masonry mortar in the 21st century

by Thomas F. Richardson, Concrete & Masonry Consultant

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only after the causes of variance in mortar has been determined, and evaluation of its effect on the masonry has been completed, can appropriate measures be established to correct the variance

The new International Building Code (IBC) and the new International Residential Code (IRC) will go through the rules process in 2000, through the education process in 2001, and be adopted by the state of Minnesota in the spring of 2002. There are substantial advantages in combining the efforts of the existing code organizations to produce a single set of codes. Code enforcement officials, architects, engineers, designers, specifiers, and contractors will be working with a consistent set of requirements throughout the United States. It is estimated that these changes can reduce the overall construction costs of most buildings by approximately 5 to 8 percent.

In Chapter 21 of the Masonry Section the IBC, Section 2103.7 Mortar states that masonry construction shall conform to ASTM C270 and "shall conform to the proportion specification of Table 2103.7A or the property specification of Table 2103.7B". You will see a new mortar called Mortar Cement in these tables. Unlike masonry cement, mortar cement can be used on any project where portland/line mortars are specified, when approved by the specifier.

The proportion or property specifications will govern as specified. When neither proportion or property specifications are specified, the proportion specifications will govern, unless data are presented to and accepted by the specifier to show that the mortar meets the requirements of the property specifications.

For many years, mortar cylinders have created many problems because they are on site for more than 24 hours, and seldom get into the laboratory moist cure room on time. Too often projects are shut down or arbitrated because some decision makers do not understand that ASTM C 270 is not a specification to determine mortar strengths through field testing. These test cylinders only give an indication of the quality control of the batching and mixing at flue job site.

In contrast, ASTM C780 is acceptable for pre-construction and construction evaluation of mortars for plain and reinforced unit masonry. This test method covers procedures for the sampling and testing of mortars for composition and for their plastic and hardened properties, either before or during their actual use in construction. The tests results obtained under this test method are not required to meet the minimum compressive values in accordance with the property specifications in Specification C270.

If, during construction, compressive strength test results vary significantly from values established during pre-construction testing, the causes of that variance must be established before any corrective action can be taken. The quality of mortar materials can be verified by repeating laboratory tests of mortar materials. Batch control may be investigated by running aggregate ratio tests or inspection of batching procedures. Information on the consistency of mortar and other test conditions can be evaluated from test reports. Tests of prism specimens can be conducted to determine performance characteristics of the masonry. Only after the causes of a variance has been determined and evaluation of its effect on the masonry has been completed can appropriate measures be established to correct the variance.

Section 2104.1 Masonry Construction requires that masonry construction comply with the requirements of Section 2104.1.1 through 2104.5 and with ACI 530.1 - 99/ASCE 6- 99/TMS 602-99. These Building Code Requirements for Masonry Structures are reported by the Masonry Standards Joint Committee (MSJC). This code covers the. design and construction of masonry structures. It is written in such form that it may be adopted by reference in a general building code. A new Section, 2104.4 Hot Weather Construction is presented for the first time.

Section 2105 Quality Assurance is used to ensure that the constructed masonry is in compliance with the construction documents. The Unit Strength Method or the Prism Test Method can determine a more satisfactory representation of the wall compressive strength. As mentioned earlier, these test methods give one a better indication of what the hardened properties of the wall are, rather than relying only on mortar cylinder test results. One must also remember that mortar cylinder strengths give only 80 percent of the cube compressive strengths achieved in the laboratory.

The MSJC Code assumes that all masonry is inspected, but permits the specifier to determine what level of inspection is required for a given project. At a minimum, a review of the contractor's proposed materials and mortar mix design for compliance with the contract documents is mandatory. However, as indicated by the old adage that advises one "not to expect what you don't inspect", it is prudent to incorporate some formal inspection requirements on substantial masonry projects. Inspection, when performed by knowledgeable, reliable personnel, is perhaps the most dependable and straightforward means of assuring that the proper mortar ingredients and proportions are being used in the batching of mortars. Procedures for measuring volume proportions of mortar materials can be observed and "real time" feedback given to those responsible for control of those proportions.

Finally, there are two areas where specifier awareness must be improved:

  1. Mortar/unit compatibility, which accommodates bond and water-tightness of the wall.
  2. The benefits of the finishing operation should be protected from improper cleaning of the masonry. 

I highly recommend the reading and study of the Appendixes in ASTM C270.

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2000 Thomas Richardson

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