Construction using concrete masonry blocks or units (CMU) is ubiquitous in the United States today, and in fact in the whole modern world. CMU blockwork is a very versatile and relatively economical building material. It is naturally strong in compression, but with reinforced, grout-filled cells, it can also withstand large shear, bending, and tensile loads imparted by lateral wind or seismic events. In this article, I will discuss the various types of CMU designs, as well as terminology, construction techniques, and application uses.
The design of CMU is typically comprised of hollow concrete “face shells” with (2) open “cells”, designed for the installation of vertical reinforcing (rebar) dowels, and filling with grout-mix concrete. However, CMU blocks may also be solid. CMU walls are customarily laid in a running-bond pattern, have welded wire fabric (WWF) truss or “ladder” type galvanized horizontal joint reinforcing, and have 3/8” thick beddings/joints of mortar all around. CMU piers are constructed of 12”, 16”, or 24” square “column” blocks with only (1) cell and have 3/8” thick mortar beds top and bottom. The most common and applicable material specifications for this type of construction are as follows:
- CMU: ASTM C90, Type 1, Normal-Weight (also available in Light-Weight)
- Grout: ASTM A476, Low Alkalai Content
- Mortar: ASTM C270, Type “S” Below-Grade and Type “M” Above-Grade
- WWF: ASTM A82, (9)-Gage
Lengths of rectangular CMU block have nominal face dimensions of either 8” or 16” long “courses”. This is intended to ensure that it provides for a type of wall construction which is modular in nature. I.e., the lengths of walls should be designed in multiples of 8”. CMU is available in nominal thickness dimensions of 4”, 6”, 8”, 10”, 12”, 14”, or 16”. The smaller 4” and 6” blocks are typically used as an Architectural veneer or façade, similar to brickwork. In fact, their cells are generally too small for reinforcing and grout-filling. The mid-sized 8” and 12” blocks are the most commonly used in modern construction. The larger 16” wide blocks are used for specialty applications requiring extra high strength or increased sound dampening. The 10” and 14” varieties are not as common nor as readily available on the market. By far the most common course size is 8”x8”x16”. No matter the size unit, however, a variety of block types are available, including: Header, bond, lintel, closer, jamb, kerf, sash, bull-nosed, beveled-end, and acoustical block. Next, I will discuss the methods of construction of both walls and piers, including the different types of reinforcement, which is determined by the design loads expected at the Project Site, and the particular in-service application.
See the Diagrams below for the following discussion on walls. As mentioned above, CMU walls are constructed by laying the block courses in a running-bond pattern. As shown, this means that each of the 16” long blocks in successive vertical rows overlaps each other by 8”. The WWF horizontal joint reinforcing is typically spaced at 16” on center vertically, meaning in every other horizontal mortar joint. The vertical reinforcing is achieved by placing 90° hooked rebar dowels, spaced at either 8”, 16”, 24”, 32”, or 48” on center horizontally. This spacing is determined by the applicable wind, seismic, and snow loads, and as the application demands. The vertical dowels are embedded in the supporting elements of the foundation base, which is reinforced cast-in-place concrete consisting of either continuous or pad footings, grade-beams, or thickened slabs. The dowels are spliced with straight lengths of rebar which extend on up vertically to about 2” from the top of the wall. Required hook sizes and splice lengths are dependent on the bar size, as governed by specification ACI 318: “Building Code Requirements for Structural Concrete”. This Specification is in turn referenced by the Building Code enforced at the building site, which in the U.S. would typically be the International Building Code (IBC) or the International Residential Code (IRC), the latest editions in effect at the time of permitting. Bar size is also determined by the design loads and demand. A bond-beam course is laid at the top of walls, which has a U-shaped trough which is filled with horizontally reinforced grout. Common applications for CMU walls are basement walls, elevated/drive-under foundation walls, and crawlspace walls. As is the case with all types of concrete, because the CMU blocks, grout, and mortar are particularly susceptible to thermal and drying shrinkage cracking, it is good practice to install expansion or control joints at least every 40’-0” horizontally in walls to arrest cracking. Where the blockwork will be below grade in-service and subjected to contact with the soil, a weatherproof sealant should be applied. Above grade the exposed faces of the blockwork should be “parge” coated with concrete stucco, or fully-coated with concrete stucco. Per ACI 318, CMU walls constructed as described above are considered to be properly detailed for seismic applications.
Next, I turn the focus to square CMU columns or piers. As mentioned, these piers are constructed of column block, which is square and has only (1) cell. The face-shells of column block have a standard thickness of 1½”, but thicker shell/smaller cell varieties are available. The typical method of reinforcement of a CMU pier is to use several vertical hooked dowels embedded in the foundation and spliced in the same manner as with walls. Again, the bar size is dependent on the loading and demand. Now here is where a crucial difference exists between seismic and non-seismic applications. For seismic applications, per the detailing specified in ACI 318, the bundle of vertical bars inside the pier cells are held together with “stirrup” ties, which are typically smaller diameter bars formed into a square shape, with the vertical bars tied onto them. These stirrups are spaced at 8” on center, or every course. The stirrups and vertical bars are tied with rebar tie wire to keep them in place and prevent them from moving. The stirrups serve to hold the bars in their optimum locations. For non-seismic applications, these stirrup ties may be spaced further apart on center, or be replaced altogether with rebar tie wire.
While I am on the topic of seismic resistance, I must make mention of the largest seismic event to ever occur in the Southeast region: The Great Charleston Earthquake of 1886. On August 31st, 1886, a huge magnitude 7.3 earthquake, with its epicenter just inland of Charleston near Summerville, rocked the entire East Coast from Florida to Toronto. It was also felt to the West as far as Nebraska, the South as far as Cuba, and the East as far as Bermuda. The City of Charleston, and in fact the entire South Carolina Lowcountry, was devastated by the quake. About 70% of the downtown district’s brick buildings and homes suffered damage, although most were later salvaged and repaired. See photographs below of the aftermath of the 1886 quake, showing buildings where the brick fell off the structures. In a quaint footnote to history, to this day many downtown brick homes and warehouses still contain earthquake bolts, where iron rods with end nuts are inserted through the entire structure to reinforce them. Back in 1886, the Charleston region and South Carolina as a whole was much more sparsely populated. Today, with the population booming in the Lowcountry, an earthquake anywhere near as strong as the one in 1886 would cause a large swath of destruction, and likely many injuries and deaths. Also, the following aging structures, which in most cases were not specifically detailed for seismic resistance, would be particularly vulnerable to damage: Bridges and overpasses, areas of filled land, older brick buildings, utility lines, hospitals, and ports.1
Therefore, because the Charleston region is in one of the largest seismic zones in the country, load-bearing unreinforced masonry such as brickwork or blockwork is strictly prohibited. Brick and façade block may be used only as a non-load-bearing veneer, and it must be attached to the backing wall structure with corrugated metal brick ties, or for critical applications, special seismic brick ties.
Hopefully, this article helps the readers take away a better understanding of the types of and techniques for CMU construction. It is a very important material, the use of which will continue to be a mainstay of modern wind and seismic resistant construction.
1 ”One Night that Changed Charleston Forever” by Robert Behre, from the Charleston Post & Courier, September 3, 2011
George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications). He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCI), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).