Development Length Location Factor 1

[EKizak]

Does anyone have some background on the 1.3 reinforcement location factor for horizontal reinforcement placed with more thna 12" of fresh concrete below the development length or splice? The code only comments that the factor was changed to 1.3 from 1.4 in 1989 but gives no reasoning for the need for the factor or why this condition requires additional development.

 

[Taro]

From MacGregor's textbook: "During the placement of concrete, water and mortar migrate vertically through the member, collecting on the underside of reinforcing bars. If the depth below the bar exceeds 12 in., sufficient mortar will collect to weaken the bond significantly."

 

[JAE]

Just a small bit of additional description to correlate with Taro - imagine a horizontal bar and the concrete sags a bit after placement - you have a small bit of void under the bottom of the bar and thus, the circumferntial bond on the bar is reduced a bit.

 

[UcfSE]

What a good post! This is one of those questions I've been wondering about and never remember to ask.

 

[miecz]

Prior to 1989, this requirement was stated in the code as a requirement for "top bars". At some point, the reference to "top bars" was removed to clarify that this is required for all horizontal bars, such as horizontal bars in retaining and foundation walls.

 

[pappyirl]

The reason it doesn't apply to bottom bars is because the hydrostatic weight of the wet concrete ensures there is sufficient compaction of the concrete at this level to ensure adequate bond.