Posts Tagged mortar

Mortar Analysis – Limitations

Mortar analysis can be done various ways with several different approaches to identify the original mortar ingredients of a formulation. However, there are limitations and replacement mortar specifications should not be based solely on laboratory analysis. Analysis requires interpretation, and there are important factors which affect the condition and performance of the mortar that cannot be established through laboratory analysis. These may include: the original water content, rate of curing, weather conditions during original construction, the method of mixing and placing the mortar, and the cleanliness and condition of the sand (Pres. Brief 2 pg. 2).

Mortar can be evaluated by simple wet-chemistry of using hydrochloric acid and water to dissolve out the binder components (calcium carbonate) leaving only the sand particles behind. The ratio of binder to sand can be determined by drying the sample first then weighing it before and after the wet-chemistry process. The problem occurs when calcium carbonate is part of the sand component which would give you a false reading of the ratio. There is also x-ray diffraction, and petrographic analysis by microscope, as well as thin-section technology, where small samples of mortar are cut into very thin sections and dies are injected into the sample showing the different components of the mortar. In addition, ASTM C1324 is a test method to determine components of hardened mortar samples.

Sand particle shape from an analysis - Historic Scotland

The most useful information that can come from a laboratory analysis is the identification of the sand by gradation and color. This allows the color and the texture of the mortar to be matched with some accuracy because sand is the largest ingredient by volume.

A simple non-technical evaluation of the masonry units and the mortar can provide information concerning the relative strength and permeability of each-critical factors in selecting the repointing mortar – while visual analysis of the historic mortar compared to the new replacement repointing mortar can be made. It’s important to match the un-weathered portions of the historic mortar in case the building will be cleaned in the future, or cleaning should be taken into account before the sample is matched.

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Pre-construction Testing and Matching

Unlike new masonry construction, restoration masonry requires matching to existing surfaces. Whether it’s a brick, stone, or mortar, samples must be submitted and often tested to determine the original material components.  So what is the best way to specify a match? Well, first let’s talk about the way it is generally done now in the restoration business. An architect writes a specification that includes the details of matching the masonry; for instance, “match the brick in color, texture, size, and physical characteristics to that of the original historic brick”….nothing wrong with that, right?

Historic brick being removed for matching purposes

Well consider the brick match needing to be located (research, calling around to suppliers, submittal of samples) after the contract has been awarded and the construction schedule is starting. The pressure to find a good and acceptable brick match is now the responsibility of the contractor who is thinking about mobilizing the site, balancing manpower to get the project done on time, and the overall responsibilities for the entire project. Question:  Is this the best time and the right person to be carrying out the important responsibility of finding a successful brick match?

The same goes for the stone or mortar match as well. Question: Is placing these decisions on the back of the contractor at the start of the project in the best interest of the project? Under this pressure mistakes can be made and searching for the best most appropriate match compromises are often made (“that’s the best we can get, or, they don’t make that any longer”). So what might be a better strategy? A relatively new movement is occurring in the architectural design world in the restoration business.

Architects are working with building owners directly and sometimes with consultants to assist them in matching historic masonry materials – prior to bid….during the design development stage of the project (often 1-year in advance of the bidding process). The brick, stone and mortar testing work are accomplished and often times pre-approved in a pre-construction test panel installed by a local mason contractor or preservation consultant. This strategy helps to eliminate delays in the construction phase of the project and it gives more time, without the pressure, to find the best available match on the market.

So the next time you are considering specifying replacement masonry materials on a historic restoration project consider this new approach to an age old problem.  It takes a little more planning on your part, and yes, the owners need to pay up front for some pre-construction test panels, installed into the actual masonry for evaluation. But in the end, the surprises related to change orders are often minimized and the team approach to getting the project done on-time and under budget becomes a reality-not just a dream. And, its money the owners will be spending anyway with the contractor after the bid award….. something to consider.

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Whitewashing / Limewashing

Whitewashing has been used for many years to cover and protect historic masonry, even before it was historic! A whitewashing application involves mixing lime putty with water in a ratio of 1:5 then vigorously stirring the material until the lime putty fully dissolves in the water. Colors can be added from earth pigments but most material was used white – thus the name. The lime (calcium hydroxide) sets slowly by absorbing carbon dioxide from the air. The chemical reaction that occurs produces crystals of calcite. These crystals are unusual because they have a double reflective index: light entering each crystal is reflected back in duplicate. This results in a wonderful surface glow that is characteristic of whitewashed surfaces and is not found in modern paint products or imitation coatings.

The application of whitewash acts more like an absorptive stain. It is not a coating so it will not peel-off. After it hardens whitewash remains vapor permeable and will not trap moisture in the wall. One of the attractive attributes is that it gradually wears off the surface of the wall over time leaving a very pleasant uneven aged look.

Many architects and designers seek this look but have had challenges because they have been using the wrong products, such as paint, to achieve the effect. If it is a traditional look you want to specify than its best to go back with the traditional material that will get you there.

The key; however, is in thinly applied coats. This facilitates the carbonation process of curing and prevents crazing and cracking. It is helpful to specify onsite application training – as most painting contractors will treat the product like paint and attempt to get the surfaces coated in paint-thickness applications. Most raw masonry surfaces require 3 to 5 applications of whitewash, then after that, just a single coat will do the trick to freshen things up later.

Additional reading: http://www.slideshare.net/speweikpreservation/speweik-limewash-returns-2000

http://ncptt.nps.gov/2011/study-on-the-durability-of-traditional-and-modified-limewash/

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Historic Romantic Mortar…

I once overheard a colleague of mine describe the process of deteriorating mortar as “romantic decay”, I guess all the years of his travels throughout Europe and the Scandinavian countries he had seen his share of crumbing bricks and mortar and had become un-alarmed about the condition.  Interestingly enough, it seems the opposite is true here in the United States.  We tend to get all worked up about crumbling mortar, especially the condition when the material turns to sand in your hand when you rub on it vigorously between the brick.

But according to the Preservation Briefs 2, “Repointing Mortar Joints in Historic Masonry Buildings” describe a different view of a good working replacement mortar. In fact, the mortar could be called a “romantic decaying mud” depending on how you look at it.

Here is what Brief 2 has to say about repointing mortar. “In creating a repointing mortar that is compatible with the masonry units, the objective is to achieve one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive and, if necessary sacrificial capacity.” We have mortar all over this country trying to sacrifice itself for the good of the masonry units by falling apart in historic masonry walls!

But unfortunately, we also have many engineers and architects, building owners and contractors doing their best to prevent the process from occurring. When the romantic decay is identified the sure tell remedy is usually a stronger mix design one that contains a large amount of portland cement to go back in with during the repairs. A new mortar without the romance and certainly no sacrifice.

As we continue our work on important historic masonry structures lets – let the mortar help us to identify the real problems – usually the water infiltration somewhere, somehow. Blame the water not the historic romantic mud!

 

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Mortar Evolution in America

The evolution of masonry mortar in America has undergone many interesting      changes over the last two centuries. The ingredients of mortar, methods of producing mortar, and specifications have all changed in some way. Many of these discoveries originated in Europe and eventually reached America many decades later. For example, Smeaton’s discovery of hydraulic lime in 1756 was not fully realized in America until the building of the Erie Canal in 1817, some 61 years later. The English discovery of portland cement by Joseph Aspdin in 1824 took 47 years before it was ever manufactured in the United States in 1871.

The development of masonry cement in the 1920s was the most interesting of mortar developments in the United States. The relationship between the cement and lime industries has been strained ever since, due to the fact that masonry cement was the first formulated mortar product that did not contain hydrated lime as a major ingredient. As a result, two sides of the mortar industry have evolved since the early 1930s. Some promote mortar products with hydrated lime, and some promote mortar products that do not contain hydrated lime.

The methods of producing cement and lime changed at the beginning of the 2oth century, allowing much more material to be fired in a shorter period of time with the use of the rotary kiln. The use of the mortar mixing machine after World War II and the introduction of the mortar silo systems after 1988 were both substantial improvements that directly influenced the methods of mixing mortar at the jobsite.

The American Society for Testing and Materials (ASTM, 1902) has been instrumental in providing the construction industry with voluntary standards on mortar products. The society has ultimately pioneered the way to standardization which has lead to better mortar products and more efficient methods of production. By taking this look into our past, it is hoped that we can gain some insight into our future.

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