Post-Installed Reinforcing in Fire Conditions
Figure
1. Example of constant temperature over the bond length of a post-installed rebar
For applications where the cover distance will vary along the length of the embedded rebar, the temperature will also vary along the length of the rebar. Figure 2 below shows an example of a concrete floor slab to wall connection with fire exposure on one side resulting in a variable temperature distribution over bond length of the of the post-installed rebar.
Figure
2. Example of variable temperature over the bond length of a post-installed rebar
1. The worst-case temperature based on the smallest cover distance can be used conservatively, or
2. It is possible to use smaller increments, or segments, l seg, and determine multiple temperatures along the length of the rebar. The designer should determine the value of l seg to be used for the application( i. e. 10 mm) and then record the worst case concrete temperature at each increment of l seg over the length of the rebar. The smaller the value of l seg will lead to more temperature checks in Figure 4.4.2.2.1a( a) but will lead to a less conservative development length in the fire condition when larger values of l seg are used.
Determine Bond Stress Related to Temperature
Once the temperature in the concrete, θ, at the rebar is determined, the next step is to determine what the bond stress is for the adhesive at that concrete cover temperature. The reduction in the bond stress under high temperatures is different for every adhesive anchor system and should be tested per the new fire provisions of AC581( 24) or AC308( 24).
Hilti has two products tested to the new AC581( 24) or AC308( 24) fire provisions. Refer to Section 3.0 of this document for the technical data for fire conditions for HIT-FP 700 R which is also provided in ESR-5891. Refer to Section 4.0 of this document for the technical data for fire conditions for HIT-RE 500 V3 which is also provided in ESR 3814.
For both products, a fire data curve is provided showing the relationship of temperature, θ, and bond stress, τ fire( θ). Refer to Figure 5 for HIT-FP 700 R and Figure 9 and Figure 10 for HIT-RE 500 V3. The figure below is from Figure 5 for HIT-FP 700 R.
The resulting determination of the temperature for calculation of the subsequent bond stress can be done in one of two ways.
Using the concrete temperature determined from Figure 4.4.2.2.1a( a) of ACI / TMS 216.1 as θ determine the corresponding bond stress under fire conditions at that temperature, τ fire( θ). This can then be determined from the graph, or by plugging θ into the provided equations for the Hilti adhesive product.
5