Traditional Masonry Archives — The Tile Files: Disaster funding to the rescue

Mortar Analysis

If the newly installed mortar is incompatible with the original building design, the building may develop a range of problems, including spalled and cracked masonry units and elevated moisture in the wall systems. According to National Park Service’s Preservation Brief 2: “Repointing Mortar Joints in Historic Masonry Buildings,” restoration mortar formulation must be as vapor-permeable and as soft as or softer than the historic mortar (measured in compressive strength). The new mix should also have greater vapor permeability and be softer than the masonry units. The sand should match the original as much as possible in color and texture.

In addition to compatible performance, the appearance of newly installed material should also be consistent with the original. The best way to select an aesthetically appropriate, high-performing restoration mortar is to first understand what was originally installed in the building.

Mortar analysis, including testing, is often an essential step in a successful mortar restoration project.

When to use mortar analysis

There is no hard and fast rule about choosing mortar analysis, but you can take some direction from the age of the building. Before 1872, lime was used as the primary binder, but occasionally, imported cement and pozzolans were also added to the mortar mix. Generally, however, mortars were lime-based, and research on local building practices and availability of materials during the period of construction can be useful in assessing whether or not testing is warranted. If you are working on a building constructed after about 1930, you will most likely find hard masonry units and harder cement-lime mortar on the exterior facades. You may also be able to obtain the original documentation, which would eliminate the need for analysis. In either case, if you are uncertain about the mortar formulation, mortar testing is advisable.

By contrast, the murkiest construction time period is from the 1870s to the 1930s. During this time cement established its position in the American market, but like any new technology it took time to gain widespread acceptance. Mortar found in masonry structures built between about 1870 and 1930 can be some variation of a mix of lime, portland cement and sand or a pure lime and sand mix. The proportions and the use of cement were largely determined by the availability of cement at the time as well as the practices of the architect or the mason, meaning that there are any number of formulas that may have been used.

Collecting a good sample

Once you’ve decided that mortar analysis is in order, you should ensure that you provide a sample that is representative of the original mortar. Usually the equivalent of a small cup or a generous handful of the material is sufficient enough to run laboratory tests. Solid mortar chunks are always better than dust, and the easiest place to extract a good-sized chunk of mortar is from the corners of the building.

Before sending the mortar to the lab, you also need to ensure that it represents the original mix, since there is always a chance that the building was previously repointed. In these cases, it is often possible to remove the weathered surface of a mortar joint to reveal a different material behind it. This is a clear sign that the visible mortar is not the original.

The exception to this rule is any decorative bead or ribbon joints or tinted face mortar that may have been applied over the bedding mortar. If there is any uncertainty about whether or not you’ve identified the original mortar, the best places to investigate are areas that are difficult to access, such as hidden areas behind porches, behind downspouts or shutters, or in sheltered areas, such as enclosures, under windows and near the roof.

If feasible, you could also remove a few masonry units and obtain a full-depth sample.

Standard procedures

The first level of laboratory investigation is visual examination. After measuring the weight and the size of mortar chunks, we look for any inconsistencies in color and texture in the sample’s cross section. This procedure assesses whether or not we have received mortar that is consistent with one single installation. Using two different mortar types in further analysis would cause errors in our data.

Next we search for lime inclusions (traces of lime putty or hot lime). We check the rate of water absorption, and we measure the hardness of the original mortar. (The ability to obtain authoritative value of the compressive strength is limited, since the samples we receive are usually small and shaped irregularly. Typically, we are only able to classify if the mortar sample reaches low, medium or high strength.)

The third step in the laboratory investigation is wet chemistry. Using an acid solution, we separate the binder from the aggregate to determine the amount of the binder relative to the complete mortar mix. The results of this calculation can be affected by the presence of calcium carbonate in the aggregate that would have been dissolved together with a binder, and this potential error should be corrected once we develop the restoration formula. The chemical reaction noted during the test can also provide sufficient information about the type of the binder and indicate whether or not there is a hydraulic component present in the mortar. An acid test can take from 30 minutes to 48 hours, and when the binder is completely dissolved, we rinse the remaining aggregate and dry it for sieve analysis.

Well-graded aggregate is essential for mortar performance, so the final step is sieving the dried aggregate and determining the particle size distribution. Results are displayed as percentage of the material that remains on each sieve. Data developed during this test show the quality and general size of the aggregate, and we classify the material as a fine, medium or coarse sand. The quality will depend on appropriate distribution of each particle throughout the material. We evaluate the sand gradation curve, and if necessary, we will recommend adjustments in the restoration formula.

All of these laboratory procedures provide information about the binder (air or hydraulic), the mortar strength, the level of water absorption, the binder-to-aggregate ratio, and the color and gradation of the original aggregate. In most cases this is enough information to develop the recommendation for the replacement mix. If it is essential to know precise cement-to-lime ratios, additional petrographic analysis is required. Microscopic investigation with a high magnification factor is used to perform a series of examinations of the thin slices of a mortar, and results provide detailed information about the mortar ingredients and allow the determination of the mortar type.

Proportions of cementitious and aggregate materials are determined so that a compatible repointing mortar can be designed.

Many of these procedures on mortar testing can be found in ASTM C1324 Test Method for Examination and Analysis of Hardened Mortar. This standardized test method provides procedures for both petrographic examination and chemical analysis of mortar, including qualifications for the petrographer and chemist.

Also, differential thermal analysis could be included as a methodology for determining the constituents and proportions of the binder.

A formula for a mix

Physical and chemical properties of historic mortar alone will not enable you to create the right replacement mortar. In order to develop the most suitable restoration formula, you must assess laboratory testing results in the context of the building’s history and condition.

Location, original date of construction, type of masonry, condition of masonry units, width of the joints, and cause of deterioration are generally the most important factors that need to be considered during the development of a proposed replacement mix. It is very easy to install an inappropriate mortar formulation using only testing results. For example, a petrographic analysis of mortar from 1920 may indicate that original mix was Type N mortar per ASTM C270 (with a proportion of one part cement, one part lime and six parts sand). It would be problematic, however, to use today’s conventional Type N for the restoration project. Mortar ingredients available today are not the same as the ones used in 1900, and modern technology yields higher, stronger binders than used in the past. The compressive strength of today’s cement can be up to 10 times higher than that of comparable cement produced in 1900. The bottom line is that our goal is to replicate the performance of the historic mortar, not the precise formula.

There is currently no obligation to analyze original materials during a restoration project, nor are there standardized procedures or guidelines to guide decision-making. Also, current building codes do not recognize historic mortars. ASTM International is currently developing a procedural standard for restoring historic masonry, but this is a long and complex process that will not yield any standards in the near future. The best guide for any project is often the building itself, so be sure that you understand its history, its construction methods and the time frame for construction as you decide whether or not to proceed with mortar testing and analysis.

Tomasz Glab is laboratory manager at U.S. Heritage Group.

TM