An experimental study of the bond behavior of twisted steel bars embedded in mortar cylinders and in the joints of masonry wallettes

Highlights

• Pull-out tests of steel bars inserted in mortar cylinders were conducted.

• Pull-out tests of steel bars inserted in masonry wallettes were conducted.

• Different bonded lengths were considered.

• Bar rotation during extraction was observed.

Abstract

Among the different strengthening techniques available to reinforce masonry buildings, the near-surface mounted technique consists in inserting reinforcing elements of high-strength material in grooves realized by removing part of the mortar in the horizontal bed joints of a masonry wall. The present study deals with two experimental campaigns on pull-out tests conducted on twisted steel bars embedded concentrically in mortar cylinders and in the mortar bed joints of masonry wallettes as in in-situ applications. In order to compare the results between cylindrical and masonry specimens, the same bonded lengths are considered.

Introduction

The main structural deficiencies of masonry buildings are related to the almost-inexistent tensile strength, as the bond strength between mortar and fired-clay brick is very low, and the limited effectiveness of the connections between the masonry elements [1], [2]. Thus, masonry buildings are susceptible to seismic actions [3], as unfortunately highlighted in the recent Italian events of L’Aquila in 2009, Emilia Romagna in 2012, and Amatrice in 2016 [4], [5], [6], [7], [8]. When fair-faced masonry buildings necessitate retrofitting, reinforced structural repointing may be a feasible solution since it does not violate the aesthetic requirements of a historical building since the reinforcement is embedded in the joints [9], [10], [11], [12], [13]. The strengthening elements can be of different shapes and materials. Fiber-reinforced polymer (FRP) thin strips or bars made of carbon (CFRP), AR glass, or basalt fibers embedded in an organic matrix are typically used, although stainless steel bars have been studied as well and will be the focus of this paper. Structural repointing is also known as near-surface mounted (NSM) technique, since the reinforcing material is inserted into 10–20 mm-deep grooves usually carved in the mortar bed joints by means of a circular saw. The reinforcing elements are embedded in the joint grooves by means of an organic material such as epoxy (typically thermosetting), cementitious mortars modified with the addition of organic additives, or cementitious-based mortars or lime-based mortars. When possible, the reinforcing elements are applied on both sides of the wall, and in some cases, they are connected by ties placed transversely in the wall [14].

The use of repointing for masonry structures is fairly common. However, only few studies in the literature have focused on the bond performance of NSM reinforcement embedded in masonry [15], [16], [17], [18], [19], [20]. The most important contributions dealing with pull-out tests of structural repointing can be roughly divided into two main groups of researchers. In the United States, most studies investigated the bond performance of carbon or glass FRP bars embedded in the bed joints of masonry specimens made of concrete blocks [21], [22]. The variables that have been considered are the bonded length, bar diameter, groove size with respect to the bar diameter, embedding material (epoxy or latex modified cementitious mortar), as well as the bar surface condition. The second major group of studies have been conducted in Australia and New Zealand and dealt with pull-out tests of CFRP strips embedded with epoxy resin in masonry specimens made of solid fired-clay bricks [16], [23], [24], [25]. In these experimental campaigns, several parameters were taken into account: the strip bonded length, the direction (parallel or orthogonal) of the reinforcing elements with respect to the direction of the bed joints, the effect of cyclic loadings, as well as the size of the strips. It should be noted that in this group of experimental campaigns the NSM CFRP strips were inserted into grooves cut through the fired-clay bricks rather than placed where the mortar joints were located.

Starting from the available experimental results on pull-out tests, simple analytical models have been developed to predict the bond properties such as bond capacity, effective bond length, and shear stress-slip relationship for concrete or fired-clay brick masonry substrates that are useful to predict the performance of structures that are reinforced with NSM bars or strips [26], [27].

To the best of the authors’ knowledge, none of the aforementioned works are related to study the bond behavior of NSM steel bars embedded in the bed joints of fired-clay brick masonry, employing as groove-filling material an inorganic matrix such as a structural mortar. To fill this gap, the aim of this work is to present the results of two extensive experimental campaigns that focus on the bond behavior of spiral-shaped stainless-steel bars. In the first campaign, in order to get a preliminary insight on the bond behavior of these bars, a series of pull-out tests are conducted on steel bars concentrically embedded in cylinders made of the same structural mortar used to embed the bars in the masonry grooves. The bars made of ultra-high strength steel are characterized by a helical shape. Several bonded lengths are considered. In the second experimental campaign, the same type of bars is inserted with the same structural mortar in the horizontal bed joints of masonry wallettes in order to reproduce more closely the in-situ application. The same bonded lengths investigated in the first experimental campaign are considered in the second set of tests for comparison.