Specification, construction-friendly detailing and quality assurance
By Andy Dalrymple, P.E.
Masonry is one of the oldest and most sought after building materials for new building construction. It can serve as both finish and structure, and is therefore extremely versatile in adding both beauty and strength to a project. For some owners, the use of masonry often is limited in projects because of concerns regarding cost, scheduling, quality assurance monitoring and performance. However, proper specification of masonry materials and development of construction-friendly details during the design process – in combination with a well-defined quality assurance program during construction – can minimize the ultimate cost and schedule duration of masonry construction, as well as ensure proper performance and a successful project.
This is the first of a series of three articles that discuss the primary areas where designers can influence the cost, performance and success of a masonry project. This article deals with specifying masonry to meet structural design strength requirements through selection and combinations of masonry units, mortar and grout coupled with preconstruction testing to efficiently use the inherent strength of masonry without undue or inadvertent conservatism. The remaining articles in the series will address:
- Detailing of structural masonry to efficiently use masonry materials and avoid constructability conflicts and;
- Development of a quality assurance program that meets both building code and project requirements efficiently without excessive and sometimes less than meaningful testing requirements.
Design Codes and Standards
The masonry design codes referenced in this article and future articles are: Building Code Requirements for Masonry Structures (TMS 402-08/ACI 530-08/ASCE 5-08), and Specification for Masonry Structures (TMS 602-08/ACI 530.1-08/ASCE 6-08).
These are the most current versions of the masonry design documents and will be incorporated by reference into the 2009 International Building Code (IBC). These standards are developed and maintained by the Masonry Standards Joint Committee (MSJC), which is an organization composed of volunteers who through background, use, and education have acquired experience in the manufacture of masonry or in the design and construction of masonry structures. The Committee is sponsored by The Masonry Society (TMS), the American Concrete Institute (ACI), and the Structural Engineering Institute of the American Society of Civil Engineers (SEI/ASCE). The Committee is charged with developing and maintaining safe, practical and efficient design provisions for masonry using the American National Standards Institute (ANSI) consensus procedures. The MSJC documents are published on a three-year cycle to coincide with the issue of the IBC editions.
Many ASTM International (ASTM) standards are referenced in the MSJC documents and in the text that follows. Table 1 lists the standard numbers referenced with the full standard title. The standard number alone is referenced in the text of this article.
Masonry Design Strength: Specification and Compliance
The required or specified design strength of masonry, f´m, is determined by the structural design requirements of the project, and it is required to be listed in the structural design documents. It is essentially equivalent to the specified compressive strength of concrete, f´c, for those more familiar with concrete design procedures, and is used to proportion masonry structural elements to resist structural design forces. The actual compressive strength of masonry is affected by the masonry units, mortar type, masonry grout and the relative strength of these materials. The discussions that follow are limited to construction incorporating clay and concrete masonry units.
As in concrete design, the masonry designer attempts to maximize the structural efficiency of the structural components by manipulating material strength, component size and steel reinforcement. As such, it often is advantageous to seek combinations of masonry units, mortar and grout to maximize the assembly compressive strength, which is defined as f´m in masonry design procedures.
Under Specification for Masonry Structures (TMS 602-08/ACI 530.1-08/ASCE 6-08), referred to hereafter as “TMS 602,” there are two methods to comply with the specified compressive strength of masonry, f´m: the Unit Strength Method, or Prism Test Method. The methods are materially different and have different advantages related to their use.
Unit Strength Method
The Unit Strength Method is based on historical test data for clay and concrete masonry units and prisms collected from various testing and research sources, which was evaluated by the MSJC Committee and tabulated into easy-to-use tables in TMS 602. The method provides a quick and relatively conservative procedure to specify and comply with the design required compressive strength of masonry, f´m. The Unit Strength Method is equally applicable to ungrouted, grouted, reinforced and unreinforced masonry construction.
Under the Unit Strength Method, f´m, is defined by the combination of masonry unit net area compressive strength and mortar type used to construct the masonry. TMS 602 contains tables for both clay and concrete masonry units that define f´m based on masonry unit net area compressive strength and mortar type. The content of these tables is reproduced herein in Tables 2 and 3.
This method is popular because masonry unit manufacturers maintain records of compressive strengths of their available products as part of their evidence of conformance with applicable ASTM standards. Thus, compressive strength information is readily available to designers through masonry manufacturers and masonry material distributors. This permits decisions related to design strength to be made quickly based on locally available products without the need for project-specific preconstruction material testing.
There are some basic requirements of TMS 602 that must be met in order to use the Unit Strength Method provisions. The requirements are as follows:
- For clay masonry:
- Units are sampled and tested to verify conformance with ASTM C62, ASTM C216, or ASTM C652 and;
- Thickness of bed joints do not exceed 5⁄8 in. (15.9 mm).
- For concrete masonry:
- Units are sampled and tested to verify conformance with ASTM C55 or ASTM C90 and;
- Thickness of bed joints do not exceed 5⁄8 in. (15.9 mm).
- For mortar used in masonry:
- Mortar conforms to ASTM C270.
- For grout used in masonry:
- Grout conforms to the requirements of ASTM C476 or
- Grout conforms to the material requirements of ASTM C476; attains the specified masonry compressive strength, f´m or 2,000 psi (13.79 MPa), whichever is greater, at 28 days when tested in accordance with ASTM C1019.
- Self-Consolidating grout must conform to the above and have a slump flow of 24 in. to 30 in. (610 to 762 mm) as determined by ASTM C1611/C1611M and have a Visual Stability Index (VSI) less than or equal to 1 as determined in accordance with ASTM C1611/C1611M, Appendix X.1.
If the above referenced requirements are not met, prism compressive strength testing per ASTM C1314 is required to establish f´m.
The average compressive strength of clay masonry units produced in the United States is slightly greater than 8,000 psi. Thus, Table 2 for clay masonry indicates that f´m for masonry constructed with an average-strength clay masonry unit would fall between 2,500 and 3,000 psi for Type M or S mortar and between 2,000 and 2,500 psi for Type N mortar. Linear interpolation between values in the tables is permitted, so the actual f´m for a clay masonry unit with a compressive strength of 8,000 psi would be 2,925 psi for Type M and S mortar and 2,440 psi for Type N mortar. The maximum f´m achievable from clay masonry using the Unit Strength Method is 3,000 psi for Type N mortar and 4,000 psi for Types M and S mortar.
The minimum net area compressive strength for loadbearing concrete masonry units produced in conformance with ASTM C90 is 1,900 psi. Thus, Table 3 for concrete masonry indicates that the f´m for a concrete masonry unit with a net area compressive strength of 1,900 psi is 1,500 psi for Type M or S mortar and 1,350 psi for Type N mortar. The maximum f´m achievable from concrete masonry using the Unit Strength Method is 3,000 psi for any mortar type.
A typical value of f´m used by structural engineers for low- to mid-rise loadbearing concrete masonry structures is 1,500 psi based on the minimum 1,900 psi compressive strength requirement contained in ASTM C90 and a Type S mortar, which typically is specified for loadbearing masonry construction. Use of this value of f´m as the basis of design often fails to utilize the full f´m attainable based on readily available standard concrete masonry unit net area compressive strengths. Most loadbearing concrete masonry units exceed the 1,900 psi minimum required net area compressive strength of ASTM C90.
Concrete masonry unit manufacturers in some regions of the country have banded together through local masonry associations and have guaranteed to maintain a readily available supply of standard size concrete masonry units that will produce a f´m of 2,000 psi using the Unit Strength Method and Type S mortar. This program provides designers with the confidence to specify higher-strength masonry units to take advantage of the higher value of f´m and makes loadbearing masonry more competitive in the overall construction market.
Typically, larger concrete masonry units (12-, 14- and 16-inch-thick units) must be manufactured with a higher-strength material mix to have sufficient internal strength to resist damage and breakage during handling and shipping. We recently worked with normal stock, 16-inch-thick concrete masonry units that had a net area compressive strength of 3,370 psi, which resulted in f´m values of 2,300 for Types M and S mortar under the Unit Strength Method, significantly more than 1,500 psi, which would result by assuming the minimum unit net area compressive strength of 1,900 psi as required to conform to ASTM C90.
Using higher compressive strength masonry units to increase f´m significantly increases the overall masonry structural design values as shown in Table 4. The overall increase in material cost incurred by specifying high-strength units on a recent project for an unreinforced concrete masonry wall system is illustrated in Table 5. Increasing the specified masonry unit strength to obtain higher design f´m often is less expensive than increasing component size or adding grout and reinforcement to increase strength. In the case of this project, adding grout and vertical reinforcement at 24 in. oc., added approximately $0.50/ft2 of wall area for an 8-inch-thick concrete masonry wall and $0.80/ft2 of wall area for a 12-inch-thick concrete wall, making increasing the unit strength to obtain a higher f´m more economical. This solution also reduced the overall construction time required to build wall sections since reinforcing steel installation and grouting are separate operations relative to laying the walls.
One should also note that grout strength is not specifically related to the values of f´m generated from the Unit Strength Method tables. Grout used in masonry under the Unit Strength Method must conform to one of two proportioning requirements:
- Proportioned in accordance with Grout Proportions by Volume table, Table 5, of ASTM C476. If so proportioned, no specified grout compressive strength is required. Or
- Grout conforms to the material requirements of ASTM C476 and is proportioned to attain the specified masonry compressive strength, f´m, or 2,000 psi (13.79 MPa), whichever is greater, at 28 days when tested in accordance with ASTM C1019.
If the prescriptive proportion by volume requirements of ASTM C476 is used to specify masonry grout, no specific grout compressive strength is required. The proportions included in ASTM C476 have historically produced masonry assembly compressive strengths, f´m, that exceed those included in the Unit Strength Method tables.
If masonry grout is specified by compressive strength per ASTM C476, the grout materials must be proportioned to produce a grout with a compressive strength that is equal to the specified f´m or a minimum of 2,000 psi, whichever is greater. Therefore, when the Unit Strength Method is used, there is no benefit in specifying grout compressive strengths that exceed this requirement.
We often see design documents based on the Unit Strength Method that specify an f´m of 1,500 psi and a masonry grout compressive strength of 3,000 psi. In this case, there is no reason to specify a grout compressive strength greater than the 2,000 psi minimum as required by the Unit Strength Method. To specify a higher compressive strength grout simply increases the overall cost of the masonry grout on the project with no increase in design value of f´m.
Prism Test Method
A masonry prism is a combination of masonry units, mortar and grout, where the masonry is intended to be grouted in service, laid in stack bond as shown in Figure 1. Masonry prisms are required to be constructed and tested in accordance with ASTM C1314 to determine the compressive strength of the prism assembly as a means of defining f´m of the combination of specific materials. Figure 2 shows a prism test in progress. Prism testing to establish masonry compressive strength often is used when:
- requirements for use of the Unit Strength Method are not met;
- high values of f´m are required by the project design, and locally available masonry units do not have sufficient net area compressive strength to develop the required f´m per the Unit Strength Method;
- the designer or contractor wishes to take advantage of the full-strength capacity of locally available materials; or
- special units or materials are required to achieve the project design requirements.
Under the Prism Test Method, f´m is defined by the testing of sets of masonry prisms constructed of the masonry materials supplied for a specific project using construction means and methods that will be employed in the masonry construction. TMS 602 requires that prism tests be conducted in accordance with ASTM C1314.
The provisions of ASTM C1314 define a set of masonry prisms as a minimum of three prisms constructed of the same materials and tested at the same age. The following also apply based on the masonry construction type: ungrouted, partially grouted, or fully grouted.
- Ungrouted masonry:
- minimum of three prisms;
- prism compressive strength test results required to meet or exceed specified f´m.
- Partially grouted masonry:
- minimum of three prisms ungrouted and three companion prisms fully grouted;
- compressive strength test results of both ungrouted and grouted prism tests are required to meet or exceed specified f´m.
- Solid grouted masonry:
- minimum of three prisms fully grouted;
- prism compressive strength test results required to meet or exceed specified f´m.
Prisms are constructed in stack bond with full mortar bedding between units. Prisms are to be constructed of a minimum of two units with a height to thickness ratio between 1.3 and 5. Prisms to be grouted are cured for a minimum of 24 hours and grouted within 48 hours after fabrication. All prisms are laboratory cured under specified controlled conditions until the time of testing.
Because most loadbearing masonry construction is partially grouted, the typical minimum number of prisms required to evaluate f´m will be six – three ungrouted and three grouted prisms. Testing of masonry prisms for quality assurance to verify f´m can become costly, depending on the size of the project. The cost to fabricate and test a set of three prisms ranges from approximately $400 to $800 depending on unit type and geographic locations.
The requirements for compression testing machines used to perform masonry prism compressive strength tests are very stringent, and most material testing laboratories that routinely test concrete cylinders for compressive strength do not have compression testing machines that comply with the requirements of ASTM C1314. Thus, it may be difficult to find a local construction materials testing laboratory that can test masonry prisms in compliance with ASTM C1314.
The primary advantage of using the Prism Test Method is that it permits the full utilization of f´m developed by the combination of the project-specific masonry materials. Table 6 presents a comparison of f´m based on the Unit Strength Method and the Prism Test Method for a recently completed project. Prisms were fabricated with the project-specific masonry units, mortar and masonry grout, and tested per ASTM C1314 to verify f´m. The project was designed as a partially grouted masonry system. Therefore, both ungrouted and grouted prisms were fabricated and tested. Prism testing resulted in an increase in f´m of approximately 23 percent over that obtained from the Unit Strength Method. This increase in f´m also translates into increases in design values for axial compression and bending compression in the structural components constructed with the masonry.
There is no accurate correlation between masonry grout compressive strength and concrete masonry prism compressive strength. There appears to be a point of diminishing gain in f´m with increasing grout compressive strength. For concrete masonry, the general rule is to attempt to match the specified grout compressive strength to the masonry unit strength to make the most efficient use of the materials.
For clay masonry, past research indicates that the strength of clay masonry prisms constructed of hollow units meeting the provisions of ASTM C652 increased with increasing masonry grout strength. Grout strength was limited to 4,000 psi in the referenced research. Thus, the inherent strength of clay masonry may be better utilized when combined with a higher compressive strength grout mix in the 4,000 psi range.
In general, ungrouted masonry prisms may exhibit a higher f´m than grouted prisms, depending on the specific combination of materials and individual material strengths. This is the reason that both ungrouted and fully grouted masonry prisms are required to confirm f´m for partially grouted masonry.
Compliance With Specified f´m
The specified compressive strength of masonry, f´m, is set by the project structural design requirements and is required by the IBC Code to be listed in the structural design documents. To achieve the most economical construction price, the contract documents should permit the contractor to show compliance with the specified f´m by either of the Unit Strength Method or Prism Test Method. This provides the contractor the most flexibility in selecting materials to comply with the design compressive strength requirements. MD
Andy Dalrymple, P.E., is principal of Whitlock Dalrymple Poston & Associates, P.C., Manassas, Va., a consulting engineering firm providing specialized structural and architectural engineering, geotechnical engineering, and construction inspection and material testing services.