Minimum shrinkage reinforcement in foundation with horizontal joint

[AhmedJ.J.]

Hi all,

I am designing a mat foundation on piles with dimensions approx (30 x 30 x 2.5 m) (100 x 100 x 8.2 ft). We decided to make a horizontal construction joint so the first pour is 1.5m (5 ft) thick and the second pour is the rest thickness. My question is; do I have to provide shrinkage reinforcement for the top face of the first lower pour? Or the mat bottom flexural reinforcement will do the job if the area exceeds 0.0018Ag (which it does)?
ACI 318-19 cl R24.4.3.2 states that "The resulting area of reinforcement may be distributed near the top or bottom of the slab, or may be distributed between the two faces of the slab as deemed appropriate for specific conditions." I don't know if I can apply this statement on mat foundation or not.

 

[Bombadilla]

I'm not familiar with the ACI, but I believe that shrinkage reinforcement in the top part of the first pour serves no purpose. Any cracks on the top surface will be sealed by the second pour. The second pour, however, will get cracking due to restraint from the first pour, analogous to shinkage cracks on the bottom parts of walls etc. Therefore, I believe the shrinkage reinforcement is better suited in the bottom of the second pour.
When adding horizontal construction joints, remember to check if vertical rebar through the construction joint is necessary to transmit the shear forces.