Two recent instances where 3.25 x 3.25 x 6.5-inch grout specimens achieved lower compressive strength at 28-days than at 7-days. Both sets were cast from 3,000-psi CMU block fill provided by a ready-mix plant at the same project. Both sets were cast in those cardboard set-of-4 molds. Both sets were returned to the lab next day, stripped,and "moist" cured for 7 / 28 days. Both sets were capped and tested. Both sets exceeded mix strength at 7-days but only reached about 90% at 28-days. Many other sets at this lab have NOT exhibited this tendency, but what could be the reasons for these two sets peculiar results? Off the top we're ruling out testing technician or equipment error, because great care was taken with the second set due to the results of the first set. Any ideas???
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Compressive Strength Testing of Grout
- Thread starter sapperDAK
- Start date
concretemasonry
Structural
I suspect there be an error in the testing (preparation/curing and timing) and possibly the results required.
I assume you are talking about a grout as described in ASTM C476 that references ASTM C190 as the testing procedure.
As far as the specifying it normal to use the masonry strength (f'm) from a 2 high hollow prism. The 3000 psi might be for a single units and not a reference to the wall or masonry strength (see ACI 530 commentary). Many engineers want the grout compressive strength to be lower than the f'm, since the grout is intended to be used for bonding the reinforcement to the masonry units and not to increase the strength of a representative masonry assembly. It all has to do with grout that is too strong gives a implied/assumed average compressive strength, when in bending the the outer fibers or the masonry units are the most highly stressed when the historic tests used in code development are used.
For pure compressive strength higher compressive strengths masonry units can be made stronger and cheaper than just dumping in grout. (I have made 8400 psi (per ASTM C140)CMUs that provided a hollow prism strength of 4800 psi, but never found a need for that high strength strength).
I assume the grout was 8" to 11" slump when sampled and samples prepared and placed in 2 rodded layers and topped off with what is need to after preliminary consolidation/absorption added later (within 30 minutes)to make the height within the 5% tolerance allowed to prepare a 2:1 ratio. It is critical that the formed grout sample be representative of the actual construction.
The samples must be cast in a form made from the same masonry units for the project with the grout cavity separated from the units by a thin, permeable material to insure a comparison to the in-place gout. Alternate methods may be used if approved by the specifier based on a history of tests if they can be used with a correlation factor applied to the results.
There are many details that are provided in, especially the Coefficient of Variation between lab and field samples. The major difference is the required higher slump for grout and the use of absorptive forms instead of non-absorptive forms that preclude the important absorption and consolidation that is the basis for the criteria used in the ACI 530 Specifications and Commentary documents that are the global basis for many codes.
In brief, it is not like simply testing a concrete cylinder, sine it is a method to approximate the performance for a masonry wall.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
I assume you are talking about a grout as described in ASTM C476 that references ASTM C190 as the testing procedure.
As far as the specifying it normal to use the masonry strength (f'm) from a 2 high hollow prism. The 3000 psi might be for a single units and not a reference to the wall or masonry strength (see ACI 530 commentary). Many engineers want the grout compressive strength to be lower than the f'm, since the grout is intended to be used for bonding the reinforcement to the masonry units and not to increase the strength of a representative masonry assembly. It all has to do with grout that is too strong gives a implied/assumed average compressive strength, when in bending the the outer fibers or the masonry units are the most highly stressed when the historic tests used in code development are used.
For pure compressive strength higher compressive strengths masonry units can be made stronger and cheaper than just dumping in grout. (I have made 8400 psi (per ASTM C140)CMUs that provided a hollow prism strength of 4800 psi, but never found a need for that high strength strength).
I assume the grout was 8" to 11" slump when sampled and samples prepared and placed in 2 rodded layers and topped off with what is need to after preliminary consolidation/absorption added later (within 30 minutes)to make the height within the 5% tolerance allowed to prepare a 2:1 ratio. It is critical that the formed grout sample be representative of the actual construction.
The samples must be cast in a form made from the same masonry units for the project with the grout cavity separated from the units by a thin, permeable material to insure a comparison to the in-place gout. Alternate methods may be used if approved by the specifier based on a history of tests if they can be used with a correlation factor applied to the results.
There are many details that are provided in, especially the Coefficient of Variation between lab and field samples. The major difference is the required higher slump for grout and the use of absorptive forms instead of non-absorptive forms that preclude the important absorption and consolidation that is the basis for the criteria used in the ACI 530 Specifications and Commentary documents that are the global basis for many codes.
In brief, it is not like simply testing a concrete cylinder, sine it is a method to approximate the performance for a masonry wall.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
darthsoilsguy2
Geotechnical
my opinion needs to first know if we are dealing with 1 7-day and 3 28-days to make one box, or 2-boxes for 3 7-days, 3 28-days, and you have 2 reserves to play with?
we gave up on those very convenient boxes a while ago, because i knew i would eventually run into somebody like concretemasonry if we kept doing it, even if actually was bad grout. The correlation charts the box supplier don't work. it won't be derived by the grout or block used on the job.
we gave up on those very convenient boxes a while ago, because i knew i would eventually run into somebody like concretemasonry if we kept doing it, even if actually was bad grout. The correlation charts the box supplier don't work. it won't be derived by the grout or block used on the job.
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