Determining Existing Masonry Structural Properties
Design professionals rely on numbers. Some of the most important numbers represent the design loads on a structure during an event with a certain probability of occurring, and the resistance of the structure given its geometry, material properties, and connections. In a new structure, these quantities are generally known, specified, and listed on project documents. Design loads are determined by building codes and jurisdiction. Material properties are specified by the designer and verified through testing.
However, when you are dealing with a century-old masonry building for which no design documents are available and multiple modifications have been made over its lifetime, how can critical values such as material properties be determined?
Investigation and Testing
Investigation and testing of the actual structure give the designer the most accurate values on which to base their design. Methods include removing samples from the structure for testing in a laboratory, testing materials in place, and fully nondestructive techniques.
The Building Code Requirements for Masonry Structures TMS 402-13/ACI530-13/ASCE5-13 (TMS 402) permits testing of masonry prisms removed from structures for verifying in-place strength. Masonry prisms can be removed from the existing structure and tested in the laboratory. However, it is critical to take special care during the removal and transportation process in order to keep the assembly intact and as similar to its in-place condition as possible.
Selection, removal, and transportation of prisms should follow the procedure of ASTM C1532, Standard Practice for Selection, Removal, and Shipment of Manufactured Masonry Units and Masonry Specimens from Existing Construction. Samples should be representative of the structure as a whole or should represent the specific location or physical condition, such as weather exposure or degree of deterioration, for which properties are to be measured. Prism specimens are commonly removed from an existing wall using a series of cuts with a diamond blade power saw. Typically, the first cut is made along the bottom surface of the prism, and shims are inserted into the open joint to support the prism’s weight while cuts are made on the top and sides. Prisms need to be confined and secured before transporting, which can be done using straps, clamps, or through-bolted top and bottom plates. If prisms are shipped, they need to be packed with adequate padding to protect them from damage.
The most common tests performed on masonry prisms removed from existing construction include compressive strength per ASTM C1314 and flexural bond strength per ASTM C1072. To accurately determine the compressive strength of the prism, the ASTM C1314 standard is explicit in describing height to thickness ratios and other parameters.
Masonry flexural bond strength is tested using an apparatus known as a bond wrench (Figure 1), which clamps on and applies torque to the top unit of a prism until the bond between the unit and the rest of the prism fails.
In-Place Testing of Masonry
As an alternative to removing and transporting masonry prisms to a laboratory for testing, in-place (in situ) methods exist for evaluating masonry compressive strength, stiffness, and flexural bond strength. Masonry compressive strength and stiffness are measured in place following ASTM C1197, Standard Test Method for In Situ Measurement of Masonry Deformability Properties Using the Flatjack Method. In this test, hydraulic bladders known as Flatjacks are inserted into slots cut in mortar joints above and below the section of masonry to be tested – typically five courses between Flatjacks. The Flatjacks are then pressurized to induce compressive stress on the masonry while surface strain is measured, either manually using dial gauges or electronically using linear variable differential transformers (LVDTs). The resulting test data can be used to generate a stress-strain curve for the masonry in place. If compressive failure of the masonry occurs during the test, then compressive strength can be measured directly. If failure does not occur during the test, masonry compressive strength can be estimated using established correlations to the measured stiffness.
Masonry shear testing can be performed in-place by three distinct methods described in ASTM C1531, Standard Test Methods for In Situ Measurement of Masonry Mortar Joint Shear Strength Index. Method A involves using Flatjacks above and below the unit to be tested to apply a known compressive stress while measuring the force required to push the unit horizontally into an adjacent head joint previously cleared of mortar. The Flatjack-controlled compressive stress can be incrementally increased while measuring the force needed to move the unit at each increment to determine the coefficient of friction between mortar and units. Method B involves the use of a hydraulic ram to move the test unit horizontally, similar to Method A. However, instead of controlling the compressive stress on the test unit, the investigator or designer must estimate and subtract the acting overburden loads from tested values. Method C is similar to Method B, except that a specially-sized Flatjack inserted into an evacuated head joint is used to apply the lateral force. The advantage to Method C is that the only material removal required is two mortar head joints, minimizing repairs after testing.
One method of masonry sample removal involves drilling to remove core samples. International Existing Building Code (IEBC) – A220.127.116.11 permits tensile splitting tests conducted on 8-inch diameter cores for qualifying masonry quality, as an alternative to shear tests. Core removal may also be useful to facilitate visual observation of wall section properties and construction quality. For multiwythe grouted brick construction, the shear bond strength of a grouted collar joint between two brick wythes may be evaluated following the method of California Test 644. While this test is sometimes also applied to grouted CMU construction, variability and inaccuracy may result from the presence of cross webs and the tapered profile of face shells.
Component Materials Testing
In some cases, removal of intact prisms may not be possible due to appearance, deterioration, or loss of integrity. Individual units may be removed from carefully selected locations throughout the structure and evaluated for properties like compressive strength and absorption for the purpose of selecting compatible replacement and repair materials. Masonry units removed from the structure may also be used to construct prisms in the laboratory to simulate original masonry for testing. Information about the original mortar is also needed to construct prisms that behave similarly to the original structure. An alternate to prism testing, the unit strength method is a means to verify the compressive strength of new masonry construction by determining the compressive strength of units and mortar type. From these two components, a conservative compressive strength for the assembly can be determined using a table that correlates assembly compressive strength with masonry unit compressive strength and mortar type. Tabulated f’m values are based on data for modern masonry units and are not intended for use with historic masonry construction.
Mortar Analysis, Characterization (Chemical, Petrography)
Determining mortar type can be useful to facilitate the unit strength method or comparison to published design values. More commonly, however, mortar analysis is performed so that repair and replacement mortar can be specified that is compatible with the original. There are a number of different analysis methods, many of which involve digesting the binder paste using acids. Further analysis methods are performed to determine binder to aggregate ratio, and chemical composition to help identify binder type.
Component Testing – Anchors
The capacity of attachments to existing masonry may be verified through testing following the requirements of ASTM E488 – 96 (2003), Test Methods for Strength of Anchors in Concrete and Masonry Elements. Anchors may be tested in tension or shear, depending on their application in the structure. ASTM E488 requires that anchor strength is calculated as the average of at least five anchors.
ASTM E488 only includes failure mechanisms that involve rupture or breaking of elements. For example, failure includes anchor pullout, fracturing of the substrate, or anchor yielding. E488 does not include any limitations on the displacement of the anchor as failure criteria. However, in historic masonry structures, anchors often exhibit very large deflections/displacements before any catastrophic failure and these large displacements often govern the design of the anchors.
Anchor displacement criteria are described in Section A107 of the IEBC and the Uniform Building Code (UBC) Standard 21-7 (Test of Anchors in Unreinforced Masonry Walls) as a measure of successful anchor performance. UBC 21-7 specifies that, when testing retrofit anchors in masonry, the test load shall be reported at a relative movement of ⅛-inch, measured between the anchor and the adjacent masonry surface.
In historic masonry structures, anchors tend to exhibit relatively large displacements without catastrophic failure. In some cases, anchors can be tested to very high loads without brittle or inelastic behavior, but the displacements become very large (at or near the capacity of the test apparatus). Therefore, it may be appropriate to apply a displacement criterion to anchors in masonry for this type of structure.
Quantity of Tests
In situ masonry tests are conducted to quantify mechanical properties necessary for structural capacity analysis and design of strengthening systems. The types of tests are based on ASTM test method requirements as described above, and the number of tests is based on requirements of ASCE 41, Seismic Rehabilitation of Existing Buildings. Depending on the use and expected performance of the building, the designer may choose either a “comprehensive” or “usual” level of knowledge. Additional tests give greater confidence that results represent in-place properties. Engineers without confidence in materials and their properties are more prone to design overly conservative solutions. Tables 1 and 2 summarize the recommended testing schedules for usual and comprehensive levels of knowledge based on ASCE 41-13 and the IEBC.
Existing masonry structures can present several challenges to structural designers due to unknown material properties and variable construction quality. Fortunately, standard test methods exist for in-place evaluation and sampling for laboratory testing, which is effective when performed by qualified personnel. A successful, thorough evaluation program will give structural designers the information they need to be confident in their design.▪