Masonry Magazine October 2000 Page. 19
Table 2.
PROPERTY SPECIFICATIONS FOR
LABORATORY-PREPARED MORTAR
(Adapted from ASTM C270)*
| Mortar specification | Mortar type | Minimum 28-day compressive strength psi (MPa) | Minimum water retention, % | Maximum air content, % Masonry cement | Mortar cement or Cement-lime |
|---|---|---|---|---|---|
| M | 2500 (17.2) | 75 | 18 | 12 |
| S | 1800 (12.4) | 75 | 18 | 12 |
| ASTM C 270 | N | 750 (5.2) | 75 | 20** | 14+ |
| O | 350 (2.4) | 75 | 20** | 141 |
* The total aggregate shall be equal to not less than 2% and not more than 3% times the sum of the volumes of the cement and lime used.
** When structural reinforcement is incorporated in masonry cement mortar, the maximum air content shall be 18%.
+ When structural reinforcement is incorporated in mortar cement or cement-lime mortar, the maximum air content shall be 12%.
It should be understood that the property requirements of ASTM C 270 are for laboratory specimens and are not for field quality control. The ASTM standard presumes that the proportions developed in the laboratory to meet the property requirements will result in satisfactory performance in the field. Field testing is not required, and if field testing of compressive strength is conducted, results are not required or expected to meet the minimum compressive strength requirements of the property specifications.
MORTAR MATERIALS
ASTM C 270 places requirements on component mortar materials (water, sand, cementitious materials, and possibly admixtures). For example, sand must meet the requirements of ASTM C 144, masonry cement must conform to ASTM C 91, mortar cement to ASTM C 1329, portland cement to ASTM C 150, and hydrated lime to ASTM C 207. It is important to understand the contribution of each component material to the performance of mortar.
Water acts as a lubricant in the plastic mortar and is required for hydration of the cement. Strength gain of mortar is not related to evaporation of water but to the chemical combination of water with cement compounds in the mortar. Since some mixing water is lost to absorptive units and evaporation, the maximum amount of water consistent with optimum workability should be added to mortar.
Masonry sand provides the basic "framework" for mortar. Sand particles are coated and lubricated by the mortar paste to provide body and flowability required in the plastic mortar and are bonded together as the paste hardens to provide required structural properties. Sand quality affects both workability characteristics of plastic mortar and properties of hardened mortar such as compressive strength, bond strength, and drying shrinkage.
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MASONRY-SEPTEMBER/OCTOBER, 2000 19