Concrete Pavement Design Allowable Stress for Flexure?

[Dinosaur]

I am a bridge designer. I was talking with a pavement designer in the office yesterday. He said pavement is designed so the flexure stress is less than 650 psi. This is a much higher stress than we use in the bridge community. Can anyone verify this design limit and or enlighten me why flexure in pavement is permitted to approach this level of stress? Thanks.

 

[Ron]

Dinosaur...well, that's not quite right. The 650 psi flexural strength (modulus of rupture) is the typical mix design strength. For allowable stress and unlimited repetitions, the actual stress should not exceed 50 percent of that value or 325 psi.

 

[rapt]

Dinosaur,

Are you talking about tensile strength of concrete in suspended structures as used by bridge designers or pavements.

Ron's explanation is correct but there is also the difference that pavemnets don't are continuously supported so do not fall down when they "fail" by cracking.

The tensile strength you use in PT bridge design is historically not really an assumption that the concrete has tensile strength. It is an nominal level of stress below which it can be assumed that cracking is controlled and crack width checks do not need to be performed. Over time, some people have begun to think of it as assuming concrete has that tensile strength but that is not the basis of the rules. The design assumption is that all suspended concrete has no tensile strength.

 

[Dinosaur]

rapt,

It is difficult for me not to consider the allowable tension stress limits in prestressed concrete as such when we use gross section properties to design and check prestressed concrete. If we used transformed sections and neglected the contribution of concrete, as we do in reinforced concrete design, then it would be clearer.

It is definitely true that the consequence of a failure of concrete pavement in tension (flexure) is less significant than the same in a concrete bridge deck. However, I am observing a dramatic number of severely deteriorated concrete pavements and I wonder if the design limits are partly to blame.

This brings me to Ron’s post concerning the “assumed mix design strength” of 650 psi with the allowable flexural stress limited in design to 325 psi. We all know concrete has a tensile capacity of approximately 10% of the compression stress. The specific design limit given in the AASHTO code for bridge design is 7.5 x root(f’c). For a 5000 psi mix design, this equates to 530 psi. This is the limit rapt mentions earlier. For comparison, other factors need to be considered.

The loading causing the 530 psi flexural stress, is a temporary live loading and is assumed to occur after all the prestress loses and after a future wearing surface of 15 psf has been placed on the deck. Also, there is a fairly substantial amount of prestressing steel within 2.5 inches of the extreme fiber where this stress occurs. Finally, there is additional factor of safety in the assumed live loading which is substantially greater than the legal load of a truck.

On the other hand, in concrete pavement there is no restoring force provided by prestressing. Mix design strength for pavement is less than prestressed members. Hopefully, the factor of safety in the magnitude of applied live loading is the same. I understand the reinforcement layer is located near midplane of the pavement so is likely much further away from the extreme fiber in bending.

I don’t want to be challenging anyone to defend the pavement design assumption. I am trying to understand the differences in the assumptions and be able to provide reasonable advice on this issue in the future. I am noticing a very large number of concrete pavement failures that will consume many of our maintenance dollars in the future. Is there something we are doing in design to cause this? I welcome any additional insight you have to share. Thanks.

 

[Ron]

Dinosaur...deterioration in concrete pavements might have little or nothing to do with the design strength. Pavement concrete (as with bridges) is some of the most adverse exposure concrete will ever get. With that in mind, the durability of the mix is extremely important. A durable mix can be achieved with a variety of design strengths, but generally under pavement design parameters, a compressive strength of less than 4000 psi is not durable enough for the exposure and use conditions.

Secondly, we have to look at the flexure of the pavement under the loading. The modulus of rupture of concrete (flexural strength) defines the resistance to flexural cracking or extreme fiber stress bending in an unreinforced section. You can get a 5000 psi concrete with a modulus of rupture of 400 psi or you can get a 5000 psi concrete with a modulus of rupture of 650 psi, depending on the parameters of the mix design. The most significant factor is the size of the coarse aggregate.

Empirical studies have shown the 50% level of stress limit to be valid. Much of the design process for ridgid and flexible pavements was based on empirical data. Subsequent analysis methods have validated these approaches, though to some degree there's some "circular reasoning" since many factors used in analytical techniques were derived from the empirical testing.

 

[rapt]

Dinosaur,

I have not used gross stresses to design and check prestressed concrete for 30 years. I use them as an indicator of how heavily stressed a section is but not as the final check. My real check is based on calculating the strains and stresses in each section based on the assumption that concrete has no tensile strength. This is the whole basis of Partial Prestressing.

There is a difference between using "an allowable tension stress limit" and saying that concrete has Tensile Strength. It is this interpretation you are making that has led to the assumption amoung designers over time that prestressed concrete is assumed to have tensile strength. That was never meant to be the case.