Masonry Magazine October 2005 Page. 17
Masonry construction has come a long way since the 1920s, when the late Axel Ohman was setting stone on Minneapolis' first skyscraper, the Foshay Tower. Mark Peschel, now an owner of the firm Ohman founded, recalls him describing that the only way to keep the masons and the masonry from freezing was to build bonfires below the scaffolding and then hope that the rising heat would keep them warm enough. When it got too cold for that strategy, Ohman went back to Norway and waited for spring.
Fortunately, today's mason contractor doesn't have to wait for warmer weather. Season-extending techniques can be as simple as heating the mortar ingredients and storing units in a protected enclosure or as sophisticated as completely enclosing the construction area in poly-draped scaffolding, pumping in heat from propane heaters, or using specially-blended ingredients. But the goals, even after more than 75 years, are the same: keep the masons and units from freezing and keep the mortar above the magic number of 40 degrees.
Codes and Technology
SOME OF THE TECHNIQUES to protect both masons and masonry materials come from either codes or technology.
Code options are designed to address the varying job site conditions and extremes experienced, thus providing flexibility for the contractor, building official and special inspector. Still, the parameters around the options are prescriptive. Between given temperatures, there are options to assure the structural integrity of the masonry, but the contractor must describe his or her approach in a plan. The crucial choice of method affects both the internal quality control program and the project-specific quality assurance plan.
For example, the code requires windbreaks with winds above 15 mph and temperatures below 25 degrees. Dan Schiffer, president of Schiffer Mason Contractors, Inc. of Holt, Mich., utilized an innovative windbreak plan when his company needed to protect its mortar-mixing area for a large project. Part of his plan involved lining up construction trailers as windbreaks on each side of the mortar mixing area, thus partially enclosing it. This allowed the space in between to serve as a virtual mortar-mixing assembly line, reducing the time from mixer to boards and aiding heat rentention of the mortar.
Technology offers other winter weather options. Today's larger LP gas heaters, with more powerful blowers, enable 24-hour heating of enclosures. To conserve as much heat as possible, some scaffold rental companies provide newer poly enclosures that fit their scaffold systems more tightly than site-adapted systems.
Miron Construction of Neenah, Wisc., uses heated enclosures with a unique heat-delivery method: a perforated fabric tube running under the scaffolding, carrying heated air from the blower down the length of the scaffold.
Another technological advancement is the fact that contractors today can access far better weather information. Many subscribe to services that provide them with up-to-the-minute weather data with the click of a mouse, while others have local weather service phone numbers on speed-dial. Either option provides information on "ambient" temperatures (e.g., right now) and "mean daily" temperatures (e.g., the average of the highest and lowest temperatures during a 24-hour period).
Tricks of the Trade
ANOTHER WAY to beat Old Man Winter is to utilize bulk-delivered dry mortar ingredients, which avoids the potential for uneven heating and scorching caused when trying to thaw a pile of damp sand. Using dry ingredients "is one of the best techniques developed for modern winter masonry construction," says Mike Cook, vice president of Bricklayers and Allied Craftworkers (BAC) Local 1 MN/ND and former supervisor at Gresser Concrete Masonry of Eagan, Minn.
Combining dry, unfrozen ingredients with heated water achieves a higher mortar temperature. In fact, heated water may be the most important ingredient, given water's capacity to retain heat and impart it to the other ingredients.
In addition, the heat from hydrating the cement brings a small temperature rise in the plastic mortar. While not a significant increase, given the relatively small volume and large area of mortar joints, it may be just enough when combined with other techniques to make a real difference. As the heated water begins hydrating the cement, the temperature rise is cumulative before leveling off, which provides some extra time before cooling begins.
Another option is to use Type III (quick-setting) cement, which increases available heat even further, or one of the vari-