Modulus of elasticity of parking garage mastic asphalt traffic topping

[ajk1]

Does anyone know the Young's modulus for hot poured mastic asphalt (as used in traffic topping on top of asphaltic waterproofing membrane)?

I ask because I want to do a Westergaard analysis of the stresses in the mastic under car wheel load, so that I can compare the effect of 5/8" thick and 3/4" thick and 1" thick mastic.

I would expect that the thicker mastic would be less stressed and would also do a better job of spreading the load down to the water-proofing membrane, so that the membrane will deform less. I would expect less squeezing deformation of the membrane will mean better performance of the overlying mastic.

 

[Canuck67]

ajk1,

I don't have an answer to your exact question, however, will add a bit of info from my experience. Often we are careful to review waterproofing membranes to ensure they meet minimum thickness specifications and if they happen to be applied too thick, it's not a problem. With mastic systems, while one still needs to comply with minimum thickness requirements for the waterproofing membrane, too thick can be detrimental as the mastic can slide around more when vehicles are starting/stopping/turning.

I would be interested to hear the outcome of your calculations, however, I believe experience and evaluation of long term performance would likely be better indicators. I have not been involved in a mastic job for quite a while, but from memory our standard specifications called for 3/4" thickness.

 

[ajk1]

To Canuck67 - I agree with everything that you say. I called 2 of the mastic asphalt contractors around here and one says they put it down at 5/8" thick; the other says even thinner. They are the only 2 companies in this area doing this work. One of the contractors says that in the parking stalls he puts 2 strips of 4' x 8' steel mesh, located in the mastic under the vehicle axles, to deal with the issue of "shoving" and cracking of the asphalt where vehicles turn the wheels before they start moving. I have not done the calculation I proposed because I still don't know the "E" for asphalt.

 

[Ron]

ajk1...I would not anticipate much attenuation of contact stresses through a 5/8" thick layer of pliable material. The elastic modulus of asphaltic concrete is typically around 300ksi. For asphalt mastic, with no aggregate in it, I would not expect the elastic modulus to be even 100ksi. For this we are looking at the compression modulus.

If you wanted to do an elastic layer analysis of the layer configuration you show, the controlling factor will likely be the Poisson's ratio of the mastic, not its elastic modulus. Since the material is viscoelastic as well, there is a time dependent function of its behavior. For example, if a car sits on the material for a while it will displace the material more that the car traveling across the material resulting in momentary loading and unloading. In short, the more time the load spends on the material, the less effective the material will be in mitigating or attenuating load to subsequent layers.

 

[Ron]

ajk1...also I do not think that Westergaard's theory would apply to your conditions.

 

[ajk1]

To Ron - ok, thanks for the advice. How long do you mean by "for a while"? The problem of "shoving" generally occurs at the front wheel locations in the parking stalls. Any given car wold not normally be there for more than 8 hours in the cases where I have seen this occur (e.g. university student parking). I have heard it attributed to the steering wheel being turned before the car started moving. Have you seen that?

 

[Ron]

ajk1...the time depends on the temperature. Determine the softening point of the material by testing.