Archive for November, 2011
Today’s blog focuses on the importance of industry memberships and associations. I have had the benefit of participating in several industry associations over the years and wanted to tell you about the ones that have helped me the most in my career and education. I have been a member of ASTM since 1992 now almost 20 years. I have witnessed the development of new standards and the revisions of older ones. I have made many friendships throughout the years of my involvement and have grown in knowledge on masonry construction and materials.
I was able to assist in the dinner event that marked the 75 year celebration of Committee C12 on Masonry Mortars in Atlanta back in 2008. I am currently on the review committee for the Yorkdale Award. I am on the Task Group ASTM C12.03.03 for Restoration Mortars that recently published the new standard ASTM C1713 for Historic Mortars. I attend meetings when I can – they are held twice per year in different cities around the country. I am a voting member of ASTM C12, C7, C15, E36, and E07. I would highly recommend your consideration in joining ASTM as a member for $75 per year it is well worth the money and time invested.
Other associations that I have joined over the years have been the Preservation Trades Network (PTN); The Stone Foundation; The Building Limes Forum (BLF); the Construction Specification Institute (CSI); US/ICOMOS; ICRI; and SWRI. Each membership is targeted toward a specific group of people, so you need to know exactly who your audience is and where you want to build contacts and continue your education. Involvement in these organizations have taught me to be open to new ways of looking at problems and solutions in my industry and have assisted me with the understanding that there are many very knowledgeable people out there ready to answer your questions if you are willing to ask.
Most investigators agree that efflorescence is caused by multiple factors in combination with the climate and environmental conditions. Views, however, on which factors are the major culprits in causing the problem are not understood or agreed upon between most experts. One thing that most all agree is that efflorescence is a soluble alkali salt, usually sodium and potassium sulfates, but expressed as Na2O and K2O equivalents, that exude from masonry as a solution. Upon drying the solution re-crystallizes as a supersaturated solution on the surface of the masonry causing a unsightly white film, coating or scum that can accumulate.
There are temporary and permanent forms of the condition. The temporary form is often called “new building bloom” and is short-lived, but there are permanent forms as well – disappearing as a result of rain only to reappear again and again for many years. It is this type of permanent efflorescence that can cause damage to masonry buildings by disintegrating mortar and spalling masonry units.
Some causes of efflorescence can be directed to the contractor and his practices at the jobsite. For instance, if brick or stone is not covered up and become saturated with water prior to installation; failure to protect unfinished walls from rain; unprotected parapet and or lack of proper flashing; lack of adequate drip edges on sills and cornices; leaky temporary gutters and downspouts; poorly filled mortar joints; chronic damp condition at grade where masonry doesn’t have a chance to dry out quickly after construction.
Brick can have varying levels of alkali salt content as well depending on the clay or shale deposit. Clay and shale deposits vary greatly in the amount of these salts. Some brick manufactures may use barium sulfate to reduce the tendency of efflorescence, but this offers no real guarantee. Generally the denser the brick the harder the unit – the less likely the efflorescence factor.
Concrete block may in some cases cause temporary efflorescence when the free lime that is liberated in the hydration of the cement carbonates on the surface of the unit. This type of efflorescence is only temporary and usually will wash off in the first rain.
Mortar materials are made from clay and shale and can also pose the ability to deliver salts to the surfaces of finished masonry walls. Portland cement and masonry cements vary greatly in their raw material make up. Cement is derived from limestone or marl of 70-90% total carbonate content, much of which is argillaceous and earthy. Some cement is made with alkali contents over 1% and in the finished product these salts will approximate these same percentages. These cements when used in conjunction with some of the malpractices previously mentioned can cause serious efflorescence.
Most non-hydraulic limes on the other hand are extremely low in soluble salts as well as sulfur content. To give you an idea, most historical limes used in building construction were made from high-calcium limestone averaging over 97% carbonate content. Total alkali content average of 0.1%. Today, most limes are made from Dolomitic limestone with even less than 0.01%. What’s interesting is that often the lime gets the blame for the efflorescence due to its color when in fact it is 10 times less likely to cause the condition than cement or even hydraulic lime. Hydraulic lime (NHL 2, NHL 3.5 and NHL 5) are still widely produced and used in Europe and some parts of the United States and can have alkali contents usually much higher, approximating cement on an average, since these limes are made from impure, siliceous limestone.
The sand aggregate may contribute to the efflorescence if the sand was not washed or if it was dredged from the areas were there might be contamination of sea water. However, most masonry sands are well-graded and have near zero potential for causing any issues related to efflorescence.
In spite of considerable research into the causes of efflorescence and the ways to eliminate it, or at least minimize its occurrence, many of the research findings are conflicting and controversial. Because there are so many different potential causes that this apparent disagreement is not surprising. Although the occurrence of efflorescence cannot be predicted, it is more prone to appear suddenly, like a “disease” in a dry period during cool weather following a sustained rainy period. It does not rear its ugly head in hot dry summer months due to rapid wetting and drying of masonry walls, but in the northern climates of the United States it is apt to occur most often in late fall or early spring after rainy periods and when evaporation is slow and temperatures are relatively low.
As we continue our learning curve on repointing mortar joints in historic masonry buildings we need to sometimes step back and re-evaluate our recommendations based on experience. One of those areas is the repointing mortar in 1/4″ layers -or lifts, usually three separate lifts in a mortar joint that has been cut out to an inch in overall depth. According to the Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings”,1998 pp.10-11 states:
“Where existing mortar has been removed to a depth of greater than 1 inch, these deeper areas should be filled first, compacting the new mortar in several layers. The back of the entire joint should be filled successively by applying approximately 1/4 inch of mortar, packing it well into the back corners. This application may extend along the wall for several feet. As soon as the mortar has reached thumb-print hardness, another 1/4 inch layer of mortar – approximately the same thickness – may be applied. Several layers will need to fill the joint flush with the outer surface of the masonry. It is important to allow each layer time to harden before the next layer is applied; most of the mortar shrinkage occurs during the hardening process and layering thus minimizes overall shrinkage.”
I do not agree with the 1/4 inch layering for several reasons. First, I believe the potential for cold joints can occur between layers. Second, the possibility of a mason being able to place his thumb back into a 3/8 inch mortar joint cut one inch in depth is not practical. And, third, I do not believe the process adds any benefit for the long-term durability and performance of the new mortar in exchange for the added cost factor to repoint a building three times as compared to one. That being said, I understand the PB2 authors point to establish methods to minimize shrinkage cracks as the mortar hardens – which this process likely does – if the mortar formulation is not correct, let me explain.
Our experience has been that most shrinkage cracks occur within the first 16 hrs after placement due to three primary reasons: 1) excess water in the mortar material; 2) the incorrect aggregate gradation in the mixture – generally the sand is not coarse enough for the width of the joint – the wider the joint the courser the particles need to be, or; 3) early evaporation of water from the joint causing a flash-set to occur. While we do specify repointing in lifts if the joint depth is greater than 1-1/4 inches. In this case, place the mortar in 3/4 inch layers, but this is generally not in every location. In most projects there is the occasional deep pocket or void but this is generally not a typical condition everywhere on the building. If you are getting into areas of two inches or more it time to re-lay the brick.