Consider this scenario: A major drugstore chain is scheduled to open another store next month, and a fire safety official inspects the new building as it nears completion. The inspector observes that the fire doors are stamped with the appropriate fire rating, then looks at the fire-rated concrete masonry walls and frowns in frustration. "How do I know that these block have the specified fire rating?" the official asks. "They aren't marked in any way."

At best, such questioning results in a paperwork delay as the appropriate documents are found and delivered. At worst, walls must be torn down, and lawsuits are filed.

A delay in opening a store can cost a chain many thousands of dollars in revenue, notes Francis A. (Doc) Shane, an architect and engineer with Kinsey, Shane & Associates of Salem, Va. "Consequently, the job of the regional manager is on the line. When the chairman says you're going to open a store on a specific day, you had better open it then."

Shane, therefore, is delighted that he can now provide his drugstore chain client--and other retailers--additional assurance that a store will open on schedule. Thanks to the efforts of an innovative Virginia block producer, he is able to specify that all fire-rated block used on a job, certified by Underwriters Laboratories Inc., bear the stamp "Classified UL --See Certificate." The stamped block cost more than unstamped CMUs, but Shane's clients have been willing to pay more upfront for this value-added service. "The stamped block gives them a whole lot of security, and it gives me security," he says. "I don't have to worry about code approval or the Certificate of Occupancy. For a chain store, going two weeks with no sales is a lot more expensive than the added cost of a stamp on the block."

Shane already has specified the UL-stamped block on several jobs. Whenever a client needs a fire-rated wall, "I'm going to specify block with a label on it," he emphasizes. Eureka!

The simple but revolutionary idea of marking individual block was the brainchild of D.E. (Don) Hogston, general manager of Roanoke, Va.-based Lightweight Block Co., a subsidiary of Solite Corp. The block producer began stamping UL-certified, fire-rated block 1½ years ago, but the innovation is not proprietary. Other block producers, such as Allied Concrete of Charlottesville, Va., have started their own stamping programs. Richard N. McDaniel, a safety engineer with the State Fire Marshal's Office in Roanoke, sparked this creative development a couple of years ago when he questioned Lightweight Block Co. about some fire-rated CMU walls that had just been constructed at the Virginia Military Institute. "We had delivered fire-rated block to the job, but we had no way to prove it," Hogston explains. The appropriate UL certificate was on file, but the block producer didn't have evidence that the masonry contractor had actually laid the specified block.

In the past, Hogston had been able to satisfy fire inspectors' queries simply by writing a letter stating that the block in question meet the requirements of UL 618 Standards of Concrete Masonry Units, which cover 8-, 10- and 12-inch concrete block that have a 2-, 3- or 4-hour fire rating (Ref. 1). But McDaniel wanted proof; a letter wasn't enough. "He was doing his job," says Hogston, who admires McDaniel's professionalism. And McDaniel has been impressed with Hogston's resourceful solution to a problem that has been dogging inspectors nationwide: Since concrete masonry traditionally is unmarked, there normally is no easy way to tell whether the block in a constructed wall is what was specified in the plans.

"We're trying to make sure that fire-rated assemblies are built in accordance with approved, reviewed drawings and specifications," McDaniel says. "If the plans call for a fire- rated gypsum assembly, then I look for a label on the wallboard. I look for labels on doors that are supposed to be fire doors. Then I come to concrete masonry walls. How do I know that the block is what you say it is? I think it's great when a block manufacturer puts a label on the block." However, McDaniel stresses, the stamp is not a UL requirement, so he can't insist that fire-rated block be stamped. And block manufacturers still must be able to produce a UL certificate for fire-rated units, whether they're stamped or not.

Underwriters Laboratories allows manufacturers the option of marking individual concrete block to identify them as UL-classified. Three different marks are permitted: the word "Classified" over the UL circle (see illustration), the abbreviation "UND.LAB.INC.CLASS" or the phrase "Classified by Underwriters Laboratories." All the labeled block must bear the words "See Certificate," to let authorities know that the stamp is in addition to--not a substitute for---the UL certificate required for fire-rated block.

Lightweight Block Co. marks each 2-, 3- and 4-hour-rated CMU "Classified UL--See Certificate" using a drum roller-type stamper, in line directly before the cuber. Hogston contacted several companies that specialize in material marking before finding one that could meet the block producer's needs. "A block is difficult to mark because it is textured and can contain moisture," he observes. After experimenting with several different inks, Lightweight Block decided to use a red acrylic ink for its optimum visibility and ability to withstand ultraviolet light. The UL-stamping process adds 25 cents to the price of an 8-inch block, but this is only about 5% of the total cost of putting a block in a wall, Hogston points out.

Each block is stamped on one face near the upper right-hand corner. The marked CMUs generally are painted after inspection. Lightweight Block currently budgets about 500,000 UL- stamped block per year out of an average total production of 3 million to 3.5 million CMUs.

A hot issue Stamping block is more than simply a way for honest block producers to reassure zealous inspectors that specific CMUs are fire-rated. The fire resistance of concrete masonry has become a hot issue nationwide because mistakes sometimes do occur.

Although all concrete masonry has superior fire resistance and stability compared to wood or gypsum wallboard, the level of fire resistance varies greatly among block. A block's fire rating depends on its equivalent thickness (the average thickness of the solid material in the unit) and the type of aggregate used, whether expanded shale, clay or slate; calcareous sand and gravel; or siliceous sand and gravel. The thermal conductivity of different minerals and aggregates varies widely--from 3 F per inch for expanded shale lightweight aggregate, to a range of 6 F to 22 F per inch or even greater for quartz (Ref. 2). If a block producer, moreover, blends high-conductivity aggregates in a block, this reduces the CMU's thermal stability and fire- resistance rating. Also critical is a block's coefficient of thermal expansion, which is significantly lower for lightweight than heavyweight masonry. Minimizing thermal expansion helps prevent forces from acting on intersecting walls, structural members or adjoining structures.

Sometimes designers don't understand this variation in fire resistance, so their specifications for fire-rated CMU walls are vague. And sometimes masonry contractors aren't aware of the significance of fire ratings; consequently, they might order standard block from a supplier when they should be ordering a specific class of block for walls with a specified fire rating.

A common misconception is that all lightweight block has a 2-hour fire rating, Hogston says. In fact, Class D-2 block must be specified to obtain a UL-certified, 2-hour-rated wall.

What's more, lightweight block can be manufactured with a 3-hour (Class C-3) or 4-hour (Class B-4) fire rating.

Even when the correct block are specified, ordered and delivered to the jobsite, mixups can occur.