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Specifying Mortar Testing

This overview covers the basics of mortar testing to help you plan testing and the get the data you need to make restoration decisions. For questions or additional information, please call us at 773-588-0800.

Clearly Articulate the Testing Goals

Before specifying mortar testing it’s essential to carefully determine what information you want to obtain from mortar analysis and how you plan to apply it. Testing options vary widely as to cost, lead time, and data produced so establishing the test goal is important to clarify early in the project. The most common reasons specifiers contact us for mortar testing are:

  • To identify the original for restoration
  • To determine if a replacement mortar is contributing to building deterioration
  • To provide documentation to support restoration plans for a historic preservation commission or other oversight group

We are regularly asked to run tests that are done simply to fulfill a documentation requirement instead of actually informing decision making, and this isn’t necessarily the best use of limited resources.  We encourage prospective customers with tight budgets to contact us prior to mandating testing in these situations, and we will be happy to discuss options.

Test Early

If mortar testing is indicated, we strongly encourage the restoration team to undertake it before creating specifications or soliciting bids. All too often testing is part of the bid package for the restoration work so that contractors provide samples for testing after they have bid on a project. By definition, this means that the winning bidder did not base his or her numbers on a specified product and instead relied on his or her experience and judgment, so in many cases, the low bidder priced out an ordinary Portland cement (OPC) mortar while anyone bidding historic lime mortar was disqualified on price. If testing then indicates that a lime mortar is the more suitable replacement mortar there is pressure to permit an OPC or to accept change orders. This scenario can be avoided if the building owner commissions the testing and the specifications are then driven by test data.

Sometimes it’s simply not possible to perform testing before issuing specifications. In these cases, we recommend including a bid allowance for both an OPC mortar and hydraulic lime mortar to allow for the final results of the mortar testing. Comparing bidders’ numbers for historic lime mortar will also give some insight into their relative experience; inexperienced bidders tend to go either very low or very high. Very low bidders usually haven’t familiarized themselves with the material costs and additional labor, and high bidders often provide extra contingencies to cover unknown factors. If you receive bids that vary dramatically you should further qualify the bidders.

Commissioning testing early in the process allows the architect and/or the building owner to choose the laboratory. This is an important consideration since testing labs vary widely in quality, capability, and experience in analyzing historic materials. Some smaller labs rely exclusively on acid digestion testing, which is a very basic analysis that offers only a general picture of mortar composition. New construction labs may not have technicians who are familiar with the idiosyncrasies of historic mortars, and some labs advertise exclusive proprietary tests that achieve results that standard ASTM tests cannot.

We are very skeptical of proprietary tests that claim they can achieve results that ASTM testing can’t since the test methods are opaque, as is the process of analyzing and interpreting the results.

As a material supplier, HFS does not perform the laboratory testing-we have a partnership with a qualified laboratory that has highly trained and skilled scientists and engineers. This independent lab does the analysis, and we interpret their findings in the context of modern restoration materials.

Be Specific

Clearly identify the ASTM procedure you want to use to reach your objective. If testing is to be performed by the contractor, identify the specific test protocol. We regularly field calls from contractors who are asked to perform testing “to identify the composition” or to “determine the compressive strength” of the mortar. These kinds of instructions are simply too vague to be meaningful, and most contractors will gravitate toward the fastest and cheapest option to fulfill these kinds of requirements. The quality and usefulness of the test results will be in proportion to the quality of the specifications.

A Note about Compressive Strength Testing

One important note: one of the most common problems we encounter are specifications that call for testing the compressive strength of mortar. There is no ASTM standard test to determine the compressive strength of an extracted historic mortar sample. The standard test for compressive strength is ASTM C-109 “Standard Test Method for Compressive Strength of Hydraulic Cement Mortars” which involves compression testing a set of two-inch cubes. Obviously, this procedure doesn’t translate to smaller, irregular samples pulled from a wall, nor does it account for the numerous other variabilities of extant historic mortar. Some testing facilities offer compressive strength testing for historic mortars but such proprietary tests rely on undisclosed methods, meaning that the results can’t be independently verified or analyzed and it’s impossible to determine how sound the methodology is.

Testing Procedures

ASTM C1324 “Analysis of Hardened Masonry Mortar”

This test is used to identify the characteristics and proportions of the original. It provides both quantitative and qualitative descriptions of the binder and it identifies the estimated proportion of binder to sand. It is recommended when the goal is to develop the most compatible replacement.

This is the most commonly specified test we offer.   This is a comprehensive laboratory analysis of historic masonry mortar that includes acid digestion, thin section analysis, and chemical analysis.  The petrographer’s report documents the visible characteristics of the mortar using thin section analysis. The cement to hydrated lime ratios and the sand to cementitious ratios are calculated using the petrography and chemical analysis findings. The lab test results include a written, photo-documented report with limited interpretation, which we then analyze the report and make restoration recommendations.

One important limitation to note about the chemical analysis portion of the procedure is that calcareous aggregates will be dissolved during the acid digestion and can skew the proportions by altering the lime to cement ratios. Petrographic findings are used to adjust the proportion calculations in these cases but if the interference is excessive it may be impossible to adjust sufficiently. If we suspect that this may be a concern we advise the client before initiating testing, and we’ll first do thin section analysis before deciding if it makes sense to proceed with chemistry. Note that if we are doing a full ASTMC-1324 we usually run the thin section and the chemistry concurrently. If we wait to do the chemistry it will add at least three weeks to the leadtime.

”Thin section analysis” can be performed alone if the volumetric proportions aren’t necessary. This test method is useful to confirm if a sample is a replacement mortar and it provides a detailed description of the binder. Thin section analysis uses optical microscopy to determine the general composition of the mortar, binder type, and sand types.

Recommended References for Mortar Analysis:

Mortar Analysis Part 1: Mortar-Making Materials | Lorraine Schnabel / APT Bulletin Practice Points

Mortar Analysis Part 2: Analytical Methods | Lorraine Schnabel / APT Bulletin Practice Points

Mortar Analysis Part 3: Buying the Right Services | Lorraine Schnabel / APT Bulletin Practice Points

ASTM C67 “Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile”

Understanding the characteristics of the masonry units themselves provides a framework for selecting mortar. The goal of the mortar is to cushion the masonry units and to transfer moisture out of the wall, and in order to do this the mortar must be lower in compressive strength and more vapor permeable than the masonry units and the existing mortar. ASTM C67 tests are often specified in conjunction with mortar testing to aid in the selection of replacement mortar and/or to identify causes of masonry deterioration.

This test determines the average compressive strength of a clay brick masonry unit based on 5 samples.

The amount of water absorption is determined by the gain in weight of a dry brick on immersion in water under specified conditions. Water absorption obviously increases as the amount of porosity increases since there must be voids (pores) for the water to be absorbed in or enter into the body of the brick. The results are based on a set of 5 samples.

The amount of water absorption is determined by the gain in weight of a dry brick on immersion in water under specified conditions. Water absorption obviously increases as the amount of porosity increases since there must be voids (pores) for the water to be absorbed in or enter into the body of the brick. The results are based on a set of 5 samples.

NOTE: These are destructive tests that require masonry units that have been removed from the building.

How to Specify Sampling

A minimum of ten (10) grams of mortar is required to perform standard mortar tests but it is always recommended that a larger sample be submitted, especially if you plan to have us match the mortar. This sample should represent one type and source of mortar, and composite samples consisting of two or more different types of mortar (such as a composite sample of older mortar and subsequent repointing or repair mortar) should be not be treated as one sample.

When removing the sample, ensure that the sample being extracted is representative of the original mortar and not a later repointing or repair materials. Mortar characteristics will vary depending on the building and type, but it’s usually necessary to cut deep into the joint to ensure an accurate sample.

When conducting ASTM C67 masonry unit tests you should plan on providing a set of five intact, undamaged units per test. They will be destroyed during testing.

Anticipating Lead Time

Laboratory mortar testing typically requires a *minimum* of 3-4 weeks, although lead times can vary depending on the current laboratory workload and the specific test procedure, and can often be longer. Please note that this lead time may be extended if there is backlog in the lab or if samples do not meet the requirements outlined above as they will need to be replaced. In some cases, testing may not be advisable if samples are insufficient in size or consistency. Once the testing is complete, the preservation staff at Henry Frerk Sons will review and analyze the report to recommend replacement mortar. This process typically takes 3-5 days. We recommend explicitly noting these lead times in specification documents since contractors generally need to obtain the results before they can put together material submittals for approval. If testing isn’t initiated promptly it can slow down the project, and no amount of calling can speed it up!

Interpreting and Applying the Test Results

We’ve run ASTM C1324 on our available replacement mortars so we can compare the test data to make recommendations, and when making recommendations we cross reference these results along with manufacturer test data for the mortars. We also consider the construction type and materials, the overall condition of the building, and the climactic conditions the building experiences.