Concrete Strength Loss (Possible??)

[aledgd]

Dear All,

I am working on a project where we are using high strength concrete (cement content 450kg/cubic meter of concrete).

I have recently received reports from a similar project saying that they have been seeing strength loss at 90-day compressive strength testing. I have not heard of concrete strength loss unless its exposed to poor environmental conditions such as highly acidic ground.

Does anyone have any information on strength loss of high strength concretes? Or is strength loss even possible (I suspected that they may have damaged their own samples)?

Many Thanks,

-aledgd

 

[Ron]

aledgd....apparent strength loss is concrete is usually attributable to a sampling or testing anomaly. It is not likely that an actual strength reduction occurs (unless there is something causing an inordinate amount of microcracking in the concrete matrix).

 

[davefitz]

I've heard of loss of strength over time if salt or chlorides were in the water used to form the concrete, in the case of geopolymer concrete.

 

[BigH]

Neville makes a passing comment (page 303 of his 4th edition - Properties of Concrete). "However, cements made since the 1930s (with lower C2S content and higher specific surface) reached their peak strength between 10 and 25 years, and thereafter undergo some retrogression of strength." (reference Washa and Wendt, Fifty year properties of concrete, J Amer. Concr.Inst., 72, pp 20-8 (1975).

This is not "short term" strength loss. On a job I had recently, our 90d strengths were about 1.5x 28 day strengths - when we didn't wash the fines (the peridotite being somewhat pozzalonic) we found gains of about 2x.