ASTM C1314 covers procedures for masonry prism
construction and testing, and is used to determine
compliance with the specified compressive strength of masonry, f'm.

ACI-530 requires that during construction f'm be verified.  This test ASTM C1314 is a direct test of f'm.

F'm may be determined by one of two means: the unit strength method or the prism test method.

The Unit Strength method:
b. Concrete masonry — Determine the
compressive strength of masonry based on the
strength of the unit and type of mortar specified
using Table 2. The following Articles must be
met:
1) Units conform to ASTM C 55 or ASTM C 90
and are sampled and tested in accordance
with ASTM C 140.
2) Thickness of bed joints does not exceed 5/8 in.
(15.9 mm).
3) For grouted masonry, the grout meets one of
the following requirements:
a) Grout conforms to ASTM C 476.
b) Grout compressive strength equals or
exceeds f 'm but compressive strength is not
less than 2000 psi (13.79 MPa). Determine
compressive strength of grout in
accordance with ASTM C 1019.

The prism test method = ASTM C1314

lostinthe triangle -

boffintech's comments are good and seem the be from ACI 530.

There is one pitfall to avoid with reinforced/grouted masonry.

Always base the f'm on the results of a hollow prism and add the appropriate grout AND associated area for the design. Of course, the grout is used to transfer loads from steel to the CMUs, but it also does add to the compressive strength of the wall. The grout should be approximately equal to the net strength of the unit compressive strength of the CMUs or slightly higher. Some codes place a limit on the grout strength, since they can have a higher E, which may distort the stress distribution.

DO NOT fall into the trap of using a grouted prism f'm value that may consist of low strength CMUs filled with high strength grout. This leads to a difference in properties and subjecting the outer fibers in compression to high stresses that may be lower than the interior grout, that do not actually take the load.

It is far more economical to specify a higher block strength (f'm) than to add excessive grout that may not be controlled. - As Jim Amhrein said, "Do not count on what you cannot see", so use quality manufactered materials that can be tested in advance of construction. It is possible to make a 4500 psi prism out of 8500 psi block and 2200 psi mortar. - The problems are finding a use for these high strength masonry assemblages and finding a lab with the machine capacity and platten thickness to do the testing, since most are only set up for 6x12 concrete cylinders.

plus, if you use grouted prisms, be prepared for some damaged backs. i agree with concretemasonry and personally suggest using ungrouted masonry prisms while testing grout prsims seperately. i did laugh my butt off once when one of my techs called me since he'd had the contractor make a grouted masonry prism with 12" block. the thing was so heavy they had to get a lift to put it in the back of his truck. needless to say, i had forgotten to tell him that we were going to test ungrouted prisms and i quickly provided a new directive. also, if you happen to have someone using onsite mixed grout, be sure to be a little dilegent with what you're going on the grout testing.

How can you get by with un-grouted prisms only?

For projects with both grouted and un-grouted walls both must be represented with C1314 tests, right?

from what i've seen, grouted prism are not usually specifically required as long as you test both the ungrouted masonry prism and grout prism. i believe it's an either/or sort of philosophy (test grouted and ungrouted masonry prism OR test ungrouted masonry prism and grout prism then refer to aci530 table)...at least that's my interpretation. at the beginning of construction, i will also make a point to test individual blocks as sort of a baseline--more as the preconstruction testing that is required for special inspections...plus, i've seen more problems with the wrong block (or deficient block) being sent to jobsites at the onset of the project.

i suppose that if the specifications specifically call out c1314 as the test method without the option to follow building codes or aci530, then someone should ask the eor if the other option is acceptable. sometimes, the eor may have a strict mindset or may simple have cut/paste from previous specs that were incomplete (in other words, there might be inadvertent omissions from the specs on their part). so it never hurts to ask...again, that's my "simple" interpretation. others might be able to explain it better and more technical than myself.

Per ASTM C1314
5.9.2 Where the corresponding construction is to be partially grouted, construct two sets of prisms; grout one set solid as described in 5.9.1 and leave the other set ungrouted.

So I don't see how you can get around grouted prisms.  What job with CMUs has zero grout?  I've never seen one.

An interesting article on CMU masonry....

"Ungrouted prisms showed greater average
compressive strength than grouted, due to the large lateral expansion of the grout under
axial compression, which led to a premature tensile splitting of the block's shells. The
grout strength did not affect the masonry compressive strength."

http://library2.usask.ca/theses/available/etd-03272006-184740/unrestricted/a_paturova.pdf

 

Step back a few steps and look at what you are building. - Forget about the old concept of filling everything up with grout because it seems to make sense. - This is especially bad on taller structures with seismic forces.

The hollow prism is a companion to the concrete cylinder since it is a measure of the materials in a structural element(a wall).

Both of these tests are compressive tests designed to test the basic materials.

Adding grout is necessary to transfer verticla/shear loads from the wall to the reinforcement that is used as necessary. Very often, the reinforcement is required as a minimum reinforcement and not really required structurally. Heavily reinforced and over-grouted areas on older designed construction are very rigid and concentrate loads instead of permitting the to be distributed to the remainder of a building. Vertical loads can easily be handled by specifying a higher hollow prism strength that can be from 2000 psi to 4500 psi if you can find a reason for the requirement and a lab that can test it.

Unfortunately, the woefull lack of understanding of masonry design in the U.S. contributes to poorly designed masonry structures from a performance standpoint. - A respected foreign enginer told me "We use use your codes, but we use them better and use modern techniques" - They even forbid clean-outs and use a video camera for periodic inspection to preserve the continuity of the masonry and instill a standard of construction.

The grout should be about the strength of the unit strength of the CMUs (or slightly higher) if you expect the stress distribution to be in agreement with the design assumptions. Relying on high stregth grout in a prism negates all strectural designs. Remember, a grout compressive strength cannot immediately transfer to a prism strength because of the testing procedure, curing and failure mode. This is also the reason you can build a 4500 psi hollow prism with 2200 psi (or so) mortar.

The intellegent use is to use common sense and engineering intellect, which is difficult with U.S. codes and other very structured traditional codes that take decade to improve.

The "interesting article" refers to a "volume bulked up" college research paper generally dealing with veritcal comprssion on a three high prism (not the norm) for compressive strength and not performance in a real building. The paper also openly admits to testing concrete masonry units not made for grouted construction, which will reduce any and all test results.

The best route is to take one of the many availablr seminars sponsored by ACI, NCMA or another educational groups regarding the proper use of ACI 530. - It will help to keep you up with the rest of the world or at least with ACI 530.

the astm describes each specific test method...code allows either masonry prism (grouted/ungrouted) OR unit and grout testing with interpolation on a table. i only test ungrouted prisms and check individual units (periodically) and grout seperately but in addition to the ungrouted prisms.

while i don't have the specific documentation in front of me and don't memorize the stuff, i believe this is described in aci530 and ibc2006. again, if the project specification describe only one particular astm test method, then the eor should approve other test procedures.

i will double check my references when i get a spare moment in the office next week.