We do not take the problem lightly when a mortar test result comes back too low. It is of the utmost concern for us to maintain a high quality product. We feel that we have done so, in spite of what the laboratory's tests show. We have seen this situation before--as have many other masonry contractors--and in our experience, the problem is in the manner the samples are taken. Very different results occur if a laboratory who understands testing masonry mortar is hired.

Experience tells us that most laboratories do not know how to take a mortar test. Most are accustomed to testing concrete, not mortar. In fact, most test result sheets are headed "Concrete Testing Report."

There really isn't any such thing as a field test when it comes to mortar. It is supposed to be tested in a lab, and then the same mix design used in the field. "Because laboratory testing is conducted under static climatic conditions, consistency retention test results reflect the relative performance of the mortar systems under test" according to ASTM C780 5.2.2. "The same general relationships would be expected to hold during testing at the construction project, except as they are influenced by jobsite weather conditions. For this reason, the specimens should be stored in a temperature, climate, and moisture controlled environment."

Faulty testing is more common than one might think. "Reliable strength results depend upon careful observance of all the specified requirements and procedures," states ASTM C780 A7.9.2.4 & ASTM C1093. The integrity of a sample can be compromised if it is taken from wet, dry, and/or old mortar, if it is not placed in the correct containers, if it is not tamped for the correct number of times, if not transported in the correct way, or not placed in the right storage. A sample is void if any one of the required procedures is not met.

Mortar tested from "sampled cubes" can, and often does, produce different results than actual "in wall mortar bonded units" Bond strength, good workability, and water retentively are important, while the significance of compressive strength of mortar is sometimes over-emphasized. The point is that the finished product, masonry units, and joints combined are more likely stronger and meet the required specifications.

Recognized and accredited laboratories have already determined the strengths of mortar, which is how they came up with the various types to begin with. For example, Type N mortar requires 1.5 bags of portland cement and 1.5 bags of lime to accomplish the right 1:1 ratio, and then using a 9-cubic foot mixer. After putting the prescribed proportions of portland cement and lime in first, the mixer would run over before you could put too much sand in it. The only item left is water. If you had too much the bricklayers would not use it, and if you had too little it would bog down the mixer. It has been proven that Type N mortar produces a 750 psi, Type S 1800 psi, and so on.

Mortar and grout meet the minimum required psi strengths because they are closely monitored during the mixing procedures. Therefore, we would like to request an accurate record of how the samples were taken, stored, and transported. We ask that we are notified when future tests are to be done in order to monitor the procedures. In this way, every precaution will be taken to assure that future tests meet specifications and the psi will be more adequate than required. Thirty years of experience tells us that low test results are generally traced back to the method of taking the samples.

The bottom line is that field testing is inadequate for determining mortar strength. According to ASTM, laboratory prepared mortar mixed with a quantity of water should produce a flow of 110 + 5%. This quantity of water is not sufficient to produce a mortar with a workable consistency suitable for laying masonry units in the field. Mortar in the field must be mixed with the maximum amount of water, consistent with workability, in order to provide sufficient water to satisfy the initial rate of absorption (suction) of the masonry units.

The properties of laboratory prepared mortar at a flow of 110 + 5%, as required by this specification, are intended to approximate the flow and properties of field prepared mortar after it has been placed in use and the suction of the masonry units has been satisfied. The properties of field prepared mortar mixed with the greater quantity of water, prior to being placed in contact with the masonry units, will differ from the property requirements in the ASTM specifications. Therefore, the property requirements cannot be used for quality control of field prepared mortar.

ASTM Test Method C780 may be used for field testing, but the results obtained under this approach are not required to meet the minimum compressive values in accordance with the property specifications in Specification C270.

We hope this information shows that mortar testing is a complicated procedure and must be done correctly. And even when every precaution is taken, the test can be faulty. We are serious in assuming our responsibility in this matter; however, the tests need to be monitored to assure credibility. And, even then we are dealing with field testing, which leaves a lot to be desired, to say the least.

Ron Willis owns Masonry Estimating and Consulting Services (www.meacs.com). He can be reached at 817-822-8595 or ron@meacs.com.