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Spring - Summer 2001 CODES AND SPECIFICATONS UNDERSTANDING AND MATERIAL AND PRODUCT STANDARDS - PART I
Provisions in 1998 California Building Code are discussed, rather than 1997 UBC Provisions, because it is a document based on 1997 UBC adopted in California and also covers State of California Amendments, which are applicable to public schools, community colleges, essential services buildings, and hospitals. The three part series is divided as follows: Part I deals with Material and Product Standards. Part II will address Design Provisions. Part III covers Testing, Inspection, and Construction. A typical concrete masonry assemblage is a complex system comprising of four primary materials; concrete masonry units, mortar, grout, and reinforcement. No unreinforced masonry is permitted in seismic zones 3 and 4 of the Uniform Building Code (1997 edition). Each of the primary materials used has significant variability in its properties, making the entire assemblage difficult to analyze, design, construct, and inspect. Testing and inspection functions are particularly important due to this inherent complexity and variability to deliver a high degree of quality in the final assemblage. From a strictly definitional point of view, concrete masonry units are considered "a product," not a material. Therefore, concrete masonry units are discussed under the category of product. MORTAR Definition for mortar is given in Section 2103.3
of the California Building Code.
Two methods are allowed for specifying mortar:
Addition of lime to mortar improves: a. Workability Please note, "lime" is a cementitious material, not an admixture or additive. The compressive strength of mortar is sometimes used as the main requirement for selecting a mortar type. Compressive strength is easy to measure, tests are well established and reliable, and can give consistent results. The importance of compressive strength of mortar though, is overemphasized. Bond strength is generally more important. Since good bond strength is also dependent on water retentivity and workability, these properties are important. Compressive strength is measured on cube or cylinder. However, the size effect needs to be considered. A mortar joint has very high width to thickness ratio. The ultimate compressive load carrying capacity of a 3/8-inch thick bed joint is deemed to be well over twice the compressive strength of a mortar cube. For these reasons, Type "S" mortar is preferable to type "M", although it has lower compressive strength. As a basic design philosophy, mortar should also be weaker than masonry units so that the cracking could occur in mortar joints instead of in the masonry units, which are repairable. GROUT Definition for grout is given in Section 2103.4. It is defined as:
We are defining them here for clarity. Grout lift is an increment of grout height within total grout pour. Grout pour is the total height of masonry wall to be grouted prior to erection of additional masonry. A grout pour consists of one or more grout lifts.
Grout can be specified by one of the three methods:
Fine Grout
Coarse Grout
Specifying grout by compressive strength at 28 days is common. Many times minimum specified compressive strength is far in excess of masonry unit strength or prism strength. This is not necessary if adequate data exists. Code allows the grout to be specified based on the data. In that case, the proportion of ingredients needs to be specified by volume. CONCRETE MASONRY UNITS (CMU'S) Primarily four types of CMU's are available; load bearing, non-load bearing, concrete brick, and segmental retaining wall units. Each conforms to a different Standard of ASTM. Our focus in this article is "load bearing units," as it is the predominant product used in structural masonry.
As one can see, the multiplier for required unit strength varies from 1.25 at the low end to 1.60 at the high end of f 'm . Linear interpolation for values of f 'm not listed in the table is allowed. REINFORCEMENT Reinforcement that is unique to concrete masonry construction is joint reinforcing made from cold drawn steel wire. This reinforcing consists of deformed longitudinal wires welded to cross wires. The joint reinforcing is placed in mortar joints between masonry courses.
It is common to have minor cracks, minor chipping, or minor imperfections in masonry walls. These should not be considered as basis for rejection of work. ASTM C90-01 Standard requires that when viewed from a distance of not less than 20 feet under diffused lighting, exposed wall construction shall not show imperfections, chipping, or cracking. This is considered a general criterion for accepting appearance. This issue of Masonry Chronicles was written by Vilas Mujumdar, Executive Director of Concrete Masonry Association of California and Nevada. |
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This
is Part I in a series of three articles to
understand and locate codes and specification
provisions related to concrete masonry design and
construction. The purpose of these articles is not
so much to provide specific design guidance, but to
direct a design engineer to various provisions,
primarily in the 1998 California Building Code.
Many other codes and standards such as ASTM,
UBC-Standards and Masonry Standards Joint Committee
(MSJC) provisions are also referenced where
appropriate.