A change in Building Code is a great time to Check-Your-Spec.
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- Specifying Concrete Masonry Units (CMU)
- Specifying Masonry Mortar
- Mortar Joints
- Specifying Masonry Grout
- Construction Tolerances
- Quality Control and Testing Program
- Movement and Control Joints
Specifying Concrete Masonry Units (CMU)
- Only specify that the CMU meet all the requirements of ASTM C90, along with color and texture. Leave the weight and method of verifying compliance with the design strength (ƒ’ₘ ) (Unit Strength or Prism method) to the Structural Sheets.
| Density Classification | Oven-Dry Density of Concrete lb/ft³ (kg/m³) |
|---|---|
| AVERAGE OF 3 UNITS | |
| Light Weight | Less than 105 (1680) |
| Medium Weight* | 105 to less than 125 (1680-2000) |
| Normal Weight | 125 (2000) or more |
ASTM C90-16a Weight Table (Density Classification Requirements Table from ASTM C90-16a)
*Medium weight concrete masonry units are the most commonly specified and constructed units in the Northwest
- When Unit Strength method is used to verify ƒ‘ₘ suggest that the structural engineer use a “standard” composite masonry strength (e.g. 2,000psi, 2,250psi, 2,500psi, 2,750psi or 3,000psi) listed in the Unit Strength Table.
- Remove “Grade” or “Type” from your concrete masonry specification. “Grade” or “Type” of masonry units are reserved for masonry veneers and do not correspond to a concrete masonry unit.
Table 2: Compressive Strength of Masonry based on the Compressive Strength of Concrete Masonry Units and Type of Mortar Used in Construction
| Net area compressive strength of concrete masonry, psi (MPa)¹ | Net area compressive stength of ASTM C90 concrete masonry units, psi (PMa) | |
|---|---|---|
| Type M or S mortar | Type N mortar | |
| 1,750 (12.07) | — | 2,000 (13.79) |
| 2,000 (13.79) | 2,000 (13.79) | 2,650 (18.27) |
| 2,250 (15.51) | 2,600 (17.93) | 3,400 (23.44) |
| 2,500 (17.24) | 3,250 (22.41) | 4,350 (28.96) |
| 2,750 (18.96) | 3,900 (26.89) | —- |
| 3,000 (20.69) | 4,500 (31.03) | —- |
¹ For units of less than 4 in. (102mm) nominal height, use 85% of the values Listed
TMS 602-16 Compressive Strengths for Concrete Masonry
(Unit Strength Table for TMS 602-16)
Specifying Masonry Mortar
- Only specify that the masonry mortar meets the requirements of ASTM C270 and any color that may be required. Leave the mortar Type (M, S, or N) to the Structural Sheets.
- Do not specify a mortar strength. Mortar meeting the requirements of ASTM C270 may be either proportioned as shown in the C270 Proportion Table, or meet the physical properties required for the Type specified (i.e. compressive strength). Mortar should never be specified to meet both the proportion and property requirements.
- Do not specify field testing in an effort to verify compressive strength of mortar used in the field. Field testing may be used to establish consistency in mortar used in the field. When field mortar testing is required, mortar must be sampled and tested prior to start of construction in accordance with ASTM C780 to establish a baseline for comparison of field-tested mortar. There are no ASTM requirements that field sampled mortar meet the strength requirements.
Mortar Joints
- Specify that mortar joints should be tooled to form a waterproof joint and a tight bond with the CMUs. A concave tooled joint is most commonly used. Other acceptable weather resistant joints may also be specified, such as V or weathered joint.
- Any mortar joints that are cracked or not bonded with the face shells of the CMUs should be removed and joints repointed prior to grout placement.
Specifying Masonry Grout
- Specify that the masonry grout must meet the requirements of ASTM C476. Leave the strength requirements to the Structural Sheets. Insist that the mix design submitted indicates compliance with ASTM C476.
- Masonry grout should have a slump between 8 and 11 inches for conventional grout. Self-consolidating grout meets the material requirements in ASTM 476. Because this mix is highly fluid, this material is qualified by measuring its slump flow and VSI using ASTM C1611.
- Where possible, indicate Type 1-2 may be replaced with Type 1L cement. The total weight of cementitious materials in the mix may be replaced with up to 40% fly ash or a combination of 70% fly ash and ground granulated blast-furnace slag (contact nwcma.org for details).
- Masonry grout should not contain integral water repellants.
Construction Tolerances
- Construction tolerances should be specified to meet the requirements of TMS 602 Article 3.3F. Note that construction tolerances are intentionally compatible with material manufacturing tolerances.
Quality Control and Testing Program
- A quality control and testing program should be specified to meet the requirements of TMS 602 Article 1.6.
- Inspection and testing frequency should be outlined on the Structural Sheets. Incorporate Tables 3 and 4 from TMS 602 Article 1.6. We suggest creating a table similar to Tables 3 and 4 and indicate project specific tests and inspections and their frequency.
Movement and Control Joints
- Movement and control joints should be specified and located on the project documents by the design team. Movement and control joint locations should never be left to the discretion of the contractor. A generic note on the Structural Sheets is not enough to comply with this requirement.
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