Posts Tagged ASTM

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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History of Mortar Types

Ever wonder how Type N mortar came to be? or Type M for that matter? Well the story goes something like this…In 1931 a group of non-mortar producers and representatives from the lime and cement industries got together and formed a committee to discuss the issue.

The issue was that mortar “types” needed to be established to distinguish high compressive strength mortars from soft flexible ones, so in 1944 the designations using A-1 (2,500 psi); A-2 (1,800 psi); B (750 psi);  C (350 psi); & D (75 psi) were adopted, with minimum compressive strength requirements specified.

In the United States, “A-1” had become synonymous with “the best” or “top quality” and some committee members felt that the designation for the higher compressive strength cement mortar was misleading. The possibility did exist that an architect desired a flexible lime mortar type for a particular project, but he might mistakenly specify the A-1 type, thinking it was the best. In an effort to avoid confusion on the subject, the committee adopted a new mortar type designation in 1954.

The new designation letters were taken from the two words, MASON WORK utilizing every other letter. The compressive strength minimums for each mortar type are still recognized in the current ASTM mortar specification C270.

(2,500 psi)  Type M  replaced  A-1

(1,800 psi)  Type S  replaced  A-2

(750 psi)  Type N  replaced  B

(350 psi)  Type O  replaced  C

(75 psi)  Type K  replaced  D

Most historic load-bearing masonry buildings have original mortars with low compressive strength, but yet are very durable (well carbonated lime mortar).  We have plenty of architectural inventory around the world that supports this statement. High compressive strength in historic masonry mortar (Type O or higher) is not a direct reflection of durability and maximum life-cycle performance.

In fact, to give you some perspective, a certain material scientist/university professor studied historic mortar for his entire career. Traveling the world he collected samples from some of the oldest historic masonry structures. Very seldom did he ever run across a historic mortar with compressive strength of over 300 psi.

As you climb the scale from Type K upward, you are adding more and more portland cement by volume. As a result, the mortar becomes less permeable, less breathable, and more inflexible in exchange for the increased compression. Historic masonry on the other hand needs mortars to accommodate building movement (flexibility), exchange moisture readily from the face of the wall (breathability), and most of all have excellent bond strength-all natural properties of lime mortar (“Type L” introduced in 1998, ref.  NPS Preservation Brief 2).

Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings” http://www.cr.nps.gov/hps/tps/briefs/brief02.htm

The History of Masonry Mortar in America 1720-1995 http://www.lulu.com/product/paperback/the-history-of-masonry-mortar-in-america-1720-1995/11271764

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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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