Development of a New Nondestructive Method for the In-Situ Determination of Mortar Strength

Abstract

The determination of the load bearing capacity of masonry in existing structures is not an easy task, even for experienced surveyors. For its assessment, it is necessary to know the compressive strength of the masonry units and mortar. Relatively good destructive and non-destructive methods were developed for the determination of the compressive strength of masonry units. However, mortar compressive strength determination is currently closer to a rough estimation rather than a deterministic approach. All of the currently used methods are either complicated, have a limited application, or are based on the surveyor’s experience only. Influence of the human factor on the results of testing is significant. The paper introduces the development of a new non-destructive method, which eliminates the influence of the human factor. The method is supposed to work as quickly and as easily as the Schmidt hammer used for concrete structure surveys. The new instrument was created where the compressive strength of mortar is assessed by hammering a steel bar into a joint with constant energy, while the depth of penetration is measured. The impact energy is provided by a steel spring with a prescribed stiffness, and the loading is provided by an automatic system similar to those used in air-soft guns.

Prediction of Mortar Compressive Strength Based on Modern Minor-Destructive Tests

“…The identical dependence was also confirmed in [ 25 ], where after 6 weeks of curing, the penetration depth was reduced by 15% (cement–lime mortar similar to G) to 21% (lime mortar similar to H). In a similar period (between the 4th and the 12th week), in the tests presented in this paper, the reduction was, respectively, 16% for mortar G and 32% for mortar H. A considerably weaker relationship was obtained in [ 28 ] between mortar compressive strength and the number of strokes.…”

“…By analyzing statistical parameters of all results presented in this paper and obtained by other authors [ 3 , 5 , 19 , 20 , 22 , 28 , 35 , 41 , 44 , 45 , 46 , 50 , 51 , 58 , 60 , 61 ], a significant difference between minor destructive tests (DPT, PT, TPT) and the destructive test is noticeable. For dispersion assessment, variation coefficients were compared in Figure 22 .…”

“…The first one, triggered by the presence of bricks, is the reduction in excessive water in the mortar mixture during hardening in the joint. This phenomenon, described in the literature as the “absorption effect”, has a significant impact on the increase in the mortar strength in the actual joints of the structure [ 28 , 53 , 54 ]. Furthermore, it contributes to the considerable reduction in the total open porosity and the reduction in the average pore radius [ 20 , 52 ].…”

“…The impact force is generated by a mass connected to a releasing spring—a mechanism similar to one in a Schmidt hammer. In practice, a few types of penetrometers were used [ 24 , 25 , 26 , 27 , 28 ], but most of them were only in laboratory tests. This article uses the first serially manufactured modern penetrometer, RSM-15, with a standard impact energy of 4.55 Nm and needle diameter of 4.5 mm ( Figure 2 ).…”

“…There are several methods [ 25 , 31 , 32 , 33 ] similar to PT that are currently being used for mortar diagnostic, e.g., “Sphere impact”, “controlled penetration” or “DRMS” (based on microdrilling resistance technique and is a modification of older PNT-G test [ 34 ]). Moreover, new methods, such as the “gun penetrometer”, are still under development [ 28 ].…”