Masons First with Lightweight Concrete Masonry Units
Words: Kevin Cavanaugh
Words: Kevin Cavanaugh, Expanded Shales, Clay and Slate Institute (ESCSI)
Photos: Expanded Shales, Clay and Slate Institute (ESCSI), Michael Rosser (Corporate Safety Services)
As you all know, the work masons and mason tenders do is physically very demanding. The repetitive lifting and placing of big and heavy natural stones, heavy precast concrete units or heavy concrete masonry units (CMU) is not only physically demanding but will often cause or exacerbate existing injuries. Put simply, masonry is hard work.
Making a mason’s task easier seems like an obvious thing to do. Some simple ways to achieve this include using exoskeletons, lifting assistance robots or equipment and lighter and easier-to-maneuver CMU. All three of these options come with cost implications that, at first glance, may seem unattractive to mason contractors. However, if the cost to use them is recouped in increased productivity, quicker job completion, fewer lost-time injuries, and reduced workers' compensation claims, why would you not use them?
This article shares some insights into how mason contractors that are using lighter and easier-to-maneuver units are also becoming more profitable and improving the morale, safety and health of their employees.
How “Light” are Lightweight Concrete Masonry Units? (Answer – Up to 46% Lighter)
In a typical day, a mason will lift and place concrete block that, in total, weigh about 3.8 tons, or the equivalent of two small SUVs. In a week, this tallies up to roughly seven Ford F350 trucks or 19 tons, and over a year equals two Airbus A380 jumbo jets that can carry 850 passengers and weigh 950 tons! These numbers are based on laying 150 normal weight, 3-web, 12” CMU that typically weigh about 50 lbs., which is an industry norm.
In most parts of the country, this same normal weight block that weighs about 50 lbs. can be made with expanded shale, clay or slate (ESCS) lightweight aggregate and will weigh only 27 lbs. This is an amazing 46% reduction. In terms of lay rates or productivity, this reduction easily allows a mason to lay an extra three block an hour, or about 25 extra block a day, which is a 17% increase in productivity. Plus, there is more good news— the total weight lifted, even though more block are installed, is also 46% lighter.
Conclusion, using lightweight block is a ‘no-brainer’, right? Or, as one of my industry colleagues often said, “Find me a mason that really wants to lay a heavier block.”
While this initial example may make using lightweight block seem like a 'no-brainer,’ there are several roadblocks that stand in the way— the biggest being the perception that lightweight block cost too much. This is not always true. In fact, most of the time, lightweight block ‘walls’, which is what the owner is buying, are less expensive.
Making More Profit Using More Expensive Lightweight Concrete Masonry Units?
Taking a deeper look at a typical masonry contractor’s costs and profits reveals that using lightweight block makes a lot of sense. Let’s look at a 10,000-block job and a mason contractor in the Great Lakes area that charges $12.50 a block to install them. The owner or GC’s cost is $125,000 (12.50 x 10,000). The mason contractor is using heavy ‘normal’ weight block.
Let’s further assume that the mason contractor has a 5/3 crew (5 masons and 3 tenders or laborers) with each mason installing 15 block an hour or 120 block per day and that the crew’s fixed cost is $115/hr per man or $920/hr for the entire crew. In this example, the crew’s cost covers everything the contractor has to pay for including equipment rental, coffee, office space, insurance, wages, etc.
This means the daily cost for this crew is $920/hr for 8 hours a day or $7360 per day. Recall, each mason is installing 120 block, or 600 for the entire crew. Based on these numbers, this job should take 16.7 days to complete (10,000 block divided by 600 block per day).
Now, let’s see the numbers when mason contractors switch to lightweight block. Each mason installs three more block an hour, going from 15 to 18. The daily total per man is now 144 block, or a crew total of 720 per day. This 10,000-block job should now take only 13.9 days, which is 2.8 fewer days than using the heavier, ‘normal’ block.
Where’s the extra profit? Assume that the lightweight block cost premium is $1.00, or $10,000 for the entire job. This extra cost is easily recovered because the initial bid was based on using heavy block and having to cover the crews’ fixed cost for 2.8 extra days, which equals $20,608 (2.8 days times $7360/day). Deducting the extra cost for the lightweight block from this $20,608 savings yields an extra profit of $10,608. The lightweight block cost premium could actually double to $2.00 per block and the contractor would break even.
The crew gets the job done quicker, all while lifting less and moves on to other jobs and bills other customers—seems like a win-win.
Improved Safety – Lifting Injuries
According to the non-profit Center for Construction Research and Training (CPWR), “Bricklayers (masons and helpers) have the highest rate of back injuries with lost workdays among construction workers.” The reasons why are probably not a surprise to any mason contractor. The CPWR identifies four key reasons why:
- Block (or masonry unit) weight
- Frequency of lifting materials and twisting
- Height of work and materials; and
- Distance of work from workers.
Using 46% lighter lightweight block clearly alleviates the first item. Using easier-to-maneuver ‘next generation block’ with fewer and/or thinner webs addresses the second item. Typical next generation block on the market today include two-web CMU that is open on one or both ends. These are sometimes referred to as ‘H-Block’ or ‘A-Block’. Units with two webs are also lighter. A 12” block with two webs is about 25% lighter than the same unit with three webs. Similarly, a two-web 8” block weighs about 10% less than a three-web 8” unit. These next generation blocks further reduce the weight masons have to lift. Again, I am reminded of what my colleague said, “Find me a mason that really wants to lay a heavier block.” Masons do not want to lift heavier units.
Reduced Silicosis Risk
Another not so obvious benefit of using lightweight CMU made with ESCS lightweight aggregates is the reduced exposure to air-borne crystalline silica. ESCS aggregates are made in a rotary kiln where temperature approaches 2000 °F. In this heated environment, the crystalline silica present in the raw shale, clay or slate is converted to an ‘amorphous’ type of silica that does not cause silicosis. Studies conducted by the Expanded, Shale Clay and Slate Institute (ESCSI) indicate that when lightweight block made with ESCS aggregates are saw-cut, airborne crystalline silica is reduced by up to 83% and are below OSHA’s Action Limit. This should come as good news to mason contractors and their employees.
Other Benefits – Architects Like to Specify Lightweight Block, Manufacturers Like to Make Them
Architects like to specify lightweight block because they have better fire resistance ratings, higher R-values (2-3 times higher), excellent acoustical properties and improved seismic performance. They can offer their clients a better wall for little or no extra cost.
Block manufacturers like to make lightweight block because they are easier on their equipment and pallets, and they can deliver more units per truckload, which reduces fossil fuel use.
More mason contractors are using lightweight block because they are better for their crews’ morale, health and safety, and because using them makes their businesses more profitable.
Want to discuss more about lightweight block and their benefits? Feel free to reach out.
About the Author
Kevin Cavanaugh works for Arcosa Lightweight based in Milwaukee, WI. He currently chairs ESCSI’s Masonry Committee, CMHA’s CMU Technical Subcommittee and ACI/TMS Joint Committee 122 on Energy Efficiency of Concrete and Masonry Systems that is charged with developing national energy code standards. He has worked for large, multi-national and small, family-owned CMU manufacturing companies and several lightweight aggregate suppliers. He enjoys interacting with people in the concrete and masonry industries, and educating owners, engineers, architects and state DOTs on the many benefits of lightweight concrete, lightweight CMU and lightweight aggregate.
Questions? Reach out to Kevin at Kevin.Cavanaugh@Arcosa.com.