casting new concrete against existing 1

[richgolang]

What are the possible effects on concrete strength when casting a relatively thin section (200mm)up against existing dry concrete blockwork?

 

[StephenA]

One effect would be that the blockwork would suck the water out of the fresh concrete. Use a bonding agent on the blockwork to seal and bond the new concrete.

 

[richgolang]

Thanks Steven,
That's exactly what happened..we were casting the top member of a ring beam up against a cavity blockwork wall adjacent to a support peir..we were able to overfill the formwork and insert poker vibrator to ensure concrete was well compacted and came up hard against the blockwork. Following day blockwork in contact with concrete showed signs of dampness, obviously draining water from the wet concrete. My concern is will this removal of water from the mix affect it's compressive strength once it has set?

 

[71corvette]

Depending on what your water cement ratio was it's very possible that the scenario you described could have affected the compressive strength. If you had a low w/c ratio and some water was pulled from the new concrete it's possibile that there may not have been enough water left in the mix to complete the hydration process.

If you're particularly concerned about strength you may want to consider having some non-destructive tests done to determine the new concretes strength after it has cured (i.e. rebound hammer tests). Although these are not overly accurate, they should give ballpark figures. You could also have some cores taken and tested in the lab. This process would yield much more accurate results but would be more expensive and is considered destructive testing.

 

[Ron]

Richgolang...the loss of water in the concrete from contact absorption is not likely enough to affect the compressive strength. You have significantly more water in the mix than is required for full hydration.

The rebound hammer is useless for determining compressive strength. It can be useful for determining variations in the concrete, but should not be used for compressive strength assessment. I agree with tinytim22 that cores should be taken for compressive strength.