P ARAMETRIC STUDY

In addition to the parameters stated in Sections ( 7.2.1) and ( 7.3.1), the effect of concrete cover depth and load ratio on the behaviour of the reinforced concrete slab at ambient and elevated temperatures were also investigated.

Slab G0F1 was examined for this purpose. Shell elements (S4R) with 4 nodes doubly curved linear reduced integration scheme, and mesh size M6x6 were also adopted in the model. The same material properties that used in modelling the slab at ambient and elevated temperatures were also were used in this section. The Hordijik’s branch was also adopted for tension softening behaviour of concrete.

The load – vertical displacement and the reinforcement temperature – vertical displacement relationships predicted from modelling the slab at ambient and elevated temperatures respectively was presented and discussed.

7.4.2 Effect of Concrete Cover Depth

One of the critical parameters in which the designer should take into consideration when designing a reinforced concrete structure is the concrete cover. The concrete cover, which is also called the concrete cover depth, refers to the concrete thickness from the external edge of the reinforcement bar to the outer concrete surface. The concrete cover depth value should meet the design specification, otherwise, it will cause some defects such as surface cracks on building components and steel corrosion, and even reduce the structure strength and durability.

The layer of concrete cover protects the rebar against fire. Since the rebar is near the concrete member surface, it is subjected to a greater temperature increase and its strength is first affected compared with the main body of concrete. It was found that the concrete cover depth has significant influence on the concrete structure ultimate capacity in the presence of fire, However, this influence decreases with an increase in the concrete cover depth [177].

[162]

The high concrete cover depth leads to increase in the depth of the flexural tension zone which may lead to larger crack width [178].

BS 8110-1[179] provides tabular data for the minimum concrete cover to reinforcement considering many factors such as: maximum size of aggregate, protection of the steel against corrosion and fire, and spalling.

In this section the effect of different values of the concrete cover was investigated. Figure 7-13 shows that increasing the cover depth up to 10 mm results in a slight decrease in the load carrying capacity for the slab at ambient temperature, however, using 13 mm cover depth results in a significant reduction in the load carrying capacity for a given maximum displacement.

At the same applied load, increasing the concrete cover depth results in an increase in the maximum vertical displacement and temperature at failure for the slab at elevated temperature, as shown in Figure 7-14 and Table 7-5.

This means more protection for the rebar against increase in temperature.

We can conclude from the results obtained from both models that 10 mm cover depth is a good choice in terms of protecting the rebar with no significant effect in the ultimate load carrying capacity.

[163]

0 5 10 15 20 25 30 35 40 45 50

0 10 20 30 40 50 60

Load (kN/m2)

Displacement (mm)

Test 5 mm 7 mm 10 mm 13 mm

Figure 7-13 Effect of the concrete cover depth (ambient temperature)

Table 7-5 Effect of the concrete cover depth

Concrete cover depth (mm) 5 7 10 13

Temperature (C^o) 875 877 879 882 Max. vertical displacement

(mm) 113 115 118 121

[164]

20 160 300 440 580 720 860 1000

Displacement (mm)

Figure 7-14 Effect of the concrete cover depth (elevated temperature)

7.4.3 Effect of the Load Ratio

The load ratio applied to the slab at the time of the fire could also effect the overall behaviour. As the greater the load applied, the higher the possibility the structure will fail as well as having a lower fire resistance [30].

As expected, at the test maximum displacement, the reinforcement temperature predicted reduced with increasing the load ratio (applied load/yield line load) as shown in Figure 7-15.

Figure 7-15 Effect of the load ratio

[165]

7.5 SUMMARY

The analysis of the small-scale slabs at ambient and elevated temperatures, conducted in this study, was presented in this chapter. Two methods were used for this purpose; the simplified method and the finite element software package ABAQUS.

The results show that the damage plasticity model is more accurate than the smeared crack model, therefore it was adopted in this study.

Sensitivity tests were carried out to ensure more accurate results. These tests have shown that a finite element size of M6X6, the fracture energy value of 0.065 N/mm, the lower bound limit of the tensile strength and the Hordijik approach for tension softening curve is suitable for modelling the slabs in ABAQUS under both ambient and elevated temperatures.

For the slabs at ambient temperature, the results showed a good correlation for the load – displacement relationship between the test and the two models up to the failure loads.

For the slab at elevated temperature, the ABAQUS model gives reasonable prediction for the temperature – displacement relationship while the simplified method gives conservative predictions for the maximum allowable vertical displacement at elevated temperature, as a result, the simplified method underestimates the temperature at which the reinforcement fracture occurs.

The parametric study shows that a 10 mm cover depth is a good choice in terms of protecting the rebar with no significant effect in the ultimate load bearing capacity. Also the study shows that at the test maximum displacement, the reinforcement temperature predicted reduced with increasing the load ratio (applied load/yield line load).

[166]

CHAPTER EIGHT

8 CONCLUSION AND FUTURE WORK