Bonded Concrete Topping for Suspended Slab

[asixth]

Hi guys

I have a rc beam design which was a 450wide x 800deep (18"x30") beam spanning 6500 (22'). Now the design load is increased by a topping slab. Because the rc beams is overstressed with the increase in loads, I thought I would bond the topping essentially increasing the depth of the beam by 200mm (8"). I was looking at a product for a bonding agent which gives a bond strength of 2MPa which I have checked for longitudinal shear and is okay.

What are the critical aspects of design when it comes to bonded topping for structural suspended slab applications?

 

[dik]

Are you increasing the beam strength to account for the added d due to the topping? or is shear an issue?

Dik

 

[hokie66]

I am reluctant to use bonding agents for topping. They require careful control of the agent setting time, which is not available for large areas. For slab topping, all that is required is a clean, moist, slightly roughened surface. For a beam, I would want mechanical means, probably drilled in bars,for taking the horizontal shear.

 

[BAretired]

You are adding a 200mm topping? Why?

BA

 

[Ron]

I agree with hokie66...you need mechanical attachment if you want the topping to act integral to the beam. This is commonly done with precast soffit beams with integral slabs poured onto them.

 

[KeerthiR]

I too agree with hokie66. What is really required is shear connectors. No. and size of shear connectors are depending on the mean roughness of the surface. A test called "Sand Patch Method" can be used to measure the mean roughness. Different roughnesses can be achieved by using either water jet or sand blasting or chisel and hammer.

KeerthiR

 

[asixth]

Thanks guys, I will use mechanical means for bonding the topping to the beam.

Dik

Shear was not an issue, the existing design had enough additional capacity.

BA

The slab needs to be raised for a mechanics workshop.

 

[rowingengineer]

Not knowing the details of your project, but an additional 200mm is a lot to expect the existing design for the footing ect to take, you have probably check all these, but I will remind you again. Depending on the end use and reason for the 200, I would look at using a void form system if possible in the slab area’s reducing the ADL.

Here are a few other things that you have probably already thought of but I will remind you anyway,

1. The existing system must be able to take the wet weight of concrete + construction loads if not possible consider propping. Propping can be problematic due to the existing structure. If the propping solution is selected I wouldn’t leave it to the propping guys, as they will not understand the problem. I would also make sure you spec the max construction loads, as the concreter is liable to think he can load your structure to the max while he works.

2. Check the shear strength of the existing slab for construction loads.

3. If the beams are continuous, you will have to consider your approach to the negative bending areas, as these can become interesting.

4. If you are going to use a bonding system ensure you consider the shrinkage behaviour of the slab. Noting that almost all existing crack will reflect into your new slab.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[apsix]

BS8110 Section 5.4 and specifically Table 5.5 gives advice on dealing with horizontal shear between composite precast and insitu members.
2.0 MPa shear is approaching the upper end of the recommended design values and requires nominal ligs across the interface as well as roughening and treatment of the surface.
Note that British concrete strength is based on test cubes, multiply by 1.2 to convert to f'c.

I can post the relevant section if required.