Jeremy Chang – 005024 Group D S Beam

Individual Discussion

Tension Beam

Group F: Tension Beam

No cracks were seen until a load of 40 bars was applied. As it can be seen in the photo above,

the cracks on the tension beam started from the bottom and slowly progressed upwards from

time to time as the load increases. Many major cracks can be seen. As soon as it hits its ultimate

failure stress of 180 bars, the bottom of the tension beam fails. This is because the bottom of

the beam was not reinforced well enough.

Jeremy Chang – 005024 Group D S Beam

Shear Beam

Group D: Shear Beam

For the shear beam, the cracks started from the sides and it slowly moves towards the middle

as the loading increases. Small cracks can be seen at the bottom of the beam unlike major

cracks in the tension beam. Its ultimate failure stress was at 175 bars, the lowest of the three

beams. Once it failed, prominent major cracks can be observed from the sides of the beam as

shown in the photo above. Not many cracks can be seen in this beam as compared to the other

beams. This happens because the beam was poorly reinforced.

Jeremy Chang – 005024 Group D S Beam

Compression Beam

Group E: Compression Beam

For this beam, the minor cracks were seen starting from the bottom of the beam. As the

experiment progressed, the cracks progressed upwards towards the top of the beam but before

reaching the top, it failed as the top of the beam was crushed. Being a compression beam, its

ultimate failure stress was expected to be the highest amongst the three beams; 320 bars. The

compression beam ‘explodes’ during failure and major cracks can be seen at the top of the

beam. This is because the bottom of the beam was over-reinforced and thus, not many major

cracks can be seen. Instead, the top of the beam is crushed during failure.

Jeremy Chang – 005024 Group D S Beam

Errors

Before conducting the experiment, the DEMEC gauge should be calibrated in order to

measure the strain. Mistakes might have been made during calibration.

When taking the readings for deflection and the strain, human error (parallax error)

should be taken into account.

The beam might also not have been placed in the right place before the experiment and

the middle of the beam might also have been marked wrongly.

Reading uncertainties occur when hydraulic jack meter fluctuates. This is due pressure

leakage of the hydraulic jack.

Improper reading of dial gauge, i.e., measurements taken at side angles

Varying moisture conditions of the concrete due to the surrounding temperature, i.e.,

extremely wet or dry during the day or time.

Assumptions

The beam was casted and prepared perfectly by the lab assistant.

The DEMEC pips were all placed correctly without any errors.

External factors, such as pre-loading is ignored.

Findings from the experiment

It is found that the higher the load applied onto the beam, the higher the value of

deflection. This tallies with the hypothesis made before the experiment. The micro-strains

measured from the first three DEMEC pips (a,b and c) were all negative values and this

represents compression in that section of the beam. The other two DEMEC pips (d and e)

clearly show tension strains in the bottom part of the beam.

Jeremy Chang – 005024 Group D S Beam

When the strains were plot in accordance to their respective DEMEC pip distances, there

were a few anomalies in the graph. The most prominent one during the first three loads

which are 20, 40 and 60 bars.

Before the experiment, design calculations were made for both the rebar cases. In the

shear beam, 2H16 rebars were used and hence, the actual calculations made for the shear

beam is compared with the design calculation of the same rebar case. It is found that the Iu

for both design and actual are almost the same value but for the I c value, the actual beam

had a higher value as shown in the gradient of the graph of P vs. Strain Gradient. This is

because the actual beam has been allowed to cure and strengthen where as the design

calculations did not take into account the increase in strength of the concrete during

curing. This is also demonstrated in the strain-gradient where the actual one has a smaller

strain gradient at PRD.

Conclusion

In a nutshell, it can be said that the theoretical prediction did not tally with the actual beam

as it did not take into account factors such as the curing process of the concrete. It is found

that the actual beam has a higher strength than that of the design and it also has a lower

strain-gradient. In terms of Load vs. Deflection, the expected results were obtained.

However, some anomalies were found in the thru-depth strains and this might be due to

errors such as human errors.

The shear beam fails too quick as it is very under reinforced. On the other hand, the overly

reinforced compression beam barely shows any warning signs and it just ‘explodes’ at the

top of the beam. Hence, when designing a beam, it should be designed in accordance to

the tension beam because there are plenty of signs before failure happens. The diagram

shows that plenty of cracks can be seen before it fails and thus, corrections and

adjustments can be made before the beam completely fails.