In order to improve the shear behavior of hollow concrete block masonry, precast concrete anti-shear blocks were proposed to enhance the shear strength of hollow concrete block masonry. Four groups of hollow concrete block masonry triplets with precast concrete anti-shear blocks were tested under shear loading, and their behaviors were compared with a control group. The results show that as the height of precast concrete anti-shear blocks increases, the shear strength of the masonry increases. The maximum shear strength of masonry triplets with precast concrete anti-shear blocks was 234.48 percent higher than that of the control specimens. The shear strength of masonry triplets was mainly determined by the failure of hollow concrete block unit. The majority of specimens exhibited double shear failures; however, these failures showed characteristics of ductile failure to a certain extent. Based on the experimental results, a equation for calculating the shear strength of masonry with precast concrete anti-shear blocks was proposed.
Masonry is one of the oldest forms of construction and has been widely utilized in both developed and developing countries due to ease of construction, availability of materials, relatively low cost of materials, and unskilled workers [
Usually, masonry is considered as a composite structure consisting of block units and mortar and is strong in compression but weak in tension [
In order to enhance the in-plane shear behavior of masonry, confined masonry (CM) structures with horizontal and vertical RC-confining elements are widely used in seismically active regions in developing countries, especially in China, due to their satisfactory behavior. However, horizontal and vertical RC-confining elements have not been widely used in rural areas of China owing to high cost and lack of skilled workers. How to find a simple and economical construction method to improve the shear strength of masonry is crucial to improve the seismic performance of masonry structures in rural areas of China.
According to the development status of rural China and the needs of rural housing construction, an extensive research program has been carried out in Tianjin Chengjian University. The purpose of this research program is to develop simple and effective structural measures to improve the hollow concrete block masonry structures without horizontal and vertical RC-confining elements. Then, precast concrete anti-shear blocks with different dimensions were proposed to enhance the shear strength of hollow concrete masonry. In this paper, a series of direct shear tests of block masonry triplets (with and without precast concrete anti-shear blocks) have been conducted to validate the effectiveness of the proposed structural strengthening solutions.
Masonry triplets involved here comprised hollow concrete blocks and cement mortar. The size of the hollow concrete block units used in this research was 190 × 190 × 390 mm, and all the hollow concrete block units were manufactured by the same company. Hollow concrete block properties were determined according to GB/T 4111-2013 [
Cement mortar consisting of cement (grade 42.5, ordinary Portland cement (OPC)) and natural river sand in the ratio 1 : 6 by weight was employed for masonry mortar. The Portland cement used in the experiment was the Chinese P.O. 42.5 grade, which contains 80–95% of cement clinker and gypsum. The physical properties of the Chinese P.O. 42.5 grade, provided by the suppliers, are as follows: the specific surface area ≥300 m2/kg; initial setting time ≥45 min; final setting time ≤10 h; 3-day compressive strength ≥17 MPa; 28-day compressive strength ≥42.5 MPa; loss of ignition ≤5%; SO3 ≤ 3.5%; and MgO ≤ 5%. The natural river sand was clean, typical fine sand found in Tianjin. The bulk unit weight of the sand was 1,598 kg/m3. Potable water was used to mix the mortar. Material properties were determined by following the specifications of GB 175-2007 “Common Portland cement” [
The strength grade of concrete used for precast concrete anti-shear blocks is C30, and the mean value of concrete compressive strength (cube) is 33.5 MPa. There are four kinds of precast concrete anti-shear blocks, which are named as PB-H/2 (Figure
PB-H/2.
PB-H/3.
PB-H/4.
PB-B/3.
The experimental program consisted of 30 masonry triplets. Each masonry prism was assembled with three block units and a full-bed mortar joint that was struck flush. The mortar bedding thickness was approximately 10 mm, and a professional mason constructed all triplets. The skilled mason exerted extra care during construction of the masonry triplets to ensure that triplets were level and plumb. All phases of the construction of the triplets were followed as specified by the Chinese code GB/T 20129-2011 “Standard for test method of basic mechanics properties of masonry” [
Photo of the specimens.
The masonry triplets are divided into 5 groups, and each of which has six test specimens. The notation CO is adopted for masonry triplets without precast concrete anti-shear blocks and used as a control group. The other four groups correspond to each of the four types of precast concrete anti-shear blocks mentioned above and have the same name as the corresponding anti-shear block. Figure
Schematic diagram of five test groups. (a) Group CO. (b) Group PB-H/2. (c) Group PB-H/3. (d) Group PB-H/4. (e) Group PB-B/3.
Masonry triplets are adopted to evaluate the shear strength along the block-mortar bed joints of masonry. There are many testing schemes for the evaluation of the shear strength in masonry. According to the boundary conditions and loading configurations, proposed in the Chinese code GB/T 50129-2011 [
The arrangement of loading device and LVDT.
The main objective of this study is to assess the enhancement in shear strength and shear deformation ability of block-mortar joint due to precast concrete anti-shear blocks. Hence, the masonry triplet shear tests were performed without axial precompression loads, and the masonry triplets were only subjected to loading parallel to the bed joint.
Six specimens of group CO (control triplets) experienced shear bond failures at block mortar interface, as shown in Figure
Typical failure modes of group CO.
The other four groups of masonry triplets constructed with precast concrete anti-shear block exhibited different failure modes in comparison to control masonry triplets. The masonry triplets with precast concrete anti-shear blocks have two obvious failure characteristics. The first is the bond failure of mortar joint; the specimens could still remain intact and maintain a certain bearing capacity, as shown in Figure
Mortar joint failure.
Shell and web cracking.
The failure of these specimens constructed with precast concrete anti-shear block indicated that the adjacent blocks and precast concrete anti-shear blocks formed an interlocking action, the block shells and webs also participated in the shear transference between adjacent blocks due to the existence of precast concrete anti-shear blocks. The shear strength of the masonry triplets constructed with precast concrete anti-shear block was not controlled by the damage of the mortar layer, but by the cracks occurring in the block shells and webs. It was shown that the force transmission mechanism of the masonry triplets constructed with precast concrete anti-shear block was different from that of the contrast specimens.
The load-displacement responses are the most important characteristic for assessing the behavior of tested specimens. The load versus displacement response of five groups of test specimens is presented in Figures
Load-displacement response of group CO.
Load-displacement response of group PB-H/4.
Load-displacement response of group PB-H/3.
Load-displacement response of group PB-H/2.
Load-displacement response of group PB-B/3.
Figure
Masonry triplets constructed with precast concrete anti-shear blocks exhibited different load-displacement responses, as shown in Figures
From Figures
Figure
Comparison of shear strength of masonry triplets.
The mean value, standard deviation, and coefficient of variation of shear strength of each group are shown in Table
Measured value of shear strength.
Group number | Mean values (MPa) | Standard deviation (MPa) | Coefficient of variation |
---|---|---|---|
CO | 0.14 | 0.01 | 0.07 |
PB-H/4 | 0.27 | 0.04 | 0.15 |
PB-H/3 | 0.33 | 0.03 | 0.09 |
PB-H/2 | 0.48 | 0.02 | 0.04 |
PB-B/3 | 0.16 | 0.02 | 0.13 |
For groups PB-H/4, PB-H/3, and PB-H/2, it can be concluded that the average value of masonry shear strength increases with the increasing height of precast concrete anti-shear blocks. According to the failure mode of the specimens, it was found that the shear strength of the masonry triplets was only related to the damage degree of the blocks, and the precast concrete anti-shear blocks remain intact. There was a good correlation between the shear strength of masonry triplets and the height of precast concrete anti-shear blocks. Therefore, an equation to the relationship between the height of precast concrete anti-shear block and the shear strength of masonry was derived using parabolic regression, as shown in Figure
Relationship curve of height of anti-shear blocks and shear strength.
The best-fit equations are as follows:
It can be seen from Figure
The above fitting formula is only applicable to hollow concrete blocks with strength grade MU7.5. It can be seen from the failure mode of the tested specimens that the failure of the masonry triplets with precast anti-shear blocks is manifested as the failure of the hollow concrete blocks themselves. The shear strength of masonry triplets with precast concrete anti-shear blocks is only related to the strength of concrete blocks themselves and the size of precast concrete anti-shear blocks. Therefore, the shear strength of the masonry with precast concrete anti-shear blocks can be improved from two aspects: on the one hand, the precast concrete anti-shear blocks can be set; on the other hand, high strength hollow concrete blocks should be used.
This paper presents an experimental program where the shear behavior of hollow concrete block masonry triplets with precast concrete anti-shear blocks is assessed.
This paper discusses the shear performance of the hollow concrete block masonry triplets with different precast concrete anti-shear blocks, compared with hollow concrete blocks masonry triplets without precast concrete anti-shear block, in shear strength and deformability.
The following points summarize the results from this study: Hollow concrete block masonry triplets with precast concrete anti-shear blocks exhibited higher gain in deformability in comparison to masonry triplets without precast concrete anti-shear blocks. The shear strength of the hollow concrete block masonry triplets could be improved by setting precast concrete anti-shear blocks into the cavity of the blocks, up to a maximum of 234.48%. The shear strength of the masonry triplets constructed with precast concrete anti-shear block was not controlled by the damage of the mortar layer, but by the cracks occurring in the block shells and webs. For a given grade of hollow concrete blocks, an equation for the relationship between the shear strength of masonry and the height of precast concrete anti-shear block was proposed using parabolic regression.
The data used to support the findings of this study are included within the article.
(i) Hollow concrete block masonry triplets with precast concrete anti-shear blocks were tested under monotonic shear loading. (ii) Hollow concrete block masonry triplets with precast concrete anti-shear blocks behaved in a ductile manner under shear loading. (iii) The proposed precast concrete anti-shear blocks could significantly enhance the strength and ductility of hollow concrete block masonry. (iv) The proposed precast concrete anti-shear blocks could provide the advantages of cost effectiveness and rapid construction.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by the Tianjin Science and Technology Commission (grant nos. 17ZXCXSF00040 and 18ZXGDGX00050) and Ministry of Science and Technology of the People’s Republic of China (grant nos. 2015BAL03B02 and 2016YFC0701508).