Masonry Magazine April 1997 Page. 37
Mortar Acceptance
Acceptance of mortar is typically based on documentation, testing, or samples that provide assurance that the mortar meets the specified requirements (example, mortar color). The extent of documentation, testing, or samples required to provide this assurance will vary depending on the project. For example, letters of certification or mill test reports from the manufacturers of component mortar materials and a statement from the contractor indicating his intent to proportion these ingredients according to the specified proportions are often sufficient for acceptance of mortar. If mortar is accepted under the property specifications of ASTM C 270, test reports indicating conformance to ASTM C 270 should be required. Such tests are to be performed on materials intended for use in construction that are taken to a laboratory, mixed and proportioned in accordance with the requirements of ASTM C 270, molded in 2-in. (50-mm) cube molds, and cured and tested according to the procedures of ASTM C 270. If mortar is specified under the proportion specifications of ASTM C 270, test reports may be required on component mortar materials to provide evidence that they conform to specification requirements, but no testing of the mortar itself is required. As previously noted, acceptance is made on the basis that approved mortar materials are proportioned in accordance with the proportion specification table of ASTM C-270.
If testing of mortar properties using ASTM C 780 test procedures will be required during construction, preconstruction test reports of mortar properties determined on mortar mixed to the proportions and consistency intended for use during construction should be required. These values can be used as a benchmark for comparison to values obtained during construction. However, compressive strength tests of mortar determined according to ASTM C 780 are not expected to meet the compressive strength requirements of ASTM C 270.
Physical samples of mortar materials or mortar specimens may be required, particularly if colored mortar is to be used. While laboratory prepared mortar specimens (typically mortar strips) may be helpful in preliminary evaluation of colored mortar materials, a field sample panel should be reviewed prior to the start of construction to confirm that the appearance of the masonry constructed (using mortar, units, and workmanship representative of what will be used during construction) is acceptable. The approved panel should be retained until the completed masonry work is accepted by the owner.
Mortar Production
After the proper mortar materials and mortar mix designs have been established, the masonry contractor must implement mortar production procedures which assure that the correct proportions are used and that consistent batching and mixing is maintained for the duration of the project.
Mortar materials are batched at the jobsite by volume. Guidelines for jobsite proportioning are also given in ASTM C 270 under the "Construction Practices" section. Cement is volume proportioned on the basis of whole bag or simple fractions of bags. For example, according to ASTM C 270, a 94-1b (42.6-kg) bag of portland cement contains one cubic foot (0.0283 m3) of portland cement by volume. Similarly, a cubic foot (0.0283 m3) of masonry cement or mortar cement by volume is taken as the amount indicated by the printed net weight on the bag. These materials are then proportioned by the mason contractor in whole or 1/2 bag increments. Hydrated lime is also proportioned by volume at the job site. However, the unit weight of hydrated lime according to ASTM C 270 is set at 40 lb/cu ft (640 kg/m3). If hydrated lime is packaged in 50-lb (22.7-kg) bags, one bag of hydrated lime contains 1 1/4 cubic foot (0.0354 m3) of material by volume.
Sand is typically proportioned by shovel. However, shovel count and sand volume can be verified using either a cubic-foot (0.0283-m3) box or other containers of known volume. For example, a five-gallon (19-L) bucket is approximately 2/3 of a cubic foot. Therefore, 4 1/2 five-gallon buckets will contain 3 cu ft (0.0849 m3) of sand. Mortar mixers that have volume-based sand measuring attachments are also commercially available. Volume proportions of sand are made on the assumption that the sand is in a damp, loose condition. Since a given volume of dry or very wet sand weighs more than damp, loose sand, care should be taken to maintain the sand in the damp, loose condition to minimize variations in proportioning of mortar.
Mix mortar materials in a mechanical mixer unless procedures for hand mixing of mortar have been approved by the specifier. For machine mixing, start mixer, engage paddles, and add materials in the following sequence:
* 2/3 to 3/4 of the required water
* 1/2 of the required sand
* masonry cement or mortar cement or hydrated lime followed by portland cement
* remainder of sand, and
* water required to reach workable consistency
Mix mortar for not less than 3 minutes and not more than 5 minutes after the last materials have been introduced into the mixer. Key steps the mason contractor can take to establish and maintain quality mortar production:
* Keep the sand pile covered to prevent wetting or drying out
* Establish mortar proportions at the beginning of the work an post the mix design on or near the mixer for everyone to follow
* Use consistent mixing procedures