Welcome to the October 2018 issue of Construction Materials. This issue covers a wide range of subject matter taking in the use of waste glass in asphalt mixes, detecting alkali silica reactivity in concrete and the use of lime stabilised bricks. As a microbiologist, I often wonder how I managed to end up in the construction industry. Imagine my delight when I discovered that the first paper in this issue had a microbiological theme in it.
Sahoo et al. (2018) investigated the potential improvements in the mechanical properties of mortar and concrete using ureolytic bacteria. Bacteria are incredibly diverse and there are many species that are capable of precipitating mineral carbonates in different environments including geological formations, soil, the marine and freshwater environments. Ureolytic bacteria are capable of calcium carbonate production as a by product of their metabolic processes such as photosynthesis, sulphate reduction and urea hydrolysis.
The construction industry uses cement and its production is a significant contributor to carbon dioxide emissions. The use of ureolytic bacteria can improve the properties of cement based products and result in less cement consumption. Sahoo et al. (2018) concluded that the species Bacillus sphaericus had the ability to precipitate minerals in cement mortar and concrete. Mechanical testing indicated that after 28 days substantial improvements in strength could be affected by the inclusion of B.sphaericus into cement mortar and concrete mixes. Bacteria improved the properties of concrete by precipitating minerals into the pores of the concrete. Microstructural analysis confirmed that the precipitation of calcite into the pores improved compressive strength and durability.
The second paper in this issue follows the theme of recycling. Sustainability is a key issue in the construction industry and the need to consume less natural resources has certainly captured the zeitgeist. Anochie-Boatang and Georges's (2018) paper focuses on the potential of using waste crushed glass as a filler material for asphalt. The study aimed to provide evidence to justify the reduction in the consumption of natural aggregates in dense-graded hot-mix asphalt. Over 900 000 tonnes of waste glass is produced in South Africa per annum. Only 300 000 tonnes is recycled and, therefore, there is an opportunity to divert 600 000 tonnes per annum from landfill to use in asphalt production. The authors concluded that there is a significant opportunity for the re-use of waste glass in asphalt production, taking pressure off the consumption of natural aggregates and the associated carbon dioxide emissions used in the extraction of these materials. Furthermore, mechanical testing indicated that asphalt containing recycled glass could have the potential to outperform conventional dense graded mix asphalt in terms of permanent deformation in service. This suggests that asphalt mixes containing recycled glass may also deliver longer service life. However the authors also identified the need for further research with a broad range of binder types, aggregate sources and varying concentrations of recycled glass.
The third paper in this issue has been prepared by Olutoge et al. (2018). The authors evaluated and compared the compressive strength of cement- and lime-stabilised lateritic bricks and blocks in order to determine what brick type provided the best overall performance including economic factors. Blocks and bricks can be essential elements in construction and have a great influence on the costs and economic viability of a construction project. The authors identify that there is shortage of conventional sandcrete blocks in Sub-Saharan Africa, in particular Nigeria. This has an adverse effect on construction costs. The shortage of sandcrete blocks and escalating prices has encouraged the search for alternative materials. The authors have focussed on the use of lateritic soils in brick and block manufacture. Laterite is a soil layer, rich in iron oxide and is abundant, readily accessible and may be obtained at comparatively low cost. Laterite containing bricks and blocks were stabilised with cement or lime and then subjected to range of mechanical tests to determine compressive strength and water absorption properties. The authors concluded that laterite soils can be used in the production of low cost cement-laterite blocks. Blocks stabilised with cement had superior mechanical properties than those stabilised with lime. Mechanical properties were related to the cement content and the authors identified that a high cement content may be cost prohibitive.
The key advantage of using manufacturing laterite bricks and blocks is that it is considerably less expensive than manufacturing conventional sandcrete blocks. The authors also advise that laterite bricks and blocks stabilised with cement are suitable for the construction of low rise buildings. However, laterite bricks and blocks stabilised with lime are only suitable for non-load bearing walls.
The final paper in this issue, submitted by Munir et al. (2018) describes the role of testing to detect the alkali-silica reactivity of concrete aggregates. The risk of alkali-silica reaction (ASR) of concrete aggregates in many parts of the world remains largely unexplored. ASR is one of the primary mechanisms of concrete damage; is a slow process and may take several years before it becomes apparent. It can cause significant expansion and occurs within the concrete mass, as opposed to its surface, and this can eventually lead to the development of significant cracking. ASR is usually confirmed by undertaking visual inspections of structures.
ASR has been studied for almost 80 years and has been reported in over 60 countries. It has caused damage in Norway's Elgeseter Bridge and affected more than 20 bridges in the Netherlands. The effects of ASR on the mechanical properties of concrete have been widely explored and it can significantly affect the modulus of elasticity and tensile strength of concrete. ASR can be detected and monitored using field techniques. Inspections typically include visual inspection with particular focus on indications of distress, such as movements and displacements and map cracking.
In this paper the authors explore the characterisation of ASR in aggregates with known slow reactivity in order to define an effective test method to quantify ASR risk. The authors review the various test methods used to detect ASR. They characterise the ASR risk in aggregates with slow reactivity in order to define a suitable test method to quantify such a risk. The authors describe the mineralogical compositions of aggregates from five different quarries Mortar bar expansion for these aggregates was tested using the guidelines of both ASTM C 227 (ASTM, 2010) and ASTM C 1260 (ASTM, 2014). The aggregates proved non-reactive under ASTM C 227 (ASTM, 2010) test conditions and ASTM C 260 identified one group of aggregates as potentially reactive. The authors conclude that aggregates with potential alkali–silica damage may be characterised as non-reactive if the appropriate test method is not adopted. They conclude that the ASTM C 1260 (ASTM, 2014) procedure is more effective in determining the reactivity potential of marginally to moderately reactive aggregates. The authors also indentify the requirement for further substantiation of their findings using the ASTM C 1293 (ASTM, 2015) concrete prism test.
