The objective of this study was to develop a test method to measure the hardness of masonry mortars with a pendulum hammer. The results of this test method are to be used as an aid in the selection of pointing mortars and for the evaluation and quality control of in-place mortars. Eight mortar formulations were investigated in the laboratory.

These tests included measures of the plastic properties of the mortars, the compressive strength, and the water vapor transmission. The laboratory tests were conducted in accordance with American Society for Testing and Materials (ASTM) standard test procedures where appropriate.

Masonry piers for pendulum hammer hardness testing were built using the same eight mortar formulations. One set of piers was constructed using reclaimed masonry units and historic construction techniques to simulate old masonry in a controlled environment. Other piers were constructed using modem materials. The results of mortar hardness measurements from these piers were compared to the properties determined in the laboratory testing program.

Field tests were conducted on historic masonry structures in Colorado and across the United States. The experience gained from the field testing and the results of the laboratory comparisons were used to develop a test method for hardness determination using the pendulum hammer. This method includes guidelines for use of the equipment and provides a method for obtaining statistically significant results. A standard test method for this technique has been drafted and submitted to ASTM for consideration as a new standard.

A literature review was performed focusing on existing techniques of evaluating historic and modem mortars, including hardness testing, penetration and pullout techniques, drilling resistance, and chemical testing. Impact heads were machined in a range of lengths for the purpose of testing mortar at depth.

The results reported herein indicate that the results of pendulum hammer testing correlate well to the mortar type and compressive strength. The hardness results do not correlate well with the results of the water vapor transmission tests nor with the mortar plastic properties. The correlation with compressive strength, economy, and the ease of use of the device make the pendulum hammer a practical device to aid in the evaluation of in-place mortar. Drawbacks to the use of the device include the need for frequent calibration and maintenance.

This research was made possible through Grant MT-2210-7-NC-13 from the National Center for Preservation Technology and Training (NCPTT).