Slab Testing: Punching Shear Analysis

8.4.1 Experimental Method

To evaluate the contribution of steel fibres to the punching shear resistance of a flat slab system three identical large scale test specimens were constructed for punching shear testing.

As with the flexural test slab the tests were aimed to verify the validity of design models and material properties determined from small scale testing, whilst revealing the structural and failure mode behaviours of a SFRC slab under punching shear conditions. The thickness of each slab was 150mm in depth, so as to be comparable with results obtained from the flexural test slab; with planar dimensions 2m x 2m to eliminate the influence of boundary conditions during the test procedure (see Figure 8.13). An indeterminate support system was applied to all three test specimens to simulate conditions under which punching shear occurs. The test load was applied to the underside of the test specimens via a 150x150x150mm stub column, allowing one to safely monitor and evaluate the crack formation and failure characteristics during the test procedure. Records were made at various load steps to give insight into the specimen behaviours until eventual failure.

Pinned temporary support

2x 60t loading jacks 1840mm apart Flexural test slab

underside

Chapter 8: Test Setup and Procedures

124 Figure 8.13 Punching shear sample plan layout and sectional support schematic

8.4.2 Sample Preparation

All three punching shear samples were constructed as per Mix 2 in composition (see Section 8.1.1). Materials were mixed using the 0.12m³ pan mixer making 6 batches (approximately 0.105m³ per batch) per specimen, adding up to approximately 0.6m³ of concrete in volume.

The fresh material was transported manually using a wheel barrow and placed into the wooden mould directly from one side throughout the casting procedure (see Figure 8.14). The concrete in its fresh state was then evenly distributed in the mould and constantly disturbed so as to reduce the potential for lamination within the cast specimen due to early setting of the lower layers. Towards the end of the casting procedure a 10mm steel mesh with bar centres at 200mm was placed on the to be tensile surface of the sample and covered with an approximate 15mm layer of fresh SFRC. The entire casting process ran over a 4 hour period.

The following day after casting of the sample concrete curing was performed using wet blankets and plastic sheeting for a period of no less than 14 days. Test samples could be lifted and moved using the 10t crane and 20mm dowel bars cast into the sample once the test sample had reached a minimum 7 day concrete strength. The slab surface was then painted using a white water/chalk mixture which made it easier to spot crack development during the applied test loading.

Chapter 8: Test Setup and Procedures

125

a) b)

Figure 8.14 a) Punching shear sample casting from one side and b) mesh reinforcing in place before placement of final SFRC topping layer

8.4.3 Test Method

Slabs were lifted and placed on temporary pinned supports and bolted to the laboratory floor slab using threaded steel bars at 920mm c/c along the test sample perimeter, forming the indeterminate support system during the test procedure. As the behaviour of a SFRC only slab panel under applied loading was expected to be dominated by a flexural behaviour a 10mm steel mesh was placed near the tension surface of the test specimen to ensure a punching shear mode of failure. Once the test sample had been lifted into place three LVDT’s were positioned in a straight line, one on each edge near the relevant support and one in the centre of the specimen (see Figure 8.15), along the tension surface to record deflections and give a representation of the deformed shape of the test sample during load application.

Due to delays regarding the setup test Specimens 1 and 2 were tested at 30day and 31day age strength respectively, whereas Specimen 3 was tested at the originally intended 28day strength. The load was applied to the stub column using a 60t Enerpac load jack with an electronic hydraulic pump (see Figure 8.16). The applied load was recorded via a 50t load cell connected to an electronic recording system, as were the deflections that were recorded using the LVDT’s. The load was applied in 50kN increments until visible cracking had

Chapter 8: Test Setup and Procedures

126 occurred after which load application was in approximate 20kN increments. At each load step the crack pattern was monitored and marked to allow a progressive analysis of the slab behaviour under increasing load. The slabs were tested until ultimate failure had occurred where the ductility of the failure mechanism was evaluated.

Figure 8.15 Punching shear test setup (top surface)

Figure 8.16 Punching shear test setup (underside)

Threaded bar support

Test sample LVDT

locations

Test sample

Threaded bar support to laboratory floor

Stub column Temporary pinned

support

60t load jack 50 t load cell

127

Chapter 9

Test Results and Discussion

The test program conducted revealed the challenges, practical issues and potential of the use of SFRC only systems in flat slab construction. The effects of fibres on the behaviour of concrete can be seen in both the fresh and hardened state behaviour, where adjustments need to be made regarding the conventional concrete methods usually implemented. The results indicate that fibres can significantly improve the post-crack behaviour of concrete through a ductile response but the material performance is highly dependent on the distribution of fibres obtained. It is thus clear that the quality control of SFRC practice needs to be maintained throughout the construction process, from mix design to slab construction and eventual service of the member constructed.

9.1 Aggregate and Fresh State Properties

In order to fully evaluate the influences regarding SFRC fresh state performance the effects of both aggregate and fibre type variance were tested. Fine and course aggregate materials underwent a grading analysis in which the changes of particle distributions could be seen.

Through flow testing the effects of the different aggregate types on fresh state performance could be identified with regards to the particular fibre type used, and it was revealed that changes to the mix design need to be made for the different fibre types used. As the aim of