Reason for porous finish?

[precast123]

Hi

In the picture is Hollow core slab 150mm. What might cause such finish in the slab? Is it water content in concrete or poor compaction?

 

[Ron]

Poor consolidation of the concrete.

 

[hokie66]

Agree that it is poor consolidation, but that is normally very good in extruded hollow core slabs. How were these slabs made? If extruded, I would look at the concrete mix design.

 

[precast123]

@ hokie66, Yes its extruded. This happened only couple of times. I think its mix design not properly followed like you said. Does strength of Hollow core slab is compromised that it must be rejected? Even if strength not compromised is it wise to use this slab, because it have gaps like holes in concrete extending almost half of the section. Water might enter in it, weakening the slab further.

 

[Ron]

Strength is compromised if voids are in the area of the prestressing tendons. Water entering the area does not compromise the concrete strength but will cause tendon corrosion if the voids are near the tendons.

 

[precast123]

Voids are not in the area of strands. Its in top half of the section. There is no slip in the strands also.