Masonry Magazine September 1975 Page. 13
NCMA-TEK
An Information series from National Concrete Masonry Association
Field Inspection
Of Engineered Concrete Masonry
Introduction
Under existing codes, design and construction of masonry may be accomplished in accordance with: (1) rules that have evolved over a long period of time; (2) engineered design, or rational analysis. Because "rules of thumb" design does not take into account many of the variables present, minimum wall thickness and fixed allowable stress requirements often results in uneconomical loadbearing wall thicknesses if building height exceeds a few stories. Engineered masonry design criteria which provides for use of high strength units and other variables, permits the designer to develop economical construction based on a thorough analysis of all forces acting on the structure. NCMA's Specification for the Design and Construction of Load Bearing Concrete Masonry, provides criteria for rational analysis and design for loadbearing block construction. This specification states that the given allowable stresses apply only where there is engineering or architectural inspection to insure that the construction, materials, and workmanship requirements of the specification are satisfied. Where inspection is not supplied, allowable stresses are drastically reduced. The purpose of this TEK is to aid the inspector in satisfying requirements for inspected masonry.
General
Good masonry construction depends upon good design, proper materials and proper workmanship. Design and specifications should be presented in such manner that the contractor will interpret them correctly. Masonry materials-units, mortar, reinforcement, etc.-must be provided that will comply with the requirements specified; and the care with which the structure is built should result in the safety factor intended and satisfaction by the owner.
The objective of field inspection is to insure that quality control is maintained throughout the duration of the construction. By observing the work of the contractor and materials used, the inspector insures that the intent of the designer and owner, as expressed by the plans and specifications, is satisfied. The inspector should use good judgement to allow the contractor reasonable freedom in techniques, so long as the required results are obtained. He should become thoroughly familiar with the plans and specifications. Since all items are not of equal concern, the inspector should meet with the engineer and architect to establish priorities concerning which parts of the construction are more important.
Materials
Concrete masonry units on the job site should be protected against wetting prior to use. Preferably they should be stored off the ground, as moisture can be absorbed from the ground as well as from rain or snow. Once wetted, units at the job site can be dried, but it may be difficult and time consuming, depending upon weather conditions.
National Concrete Masonry Association
Concrete Units
Field inspection of masonry units includes determining that units are sound and free of cracks or other defects that would interfere with the proper placing of the unit or impair the strength or permanence of the construction. Minor chipping is not grounds for rejection unless clearly stated in the specification. Units selected at the job site for laboratory testing should be picked at random so that the sample is representative. An indication of the moisture content of the units can be obtained by breaking a unit and looking for moisture on the newly fractured face.
Mortar proportions for the various types of mortar are regulated by standard specifications. Each of the principal constituents of mortar makes a definite contribution to its performance. Portland cement contributes to strength and durability. Lime gives the mortar workability, water retentivity, and elasticity. Both cement and lime contribute to bond strength. Sand acts as a filler which also contributes to the strength of the mix and enables the unset mortar to retain its shape and thickness under several courses of block, Water is the mixing agent which gives workability and begins the cementing action.
Once the mortar proportions are established, it is important that these proportions be accurately and consistently maintained throughout the duration of the project. Batch size should be such that cement can be used in full sack quantities. Sand should be batched more accurately than by the "shovelsfull" method. For