Reinforced masonry retaining wall

[engstructural]

I have inspected a recently constructed retaining wall build using hollow core blocks 215mm thk and cores reinforced using 35N/mm2 concrete with T12 bars.

There are a number of issues with the wall one of which I would appreciate some additional points of view on.

The concrete in the cores should be 35N/mm2 with the reinforcenent approx. 30MM from the rear of the core (towards the retaining face).

on inspecting the concrete it appears more like a dry sandy mix was used. The concrete can be broken and crushed by hand. This is obviously weaker concrete.

The question is; other than poor quality could there be any other reason for the weakening of the concrete (blocks soaking up water from the mix etc)?.

 

[Lion06]

First, it should be grout, not concrete.
While it is true that the blocks do soak up water, the grout prisms are made using block to replicate this fact and to get a more accurate indication of what the in-place strength of the grout will be.

 

[cvg]

with all other things equal, low water cement ratio generally results in higher strength grout. It sounds like your contractor added insufficient cement.

 

[concretemasonry]

There should definitely have been grout used and not concrete. It is not necessary to have high strength in the grout. The reason for the grout is to transfer the load from the walls surface into the rebar and to provide a good bond. Grout, by itself in unreinforced masonry provides little increase in the flexural strength of the wall because it is not near the outer fibers. That is why there are limits on the strength of grout relative to the strength of the masonry units.

The slump of concrete usually is 3-4", while grout must be between 8" and 11".

During consolidation and curing it is intended that the block absorb the extra water needed to get the correct placement.

By your decription you mave have found that mortar was used in place of grout or the grout mas incorrectly made to concrete standards for slump. Since you did not fond any coarse aggregate un the cores it probably was not concrete.

Dick

 

[JAE]

I would agree with everything that concretemasonry says.

8-11" of slump is correct. A low water/cement ratio for grout in masonry isn't an issue as it is with concrete.

With weak grout within the cores, you may have a serious issue with the wall. Lack of bond strength on the rebar, poor compressive strength, etc. could definitely compromise the overall intended strength of the wall.