Browsing by Author "Alexander, Mark"

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    Open Access
    A Practical Carbonation Model for Service Life Design of Reinforced Concrete Structures
    (2021) Harold, Romuald Fotso Lele; Beushausen, Hans-Dieter; Alexander, Mark
    The increase in atmospheric carbon dioxide concentration due to global warming has a direct impact on the amount of carbonating concrete structures. For the past years, numerous studies have been done in South Africa on the subject and models developed to predict carbonation in concrete structures. Despite the large amount of resources and research effort put into developing these models, the translation from theory to practice represents a great challenge for design engineers in the field of durability design. This study presents a design tool based on existing models for use in practical applications. The proposed design tool assists in computing the service life of carbonating concrete structures and provides reliability values associated with the service life. It accounts for different binder compositions and binder types, as well as different locations and environmental land uses in South Africa. The validation of the design tool was done by comparing the service life prediction results to existing models, which generally showed good agreement. The developed design tool can be applied for predicting the long-term performance of new RC structures as well as improving the basis for quality assessment of existing, newly built RC structures. For the design of new structures, the designer is required to make certain assumptions concerning the information to be used for the simulation. These include values for the binder type, binder content, OPI, cover depth, land use and exposure parameters. For the quality control of new structures, the way in which the model parameters are obtained differs from that of new structures. As the structure already exists, both the concrete quality, cover depth and environmental loading can be measured directly on the structure with appropriate testing procedures. The outcome of applying the design tool for the analysis of concrete produced for the Gauteng Freeway Improvement project (GFIP) is also presented, with a case study of precast and in-situ structures chosen for the analysis.
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    A review on the efficiency of different supplementary cementitious materials as a partial replacement for Portland cement in concrete
    (2022) Taiwo, Ridwan A; Alexander, Mark; Leo, Emmanuel
    The effects of global warming and climate change are important and have attracted the attention of many researchers. Global warming is a result of the presence of increasing amounts of greenhouse gases in the atmosphere. Carbon dioxide, which is largely emitted into the atmosphere during the manufacture of cement clinker, is one of the greenhouse gases. Hence, researchers have explored the use of some waste materials and naturally occurring minerals as a partial replacement for cement in concrete. These materials are often referred to as supplementary cementitious materials (SCMs). Apart from the potential benefits of these SCMs for the properties of concrete, they also bring about a reduction in the amount of waste in landfill sites, as these wastes can cause land, water, and air pollution, thereby posing threats to human health. However, despite the potential benefits of SCMs in the cement and construction industry, they have not been fully utilized especially in developing countries in Africa. This may be due to low awareness of the potential benefits of SCMs among the stakeholders in the construction industry, and also limited availability. Nevertheless, due to extensive research into the usability of different materials as SCM, various materials are available in the construction market as binder systems. Thus, selecting the appropriate binder system to get the desired result for a particular concrete might be difficult for construction personnel. Hence, this study presents a review of the effects of various SCMs on the mechanical and durability properties of concrete. Six SCMs are reviewed. These SCMs include fly ash, silica fume, which are industry by-products; metakaolin, limestone calcined clay, which are naturally occurring minerals; rice husk ash, which is an agricultural waste material; and limestone-fly ash, which is a combination of an industrial by-product and a naturally occurring material. Firstly, an overview of the mechanical and durability properties of concrete is presented. This includes the presentation of general factors affecting the mechanical and durability properties of concrete. Subsequently, the effect of the various SCMs on mechanical (such as strength, elastic modulus, creep, and shrinkage) and durability properties (freeze-thaw, acid attack, sulphate attack, chloride-induced corrosion, carbonation-induced corrosion, and alkali-silica reaction) of concrete are presented. The review shows that the inclusion of appropriate dosage of these SCMs in concrete or mortar enhances their properties. Certain limitations of these SCMs are also discussed. This study also identifies areas of further research in relation to the properties of concrete produced with the SCMs.
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    A step towards standardising accelerated corrosion tests on laboratory reinforced concrete specimens
    (2012) Malumbela, Goitseone; Moyo, Pilate; Alexander, Mark
    Natural steel corrosion of reinforced concrete (RC) structures is a slow process which researchers find necessary to accelerate in laboratory tests to obtain needed damage in a short time. Regrettably, there is no standard procedure for accelerating steel corrosion in RC specimens. Researchers therefore continue to use various techniques to accelerate it. Unfortunately, structural damage and rate of steel corrosion are dependent on the accelerated corrosion technique used. Despite that, results obtained by researchers are applied by structural engineers and asset managers to in-service structures. This paper reviews previous techniques used to accelerate steel corrosion. Where possible it proposes standard procedures to accelerate steel corrosion. In other instances it points out needed further research. One of the procedures recommended in the paper is to contaminate selected faces of RC specimens with chlorides, as opposed to immersing samples in NaCl solution or adding chlorides to concrete mixes. It is also recommended to allow specimens to sufficiently dry during steel corrosion so as to promote steel corrosion.
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    Open Access
    Alkali-Aggregate reaction in Western Cape concrete
    (2018) Mahomed, Zubair Lall; Alexander, Mark
    Alkali-aggregate reaction, AAR, was first discovered in 1938 by Stanton in the USA. Subsequently, researchers across the globe have reported incidences of the reaction with different aggregates in their respective countries. The reaction entails the interaction between reactive silica found in aggregates and alkali in the pore solution of concrete. Through research, the reaction has been categorised into three main classes depending on the type of aggregate used. Alkali-silica reaction, ASR, being one of those classes, is the most common one and is the primary concern in the local concrete industry in the Western Cape, where reactive greywacke aggregates are used. In South Africa, the problem has often been dealt with using low alkali cement. However, those low alkali resources have been depleted and more alkali-rich resources are now being used in the production of cement. This completes the three requirements needed for ASR reaction to occur, namely a high alkali source, presence of reactive silica and moisture conditions. Furthermore, the introduction of greywacke crusher sand as a partial substitute to natural sands in local concrete mixes, implies that more reactive silica is available in the mixes. The research aims at finding whether the current concrete mixes are prone to alkali silica reaction and how to mitigate this expansion using cement extenders, which is the most common ASR mitigation measure. The long-term performance test, which allows testing of concrete, generally takes a minimum of 6-12 months to complete. As such, attention was turned towards the use of an accelerated mortar bar test (AMBT), which is generally used as an indicator test in the preliminary stages of the testing. However, the AMBT test imposes material limitations such as cement type and aggregate grading. Consequently, modifications were made to the AMBT test to allow for the concurrent use of reactive fine aggregates and coarse aggregate as well as a commercial cement. The first stage of this project involved the use of a modified AAR-2 AMBT test and was subdivided into three phases. Phase A was centred around investigating the use of reactive fine aggregates and reactive coarse aggregates in conjunction. For this purpose, 40% of the total aggregate blend by mass was constituted of a sand blend having both reactive (greywacke) and non-reactive (Philippi dune sand) components, while the remaining aggregate portion was a 9.5 mm greywacke coarse aggregate. The reactive fine aggregate level was varied in the sand blend and the ASR expansion recorded. A limited pessimum effect was observed at around 40-60% reactive greywacke by mass in the sand blend, whereby the expansion recorded peaked. Phase B of Stage 1 then involved the use of a 50/50 greywacke crusher sand/Philippi dune sand in the sand blend as a base mix. Cement extenders were then substituted in different levels for the cement. For this work, common replacement levels of 20, 30 and 40 percent fly ash and 40, 50 and 60 percent corex slag were used. It was found that all the mixes mitigated the ASR expansion to acceptable levels, that is below the 0.10% expansion, while increasing cement extender levels reduced the expansion further. It was also found that fly ash was more effective at reducing ASR than corex slag. Phase C of Stage 1 involved identifying the mechanisms behind which cement extenders mitigate ASR. Subsequently, the mixes used in Phase B were replicated with the exception that an inert limestone filler, “Kulubrite 10”, was used instead of the reactive cement extenders. It was observed that the limestone filler does reduce the expansion but to a much lesser extent than the reactive cement extenders. This implied that the cement extenders not only dilute the alkali content but also undergo further reaction which removes more alkali from the pore solution. The second stage of the project dealt with the influence of ASR gel formation on compressive strength. Compressive strength tests were performed on 2 sets of cubes for each mix, which were exposed to different curing conditions, namely a water bath at 22-25 ̊C and an alkaline solution of 1M NaOH at 80 ̊C. It was observed that there is reduction in strength as the expansion increases. Scanning electron microscopy, SEM, performed in Stage 3, of the samples confirmed that this phenomenon is due to the increased number of cracks as the expansion increases. Other subsidiary tests conducted in Stage 3, such as light microscopy and EDS, resulted in inconclusive results and need to be further investigated. Lastly, Stage 3 involved conducting long-term testing using a modified version of the AAR-4 test and field performance test. Five ‘real-life’ concrete mixes, based on the mixes in Stage 1, were cast and are still under observation. The initial measurements on the AAR-4 samples showed no sign of expansion as of 15 weeks of testing. This was thought to be due to the un-boosted alkali content of the cement, 0.7 % Na2O eq, which may have not been enough to start the reaction. The preliminary results of the field testing at 15 weeks of age showed that apparent shrinkage was occurring, likely due to the environmental influences over this period (summer months). This could be attributed to the fact that the ASR gel formation mechanism is still in its early stages in those specimens or has not started yet. The final results of these tests, at 6 months and 2 years respectively, are however needed to confirm whether the modifications made in Stage 1 of this research resulted in a good approximation of what is to be expected from the use of reactive greywacke fine and coarse aggregates in conjunction. In general, it can be concluded that the concurrent use of reactive greywacke crusher sand and reactive greywacke coarse aggregate in concrete mixes, would not be deleterious to structures. Nevertheless, it is advised that a minimum of 20% fly ash or 40% ground granulated corex slag by mass of the total binder content is used, as per the current conventional precautions.
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    Assessment of alkali aggregate reaction avoidance measures and alkali aggregate reaction tests worldwide
    (2021) Mwatile, Martha Ndinelao; Alexander, Mark
    Alkali Aggregate Reaction (AAR) is a deterioration mechanism which affects concrete structures all over the world. Different parts of the world employ various mitigation and control measures for AAR damage. Different tests are also performed worldwide to assess AAR. With the variety of AAR avoidance measures and AAR tests performed worldwide, it is necessary to have a thorough compilation and critical assessment of these AAR avoidance measures and AAR tests, which may be of assistance to engineers and other professionals who are involved in structural and material design of concrete structures or in the construction, quality control and condition monitoring and assessment of concrete structures. This dissertation aims to outline the types of AAR and the mechanisms associated with them, and to highlight case studies of AAR incidences around the world. This dissertation further aims to provide a comprehensive compilation and analysis of various AAR avoidance measures as well as AAR tests that are performed worldwide. Commonalities and differences will be highlighted between the different case studies, and critical analyses will be done on the AAR avoidance measures and AAR tests that will be discussed. There are three main types of AAR, distinguishable by the aggregate source. These are: AlkaliSilica Reaction (ASR), Alkali-Silicate Reaction and Alkali-Carbonate Rock Reaction (ACR). Since AAR is a type of internal chemical damage to concrete, it can be avoided by engineering design and by carefully selecting the concrete construction materials. In order for damaging AAR to occur in concrete, the following conditions need to be met: • Reactive silica in the aggregates should be present • Alkali, which is primarily from Portland cement, should be of a sufficient concentration • There should be sufficient moisture in the concrete • Portlandite should be in a sufficient concentration (this is specifically for ACR) To prevent the occurrence of AAR in concrete, one or more of the conditions above should be eliminated, except for the case of ASR in which one or more of the first three conditions should be eliminated. Since this dissertation mainly focuses on ASR, only the first three conditions will be considered as these are the only conditions for the occurrence of ASR. Various testing methods are employed all over the world to assess AAR. These tests include tests performed to assess whether certain aggregates are susceptible to AAR; tests to assess the performance of specific concrete mixes and thus determine if they are susceptible to AAR, and also tests performed to assess the occurrence and extent of AAR in existing concrete structures.
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    Bamboo construction as a sustainable building technology from a structural and materials engineering perspective
    (2021) Ross, Sheila; Alexander, Mark
    The objective of this dissertation is to determine whether bamboo culms or stems are suitable for use as a construction material for permanent structures, from an engineering as well as a sustainability perspective. A secondary objective is to establish whether this would be a suitable building technology for South Africa. The research is desk-top in nature and involves reviewing articles in online journals and publications and collating the information into a coherent form. Eleven species of bamboo commonly used in construction worldwide are selected for investigation of their material and engineering properties. Based on the variations found in the various species of bamboo, approaches to engineering modelling and design of bamboo structures are detailed and discussed. Furthermore, four case studies are presented that illustrate the various areas where bamboo construction is currently used. Finally, research is done regarding the level of the South African bamboo industry, including whether a bamboo species currently exists in South Africa that would be suitable for construction purposes. The preliminary literature review indicated that there is a lack of scientific or structural information regarding bamboo as a building material from an engineering or materials perspective, both globally as well as in South Africa. Although a substantial amount of information was subsequently found on the various aspects of bamboo as a structural material, the information varies widely between sources, which is ascribed to variations in test methods as well as to the location of the species being tested. The design codes and standards from various countries for bamboo design and construction are briefly reviewed. For countries where there is currently no bamboo design code or standard, such as South Africa, it is recommended that the International Standard, ISO 22156, be used as a design basis, using specific material properties relevant to the local species. Many publications state that bamboo is a sustainable building material, comparing favourably with other building materials. However, quantitative proof was found to be either lacking or unclear. Different methods were used in the publications to evaluate bamboo from an environmental perspective, making it difficult to compare and evaluate the different reports and results. However, despite the different methods, it appears that bamboo can be regarded as an environmentally favourable material, provided that local species are used in construction. The four case studies presented demonstrate that bamboo culms are suitable for use in large structures, such as bridges or trusses, as well as for smaller structures such as buildings or houses. They also illustrate the issues that can occur if the design intent is not understood or correctly carried out during construction. South Africa has one bamboo species considered suitable for construction, namely Bambusa balcooa, which is grown in various parts of the country. The properties of the South African plants have not been established as yet. However, theoretical engineering and material properties as determined elsewhere in the world indicate that this is a viable construction material. Further areas of research are the establishment of the material and engineering properties of the local South African bamboo species Bambusa balcooa, as well as further research into the behaviour of bamboo in fire conditions.
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    Biogenic acid corrosion of sewer concretes with different binders: in-situ and model studies, with advancement of the life factor prediction method
    (2023) Bakera, Alice Titus; Alexander, Mark; Beushausen Hans-Dieter
    Biogenic Acid Corrosion (BAC) is the biodeterioration of concrete caused by biological and chemical activities of bacteria that grow in an acidic environment. Such environments are typical in sewer systems, which collect wastewater from households, industries, and urban and storm water runoff, and convey this to wastewater treatment plants. Since these sewer systems transport large volumes of sewage and are invariably buried under the ground, concrete pipes are customarily used. Concrete is widely known as a robust, flexible, and durable material in many aggressive environments, yet it can suffer from severe sewer corrosion. Numerous techniques have been employed to eliminate or control the problem, including applying chemical or biological agents that decrease acid production, and surface treatment techniques that inhibit chemical attack and acid penetrability into the concrete. However, these techniques are expensive and variably effective, and some may lead to a loss of structural integrity and performance during the long service life of more than 50 years, due to a lack of long-term performance testing before they are introduced into the market. The most reliable and affordable approach remains to modify concrete by incorporating readily available binders with high chemical and physic mechanical potential in resisting corrosion. Numerous studies have approached this technique using Portland Cement (PC) based systems with Supplementary Cementitious Materials (SCMs). However, other binder systems such as Calcium Aluminate Cement (CAC) and Calcium Sulpho-Aluminate cement (CSA) have been less studied, despite showing higher potential in suppressing biogenic corrosion. Furthermore, most biogenic prediction models cannot effectively predict the performance of these binders in a sewer environment, especially when they are incorporated with different aggregates. For instance, the Life Factor Method (LFM), which is a common and often preferred method in sewer concrete design, is not formulated to enable the prediction of the corrosion rate of concrete comprising CAC, or specific PCbased systems with SCMs such as slag, fly ash, and silica fume. The LFM model consists of two parts; the acid environment generation part, and the acid resistance part, also sometimes called the ‘material factor', which defines the potential of concrete to resist corrosion due to acid. However, the current material factor in the model involves only an alkalinity factor (total calcium oxide content in concrete) as a significant resistance provider, while not considering other chemical compositions that significantly contribute to providing concrete with acid resistance. A previous amendment of the model at the University of Cape Town (UCT) resulted in a 'material factor' approach that was highly empirical and complex, and therefore less practical and comprehensive in application. The model also could not handle modern binder systems in conjunction with aggregates, and the criteria for its refinement were somewhat self-contradictory. Therefore, this study aimed to fundamentally re-think and improve the LFM model to cover a broader range of binder systems and aggregate types with their performance in different sewer conditions. Firstly, the deterioration mechanisms of concretes with different binder systems and different types of aggregates were studied in different live sewer environments. Secondly, the mechanism of deterioration of the binder systems was further studied using a reactive transport modelling approach to understand the critical phases that govern the deterioration. Using the information from the first two goals, the LFM model was then improved and advanced to cover a wider range of binder systems, aggregate types, and sewer environments. The study characterised the behaviour of three sewer sites in the Cape Town Metro (i.e., Langa Pump station (LPS) manhole, Northern Area Sewer manhole 19 (NAS M19) and manhole 54 (NAS M54)), monitoring techniques (i.e., visual observations, mass and thickness change, and concrete surface pH), The study also evaluated the influence of different concrete mixes using concrete microstructural analysis (Scanning Electron Microscopy (SEM), Quantitative Evaluation of Minerals (QEMSCAN), and X-Ray Diffraction (XRD) analysis), and reactive transport modelling (i.e., HYTEC modelling tool). 2 The concrete mixes were grouped into two batches: LH concrete and UCT concrete mixes. The LH concrete mixes consisted of four binder systems, i.e., a blend of 80% Sulphate Resisting Portland Cement and 20% Fly Ash (SRPC+FA), a similar blend with 11% iron-based additive (SRPC+FA+HC), a blend of 80% SRPC and 20% ground Limestone (SRPC+LS), and CSA. These mixes were cast with calcite and siliceous aggregates by Lafarge Holcim (LH) in Lyon, France and delivered to the UCT laboratory for sewer exposure. Before exposure, it was observed that the mixes exhibited significant compaction voids, but their condition was such as to permit sewer exposure with the expectation of gathering useful information on these mixes. The UCT concrete mixes were prepared at UCT with local dolomite and siliceous aggregates, but using the same LH binders, i.e., SRPC+FA, SRPC+FA+HC, with some additional local binders, i.e. Portland cement blended with limestone (CEM II A-L), a blend of 50% CAC and 50% SRPC, CSA, and CAC. The LH concrete mixes were exposed to all three sites, while the UCT concrete mixes were exposed only to the LPS and NAS M19 after observing that NAS M54 was minimally aggressive. In terms of reactive transport modelling, only UCT concrete mixes were studied. Regarding sewer characterisation, the LPS manhole has the most aggressive environment, followed by NAS M19 and NAS M54. The aggressivity of the LPS was due to its high H2S gas concentration and sewer hydraulic actions as it receives wastewater from a pump station. NAS M19 is located in the midsection of the sewer line and collects a mixture of domestic and industrial wastewater. It experiences occasional flooding, mainly during the winter season. NAS M54 is an upstream manhole with maximum gas concentrations below 10 ppm. Therefore, concrete mixes exposed at the LPS exhibited more severe corrosion than at NAS M19, while at NAS M54, only minor signs of corrosion were observed after two years. Also, it was observed that the aggressivity of the sewers varied with the seasons, with higher gas production during hot periods. BAC monitoring and concrete microstructural analyses indicated that the Portland-based concretes (SRPC-based and CEM II A-L) experienced more severe deterioration compared to alumina-based concretes (CAC-based and CSA). CAC performed the best, followed by a blend of CAC+ SRPC concrete and then CSA, due to the formation of gibbsite and the high neutralisation potential provided by alumina bearing phases over calcium oxide-bearing phases such as calcium silicate hydrates and portlandite. In terms of Portland-based concretes, it was observed that blending SRPC concrete with fly ash improved the resistance potential over CEM II A-L, iron-based additives had little influence, and ground limestone in conjunction with calcite aggregates provided more acid-soluble material to neutralise the acid. In terms of aggregate performance, siliceous aggregates do not react with acid, and as a result, they eventually detach from the exposed surface. Dolomite and calcite aggregates dissolve in acid, and with magnesium carbonate, the rate of dolomite dissolution was slightly higher than calcite. The corrosion rate of the concrete depended on the relationship between the rate of deterioration of the cement matrix and the rate of deterioration of the aggregate matrix. Thus, concrete with similar cement and aggregate deterioration rates has a uniform corrosion front and a slower rate of corrosion, and vice versa. The LFM model was modified with the information from the BAC monitoring and concrete microstructural analysis. The revised model has two key parameters: the sewer environment factor and the material resistance factor; the latter includes the acid neutralisation factor of the binder system and the aggregate reactivity factor. The sewer environmental factor evaluates the rate of acid generated on the exposed concrete surface while considering various factors associated with H2S gas adsorption and oxidation. The material resistance factor, on the other hand, evaluates the quantity of acid to be neutralised by a specific volume of exposed concrete while considering the influence of binder and aggregate. Ultimately, a ratio of sewer environmental and material resistance factors can assist in predicting and providing the corrosion rate of any concrete mix with any binder and aggregate types when subjected to a sewer environment. The corrosion rates predicted by this model correlated well with field-measured corrosion rates both in this study and in previous sewer studies at UCT. Therefore, this study provides engineers with a relatively simple tool for predicting the corrosion rate of sewer concrete, with recommendations for selecting the most durable sewer concrete mix designs.
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    Concrete carbonation prediction for varying environmental exposure conditions
    (2020) Gopinath, Rakesh; Alexander, Mark; Beushausen, Hans
    The Durability Index (DI) approach has been developed in South Africa, in order to improve the durability performance of reinforced concrete structures. The DI approach is based on durability index tests, which are linked to transport mechanisms related to particular deterioration processes (Alexander et al., 1999a). Carbonation of concrete is governed, inter alia, by the microstructure and the transport characteristics of the concrete. A carbonation model with permeability coefficient (k) from the Oxygen Permeability Index (OPI) test as the key material variable was developed by Salvoldi (2010) using accelerated carbonation test data. The main aim of this research is to further develop the carbonation model by adopting the modelling framework of Salvoldi (2010) using natural carbonation data. For the experimental work, a total 48 different concrete mixes were produced by with different water: binder ratios (w/b), cement types, cement extender (addition) type and curing regime. The OPI test was conducted on all the concretes, and their corresponding permeability coefficients were determined. A set of 48 concrete specimens were exposed to five different sites for natural carbonation, and carbonation depths were measured periodically. Based on the modelling framework of Salvoldi (2010) and using the natural carbonation data between 150- 850 days, a model predicting the depth of natural carbonation was developed. However, in the case of concrete exposed to rain, drying/wetting is a major factor influencing the rate of carbonation. Therefore, the carbonation model was further modified taking into account the influence of drying/wetting cycles, by coupling it with a moisture model. For the development of the moisture model, the concrete specimens were exposed to a laboratory environment maintained at constant temperature and relative humidity (RH). The internal RH of the concrete specimens at varying depth was measured at different time intervals. Based on the measured RH data, the moisture model was also developed with ‘k' from the OPI test as the key input parameter. The moisture model was then coupled with the carbonation model developed. This provides an integrated and powerful solution for predicting carbonation of concrete both sheltered and exposed to rain by using only one main material input parameter ‘k', which is one of the major contributions of this research.
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    Concrete mixes for durable marine structures
    (2003) Alexander, Mark; MacKechnie, James
    The marine environment provides a severe test of the durability of reinforced concrete structures. Predictions of durability need to consider the complex interactions between environment, materials and structure that affect long-term performance of marine structures. An empirical chloride prediction model has been developed for chloride ingress into marine concretes. The prediction model was formulated from the relationship between early-age chloride conductivity test results and medium- and long-term observations of the performance of concrete in different marine environments. This paper presents some of the practical results of this approach in terms of design limits and mix design recommendations. Design guidance is given such that a matrix of factors may be optimised, including water/binder ratio, binder type, cover depth, environment, and construction practice, in order to produce durable marine concrete structures. Preferred concrete mixes for marine applications are also given, showing the advantage of concrete containing supplementary cementitious materials. However, good material selection and design are not sufficient to ensure durability, and recommendations are made for a system of performance specifications to ensure durable marine structures.
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    Development of low-clinker concrete: partial replacement of cement with calcined clay and limestone, based on selected African raw materials
    (2022) Leo, Emmanuel Safari; Alexander, Mark; Beushausen, Hans
    The most promising option for lowering the cost and environmental impact of cement is the use of blended cement. The well-known supplementary cementitious materials such as slag and fly ash are limited in most African countries. An attractive option is to produce LC3 binders, which consist of ground limestone, calcined kaolinite clay, and cement. In this study, the suitability of kaolinite clays for use in LC3 binders from selected deposits in South Africa and Tanzania was assessed. Four samples of clays were selected for the experimental work. The selection was based on the kaolinite content of the clay. Further, LC3 mixes with clinker content in the range of 40% to 70% were designed using the mixture design 3-factors approach. The compressive strength of mortar with a water/binder ratio of 0.4 was used as the performance parameter for optimising proportions. Two reference mixes were considered: a mix with 100% CEM II/A-L 52.5 N, and a recommended mix for South African marine environments with 50% cement replaced by ground-granulated blast-furnace slag. In general, the results suggest that, for optimum performance, regardless of the type of clay, the lowest practical clinker content is 55%, at which the amount of calcined clay is 35% and the amount of limestone is 10%. However, on the knowledge that high compressive strength does not automatically represent concrete with excellent durability, two other LC3 proportions were also considered for the concrete work: one with 65% clinker, 25% calcined clay and 10% limestone and the other with 45% clinker, 40% calcined clay and 15% limestone. The performance properties of the concrete mixes considered were workability, strength development, durability indexes, chloride diffusion characteristics, electrical resistivity, carbonation resistance, shrinkage, and potential for early age cracking. Overall, it can be concluded that the selected LC3 concrete mixes perform similarly or better than the reference mixes. The results indicate that, apart from kaolinite content, factors that influence the performance of the system include other minerals present in the clay, the filler effect, pozzolanic reaction, formation of carboaluminate phases, stabilisation of the ettringite phase, and the internal surface area of the clay.
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    Heat transfer through anaerobic digester concrete tank walls
    (2018) Davis, Owen S; Alexander, Mark
    This dissertation is a study of the heat transfer through concrete walls in anaerobic digester facilities. In the biogas industry, the term “heat loss” is synonymous with heat transfer. The dissertation identifies the reasons why heat is critical in the operations of these facilities. Concrete has traditionally been a material used for the retention of liquid-based products and additionally provides good thermal insulating properties. It combines the benefits of being relatively cost effective for the construction of large tanks and requiring low maintenance during the operational life span. The research focuses on the thermal properties of the various constituents of concrete and the influence these have on the overall thermal properties of the concrete tank. The constituents forming part of the study are cement, corex slag, water, fine and coarse aggregate. The study showed that the aggregates have a greater influence on the thermal conductivity than the other constituents. It also showed that the mineral composition of the aggregates has a greater effect on the thermal conductivity than the porosity of the aggregates. The study also looked at the influence of the interfacial transition zone around the aggregate and this was found to be not significant and generally can be ignored as a contributor to the thermal conductivity of normal / structural concrete. The effects of the porosity of the binder paste does affect the thermal conductivity specifically when aqueous solutions are being retained. The capillary pores of the paste can be filled with liquid (mainly water) and less with air. Due to water having a higher thermal conductivity than air, the thermal conductivity of the binder paste is significantly increased. The effects of reinforcement on the thermal conductivity of concrete was also investigated. Different types and arrangements of reinforcement could have a big influence. Steel fibres and reinforcement if aligned in the direction of the thermal gradient will greatly increase the thermal conductivity. However, it was found that the reinforcement used in the sample wall did not increase the thermal conductivity significantly as it was mainly aligned perpendicularly to the direction of the thermal gradient. Similarly, no steel fibres were used in the concrete. Once the thermal conductivities of the constituent materials were determined, the effective thermal conductivity of the concrete could be calculated using the effective medium theory. The subsequent heat losses, which are a function of the thermal conductivity, the temperature gradient between the internal and external faces of the concrete wall, the contact surface area and the heat transfer coefficient, could be calculated as a function of time. The New Horizons Waste to Energy Project in Cape Town was used as a reference project and the research was based on the materials used in the construction of the concrete anaerobic tanks. The project was also used for the measurement of the temperature gradients and subsequent calculation of actual heat losses at various points along the concrete walls. Furthermore, a computational model was developed using Abaqus to compare the results with those derived from the theoretical model. The heat loss from the computational model compares very well with that of the theoretical model.
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    Performance of geopolymer concrete subjected to mineral acid corrosion and related to microbially-induced corrosion (MIC) of concrete in sewers
    (2021) Dlamini, Mandla; Alexander, Mark
    worse than degradation at the crown of the sewer pipe. Furthermore, results from this study show that high resistance under the static acid corrosion exposure condition cannot be extended to mean high resistance under the erosion-corrosion exposure condition for some concrete mixes. In this study, the static HCl test and the dynamic HCl test were used to measure the resistance of concrete mixes under the static corrosion exposure condition and erosion-corrosion exposure condition respectively. However, concretes that exhibited high resistance to the erosion-corrosion exposure condition were consistent in exhibiting high resistance to the static corrosion exposure condition. This finding is consistent with the sequence of corrosion processes in MIC, wherein dissolution of the concrete components occurs before the precipitation of corrosion products. Therefore, it expected that high resistance in the dynamic acid test (i.e. resistance to dissolution) implies high resistance in the static test, which measures the combined resistance of dissolution and resistance emanating from corrosion products. Both static and dynamic acid corrosion tests revealed that the geopolymer concretes tested in this study outperformed PC and CAC concretes. Results from the static HCl test showed that GP-ferro-quartz concrete, the most durable concrete specimen, provided a 69-fold improvement in resistance when compared to PC-dolomite mixes (control #1) and a 4.72-fold improvement in resistance when compared to CAC-dolomite mixes (control #2). Results from the dynamic HCl test show that the GP-ferro-quartz mix provided a 180-fold increase in resistance when compared to the PC-dolomite mix and a 275-fold increase when compared to CAC-dolomite mix. The CACdolomite mix was found to have the lowest resistance to the erosive-corrosive exposure conditions of the dynamic HCl test. Thus, in terms of the concrete MIC resistance properties identified in this study, it is suggested that the CAC-dolomite mix had poor kinetic resistance to dissolution. However, under the static acid test (static corrosion exposure condition), the CAC-dolomite mix performed better than the PC-dolomite mix and GP-dolomite mix. CAC-dolomite concrete performed inferiorly only to the set of GP-siliceous-aggregate mixes in the static HCl test. The difference in the performance of CAC-dolomite concrete performance between the static and dynamic test is largely attributed to the formation of alumina gel, an acid corrosion product of CAC hardened paste, which envelopes the concrete specimen and reduces the rate of surface corrosion in the static HCl test. However, under v the dynamic HCl test, the gel layer is brushed off the surface of the concrete specimen rendering it ineffective in protecting the concrete specimen from corrosion. Previous research on the acid attack of concrete posits that the chemical make-up of concrete materials has a strong bearing on corrosion behaviour. To this end, various measures have been suggested such as the ratio of calcium to silicon (Ca/Si) in concrete. The approach utilised in this study was to calculate the “basicity value” which provides the ratio of major basic to acidic oxides found in the concrete. XRF analysis of the hardened cement pastes and the 5 aggregate types used in the experiments enabled the calculation of basicity values. The combined basicity value for concrete specimens was determined by proportionally summing (according to mass) the basicity values of the aggregate and hardened cement paste parts. A strongly correlated linear relationship between the basicity value of concrete and the corrosion rate from the dynamic HCl test was established. This empirical relationship warrants further investigation and verification, as it would, in principle provide a means to estimate the dissolution rate of concrete by calculating its basicity instead of undertaking laboratory acid tests. Basicity was also found to be useful in determining the corrosion compatibility of binder type and aggregate types. It was found that the difference between the basicity value of hardened cement paste and the basicity value of the aggregate was useful in determining the type and extent of preferential corrosion of a concrete specimen tested under the dynamic HCl test. For ease of reading, this difference was called the “basicity differential”. By visually assessing corroded concrete specimens from the dynamic HCl test, it is was possible to determine whether the hardened cement paste or aggregate component was preferentially corroded, and to gauge the extent of preferential corrosion visually. GP-ferro-quartz and GP-granite concretes had the lowest levels of preferential corrosion which corresponded to their low basicity differential values. In contrast, CAC-dolomite concrete had the highest basicity discrepancy which corresponded visually to a high preferential corrosion of the hardened cement paste. Mineralogical analysis via XRD, found that the hardened cement pastes of the three binder types consisted mainly of amorphous phases (>70%). The crystalline phase of the geopolymer hardened cement paste was mostly constituted by insoluble minerals such as mullite. This partially explains the higher corrosion resistance of geopolymer concretes. However, a more comprehensive explanation needs to include analysis of the amorphous phases, which fell outside the scope of this study. SEM analysis of HCl corroded geopolymer hardened cement paste found that fly ash spheres embedded within the geopolymer matrix were preferentially corroded. This indicates that fly ash content negatively affected the rate of corrosion of the geopolymer hardened cement paste. Furthermore, SEM analysis showed that the geopolymer matrix surrounding the fly ash spheres was relatively intact.
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    Powder packing optimisation for clinker reduction in concrete
    (2018) Holmes, Matthew; Alexander, Mark
    Globally, concrete is the most used construction material. Its embodied energy is relatively low, yet due to the vast quantities that are produced annually, it has substantial greenhouse gas (GHG) emissions associated with it. Of the concrete constituents, the manufacture of clinker - the basis of all conventional cements - contributes the most significant emissions. Therefore, to reduce the emissions associated with concrete manufacture, there has been extensive research into how clinker content can be reduced without compromising desired concrete properties. Existing methods for clinker reduction have, however, only allowed clinker replacement to a limited extent. This research investigated the more efficient use of clinker to minimise clinker content required to achieve desired mechanical and durability properties of concrete. The optimisation of powder (materials < 125 µm) packing, using filler materials with varying fineness, was identified to potentially increase clinker efficiency. The optimisation undertaken was the maximisation of powder packing density but without adversely affecting workability. The investigation entailed the application of analytical particle packing density models as well as experimental investigation. Two particle packing models, the Compaction Interaction Packing Model (CIPM) and the Modified Andreasen and Andersen Curve (MAAC) were applied. Various methods for determining the packing density of powder combinations were investigated which informed the use of the mixing energy test to provide experimental packing density data for the modelling procedures. The CIPM was used to optimise the powder phases of concrete as it incorporated the effect of surface forces on powder packing and the MAAC was used to complete the optimisation of fine and coarse aggregate materials. It was necessary to calibrate the CIPM through the selection of various model constants, based on the minimisation of the average error associated with predicted packing density. Despite the incorporation of surface force effects, the CIPM did not predict the trend in packing density observed for various experimental powder combinations with consistent accuracy. Combinations of cement with limestone of high and low fineness (relative to cement) were most accurately predicted but combinations with limestones of similar fineness to cement were less accurate. It was therefore apparent that the model inadequately accounted for the effects of varying particle size and the corresponding influence of surface forces on these particles. However, for practicality, model constants which minimised overall error were used to determine powder combinations enabling maximum packing density for use in optimised concrete mix design. Concrete mixes were designed in 2 phases. Initially water content was fixed, and limestone content was successively increased to 40 vol. % (Phase 1). Despite the formation of mixtures according to maximum packing density, the results showed that optimisation of packing density with a fixed water content was insufficient to reduce clinker content without adversely affecting compressive strength. However, workability was maintained without excessive superplasticiser (SP) dosage and oxygen permeability, water sorptivity and accelerated drying shrinkage were either improved or not adversely affected. This was attributed to the ability of fine fillers to prevent interconnectivity of the pore structure and the decreased volume of gel hydration products leading to reduced drying shrinkage. Compressive strength was tested for a binary (cement/limestone) and ternary (cement /limestone/fly ash (FA)) binder blend for Phase 2 in conjunction with a substantially reduced water content. Workability was adversely affected and both mixes required high SP doses, however, the FA blend required a relatively lower dose. Compressive strength was again decreased relative to the reference mix but when comparing Phase 1 and 2 mixes with predicted strength for equivalent w/c ratios, compressive strength was relatively unchanged, inferring little benefit of packing optimisation. However, binder efficiency indices (‘bi’) (between 5.3 and 6.9 kg/m3 /MPa) were reduced relative to data from previous investigations with similar strength class (between 10 to 20 kg/m3 /MPa), inferring increased binder performance. Powder packing optimisation thereby has the potential to enable clinker reduction, particularly for lower strength grade concrete, without adversely affecting compressive strength. Furthermore, the relatively unaffected durability indicators portray the beneficial effects of powder packing optimisation on increasing the impenetrability of concrete microstructure and it potential use in applications where durability is of importance. These findings also pointed to further possible reductions in the binder efficiency index below 5 kg/m3 /MPa if water content is further reduced (to maintain low water: cement ratio) and reactive SCMs are incorporated. However, further investigation and understanding of the fundamentals of powder packing is necessary to achieve a fully predictive process of low-clinker concrete mix design that can be universally applicable.
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    Open Access
    Practical application of the torrent permeability test method with the South African durability index approach
    (2025) Alfred, Sean; Alexander, Mark; Beushausen, Hans-Dieter
    Tests that assess the potential durability of concrete offer robust evaluation approaches for predicting its performance. The South African Durability Index Approach (SADIA) and the Torrent method are used in South Africa and Switzerland, respectively, to assess the potential concrete durability of the cover layer. Specifically, the Oxygen Permeability Index (OPI) test and the Torrent Permeability Test (TPT) are widely recognized as reliable techniques for evaluating the gas permeability of concrete. Studies have demonstrated the sensitivity of these methods to various factors that affect the properties of hardened concrete. Recent research has shown a strong correlation between the OPI and TPT methods in simulated environmental conditions. A combined approach using both methods has been suggested to improve existing practices for assessing concrete durability. However, the correlation has not been confirmed for site concrete elements, and practical guidelines have not yet been established. This research aimed to develop practical guidelines for an integrated durability assessment strategy involving the OPI and the TPT methods on in-situ concrete elements. Five mixes consisting of two water-to-cement (w/c) ratios (0.5 and 0.6), two concrete grades (30 MPa and 40 MPa), and three binder types (100% CEM I 52.5N, CEM II/A-L 52.5N, and 70/30 CEM I + fly ash) were used to manufacture precast freeway median barriers and representative test panels. The concrete elements' early-age (28 – 56 days) gas permeability characteristics were tested under various environmental exposure conditions (summer and winter) in Cape Town, South Africa. The OPI and TPT methods generally showed good sensitivity to the test variables (w/c ratio, binder type, age, and environmental exposure). There was a strong correlation between the two methods in the summer data but a weak association in the winter data. This difference was attributed to the pore-blocking effect of moisture on the Torrent test, emphasizing how surface moisture may affect the measured results even within permissible moisture levels. Nonetheless, the site correlation in the summer confirmed findings from previous studies. Given this study's findings, it was determined that a combined approach using the Torrent and OPI methods can be implemented based on site-specific conditions. The initial assessment of the structure can be conducted using the Torrent test and, if required, substantiated by the OPI test in moist conditions. It is recommended that moisture correction methods be used to adjust the measured Torrent results in very dry conditions using reference surface moisture values. However, to consolidate the proposed combined practical approach, the results of this research need to be validated through a more extensive study using larger sample sizes, a more comprehensive range of concretes, and varying environmental conditions.
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    Properties of Western Cape concrete with metakaolin
    (2018) Bakera, Alice Titus; Alexander, Mark
    More than four billion of tons of cement are produced annually for construction purposes in the world. This is associated with high production costs and environmental pollution, since cement manufacturing requires fuel to run the processes, and emits carbon dioxide (CO2) during the burning of raw materials. With these problems, the use of Supplementary Cementitious Materials (SCMs) is found to be an appropriate solution, as it contributes to the reduction of cement used in concrete by partially replacing cement with SCMs. However, a challenge arises as to the availability of SCMs. This is mainly because many SCMs such as slag, fly ash, and silica fume are industrial by-products. This means that any fluctuation in the production of the primary products (steel, electrical power, and silicon metal) directly affects their availability. Therefore, an alternative SCM such as metakaolin, which is obtained from calcination of kaolin clay, is a potential solution to SCM availability fluctuation, especially in the Western Cape construction industry which depends on the use of Corex slag (GGCS) from a source that can be variable. This raises the need to investigate the properties of concrete with metakaolin, in order to assess its potential for use as an alternative SCM in the Western Cape. The properties of concrete with a locally available metakaolin were studied with the following specific objectives; i) to characterize metakaolin in terms of its morphology and pozzolanic activity, ii) to assess the influence of metakaolin on fresh and hardened concrete properties, and to compare these with the properties of concrete with GGCS, iii) to assess the influence of metakaolin on the deformation behaviour of concrete, iv) to evaluate the durability properties of concrete containing metakaolin by assessing its potential to mitigate Alkali Silica Reaction (ASR), and to reduce concrete penetrability. The study followed two methodologies; a substantial and critical literature review of the use of metakaolin in concrete, and laboratory investigations on the performance of metakaolin in mortar and concrete. In the literature review, metakaolin was shown to enhance the properties of concrete. However, various contradictions were highlighted on the influence of metakaolin on certain properties of concrete, such as setting times and tensile splitting strength. Moreover, there were limited studies on the deformation behaviour (especially creep) of concrete with metakaolin, as well as the potential of metakaolin to mitigate ASR. Besides, the characterization of Western Cape concrete with metakaolin had not been extensively studied. Therefore, these gaps raised a need for experimental investigations. The experimental investigations involved six categories; i) Morphology of metakaolin using Scanning Electron Microscopy (SEM) analysis, ii) pozzolanic reactivity using three tests; strength activity index test, heat of hydration test (semi-adiabatic and isothermal calorimetry tests), and thermogravimetric analysis (TGA), iii) fresh properties of concrete assessed by setting time and workability, iv) hardened properties of concrete using compressive strength, tensile splitting strength, and porosity using Mercury Intrusion Porosimetry (MIP), v) deformation behaviour of concrete using static elastic modulus, and creep and shrinkage tests, and finally, vi) durability properties of concretes assessed by Durability Index (DI) tests, accelerated mortar bar test (ASR), accelerated carbonation test, and chloride bulk diffusion test. Special mortars were designed for pozzolanic activity and accelerated mortar bar tests, while three groups of concretes i.e. with 0.4, 0.5, and 0.6 water/binder (w/b) ratios with five different replacement rates of SCMs (0%, 10%, 15%, and 20% metakaolin, and 50% GGCS) were designed and cast for hardened properties and durability tests. Experimental results showed that metakaolin had a significant influence on Western Cape concrete properties. It was found that metakaolin had a high pozzolanic activity. Metakaolin in concrete was found to increase setting times while decreasing workability. Compressive and tensile splitting strengths were enhanced by metakaolin, with the highest strengths at 20% replacement level with an increase of approximately 47% and 41% in comparison with the controls, respectively. Metakaolin was found to have a greater influence on concrete with higher w/b ratio. Metakaolin was also found to alter the microstructure of concrete by refining the pores and minimizing their connectivity. The deformation behaviour was also affected by metakaolin. It was found that metakaolin increased the elastic modulus of concrete while decreasing creep and drying shrinkage of concrete. With increasing metakaolin content, the durability of concrete in terms of transport properties, and resistance against deleterious chemical processes was improved. It was concluded that the addition of metakaolin helped to produce concrete with excellent quality. In comparison to GGCS, it was found that in most of the investigated concrete properties, metakaolin outperformed GGCS at equivalent mix proportions. Technically, metakaolin can therefore be used as a substitute for GGCS in concrete in the Western Cape, thereby contributing a solution to the potential scarcity of SCMs, and potentially reducing environmental effects. Nevertheless, challenges remain on the cost effectiveness, and the willingness and awareness of the construction industry in adapting its use.
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    The design of a data model (DM) for managing durability index (DI) results for national road infrastructure
    (2019) Govender, Daniel; Alexander, Mark
    As part of a R 1.14 Billion 64-month concrete construction mega-project which began in May 2013, the Mt Edgecombe Interchange, comprising two incrementally launched bridges, the longest at 948 metres long and the other at 440 metres which joins uMhlanga and the N2 North, necessitates the demand to have adequate systems in place to measure durability compliance. Construction contracts of this nature exhibit thousands of test results that need to be assessed for variability, outliers and compliance for quality assurance in line with current performance-based specifications such as those contained in COTO (2018a; 2018b) derived from COLTO (1998) which requires judgement based on statistical principles. Since the inception of Durability Index (DI) performance-based specifications in 2008, over 12000 DI test results or determinations have accumulated within a repository at the University of Cape Town. As such, the performance-based approach in South Africa is now a decade into maturity and considerable amounts of actual site data are collected daily, and significant for refinements of the DI values in performance-based specifications, the long-term monitoring of Reinforced Concrete (RC) structures in a full-scale environment along with other research and development (R&D) initiatives. Data modelling can be defined as the process of designing a data model (DM) for data to be stored in a database. Commonly, a DM can be designated into three main types. A conceptual DM defines what the system contains; a logical DM defines how the system should be executed regardless of the Database Management System (DBMS); and a physical DM describes how the system will be executed using a specific DBMS system. The main objective of this study is to design a data model (DM) that is essentially a conceptual and logical representation of the physical database required to ensure durability compliance for RC structures. Database design principles are needed to execute a good database design and guide the entire process. Duplicate information or redundant data consume unnecessary storage as well as increase the probability of errors and inconsistencies. Therefore, the subdivision of the data within the conceptual data model (DM) into distinct groups or topics, which are broken down further into subject based tables, will help eliminate redundant data. The data contained within the database must be correct and complete. Incorrect or incomplete information will result in reports with mistakes and as such, any decisions based on the data will be misinformed. Therefore, the database must support and ensure the accuracy and integrity of the information as well as accommodate data processing and reporting requirements. An explanation and critique of the current durability specification has also been presented since information is required on how to join information in the database tables to create meaningful output. The conceptual data model (DM) established the basic concepts and the scope for the physical database through designing a modular structure or general layout for the database. This process established the entities or data objects (distinct groups), their attributes (properties of distinct groups) and their relationship (dependency of association between groups). The logical database design phase is divided into two main steps. In the first step, a data model (DM) is created to ensure minimal redundancy and capability for supporting user transactions. The output of this step is the creation of a logical data model (DM), which is a complete and accurate representation of the topics that are to be supported by the database. In the second step, the Entity Relationship Diagram (ERD) is mapped to a set of tables. The structure of each table is checked using normalization. Normalization is an effective means of ensuring that the tables are structurally consistent, logical, with minimal redundancy. The tables were also checked to ensure that they are capable of supporting the required transactions and the required integrity constraints on the database were defined The logical data model (DM) then added extra information to the conceptual data model (DM) elements through defining the database tables or basic information required for the physical database. This process established the structure of the data elements, set relationships between them and provided foundation to form the base for the physical database. A prototype is presented of the designed data model (DM) founded on 53 basic information database tables. The breakdown of database tables for the six modules is split according to references (1), concrete composition (13), execution (4), environment (7), specimens (2) and material tests (26). Correlations between different input parameters were identified which added further information to the logical data model (DM) elements by strengthening the relations between the topics. The extraction of information or output parameters according to specification limits was conducted through analysing data from five different projects which served as input for a total of 1054 DI test results or 4216 determinations. The results were used to conduct parametric studies on the DI values which predominantly affects concrete durability in RC structures. Lastly, a method is proposed using joint probability density functions of Durability Index (DI) test results and the achieved cover depth to calculate the probability that both random variables are out of specification limits.
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    Open Access
    The effectiveness of a percussion drill method for making concrete cube samples to assess the characteristics of precast zero-slump concrete
    (2023) Du Plessis, Pieter; Alexander, Mark
    The precast industry in South Africa consumes about 28% of the total cement produced, and a large proportion of its concrete is prepared with zero-slump concrete mixes. Precast finished products are load tested to confirm compliance, but the zero-slump concrete is often not tested by cube or cylinder samples, as is the case with ready-mix concrete. There is an industry practice of using the percussive action of a rotary percussion drill to compact control samples when necessary. However, variation in the compaction of the specimens results in variation in the density and compressive strength results. Therefore, there is a need for a simple, standardised quality control method that considers the compaction achieved by precast machines. A suitable procedure was pursued with the following specific objectives; i) to formulate a practical and economical procedure for quality control based on the concrete density achieved by precast machines, and ii) to apply this method (i.e., percussion drill method) to mix optimisation with the specific objective of partial replacement of the cement in a specific factory mix with a suitable fine filler in order to reduce cost and the carbon footprint of a specific precast facility. The study methodology involved a literature review on available quality control methods, and a laboratory investigation that combined the percussion drill method with a target density method. The laboratory investigation produced representative samples of an industry mix based on density tests on core samples of the same mix produced by different precast machines. The method was then used to test different proportions of cement replacements by volume with a fine filler. The purpose of the partial cement replacement by fine filler mixes was to assess the effectiveness of using the percussion drill method in mix optimisation. The experimental investigation involved determining the density of concrete produced by three different precast machines using the same industry mix. The density was measured for samples in different moisture conditions, i.e., the as-received state, oven-dried state, saturated state and in an environmentally controlled room. Based on the results, it was recommended to use the as-received density for quality control and mix optimisation at precast production facilities when measuring target density using the percussion drill method. The specific factory mix used in producing the precast elements was employed in the laboratory to produce cube samples using the percussion drill method. The specific factory mix had a water-to-binder ratio of 0.25, a very stiff and dry mix. Therefore, the percussive action of a percussion drill was used for compaction to achieve a target density. The aim was to achieve a compressive strength of 30 MPa at 18 hours and 50 MPa at 28 days. The compressive strength achieved was 1.9 MPa at green state, 40.7 MPa at 18 hours, and 66.5 MPa at 28 days, higher than the target strengths. The green state strength was required to assess the ability of a mix to retain its shape without formwork after extrusion. Thus, the specific factory mix provided a benchmark value. In achieving the second objective, quartz flour was selected as a fine filler and replaced cement in the concrete at replacement rates of 20%, 30%, and 40% by volume of cement. This selection was based on its lower cost and carbon footprint compared to Portland cement and the fact that the particle size and texture are similar to Portland cement. CEM I 42.5 R was used in this objective instead of CEM II / A-L 42.5 N used in the first objective due to its potential to provide sufficient early age strength. The results indicate that the 20% replacement rate outperforms the specific factory mix and other replacement rates in strength at all three critical time intervals. This was either due to the quartz filling effect, which improved the concrete compaction, or a higher early hydration rate of the CEM I 42.5 R than the CEM II / A-L 42.5 N, or a combination of two. The lower strength performance of 30% and 40% replacement rate was related to the fact that quartz flour is inert, which reduces compressive strength with further cement reduction. However, the green state, 18-hour, and 28 days compressive strengths of the 40% replacement mix were still above the minimum requirements. Therefore, the 40% replacement mix was appropriate for industry application. However, it must be noted that these results were obtained in a controlled laboratory environment, which is not replicated at the specific precast facility. Therefore, the recommended trial replacement volume of cement by quartz flour is 33% for an industry trial to anticipate less ideal curing conditions at the production facility, resulting in a net CO2 emission reduction of about 113.1 kg /m3 of compacted concrete. Generally, the results indicate that the percussion drill method can effectively provide adequate quality control measures, and mix optimisation if compaction achieves a predetermined density.
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    The influence of concrete mix composition and environmental exposure on long-term chloride ingress in concrete
    (2022) Heiyantuduwa-Beushausen, Rukshani Heiyantuduwa; Alexander, Mark
    The parameters influencing long-term chloride ingress in concrete structures exposed to different environmental conditions in South Africa were investigated, aiming at improving service life prediction models for reinforced concrete structures situated in marine exposure classes. Time-dependent chloride ingress was measured over a period of roughly 4 years with respect to the influences of binder type, w/b ratio and environmental conditions, exposing samples to submerged, tidal, splash/spray and airborne chloride exposure in the Atlantic Ocean in Cape Town and the Indian Ocean in Durban. As a comparison, chloride ingress was measured under laboratory-controlled conditions. The data was analysed to identify the parameters used in the modelling of long-term chloride ingress into concrete using Fick's laws of diffusion. The time dependency of apparent diffusion coefficients and chloride surface concentrations was identified and used in the long-term service life prediction with respect to the various experimental parameters. Based on the model predictions, the combined influences of binder type, w/b ratio, exposure condition and cover depth on the expected service life duration of RC structures in South African marine environments were investigated quantitively.