concrete rebar placement factor

huangyhg

concrete rebar placement factor
in the canadian concrete code, there is a modification factor for the development length that is dependent on whether or not the rebar is "top" tension steel or "bottom" tension steel. i understand how to differentiate between the two, but what is the reason for this modification factor?
the code simply states that the factor was reduced from a previous edition, but gives no explanation as to why "top" steel requires a larger development length.
thanks,
mike
this difference occurs as well in the spanish ehe code.
that sounds a little different than our code. it states that top steel is:
"so placed that more than 300mm of fresh concrete is cast in the member below the development length or splice."
so in a wall, the horizontal rebar could be 12" above the footing (with say 7 ft of concrete above it)and be considered top bar, thereby requiring extra development length.
i think someone said once that it had something to do with air bubbles genereated in the curing process, but i can't re  
i also heard that it is due to air bubbles forming on the underside of top bars, thus reducing bond perimeter.
daveatkins
fresh concrete does not have a uniform constitution through its thickness after being placed.  water and air entrapped below the reinforcing bar decreases bond more for "upper" bars than for bottom bars (bottom 12").
also....
with extra concrete below the bar, there is the potential for the semi-set concrete to sluff downward, creating small voids directly below the bar, thus reducing its bond.