Horizontal Construction Joints

[Zulak]

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.

 

[ishvaaag]

As long as the concreting operations progress normally (the slab is poured soon (1 to 2 days) after the grade beams and the interface is got rough through whatever means, there should be almost no problem in doing that. In some area of Spain (Galicia I think to remember) this was almost standard practice some years ago even for over the ground level beams deeper thant the slab.

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.

 

[RiBeneke]

I have been involved with the use of a method which we called 'greencutting'. It is probable similar to what you mention, and was used on a hydropower project with large concrete structures cast in sections. We made up nozzles from pieces of 25mm (1'') steel pipe about 1.5 m long, with valves and a t-piece at the top. The nozzle was fed simultaneously with compressed air (7 Bar 100 psi) and with water at a similar pressure. The air and water mixed turbulently in the pipe, and the jet was directed onto the surface of the concrete about 6 hours (this time delay must be found by experiment) after the concrete had been cast. This process very effectively removed laitance and loosely bound aggregate from the surface, and allowed for a good construction joint between casts.

Richard Beneke

 

[Qshake]

I have seen a number of methods used to achieve a good, rough joint surface. However, in many cases, unless it is specifically designed for some roughness above the level normally required for concrete construction, then nothing really need be done besides to clean the surface free of laitance. The rebar extending into the slab should also be cleaned and a conscientous contractor/laborer would do so immediately following the pour as it is manyfolds easier.

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.

 

[phuduhudu]

I have also allowed contractors to do what you describe with a beam and slab system. I am not sure that roughness is as much of an issue as making sure laitance and loose or powdery material is not present at the construction joint. The presence of the stirrups will give plenty of bond too. So I would agree with Qshake that the normal standards for CJs should be observed. Carl Bauer

http://www.bauerconsultbotswana.com

 

[JAE]

With a horizontal construction joint through the beam you have to understand that you have a potential slip plane that must be resisted by shear friction along the length of the beam.

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.

 

[ishvaaag]

The approach by JAE is sound in being conservative, and maybe unneeded since the total shear at the horizontal plane is dealt with by the ordinary stirrups, as long no relative displacement occurs. The shear at the horizontal and vertical interfaces are part both of the same behaviour being dealt with stirrups and part of the longitudinal steel. In any case I appreciate the insight in discerning the 2 concepts for a practical sum when the quality of the horizontal interface is being put in question, even more if the stirrup grandtotal is only scarcely increased.

 

[JAE]

ishvaaag - yes, I know its conservative because, as you say, the stirrups resist the shear effect, which is the same in the vertical/horizontal direction. However, we have to attribute the bad effect of the joint, which is no longer monolithic concrete but is now a weaker slip plane. We just decided to be conservative here to account for the added potential for slip and another effect: the possibility that the slab, in compression, might buckle up off the beam starting at an inflection point. I know our method isn't the most scientific, but we feel good about it.