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astm c90 in 1981 - strenght of block
search doesn't seem to be working today. does anyone know the strength of concrete masonry units per astm c90 back in 1981. currently the average for 3 is 1900 psi. we have an old specification that states the net area compressive strength of the masonry unit is 1000 psi. we are wondering if that is a typo and should say gross area.
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c90-81: grade n-i minimum compressive strength for average of 3 units was 1000 psi on the average gross area.
there is not typo.
the current required strngth is 1900 psi based on the net area. normally, the strengths are about 25 to 50% over that since manufacturing and process requirements result in the higher strengths. it is difficult to not meet the astm c90 requirements if reasonable aggregates are used. i have seen 8x8x16 hollow units (52% solid) test over 8000 psi based on the net area.
the olde requirements (essentially going back to the 1930s) required the block to test 1000 psi(average of 3 or 5 units depending on the year) on the gross area. this permitted substituting more area and lower concrete strengths to make up for inferior materials and processes.
the new standard of 1900 psi was created i the late 1980s to essentially convert the old standard to an equivalent unit strength without permitting using excess poor quality concrete to acheive the same unit properties.
this conversion was done to permit the use and conversion of all the old testing, reseach and standards on the old minimum standard products.
structurally, you can still take advantage of the ability to specify much higher strength units than the minimum astm c90 requirements. this would be done under the code and specifications provision of aci 530. this permitts the assumption of prism strengths (f'm) (the parallel to concrete cylinders - fc), based on actual two block prisms or the use of the table relating actual net unit compressive strengths to the allowable prism strength (f'm) that is used in many of the equations contained in the three option design methods of aci 530.
i have seen 15 to 22 story loadbearing buildings built using 6" cmus. the engineering was supurb, the drawings clear and concise and the practical construction was excellent. usually 4 different strengths were used for different floor levels on the 10 to 20 buildings under construction at different stages at the same time on the same site.
i hope this helps to unravel the mystery of the old gross area strength that may still lurk in some olde boiler plate specs.

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