Horizontal Construction Joints
Horizontal Construction Joints
(OP)
I designed a structural floor slab and grade beam system founded on piles, and detailed the slab and grade beam as a monolithic pour. The contractor has asked if he could separate the pours, by pouring the grade beams first, and then coming back and doing the slab later. The top bars of the grade beam are in the slab, so there will be a horizontal construction joint at the slab-to-beam interface. I've heard of hydro-blasting the "green" concrete of the grade beam to create a rough surface. Does anyone have any experience with this method? If not, what other acceptable methods are typical? Job is near Boston, Ma.






RE: Horizontal Construction Joints
Of course the normal number of stirrups need be in place.
I think I have made quite exactly what you say in a pair of (small) occasions (rc joists instead of a slab) without any consequence whatsoever.
RE: Horizontal Construction Joints
Richard Beneke
RE: Horizontal Construction Joints
If a rougher surface is required, then I agree with the methods mentioned earlier by RiBeneke and would only add that sand blasting is also frequently used.
RE: Horizontal Construction Joints
Carl Bauer
www.bauerconsultbotswana.com
RE: Horizontal Construction Joints
ACI Chapter 11 gives you the shear friction calculations and essentially you are resisting this slip through the vertical stirrups of your grade beam. These same stirrups are also resisting your vertical shear. What we've done in the past is first calculate your stirrups for vertical shear. Then, separately, calculate the shear force (factored) along the horizontal plane. The shear friction method in ACI depends upon the surface condition. We usually specify a 1/4" roughened surface (cleaned as per Qshake above). The shear friction Av is added to the vertical shear Av which gives you a slightly tighter stirrup spacing that you would have without the joint.
Note also that your stirrups, technically, have to extend the full hook development length above your horizontal joint.
RE: Horizontal Construction Joints
RE: Horizontal Construction Joints