Flexural strength characteristics of non-load bearing masonry walls in Kuwait

The controlling factor in designing non-load bearing masonry walls, such as those used in Kuwait, is the lateral resistance to wind loads. To ensure safety of the walls, data is needed on the flexural strength characteristics of walls constructed with locally-available materials. The flexural strength of masonry walls constructed with autoclaved aerated-concrete blocks, sand-cement concrete blocks or calcium silicate bricks was evaluated in a test program that involved testing small-scale walls or wallettes. The tests were performed in accordance with the British Standard for unreinforced masonry. The autoclaved aerated-concrete block wallettes were constructed with epoxy glue mortar, whereas the concrete block and calcium silicate brick walletters were constructed with sand-cement mortar. Two stages of testing were undertaken to evaluate bending parallel to bed joints and bending perpendicular to bed joints. The flexural strengths required by British and American codes exceed the strengths of the concrete block and calcium silicate brick walls used in Kuwait, implying that the allowable tensile stress requirements of these codes are not safe for assessing the lateral resistance of the walls. The format used for the autoclaved aerated-concrete block wallters, which is identical to the standardized format for concrete block wallettes in the British standard, is suitable for determining the flexural strength of full-size autoclaved aerated-concrete block walls.     

Flexural bond strength of natural hydraulic lime mortar and clay brick

This paper measures the bond strength of natural hydraulic-lime (NHL) mortars, to further characterise their properties and enhance their use in building. An additional objective is to correlate bond strength with mortar hydraulicity, water content, workability and water retention, to develop mortars of high bond strength that would improve the quality of masonry. To this aim, the flexural bond strength of masonry, built with mortars of three hydraulic strengths-each including the water amount required to attain three specific flows (165, 185 and 195 mm), was measured with the bond wrench test. The results suggest that NHL mortars possess high water retention, and this enables a strong bond that compares well to that of Portland cement and cement/lime mortars. The results also indicate that bond strength is not determined by the binder’s hydraulic strength, but it increases proportionally to the mortar’s water retention. The paper concludes that for the NHL5 mortars, the 185 mm flow results in the strongest bond, simultaneously providing the highest water retention and best workability. However, for the lower strengths (NHL 2 and NHL 3.5 mortars), the water content required to attain the flows that provide an optimum workability (165 and 165–185 mm, respectively) does not lead to the strongest bond, but it is the highest flow values that provide the NHL2 and NHL3.5 mortars with the strongest bond and, in most instances, the highest water retention.     

Autoclaved aerated concrete (AAC) rubble for new recycling building products: In dry premixed mortars for masonry,in masonry blocks and in lightweight blocks

In cooperation of Bremen Institute for Materials Testing (MPA Bremen), a Department of Leibniz Institute for Materials Engineering IWT, Bremen University of Applied Sciences and the Research Association RWB Bremen building products for masonry structures were developed on the basis of AAC rubble from C&D wastes. Granulates from processed AAC rubble were introduced as aggregates in dry premixed masonry mortars, in masonry blocks and lightweight building blocks and elements to replace completely natural aggregates. These recycling products exhibit beneficial technical properties, at the same time large volumes of AAC wastes may be re‐used. On the basis of the achieved R&D‐results from laboratory experiments, trial batches of dry premixed mortar and masonry blocks were produced in the building materials industry on their available industrial equipment, minor adjustments in the mix composition were necessary. After a sufficient amount of dry premixed mortar and masonry blocks were produced, the recycling products were used to erect indoor masonry walls in a building project in Bremen.     

On the in-plane shear response of the high bond strength concrete masonry walls

Conventional unreinforced masonry walls subject to in-plane shear loading fail due to exceedance of shear and tensile bond strengths. This paper examines whether or not the in-plane shear capacity of masonry walls would increase with the increase in the bond strengths through experimental and numerical investigations. For these investigations, shear walls were built with high bond strength polymer cement mortar; they were applied in thin layers of 2 mm thickness each. Material tests were carried out to characterise the bond and the compressive strengths of the high bond strength thin layer mortared masonry; the bond strengths were found approximately double that of the conventional 10 mm thick cement mortars. The shear walls, however, exhibited significantly lower capacity (contrasting the expectation) and displayed base course sliding mode of failure. To ascertain the validity of the experimental results, a combined surface contact—interface element micro finite element (FE) modelling technique was formulated; the results adequately reproduced the experimental datasets. The validated FE model was then applied to examine the effect of the aspect ratios and pre-compression levels to the failure modes, deformation and strength of the high bond strength shear walls and is shown that once the pre-compression exceeds 15% of the masonry compressive strength, the base sliding failure mode changes to the diagonal cracking mode with corresponding increase in in-plane shear capacity. Therefore, it is concluded that the increase the bond strength without regard to pre-compression could adversely affect the safety of the high bond strength unreinforced masonry shear walls.     

Mechanical behavior of masonry assemblages manufactured with recycled-aggregate mortars

Mortars containing recycled aggregate, instead of quartz sand, were characterized to find an alternative application for the fine recycled-aggregate fraction coming from building debris processing. Tests on bond strength of mortar to masonry units were carried out, as well as tests on compressive and shear strengths of masonry assemblages. The results obtained were related to the mechanical properties of mortars and brick. On the basis of the characterization results and performance evaluations, recycled-aggregate mortar appears to be superior to ordinary mortars in terms of mortar–brick bond strength and shear strength of masonry assemblages. This improved performance is of particular interest for the masonry structures in zones of seismic activity. In addition, the use of fine recycled aggregate is in accordance with the sustainable development concept, where recycling of building rubble plays a key role in ending the building life cycle.     

再生粉体对干混砌筑砂浆性能的影响

掺加一定量的再生粉体对细骨料分别为机制砂和再生砂的干混砌筑砂浆进行改性,研究不同掺量的再生粉体对干混砌筑砂浆基本性能、力学性能及抗冻性能的影响。结果表明:随着再生粉体掺量的增加,砂浆的各项性能均有所降低,且对2种不同细骨料的砂浆影响基本一致;掺加再生粉体的质量分数为30%的砂浆抗压强度相比未掺加再生粉体的抗压强度最大降低幅度为44.2%;再生粉体对干混砌筑砂浆抗冻性能的影响较为明显,且最大掺量不宜超过10%,细骨料为机制砂的砂浆可采用1∶3和1∶4的胶砂质量比,而细骨料为再生砂的砂浆宜选用1∶3的胶砂质量比。     

Seismic strength of adobe masonry

The most important soil characteristies from the viewpoint of the strength of adobe masonry are first studied. Subsequently, based on the acquired knowledge, the effect of some natural additives to the soil is investigated. Simple field tests, devised to identify the most adequate materials for adobe construction and to be easily transmitted to the potential adobe builder are finally proposed.     

Properties of some cement stabilised compressed earth blocks and mortars

Findings from an on-going investigation into the effects of soil properties and cement content on physical characteristics of compressed earth blocks and soil mortars are presented. A series of test blocks were fabricated using a range of composite soils, stabilised with 5% and 10% cement, and compacted with a manual press. Results for saturated compressive strength, drying shrinkage, wetting/drying durability, and water absorption testing are presented in the paper. In conjunction with the block tests, workability and compressive strength characteristics of suitable soil: cement and cement: lime: sand mortars were also studied. Mortar consistency was assessed using cone penetrometer and slump tests. Water retention properties of the mortars were also measured. For a given compactive effort, the strength, drying shrinkage, and durability characteristics of the compressed earth blocks improved with increasing cement and reducing clay content. Slump testing proved the most reliable means of assessing soil: cement mortar consistency. Both the flow table and cone penetrometer tests were found to be unsuitable. Water retention properties of soil: cement mortars appear well-suited to typical unit water absorption characteristics. Mortar strengths were closely related to cement and clay contents, but as expected were less than the average unit strengths.     

Prediction of solid block masonry prism compressive strength using FE model

Masonry strength is dependent upon characteristics of the masonry unit, the mortar and the bond between them. Empirical formulae as well as analytical and finite element (FE) models have been developed to predict structural behaviour of masonry. This paper is focused on developing a three dimensional non-linear FE model based on micro-modelling approach to predict masonry prism compressive strength and crack pattern. The proposed FE model uses multi-linear stress–strain relationships to model the non-linear behaviour of solid masonry unit and the mortar. Willam–Warnke’s five parameter failure theory developed for modelling the tri-axial behaviour of concrete has been adopted to model the failure of masonry materials. The post failure regime has been modelled by applying orthotropic constitutive equations based on the smeared crack approach. Compressive strength of the masonry prism predicted by the proposed FE model has been compared with experimental values as well as the values predicted by other failure theories and Eurocode formula. The crack pattern predicted by the FE model shows vertical splitting cracks in the prism. The FE model predicts the ultimate failure compressive stress close to 85% of the mean experimental compressive strength value.     

Durability of lightweight masonry mortars made with white recycled polyurethane foam

Masonry mortars made with Portland cement, sand, water and white recycled polyurethane foam from industrial waste are examined in this study. Different mixtures were firstly prepared through the substitution of different amounts of sand by equivalent volumes of polyurethane and then, with different ratios of cement/aggregates. The comparative study was carried out on the effect that different ageing tests have on the mechanical properties of these mortars under flexion and compression. For this purpose, the samples were exposed to different corrosion and hardness tests: resistance to dry heat, hot water, salt spray test and Kesternich testing. After ageing, a small reduction in compressive strength was observed. However, in all the samples, the strength values were sufficiently high to consider that these types of recycled materials remain practically unaffected when compared with the reference specimens. Finally, alkali-silica reaction tests were performed to determine the chemical stability of these mortars.     

Flexural strength and cracking behavior of hybrid strength concrete beams

The flexural and cracking behavior of hybrid strength concrete beams cast with two concrete compressive strengths of 20 and 70 MPa were compared with 20 MPa normal and 70 MPa high strength beams. The hybrid beams showed an improvement in the load carrying capacity at cracking, yielding and ultimate loading as compared to normal strength beams. The increase in load carrying capacity was (1.80–70.8%) higher than normal strength beams and only (3.3–9.8%) lower than corresponding high compressive strength beams. Also from experimental results the crack spacing of hybrid beams were between those of normal strength and high strength beams, but the crack width in the hybrid beams were narrower than both types of beams at all loading stages. At service and ultimate loading stages, the crack width in the hybrid beams were 19.5–26.0% narrower than those of corresponding normal strength beams, and 9.2–15.1% narrower than high strength beams.     

Durability of lightweight masonry mortars made with white recycled polyurethane foam

Masonry mortars made with Portland cement, sand, water and white recycled polyurethane foam from industrial waste are examined in this study. Different mixtures were firstly prepared through the substitution of different amounts of sand by equivalent volumes of polyurethane and then, with different ratios of cement/aggregates. The comparative study was carried out on the effect that different ageing tests have on the mechanical properties of these mortars under flexion and compression. For this purpose, the samples were exposed to different corrosion and hardness tests: resistance to dry heat, hot water, salt spray test and Kesternich testing. After ageing, a small reduction in compressive strength was observed. However, in all the samples, the strength values were sufficiently high to consider that these types of recycled materials remain practically unaffected when compared with the reference specimens. Finally, alkali-silica reaction tests were performed to determine the chemical stability of these mortars.     

Hygrothermal durability of bond in FRP-strengthened masonry

Fiber reinforced polymers (FRPs) are accepted as an efficient material for external strengthening of masonry structures. Previous researches have shown that the bond between FRP and the substrate plays an important role in the effectiveness of this strengthening technique. Extensive investigations have been devoted to the characterization of the short-term bond behavior, while its durability and long-term performance requires further studies. In this regard, a full experimental program for investigating the environmental durability of bond in FRP-strengthened masonry is crucial for understanding the degrading mechanisms. This paper presents the results of an experimental program aimed at investigating the hygrothermal durability of bond in FRP-strengthened bricks. Accelerated ageing tests were performed on the FRP-strengthened brick elements and the bond degradation was periodically investigated by visual inspection and by conventional single-lap shear bond tests. The changes in the properties of material constituents have also been monitored. The obtained results are presented and critically discussed.     

Properties of lime stabilised steam-cured blocks for masonry

The paper addresses certain issues pertaining to the technology of lime-stabilised steam-cured blocks used for masonry construction. Properties of lime-stabilised steam-cured blocks using expansive soils and tank bed soils have been examined. Influence of parameters like steam curing period, lime content and fly ash content on wet strength of blocks is studied. Steam curing of lime stabilised blocks at 80°C for about 20 hours at atmospheric pressure leads to considerably higher strengths when compared with curing under wet cloth at ambient temperatures. Clay-fly ash fractions of the mix control the optimum lime content yielding maximum strength. Long-term strength behaviour of steam-cured blocks has been monitored. The results indicate a favourable lime-clay ratio for stable long-term strength. A small-scale steam cured block production system has been designed and implemented to construct a load bearing masonry structure, thus demonstrating the potential of steam-cured block as a material for masonry construction.     

Flexural strength analysis of brittle materials

The relation between flaw density and strength of ceramic or brittle materials were derived and applied to a commonly used testing method of four-point bending for determining the strengths of ceramic or brittle materials. Previous analysis failed to include the failures outside or at the inner loading positions for four-point test data. The present approach elevates such a constrain so that the relation applies to failures occurs at any points between the outer loading positions for four-point test data.     

Flexural strength and fracture toughness of sea ice

A series of mid-winter experiments were carried out on the ice in the rubble field around Tarsiut Island in the Beaufort Sea. The tests included grain structure determinations, salinity and density of the ice, small beam flexural strength and fracture toughness. Typical values for flexural strength and fracture toughness were 0.6–1.0 MPa and 100–140 kPa m^{$\text{1}\text{2}$} respectively. Both properties were dependent on brine volume and depth in the ice sheet. In comparing these results with identical tests on finegrained freshwater ice it was found that for comparable loading conditions, the strength of the sea ice was significantly lower than the strength of the freshwater ice, whereas the fracture toughness of the sea ice was higher than the fracture toughness of the freshwater ice.     

Rilem TC 203-RHM: Repair mortars for historic masonry. Testing of hardened mortars,a process of questioning and interpreting

This paper presents an approach to the use and interpretation of tests on mortar samples when restoring historic masonry. It is largely based on the work performed by the former RILEM technical committee 167-COM, Characterisation of old mortars, closed in 2003, and the ongoing committee 203-RHM, Repair mortars for historic masonry. The focus of the present paper is on the decision process: what to test and how to interpret the test results.     

Flexural strength of steel fibre-reinforced cement composites

The paper considers two existing theories for the flexural strength of steel fibre-reinforced cement composites, which are based on the post-cracking strength concept and on the rule of mixtures, respectively. It is shown that the former theory has serious limitations because of the omission of the matrix strength contribution to the ultimate composite strength. The rule of mixtures is strictly valid for composites under a direct tensile state of stress and its extension to the flexural state of stress has not been theoretically verified. An expression for the flexural strength of steel fibre-reinforced cement composites has been derived, based on an assumed stress block and fundamental principles of flexural mechanics. The derived expression for flexural strength is shown to be valid to the experimental results of this investigation and to the flexural strength data available from previous research. In the experimental part of this investigation, theV _f(l/d) ratio of fibres in a wide range of cement matrices was kept constant. Variations in composite strength were achieved by different mix proportions of the matrix and by long-term curing both under marine exposure and under laboratory curing. In the experimental results from previous research, however, changes in composite strength were caused by differentV _f(l/d) ratios of fibres.     

Increased strength and related mechanisms for mortars at cryogenic temperatures

Properties of cement-based materials at cryogenic temperatures are quite different from those at room temperatures. The strength of mortars at cryogenic temperatures was experimentally studied and an empirical model was established. The freezing thermodynamic process of pore water and pore size distribution in mortars were characterized by differential scanning calorimeter (DSC) and thermoporometry (TPM), respectively. The relationship between the increased cryogenic strength and pore ice formation was discussed. The results showed that flexural strength of mortars increased at a higher rate than compressive strength. Water content and initial strength at room temperatures were the main factors influencing the cryogenic strength. Higher water content and higher initial strength resulted in higher cryogenic strength. Ice formation in pores is one of the main reasons for the mortar’s cryogenic strength increase. Nearly half of the water remained unfrozen in pores with radius less than 40 nm at −40 °C. Both ice formed in capillary pores and gel pores contributes to the strength increase observed at cryogenic temperatures.