-off of concrete cover. The damage from shear cracks that were formed and propagated at the ends of NSM-CFRP causing the peeling-off of the sides concrete cover were much bigger than those that had appeared in specimen TB-3-30. This was due to the length of NSM-CFRP that were in a further distance from the top and bottom of the specimen, which led the crack to face a non-strengthened area. As a result, the specimens had lower resistance to shear crack propagation because they were unable to stop the formation of cracks. It may be interpreted that the failure mode of shear instead of flexure in these two specimens

were due to insufficient bonded length of NSM-CFRP, resulting in insufficient shear strengthening effectiveness.

FIGURE 4.8: Failure mode of TB-4-50

FIGURE 4.9: Failure mode of TB-5-70

CHAPTER 5

CONCLUSION AND RECOMMENDATION

The experimental investigation on the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar created in the lab by physically folding CFRP sheets over itself was able to address the research gap of the experimental study by Al-Zubi et. al. (2021) and Al-Rjoub et.

al. (2019) where:

• The strengthening effectiveness in terms of ultimate loads and maximum mid-span deflections of the strengthened specimens increased as the bonded length of NSM-CFRP increased.

• NSM-CFRP bonded with different lengths for 1-0, 2-10, 3-30, TB-4-50, and TB-5-70, the load strengthening ratio increased by 48.93%, 47.00%, 40.36%, 21.54%, and 8.95% respectively, while the ductility improving ratio increased by 83.47%, 66.53%, 43.39%, 9.30%, and 2.48% respectively.

• The failure modes of all the strengthened and un-strengthened shear-deficient beams were discussed in terms of the shear and flexural cracks formation and propagation.

For further studies and to produce more accurate result on the effect of varied NSM-FRP bonded length for the shear strengthening of RC beam by using manually made CFRP rectangular bar created in the lab by physically folding CFRP sheets over itself, several recommendations are suggested:

• To design a shear-deficient RC beam only at the critical shear span (CSS) by using software including justification by manual design calculation as per Eurocode 2 to obtain a more accurate representation of a real-life situation of RC beams with low shear strength.

• To utilize an advanced testing machine that is able to automatically determine the load at first crack of tested specimens.

• To validate the experimental results with analytical investigation using formulas that can be found in literatures.

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APPENDICES

1.1 Design Mix 1

1.2 Design Mix 2

1.3 Beam Design Calculation

1.4 Cube Test Results for Design Mix 1 (7 days Wet Cured)

Cube 1:

Cube 2:

Cube 3:

In document Dissertation submitted in partial fulfilment of the requirements for the (halaman 31-61)