Repaired Panel – Load Application

Having learnt form the initial testing the LVDTs were set up so that the end could move more freely on top of the concrete when in-plane movement of the panel took place – see Fig 7.7. Similar to the load applications on the virgin test panel some of the supports presented problems. This can be clearly seen in Fig 7.8.

Fig 7.7 Corner LVDT setup

Fig 7.8 Column load vs. Corner Support Deflections

From Fig 7.8 the following deductions can be made; firstly, Corner 3 shows erratic behaviour due to friction between the supporting rod and the concrete. Secondly, it seems as if there were some interference with the LVDT at Corner 1. This instrument shows a substantial deflection at ±200kN, while the LVDTs at Corners 2 and 4 do not.

In order to calculate an average corner deflection the measurements at Corner 1 was scrutinized and adjusted to remove the unwanted jump in the deflection and the measurements at Corner 3 were ignored.

Fig 7.9 Column load vs. Adjusted Corner Support Deflections

Fig 7.10 Column load vs. Relative Middle Deflection

From Fig 7.10 the following can be concluded:

 0mm to ±2.0mm deflection, 0kN to ±200kN column load

Initially the response of the slab was parabolic, settling to a linear trend. At approximately 200kN the first of the original flexural cracks started to open up – Fig 7.11

Fig 7.11 Reappearance of original cracks

 ±2mm to ±6.0mm deflection, ±200kN to ±400kN column load

Due to observers marking cracks on the slab some movement takes place, causing the erratic behaviour seen on the curve at 200kN, 300kN and 400kN.

Virtually all the flexural cracks have opened up through the chalk locating on the slab at this stage. The crack pattern at approximately 400kN is shown in Fig 7.12.

Fig 7.12 Crack pattern at ±400kN

A very important crack appeared at this stage of the test. A shear crack was starting to appear on the concrete surface outside the newly installed third perimeter of shear reinforcing. This happened without the formation of new shear cracks within the shear-reinforced zone – Fig 7.13.

Fig 7.13 Appearance of new shear cracks outside the perimeter of dowel bars

 ±6mm to ±10mm deflection, ±400kN to ±550kN column load

After the appearance of the shear crack outside the last perimeter of shear reinforcing, the angle of response on the load-deflection curve decreased. Cracking was audible inside the slab, without significant new flexural or shear cracks appearing on the concrete surface. However, it seems as if the cracking started to migrate towards the supports along the tensile reinforcing. This probably happened due to stress concentrations at the middle supports between Corners 4 & 1 as well as Corners 1 & 2. Delamination of the concrete at theses supports was clearly evident, as seen in Fig. 7.14.

Fig 7.14 Delamination due to cracking migrating to the supports

 ±10mm to ±12.5mm deflection, ±550kN to ±450kN column load The load carrying capacity of the slab reached a peak at

approximately 550kN and the deflection started to increase accompanied by a lessening load carrying capacity.

It was decided to stop the test. The support conditions had to be altered before the panel could be tested to destruction.

Up to this point the new shear crack had become significantly visible and the slab seemed to be increasing in thickness.

To get better insight into the behaviour of the repaired slab in comparison with the original, the measurements of the different tests can be added together. Fig 7.15 shows the load deflection behaviour of the repaired slab when it is added to the response of the undamaged specimen.

Fig 7.15 Column Load vs. Relative Middle Deflection

Original Panel & Repaired Panel

7.3.4. Repaired Panel – Load Application 2

Due to the installation of additional shear reinforcing the shear cracking was forced to migrate away from the column, causing shear cracks to grow to the surface outside the shear reinforced zone.

In essence this proves that the installation of additional reinforcing is an effective way of countering punching shear failure. However, due to the observed migration of cracking, the support conditions of the slab were interfering with the mode of failure. Delamination started to occur at the small bearing plates.

It was thought best to increase the area of load transfer at the supports before any further testing took place – Fig 7.16.

(a) (b)

Fig 7.16 Revised support conditions: (a) Original support, (b) Revised Support

Due to the friction in the supports reloading of the repaired panel caused erratic movements on the load-deflection plots of the corner supports – as seen in Fig 7.17.

Fig 7.17 Column load vs. Corner Support Deflections

In order to calculate the average support deflection the measurements of only Corners 2 and 4 were used. The values measured at Corner 3 are too erratic due to the friction between the tie rod and the slab. The deflection behaviour of Corner 1 is most likely due to the delamination and increased slab thickness observed in that quarter of the panel.

Fig 7.18 Column load vs. Corner Support Deflections

Fig 7.19 Column load vs. Relative Middle Deflection

From Fig 7.19 the following can be concluded:

 0mm to ±7.0mm deflection, 0kN to ±400kN column load

Reloading caused the slab to behave with an initial parabolic curve settling to a linear response from ±100kN onwards. The two most important observations for this portion of the test were; firstly, that delamination of the concrete continued at the supports, despite the enlarged support area – see Fig 7.20. Secondly, extensive cracking within the slab was audible without any major crack appearances or growth on the slab surface.

Fig 7.20 Delamination at middle support

 ±10mm to ±15mm deflection, ±400kN to ±400kN column load At approximately 400kN the slope of the load-deflection curve decreased dramatically. A peak value can be observed at

approximately 425kN – Fig 7.19. Post peak the load carrying capacity of the slab started to decrease with increased deflection.

On the top surface of the slab, as seen in Fig 7.21, the newly formed shear crack was growing around the last row of shear reinforcing. The direction of growth is indicated with dotted arrows.

On the soffit of the slab punching of the column could be clearly seen as well as concrete spalling below the original shear clips – see Fig

7.22.

Fig 7.22 Concrete spalling and punching of the column

 ±15mm to ±36mm deflection

Punching shear failure of the slab has clearly taken place and it was decided to sustain the load application. With continued pumping of the jacks the slab resistance remained fairly constant with very high and increasing middle deflections.

The behaviour of the slab is similar to bending failure behaviour in reinforced concrete.

Delamination of the cover concrete became highly defined on the one end of the slab. Accompanying this, the slab thickness started to increase as a cone of concrete was being pushed out of the original slab. This can be clearly seen in Fig 7.23 & Fig 7.24.

Fig 7.23 Continued delamination and shear cracking

Fig 7.24 – continued Bulging of the slab

Further pumping was stopped and the instrumentation removed. The jacks were kept extended to support the slab in its bulging form.

In Fig 7.25 the compilation of all the load-deflection curves can be seen.

Fig 7.25 Column Load vs. Relative Middle Deflection