The problem with WWF (mesh) is that when the concrete is being placed the workers are stepping on the WWF and their weight pushes it to the bottom. With the weight of the concrete and its density the WWF does not"rebound" to its originally placed position. This same problem exists for rebar IF and WHEN there is no adequate chairs or spacing of chairs to hold it up as the workers step on it. If the rebar spacing is at least 12" on center each way they may be able to miss pushing it down, but as they are trying to place the concrete and moving about with shovels, screeds or even pump hoses they are not concerned with the final placement of the rebar unless the inspector or the engineer is onsite to review with them the importance of not relocating the reinforcement during placement.
When I specify reinforcement I state that the placement of chairs, or what I prefer is concrete blocks having the same f'c as the concrete to be placed, used as chairs, be located such that the maximum deflection in the bars (or WWF) is no greater than 1.5" when supporting the weight of the workers. This ensures that a) the workers can walk on the reinforcement without pushing it all the way down, and b) provides for adequate stiffness so that as they the workers pass over, the reinforcement will spring back to the original location.
In the documents referred to by others you will find that there is both positive and negative bending in the slab. Therefore reinforcement placed at the mid-depth accomodates both. The positive moment is directly below the wheel load and the negative moment occurs between wheels on the same axle. Similar analyses are used for high storage rack post spacing as well as fork lift traffic and their related loads.
One last observation. I reviewed a construction failure where the contractor had pushed the WWF down to the bottom of the slab. It was a reinforced slope pavement under a bridge. Besides walking on the WWF and pushing it down, the contractor did not notify my firm that they were going to make the pour so we were not present on site to instruct them to pull the WWF back into position ( as we had on a previous pour). Well they also did not follow our plans that instructed them to turn the side walls down and embed a minimum of two feet into insitu soil. This bridge was over a creek that had been flooding regulary - so much so the original rip rap slope protection had been washed away a few months earlier due to a 100 year storm event. This contractor refused to watch the weather reports and did not adequately protect his work. In a few days after placing the reinforced concrete slope pavement it rained, the creek waters rose and eroded the side of the slope pavement, water then piped underneath the pavement and washed all the soil from under the pavement. Gaps as large as two feet deep by 12 to 20 feet wide were observed. We made them tear out the slope pavement. Now the reason for this story: I was there to observe the demolition and thought that it would go quickly since the reinforcement was not located in concrete as specified and it was a nominal amount. They used a medium size bull dozer to attempt to break up the slab. The reinforcement had been completely pushed down to the bottom of the slab. This could be seen after the waters receded; remember the free span of this slab is now as much as 20 feet wide with a 2 foot gap beneath. Th dozer could not break up the concrete from the side so the operator climbed the "cat" up on top of the slab. Now I don't know what the weight of this unit was but it had to be at least a few tons. The slab did not break; It took them most of the day to ripp that "inadequately" reinforced slab apart. The reinforcement was only for temperature steel as it was not designed for beam action or even traffic of any kind common to typical pavements. I do not use this example to state that it is acceptable to allow the pushing of the reinforcement down to the bottom. In time the reinforcement, in contact with the subgrade would have corroded and further reduced the tensile capacity of the section. However, the slab was extremely strong at that point in its life.
I'm sure that contractor will never believe that all the care engineers emphasize with respect to reinforcement placement is NOT justified based on the level of difficulty they had removing that particular slope pavement. They might not care but our clients will when after a few years cracks start to develop in their pavements (due to improperly placed reinforcement). When that occurs who are they going to call - most likely not the contractor but the engineer that provided the plans.
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