Effect of different strengthening materials on tensile behaviour of plasters and renders

The purpose of strengthened plasters (interior applications) and renders (exterior applications) in this paper is to control cracks at the interface between the main structure and the infill wall rather than strengthen masonry and reinforced concrete structures. Cracks in plasters and renders have caused severe issues that need to be resolved urgently. Cracks affect not only the aesthetics but also the normal use of a building. The presence of cracks changes the perceptions of the owners of a structure, forcing them to question its safety and even take legal action against its developer. Consequently, the present paper discusses the effects of different strengthening materials on the tensile behaviour of plasters and renders, particularly on the first cracking. One test group without strengthening materials and six test groups with strengthening materials were tested under uniaxial tensile loads. Each experimental group included the same six specimens. A glass fibre-reinforced polymer (GFRP) grid, two types of basalt fibre-reinforced polymer (BFRP) grids, and three types of welded wire meshes (WWMs) were selected as the strengthening materials in the tests. Four types of plasters and renders were used: cement mortar, anti-crack mortar, gypsum plaster, and thermal insulation mortar. The crack pattern, stain–stress relationship, first cracking strain, energy dissipation capacity in the uncracked stage, and microstructure were investigated to gain better insight into the tensile behaviour of the plasters and renders. It was found that a strengthening material with a small mesh size resulted in numerous cracks and a narrow crack spacing. The stress–strain curves of the strengthened specimens could be divided into uncracked and cracked stages. The higher slopes in both stages of the strengthened specimens could be attributed to the higher longitudinal strain coefficient, smaller opening size, and larger cross-sectional area of each strand of the strengthening materials. The first cracking strain of the unreinforced specimen was smaller than that of a strengthened specimen. Stronger bond strength between plaster/render and strengthening material, smaller mesh size and greater cross-sectional area of each strand of strengthening material led to the larger first cracking strain. The energy dissipation capacity in the uncracked stage of the strengthened thermal insulation mortar was the least. The energy dissipation capabilities in the uncracked stage of the plasters/renders reinforced with BFRP grid-2 and WWM-3 were the second-highest and highest, respectively. The bond strengths of the GFRP and BFRP grids to the plasters/renders were stronger than those of the WWMs. The bond strengths between the gypsum plaster and the reinforcing materials were the highest, whereas those between the thermal insulation mortar and the strengthening materials were the lowest. Additionally, stress–strain relationships were proposed for the strengthened plasters and renders; the results were found to be in good agreement with the test results.