Archive for September, 2011

Finding Structural Clay Tile

Structural clay tile exposed after chair rail was removed

Structural clay tile sometimes called “speed-tile”, “partition block” or “back-up clay block” were extensively used as a backup material in schools, government buildings, airports and even high-end residential properties at the turn of the last century up until the 1940s. Designed to eliminate the labor cost of laying standard size brick units, structural clay tile could be set quickly by a mason as they were light weight, while at the same time being fireproof.  In addition, the electricians could run their conduit pipes inside the web spacing of the units offering the plasterer a nice substrate to work with in applying his materials later in the project.  The units used for interior partition walls were generally 4 to 6 inches wide by 12 inches in height by 12 inches in length and vertically scored with 1/4 to 3/8 inch grooves on the face to receive the plaster scratch coat.

The material fell out of use after the invention of CMU’s (Concrete Masonry Units), concrete block. In the restoration business we always strive to match the original materials in our efforts to deliver a project. Whether it is filling in a door opening, or rebuilding an interior wall section it is important to locate the companies that still manufacture structural clay tile.

In a recent project we were glad that our social network paid off in LinkedIn.  A group called “Brick and Mortar – for real” which currently has approximately 750 members came to the rescue. I posted the question as to where I could locate a manufacture of this material and a few days later had several to choose from.

The contractor at the site was not in any mood to do any research himself and was ready to just install concrete block into an existing structural clay tile wall and plaster over it – and be done. I was not comfortable with this approach as I remember from my workshop classes that concrete shrinks and clay masonry expands – so I knew we were just asking for cracks to eventually mirror through the new plaster wall surfaces at the transition points.

I wanted to share this project and success we had in locating the original structural clay tile in this blog – just in case you happen to run into a project where someone says…..”You can’t get that anymore”.

Here are your sources: Superior Clay Tile Corporation, Uhrichsville, Ohio 800-848-6166. This company even has the old fashioned double slant wall coping with the flange for your parapet wall projects. Sandhuhl Clay Works, Inc., Spencerville, Ohio 419-657-2905 and Elgin Butler, Austin, Texas 512-453-7366.

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Stone Repair Mortar vs the Real Thing?

Substitute stone repair material being applied to limestone

Fixing the faces of the stone using stone repair mortar has become a huge industry in the United States in the past 20 years. Manufacturer’s of these products all have their reasons why theirs is the best and most compatible to the original stone substrates. Each company guards its proprietary mixes with closely kept secrets as to what and why theirs is the best on the market. Some companies claim acrylics in the formulas, where others claim natural minerals – whatever that means.  Most offer custom color matching, some texture matching. Either way, the products are here to stay and will be an option on most restoration projects large and small.

When is the best time to consider using stone repair mortar? Let’s first discuss the term – “repair mortar” not to be confused with “repointing” or “rebuilding mortar” that is used between stone and defined as mortar joints. A better term might be “substitute stone repair material” because that’s what it is – a substitute for the real thing. By definition a substitute stone repair material is a material that is used to patch damage or deteriorated surface masonry units’ insitu. When a stone is deteriorated beyond its original surface texture or carved feature it’s time to consider stone replacement or substitute stone repair patching.

Each condition and situation is unique to each building or monument and care must be taken in making/specifying this important decision. Cost of replacement may be problematic and even unrealistic. The size of the repair area may be so small in comparison to that of the entire stone that the substitute stone repair material may be justified. But the decision should also be compared to the old fashion Dutchman repair approach as an additional option. It seems that the repair material products are so easy to use that many situations that could merit a stone Dutchman repair simply get the “patch”.

On a recent project at the University of Wisconsin we were able to redress the deteriorated stone surfaces instead of applying a substitute stone repair material. The deterioration patterns were such that the surfaces had spalled and chipped away over the years leaving textures and stone massing voids from the profile of the walls.  In many cases we were able to remove the damaged stone units redress them on a sand table, duplicate the original tools marks with hammer and stone chisels, and re-install the stone back to its original historic plumb lines of the wall. This was the best preservation practice approach for this project and one that limited the use of the substitute stone repair materials to the building. We did use a small amount of the material but only when all other stone treatment options were exhausted, which meant most substitute stone repair patches were on spots smaller than 1-inch in size.

Redressing Stone Training:

Companies that currently sell substitute stone repair materials are: Cathedral Stone Products, Edison Coatings, Transmineral, Pennsylvania Lime Works, U.S. Heritage Group, Conproco, Keim, and Virginia Lime Works

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Matching Historic Mortar

Best way I know of to match a historic mortar is to first identify whether you are dealing with a cement-based binder or a straight lime binder. You can do this by dissolving a sample of original historic mortar in a solution of hydrochloric acid and some water. Watch the reaction of the material as the solution makes its first contact. It’s best to place the material into the solution rather than the solution into the material for best results. If it is a lime binder the material will break down quickly and form foam at the top of the material, bubbling and hissing as the calcium breaks down. If the solution just sits there with no reaction, only a few bubbles – but know foam or hissing action you have a portland cement mortar.

Allow the materials to soak in this solution until all the binder materials are gone.  You can check and see this visually by looking at what is left – the aggregate of the mortar.  It should be clean and free of any particles of binder still attached. Lime binder mortar can dissolve quickly, sometimes in a matter of hours. Cement binders can take up to several days to dissolve down. Now comes the fun part – identifying the aggregate or sand in the mortar.

Sand particle distribution after sieving (courtesy of Historic Scotland)

Drain off the solution through filter paper to collect the fines.  Dry the material in an oven at around 200F on a hot tray. Weigh the material into an even gram amount then run the sand through the series of ASTM E11 sieves specified through ASTM C144 and calculate the percentage of grain particles on each sieve. Then create a sand gradation chart which depicts to sand particle size, shape and color to make for an easy way to match the material.

Chances are if you match the sand and binder materials in a historic mortar you should not need to assistance that oxide pigments can provide. However, these materials are used in the industry to assist masons in matching mortars regularly. We typically stick with a 1 part binder to 2.5 parts sand in our formulas as this was the standard in the industry for the exception of butterjoint brickwork which is often a mixture of one part binder to one part sand by volume.

The performance characteristics of a historic mortar; bond strength, flexibility, breathability, vapor permeability, and compressive strength (in that order of importance) will typically fall into place if the necessary time and testing of the original material is carried out. Most historic lime mortars are very durable and well carbonated and worth replicating. It is not best practice to trump a historic mortar formulation and go to the next higher mortar type, i.e., Type L to a Type O for example. It is best to fix the problem of why the historic mortar deteriorated in the first place – more than likely a water related issue.

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High Compressive Strength – Not a New Discussion

Testing masonry materials for durability and performance has been going on for some time. It is important to study and test materials prior to construction of an actual wall to prevent wasted material, time and labor. Some of the earliest recorded tests performed on masonry mortar ingredients were carried out by the U.S. Army Corps of Engineers in the construction of fortifications during the early part of the 19th century.

The durability of masonry buildings relied heavily upon the past performance of the actual structures and the master mason’s experience with the individual materials. Much of the heritage knowledge of making good mortar was passed down from generation to generation through the trades. Testing mortar ingredients historically involved the masons working with the architects in a team approach for common understanding. However, signs of change began to appear as early as the 1890s.

Uriah Cummings - 1898

Uriah Cummings writes in his book, “American Cements,” which was first published in 1898. “With their former teaching and experience on the one hand, and the testing machine on the other, the question was not long in doubt. The machine was victorious, and henceforth all judgment founded on experience was laid aside and they became blind believers in the tensile strain tests. What matter though they were continually befogged by the frequent, unreasonable, and capricious pranks of the machine, they had found a god, and were determined to worship it.  And so it came to be established as a fixed belief among engineers and architects that the best cement was the one which tested the highest, and the manufacturer had no alternative but to strive to make his product test as high as possible.”

Seems from the tone of Mr. Cummings writing that he knew the industry was going in the wrong direction toward high compression. Is it high compression that destroys historic masonry? Well indirectly it does. Most mortars that are very high in compressive strength are very low in vapor permeability. The ability a mortar has to capture and release water easily through evaporation. What tends to happen in a historic masonry wall is moisture infiltrates by various ways; rising damp, poor roof/parapet/flashing details; driving rain; capillary action through cracks among other ways. The water needs to escape from inside the walls through the mortar joints ideally keeping everything dry from the inside out. Hard, high strength mortar prevents water from escaping thus trapping it inside the wall potentially causing damage to the masonry units of brick and stone as well as terra cotta over the course of time. It’s always better to insist on a lower compressive strength lime mortar that readily breathes with the masonry allowing quick evaporation of water, and in addition, provides the natural flexibility needed for traditional load-bearing masonry walls to perform at their best.

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Hot Rocks and Compressive Strength

Looking down into a rotary kiln - the limestone can be seen on the right

Ever wonder why portland cement gets so hard? Why it is so high in compressive strength? This blog post is a continuation from an earlier one from last week titled “Hot Rocks” which most people seemed to enjoy. Early lime kilns could operate only up to around 1600F — just hot enough to activate the clay, turn the limestone (calcium carbonate) to quicklime (calcium oxide) and combine them both to form belite (dicalcium oxide) to make hydraulic lime.

Joseph Aspdin (1824) had achieved a slightly higher temperature of around 2200F forming some liquid phase and combining the belite with the remaining quicklime to create alite (tricalcium silicate), the base compound for portland cement. The vertical shaft kilns could burn slightly hotter, but temperatures were mainly kept below the maximum heat achievable to limit the liquid phase and prevent the danger of a kiln blockage, a situation created when the molten mass of materials cooled within the kiln. Kiln blockage was something kiln operators historically and understandably feared; crawling down into a shaft kiln and chopping away with a hammer and chisel at a molten mass of material was to be avoided at all costs.

The rotary kiln, however, not only helped to overcome the kiln blockage challenge, but it also allowed the higher temperatures necessary to produce portland cement with a much higher compressive strength. Before the rotary kiln was invented (1889) most kilns were constructed vertically, loaded with alternating layers of limestone and fuel (wood and/or coal or both) until the materials reached the top. A fire was started at the base of the kiln and allowed to burn for a day or two depending on how large the kiln was. The entire kiln was cooled down another few days until the materials were discharged from the bottom. The process would start over again for the next batch, a very labor intensive process. The rotary kiln also allowed for continuous feeding of the kiln without the usual starting and stopping and could run 365 days per year without interruption in production.

So just how hard is portland cement? Well, in a paper presented at the American Lime Conference in Lynchburg, Virginia in March 2003, Paul Livesey of Castle Cement presented the following information about the answer to this question. When matching a “portland cement” mortar from the period between 1871 and 1920 then, we should not be confused by the terminology. Portland cement of 1871 was a different material from that of 1920 which, in turn, is totally different from portland cement today.

Portland cement products produced at the turn of the last century were fired at temperatures from 2300F to 2600F. In comparison manufacturers making portland cement in this century fire at temperatures ranging from 2800F to 3000F. The differences in mineralogy and the effect that burning at higher temperatures has on strength development are well known. For example, in 1871, portland cement tested in the 1800 psi range; today’s portland cement are in the range of 8,000 psi, an increase of 344 percent in compressive strength. Indeed, the portland cement of 1871 bears more relation to modern, higher-strength hydraulic lime (NHL 5, 1000 to 1500 psi) than it does to modern portland cement.

And when making decisions on the appropriate mortar match for historic buildings remember that the National Park Service as well as leading American experts agree that it is always better to err on the side of a lower strength mortar replacement in order to protect the historic masonry materials. Even adding a small amount of modern portland cement can have a significant affect of increasing strength when maybe you did not intend to in the first place.

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Historic Portland Cement

Roadside Marker in Lehigh Valley, PA

It was David O. Saylor back in 1871 that produced the first portland cement in the United States in Lehigh Valley, Pennsylvania.  Between 1871 and 1920, American portland cement production skyrocketed, due in part to the increasing demand for automobiles and the attendant need for roadways during this period.  The years 1871 to 1920 also saw a major change from “traditional” manufacture of cement to a more technically aware, science-based industry.

As a consequence, it was inevitable that cement products that reflected both traditional and modern scientific production methods were on the market at the same time. Thus, anyone looking to match a historic portland cement mortar from the time period between 1871 and 1920 will benefit from considering the evolution of cement production technology during this period.

As popularity of portland cement grew over the period, so did its compressive strength, from 1800 psi in 1871 to 3000 psi by 1920, an astounding 110 percent increase. An examination of the limestone and clay used to produce portland cement, however, shows that they changed very little after 1871. What, then, had changed? Mainly it was the production process and the ability to fire the raw materials to consistently higher temperature.

The main technological breakthrough came with the invention of the rotary kiln, which was first used commercially in Lehigh Valley, Pennsylvania in 1889. Initial trials by Ransome and Stoke in England had lacked the necessary financial backing to succeed.  In the end, it was left to Seaman and Hurry in the United States to make the final technical refinements that could produce sufficient temperatures and efficiency to unleash the massive portland cement industry we know today.

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Measuring and Mixing Historic Mortar

Various Sands for Comparision / Matching Historic Mortar

The traditional volume mix design of 1 part lime putty to 3 parts sand may be insidious to follow straight up without more details. First, mix designs historically used quicklime as the 1 part of lime mixed to the 3 parts of sand by volume. Quicklime when it is slaked with water will increase volumetrically 70-100 percent – or basically double its size. This fact would reduce the sand content closer to that of a 1 part lime to 1.5 parts sand – a much sticker richer mix design.

Secondly, sand must be measured in a damp loose condition according to ASTM C270 when mixing mortar. Dried sand will bulk up to 30 percent and grow volumetrically by the addition of a small amount of water. This can send your mortar mix designs at the construction site off the specified requirements.

I recommend everyone take a moment to read Gerard Lynch’s article on the subject it is well done.

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Qualified Masons…. On-site Training

Preparing for a Stone Dutchman Repair during a ASTM E2659-09 Historic Masonry Training at North Hall, ca. 1851 – University of Wisconsin, Madison

Most specifications call for masons to have five (5) years in historic masonry restoration experience in a similar project and scope of work. The problem is most mason contractors don’t often have the experience in the particular specification materials or methods of approach to specific projects. A new movement in recent years has been to assist the masons in gaining the necessary knowledge through on-site contractor training programs. These programs are specifically designed for the masons and teach them the workmanship skills required for specific masonry restoration treatments that may not be commonplace. For example, a crew may learn the techniques of Dispersed Lime Injection (DHL) or the installation and curing of lime putty mortar for repointing.

Architects and building owners are encouraged to attend these sessions. While they do not receive a certificate for the application, they do receive a supervisory role certificate in order to participate in quality assurance inspections during the construction phase.

The training is built from the framework provided in ASTM Designation E2659-09e1 Standard Practice for Certificate Programs. Each training event is carried out at the project site with the masons performing the work. The training programs allow owners and architects to identify qualified masons based upon delivering an acceptable test panel of each specified treatment. Each mason must meet certain criteria and pass a written test defined in the training program plan in order to receive an E2659-09e1 project certificate. An oversight committee is formed for each project representing the primary stakeholders. The primary stakeholders are typically the owner, architect and the preservation officer.

The project training program plan is developed by SPC personnel in collaboration with the project architect during the design development stage. The SPC training program dovetails into the project specifications and supports the quality assurance of the overall project. The project training program plan is submitted to the oversight committee for review and approval. The SPC certificate issuer is a qualified historic masonry specialist having designated authority charged to administer the training. The oversight committee typically requires that all masons and supervisors participate in a series of learning events designed to assist him or her in achieving the learning outcomes within a defined scope prior to working on the project.

Specifying SPC ASTM E2659-09e1 Project Certificate Training helps to ensure delivery of the highest quality craftsmanship and maximum life-cycle performance of the repairs.  All learning events in each training component comply with project specification requirements. The SPC training programs preserve the historic integrity of projects by assuring quality applications and installations of specified materials.

Because we work on National Landmark Buildings all SPC training components and learning events comply with all federal, state, and local building code requirements and preservation guidelines set forth in the Secretary of the Interior’s Standards for Rehabilitation.

To view training in progress visit: SPC ASTM E2659-09 Historic Stone Masonry Training – Remove, Redress, and Return Stone

To receive a copy of a template specification that includes training write to:

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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”

The History of Masonry Mortar in America 1720-1995

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Lime Putty Mortar – Buckshot

Lime Mortar Buckshot - the Wrong Way to Mix

Let me save you some time and trouble if you are considering specifying lime putty (ASTM C1489-01) for your next historic masonry restoration project. Forget about the standard way of mixing mortar with a gas-powered paddle mixer or drum type machine used in new masonry construction. These machines require the mortar ingredients to have a high rate of flow by adding enough water into the mixer to keep everything moving and mixing thoroughly. Not so with lime putty. This material is generally 50 percent water and 50 percent solid (looks like thick cream cheese) and requires a mixer that provides pressure or a kneading action to evenly incorporate the sand particles into the material.

Mixing lime putty and sand together works well when mixed by hand with a mortar hoe and shovel as you can place pressure into the mix by pressing down during the process. Ramming rods made from wood with handles also work well to beat the mortar into submission forcing the sand particles into the lime putty.

What is interesting about mixing lime putty mortar, if you have never had the pleasure to do so, is that it requires no additional water once properly mixed. There is enough water in the lime putty to create a good workable mixture that can be used for repointing. For years we have used a vertical shaft mixer that whips the material into form from the outside-in once all the ingredients are in the shaft mixer. So whatever you decide to do on that next historic masonry restoration project, if it involves lime putty, be ready for some good-old-fashion hand mixing or get ready for some buckshot of lime putty balls coated with sand!

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