Rebar Testing 1

[RickWells]

We have an existing structure designed based on 3000 psi concrete and Grade 40 rebar. Analysis of the structure indicates that some members may be undersized if we consider the design concrete mix strength and design rebar yield strength. Original 28 and 90 day concrete break strengths are available and we can make a statistical analysis and justify a higher concrete strength. Original mill certs for the grade 40 rebar are not available. How many samples of the rebardo we need to test to make a valid determination of the actual strength of the rebar in the structure.

 

[paddingtongreen]

If it is an old structure, the concrete probably gained strength between then and now, you could take a core and test it. As for the rebar, from deep in my memory, I think it's three tests but I can't give you an authority, and things may have changed since I last did anything like that.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[bridgebuster]

Look at ACI 437R-03 - Strength Evaluation of Exisitng Concrete Buildings.

Which in part says:

"Sampling techniques—When the yield strength of
embedded reinforcing steel is determined by testing, the
recommendations listed below should be followed:
• Tension test specimens shall be the full section of the
bar (ASTM A 370—Annex 9). Requirements for
specimen length, preparation, testing, and determination
of the yield strength are provided by ASTM A 370.
• In the event that bar samples meeting the length
requirements of ASTM A 370 (Annex 9) cannot be
obtained, samples may be prepared (machined) according
to the general requirements of ASTM A 370 for testing
and determination of mechanical properties.
• Samples should be removed at locations of minimum stress in the reinforcement.
To avoid excessive reduction in member strength, no
two samples should be removed from the same cross
section (location) of a structural member.
• Locations of samples in continuous concrete construction
should be separated by at least the development length
of the reinforcement to avoid excessive weakening of the
member.
• For single structural elements having a span of less than
25 ft (7.5 m) or a loaded area of less than 625 ft2 (60 m2),
at least one sample should be taken from the main
longitudinal reinforcement (not stirrups or ties).
• For longer spans or larger loaded areas, more samples
should be taken from locations well distributed through
the portion being investigated to determine whether the
same strength of steel was used throughout the structure.
• Sampling of prestressed reinforcement, whether from
bonded or unbonded systems, is a complex undertaking
and beyond the scope of this report. Some discussion of
extraction of unbonded single-strand tendons for testing
can be found in ACI 423.4R."

Also:

"Reinforcing bars manufactured before 1947 are sometimes
smooth or have deformation patterns not meeting
modern requirements and, as a result, the bond and development
of these bars could be significantly different from those
of modern reinforcement CRSI (2001). Similarly, changes to
details and assumptions for standard hooks can affect the
development of hooked bars in older structures. For structures
with reinforcing bars manufactured before 1947, CRSI
(2001) conservatively recommends assuming that the
required development length is twice that based on current code provisions. Concrete deterioration will also increase the
development length of reinforcement."

 

[dik]

and really early structures may have concrete strengths of 2000 psi or so and rebar could be 30-35 ksi, yield...

Dik