Concrete Strength

[gosai]

I have one project for addition and alteration. The project require to place concrete on top of old concrete. The existing structure was approximately built in 1937. The concrete wall placed has no reinforcement. I tried to take cores from existing structure to check comprssive strength. During coring process the concrete start falling apart in pieces and i noticed 1 to 2mm thick cracks in broken cores

What cause concrete cracks other than age?

Thanks to all in advance

 

[frv]

well.. not having reinforcing sure doesn't help.

ASR is another possibility.

 

[Ron]

Cracks in concrete can be caused by numerous things. The most common are drying shrinkage and overstress. If you are experiencing crumbling of the concrete during coring, you likely have low strength that could be caused by a variety of factors as well. For instance, the concrete might have had a low strength at the time of placement. It could have low strength because of deterioration. The most likely deterioration would be loss of calcium through leaching (efflorescence or other solutioning of materials), carbonation, or some environmental attack. ASR as frv mentioned, can occur but is usually more destructive and apparent.

Can you post a photo or two of your cores?

 

[BAretired]

ASR is Alkali Silica Reaction?

BA

 

[hokie66]

Many early concrete structures were built with little cement, a lot of water, poorly graded aggregates, little consolidation...the list goes on. So instead of strengths in the structural range we use today, say 25 to 60 MPa, strengths in these structures are commonly found to be 5 to 10 MPa. Sure, deterioration can occur as noted by others, but if in a protected internal environment, the most likely answer is that the structure was originally built with poor quality concrete.

 

[Ron]

BA..yes. Sorry, shouldn't have carried the acronym.

gosai....you should get a petrographic examination of the concrete done to determine the probable cause of the problems. You need to know that before you can determine a repair/remediation scheme.

hokie66...I agree, that's the most likely cause. Follows the addage that good concrete is made from aggregates, cement, and water....bad concrete is made of the same stuff.

 

[BigH]

Sometimes, we focus on the "concrete" rather than on the workmanship in transporting and placing the concrete. It could be that the concrete was poorly consolidated, vuggy, entrapped air voids, water left in the transit mixer that gets mixed in with the new batch, and the like. This is something that seems to be forgotten by many . . . Most of this should be picked up by QC but many forget that QC doesn't operate the vibrators, doesn't operate the buckets delivering the concrete, etc.

 

[hokie66]

QC didn't exist in 1937. Vuggy?

 

[Ron]

"Vuggy" is a petrographic term describing an aggregate with, particularly, elongated or piping voids, usually with a deposition of another mineral in the lining. An example would be fossiliferous limestone with crystal growth in the void spaces. Vuggy aggregates are generally weaker than solid aggregates, so using them in concrete is not a good thing.

When used to describe the concrete matrix, it generally means that there are piping voids, usually from water gain and/or lack of consolidation.

 

[gosai]

Please find attached photograph of core

 

[concretemasonry]

That is some tough looking and angular large (very large) aggregate for for proper placement. Not thoroughly consolidated either because of the harshness of the mix. Probably uncontrolled by today's standards.

I doubt it was hauled in a transit mixer in 1937 at the time unless it was in a large city.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[BAretired]

I would be reluctant to place new concrete on an unreinforced wall with questionable strength. Unless the new construction is minimal, perhaps the existing wall and its foundation should be replaced.

BA