Browsing by Author "Beushausen, Hans"
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- ItemOpen AccessAbrasion resistance of IV-RCC used to construct spillway concrete steps of South African dams(2018) Poyo, Myezo; Beushausen, HansThis dissertation outlines the investigation on abrasion resistance of roller compacted concrete designed for the spillway steps of dams owned by the Department of Water and Sanitation. Literature on abrasion resistance of concrete has been reviewed and factors affecting abrasion resistance are discussed. Among these include hardness of concrete, aggregate/paste bond, type of aggregates, concrete compressive strength, curing, surface treatment and the quality of the concrete surface which is influenced by the finishing method. Available test methods as outlined by ASTM International Standards are also discussed in the literature review but only two methods (sandblasting method and wire brush method) were used in this research. These have been chosen because of their availability (at the UCT Civil Engineering Laboratory) and relevancy to this research. The sandblasting method is the most relevant in this research because it stimulates similar abrasion action as waterborne action on the concrete surface. The other method has been chosen for its availability in the UCT lab, to compare its results with the sandblasting results, and to evaluate whether if it can be used to assess abrasion resistance of hydraulic structures. A two-way experimental approach was adopted to achieve results for this research. The first approach was a laboratory investigation, where concrete cubes were produced under controlled conditions and tested for abrasion resistance, compressive strength, oxygen permeability index (OPI) and Sorptivity test. The durability index tests were conducted to get a general evaluation of the concrete’s microstructure. If the OPI values are acceptable (more than 9) then the concrete’s general microstructure would be accepted to be durable. Materials (aggregates) from De Hoop Dam and those readily available at the UCT laboratory were used to cast different types of concrete to be investigated and compared. The types of concrete included conventional concrete, fly ash concrete and slag (GGBS) concrete. DWS uses fly ash in the majority of their mass concrete projects due to its pozzolanic properties which help slowing down the heat of hydration. The slag was included in these mixes in order to have available results when it is used in future projects as it is highly likely to be used for areas in the KwaZulu-Natal province which have a less reactive type of slag in terms of heat of hydration. The raising of the Hazelmere Dam, close to Durban, is currently using GGBS as an extender. The second approach of this research was based on abrasion testing of cores drilled from the stepped wall of De Hoop Dam. These cores were drilled and taken to UCT laboratory for investigations with the same test methods used for laboratory cubes. A comparison was drawn between the test results of the laboratory made cubes and those drilled from the dam spillway, and a correlation was investigated between these two. Furthermore, the sandblast test method was compared with the wire brush method to assess which one produces more reliable results. The abrasion results showed small differences in abrasion resistance performance between the conventional concrete and the roller compacted concrete mixes. In fact when the percentage of the extender was adjusted from 70% to 65% these two concrete mixes performed equally. It was confirmed that beyond 45% replacement of Portland cement with fly ash, there is no improvement in abrasion resistance of concrete. However, there were no significant differences between abrasion resistant values of concrete with 45% or 65% fly ash replacements. It was observed that abrasion resistance of concrete was compromised beyond 65% fly ash replacement. Aggregates with sub-angular shape and rough texture have proven to yield high abrasion resistant concrete. The concrete made with blends of GGBS and Portland cement showed slightly improvement in abrasion resistance of concrete when compared to fly ash concrete. The cores drilled from the structure had abrasion resistance values 3 times more than the laboratory produced cubes and this was consistent with both the sandblasting and wire brush method, indicating that the actual structure has adequate abrasion resistance. The sandblasting method has proven to be reliable and more sensitive and it is recommended for assessment of abrasion resistance of hydraulic structures, while the wire brush method can be used for general quality control. Recommendations based on the results and literature study will be made available to the Department of Water and Sanitation to update their specifications, which was one of the key goals of this research.
- ItemOpen AccessAn analysis of the predominant causes of deterioration of concrete structures in South Africa(2018) Mashanda, Darison; Beushausen, HansConcrete deteriorates due to, but not limited to the ingress of deleterious substances which react with the cement matrix, reinforcing bars corrosion, mechanical effects, physical effects, structural damages, poor construction practices. All these factors individually or combined, ultimately reduce the expected service lives of the concrete structures. The trends vary with different exposure conditions and geographical locations, and a reference guide is required in South African context. A total of twenty-four concrete structures were visually assessed by different University of Cape Town (UCT) scholars and findings were captured in project reports. The reports of these assessments were analysed in this research to identify the main causes of concrete deterioration and severity of damages in the three provinces considered in South Africa, whilst linking these to environmental exposure conditions and geographical location. It is important to elucidate that deterioration mechanisms and trends were drawn from the limited number of visual assessment reports, and the mechanisms assumed might not have been necessarily correct. The rating of the defects was done using the DER-U rating system, a method available for bridges and retaining walls. DER-U rating system was developed for buildings, exploiting the available rating system for bridges as there is no available established rating system for buildings, and the author considered it an important tool for the preliminary evaluation taking note of all limitations. However, reinforcing bars corrosion has been found to be the most prominent deterioration mechanism on structures assessed and severity was high on the structures located in the Western and Eastern Cape provinces, and was exacerbated by the inadequate cover provided on most structures. Furthermore, it was also noted that the severity of the damage increased with age of a structure. Although petrographic analysis as an additional investigation was required to ascertain Alkali-Silica Reaction (ASR), damage was observed in the Western Cape and Gauteng provinces. Even though the occurrence was low, it still required special attention as the effects are usually disastrous and very expensive to maintain the affected structures. Leaching was observed on all the bridge structures assessed though it was more prominent on the structures situated at the coast. Plastic and drying shrinkage cracks were observed on all structures in the Gauteng province and it has been noted from the literature that shrinkage cracks were exacerbated by very high seasonal temperatures in these provinces. Abrasion was high on all structures on the tidal zones and the elements of structures located in the water courses. The proposed in-situ and laboratory tests have been discussed in this report and they are recommended for full-scale condition assessments to complement the visual assessments in an endeavour to ascertain the mechanisms identified. Evidence of poor maintenance practices was observed in the Eastern Cape province where delamination and spalling were observed on freshly repainted structures. As a result, in South Africa there is undoubtedly, a constant need of developing and employing effective and efficient tools to ensure quality is not compromised. Design engineers must always take into cognisance the exposure conditions and ensure strict quality control measures during the construction phase. Maintenance engineers should take into consideration the location of the structure and deterioration mechanisms in the specific areas when determining the maintenance strategies. The clients should always employ knowledgeable design and maintenance engineers, to ensure durable structures are erected and correctly maintained.
- ItemOpen AccessAn investigation into the effects of early propping removal on the deflection of reinforced concrete beams(2018) Rockstroh, Benjamin Andreas; Beushausen, HansIn today’s fast paced construction industry, there is an ever present need to increase productivity and to complete projects as quickly as possible. Reinforced concrete is a popular and widely used construction material. However it has the unfortunate drawback in that the concrete requires time to set and gain sufficient strength before loads may be applied and the formwork and props can be removed. It is therefore desirable to keep propping times to a minimum. If the propping is removed too early, there is a risk of the member deflecting excessively and exceeding the maximum allowable limits, or in severe cases it could even lead to a structural failure or collapse. The SANS 2001 code provides recommended propping times for beams and slabs, which can be used as a guideline by building contractors and structural designers. These propping times present a universal approach, which does not consider all the factors that affect deflection. This simplified approach may be considered to be conservative as shorter propping durations could be possible without a loss in performance. The aim of this dissertation is to look into the effects of early propping removal on the longterm deflections of concrete members. This was done by modelling the deflection of a typical reinforced concrete beam at different ages of loading, using three code-based deflection calculation methods. The codes that were used are the South African National Standard (SANS), Eurocode (EC2) and American Concrete Institute code (ACI 318). A detailed literature-based investigation was conducted to determine the factors which affect deflection in reinforced concrete members, as well as the theory behind the code-based deflection calculation procedures. This was followed by the modelling of deflections using the abovementioned methods. Three case studies were performed to determine the effects of early propping removal under different scenarios. The first case study only deals with the effects of early age loading on long-term deflection. As an added point of interest, two different concrete mixes were used, made with two different types of cement. The second case study compares the effect that different levels of relative humidity have on the long term deflection at early ages of loading. Lastly, the effects of concrete strength on long-term deflections at early ages of loading was modelled. The results of the first case study indicated that a reduction in propping time is possible without causing excessive deflections. In the second and third case study is was observed that both the relative humidity and concrete strength respectively have an effect on the long term deflection and therefore also influence the propping time. The study concluded that based on the obtained estimated deflection values using the codebased methods, the propping times provided in the SANS 2001 code may in certain applications be conservative. According to the results obtained from the code-based deflection calculation procedures, it is possible to reduce the propping duration. It was suggested that an alternative method should be developed which would allow structural designers to determine the required propping time more accurately.
- ItemOpen AccessApplying the DER rating system for the visual assessment of defects on concrete dams(2021) Muhenje, Mariura; Beushausen, Hans-Dieter; Beushausen, Hans; Ndawula, JoanittaNamWater is responsible for the management and maintenance of Namibia's main dams, amongst the many other infrastructure assets under their ownership. They have, as a result, devised asset management policies and practices with the aim of reaping the benefits of asset management. Additionally, they have aligned themselves with the current approaches to dam safety management in managing the dams. These current approaches do however not provide for a guided and standard approach when it comes to the visual assessment of surface defects on the dam elements. This may result in varying assessment outcomes from different individuals based on their differences in training and experience. The DER rating system used for the rating of defects on road structures, as input into the STRUMAN BMS used by SANRAL and other entities, provides for a standard approach to visually assessing the defects on these road structures. A study by Gombele (2017:79) has also demonstrated the possibility of using the DER rating system for the assessment of defects on cooling towers in a power generation environment. Additionally, the rating of defects can also play a role in dam risk determination, as demonstrated in the CIRIA 1 Project RP568 risk assessment methodology (Morris, Hewlett and Elliott, 2000:15). In quest for a standard and guided approach for visually assessing surface defects on dam elements, this study applied the DER rating on selected elements of three NamWater dams. The approach was by initially identifying dam elements that are deemed equivalent to the bridge items in the TMH19: Part A. There are variations in the design and functions of the dam elements versus those of bridge elements and thus the relevance of the defects may also vary for the two structure types. Thus, the focus of this study was on using the DER rating system to only assess the surface defects on the dam elements. This may be useful for the initial phase of a condition assessment for dams whereby it can provide a quantifiable indication of durability issues. While the study was able to demonstrate the possibility of using the DER rating system to assess defects such as cracks, spalling and erosion on the surfaces of dam elements, gaps where identified in its applicability. Only 33% of the bridge items in the TMH19: Part A were deemed relevant for the visual assessment of the dam items, coupled with the significant amount of U (unable to inspect) ratings given during the assessment. Furthermore, of the identified defects, a significant amount was given a low Relevancy rating (R) meaning that they are of a low relevance to the structural integrity of the dam structure. This may be due to the fact that the guiding tables used are originally for bridge items and thus not entirely suitable for dams. For the DER rating system to be applied extensively to dam items, defects that are specific to dam elements will therefore need to be incorporated into the guiding tables. Additionally, the weighting of the ratings for certain defects would need to be revised to specifically align with the consequence of the defect on the dam item. This process may require the compilation of a database of historical defects, guided by expert engineering judgement, to provide for guiding tables that are specific to dams. Assessment of more dams that vary in age, type, and performance may also be required to get a more diversified outcome of the applicability of the DER rating system on dams.
- ItemOpen AccessCarbonation and permeability characteristics of modern South African concretes(2018) Omar, Nabeel; Beushausen, HansThe world’s exponential growth in urbanisation has placed significant pressure on the construction industry to support development by expanding its provision of infrastructure. There is expected to be a rapid increase in the consumption of structural concrete to meet the associated requirements. This increase in concrete consumption has adverse effects on the environment. Firstly, the production of cement, one of the main components of concrete, is regarded as a system of energy-intensive processes. Secondly, the production of Portland cement (PC) releases a substantial amount of greenhouse gases (such as carbon dioxide), which in turn contributes to the global warming phenomenon. In addition to the change in demand for concrete over time, its composition and mix proportions have indeed also undergone a significant evolution. Concrete is becoming more sophisticated and complex. The construction industry has introduced mineral admixtures as partial replacement of PC in the attempt to mitigate the negative environmental impact of cement production. The use of mineral admixtures has positive economic and environmental benefits. In the context of concrete durability, the use of mineral admixtures has the potential to improve the performance of concrete by mitigating the deterioration processes occurring in concrete structures, such as reinforcement corrosion. Reinforcement corrosion is one of the most pervasive concerns within the construction industry. Carbonation is considered as of the main causes contributing to the corrosion phenomenon. The carbonation mechanism entails the reaction between atmospheric carbon dioxide and the cement paste and leads to an altered chemistry within concrete, which eventually causes the depassivation of steel reinforcement. The deterioration of the concrete caused by carbonation can be predicted using the oxygen permeability index (OPI) test results as an input parameter in the appropriate carbonation prediction model. While South Africa has developed carbonation durability prediction models that can predict the performance of conventional concrete mixes (concrete containing 30% fly ash, 50% slag, 10% silica fume) relatively well, this formulation of the carbonation model was instituted approximately twenty years ago and is considered outdated. Therefore, this research seeks to investigate whether the previously established correlation between carbonation and oxygen permeability is still relevant for modern South African concretes. In this study, concrete constituting of different mineral admixtures at varying PC replacement levels or the use of chemical admixtures is defined as modern concrete. The experimental work included investigating the permeability and carbonation performance of modern concretes made with modern binder types at varying binder replacement levels and binder combinations, including binary and ternary cement blends at two water:binder ratios of 0,50 and 0,65. This included addition of fly ash (FA) (20%-50% in 10% increments), blast furnace slag (BS) (20%- 60% in 10% increments), Corex slag (CS) (20%-60% in 20% increments), and limestone (10% and 20%). For ternary blends, the concrete was limited to three mixes, that is, 5% SF with either 25% FA, 25% BS or 25% CS. Furthermore, two commercial blended cement products were tested namely CEM II A-L, and CEM Il B-M (L-S) 42,5N, referred to as A-L and B-M. A-L and B-M cement nominally contain 8% L, and 8% L coupled with 25% CS respectively. The OPI test was conducted after 28-days of wet curing. The accelerated carbonation tests were conducted using a phenolphathein indicator solution at 6, 9 and 12 weeks of exposure. Prior the testing, the samples were wet cured for seven days and underwent a preconditioning regime in attempt to minimise the influence of the internal moisture of the concrete affecting the carbonation depth results. A statistical analysis was done on both OPI and the accelerated carbonation results to determine the significance in results with the increase in binder replacement percentage, different binders of the same binder replacement percentage and significance of using ternary mixes in comparison to binary mixes. In conclusion it was found that, generally, mineral admixtures had a statistical insignificant influence on the permeability. This can be attributed to the fact that the control mixes already possessed a high permeability performance i.e. concretes exhibiting relatively low permeabilities. Therefore, the inclusion of a mineral admixture would result in a minor influence on the performance. Regarding carbonation depths, the inclusion of mineral admixtures resulted in a decrease in carbonation performance, as expected. This is attributed to the dilution effect and the pozzolanic effect to some degree, which decreased the amount of carbonatable material that is calcium hydroxide, subsequently decreasing the concrete’s resistance to carbonation. Finally, reasonable correlations were identified between carbonation depth and permeability when all concrete mixes were considered. The direction of the trend showed a positive and negative association when the carbonation coefficient was plotted against k-permeability and OPI respectively. Further investigation of the correlation between carbonation depth and a singular binder type regardless of the replacement level showed an increased in correlation strength between permeability and carbonation. It was concluded that using this approach may provide reasonable correlations for carbonation prediction modelling. However, more testing would be required to confirm the previous statement.
- ItemOpen AccessConcrete carbonation prediction for varying environmental exposure conditions(2020) Gopinath, Rakesh; Alexander, Mark; Beushausen, HansThe 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.
- ItemOpen AccessCritical literature review on concrete crack repairs(2018) Hove, Grandeur Tofara; Beushausen, HansThis dissertation focuses on an in-depth review of literature on concrete crack repairs. Deterioration of concrete affects both the aesthetics and the integrity of structures. As a result, there is need to repair such defects in order to restore the aesthetics and the integrity of concrete structures. This research discusses concrete crack repairs taking into consideration the mechanisms resulting in concrete cracking, techniques to determine concrete cracking, practices for prevention of concrete cracking and concrete crack repair techniques. Causes of cracking include alkali silica reactions (ASR), steel reinforcement corrosion, shrinkage, thermal variations, foundation movements, soil settlement, vegetation effects and cracking related to earthquale effects. Non-destructive and destructive techniques to determine conrete cracking shall be discussed. Destructive techniques usually involve core drilling of samples whilst non-destructive techniques include visual inspections, ultrasonic pulse velocity test, acoustic emission, spectral analysis of surface waves, modal analysis, petrographic analysis and infrared thermography. Several concrete crack repair techniques are to be discussed such as epoxy injection, routing and sealing, near surface reinforcement, additional reinforcement, gravity filling, grouting, dry packing, crack arrest, polymer impregnation, overlay and surface treatments, crack filling, crack sealing, blanketing, stitching and external stressing. These techniques are applied differently depending on the nature and cause of the concrete cracks. Further discussions will be on various case studies around the world on concrete crack repair which demonstrate the application of various concrete crack repair techniques to different types of concrete cracking scenarios. The case studies also highlight recent developments in technology, repair materials, application fields for various techniques and limitations to concrete crack repair. Some of the case studies discussed include concrete crack repair due to ASR damage on transport infrastructure in USA, ASR effects and crack repairs on a two storey building in California (USA), ASR effects and crack repairs to concrete structures in Hokuriku District in Japan, ASR effects and crack repairs to a gravity dam in India, crack repairs at Buttermarket Shopping Center due to shrinkage cracking and concrete crack repairs in Cheshire due to thermal effects.
- ItemOpen AccessDevelopment 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, HansThe 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.
- ItemOpen AccessEffect of anti-corrosive coatings on the bond between corrosion-damaged rebar and concrete repair materials(2022) Moolla, Muhammad Ameen; Beushausen, Hans; Jarratt, NicholasReinforcement corrosion is the largest contributor to the deterioration of existing RC structures worldwide. The aggressive electrochemical process causes destruction to the steel by a loss in cross section and rib height, which in turn affects the bond between steel and concrete and consequently the structural performance of reinforced concrete structures. During a repair strategy, when the patch repair method is considered, one of the steps usually involves the application of a protective coating to the corroded and subsequently cleaned steel. It is well accepted that protective coatings offer corrosion resistance to the steel in reinforced concrete repaired elements. However, information on how well the coated rebar bonds to the new surrounding repair concrete is limited. Hence, developing an understanding on the bond performance of repaired coated rebar is important for engineers in the construction repair industry. One commercially available epoxy-modified, cementitious coating material was considered in this study, Sika ® Armatec ® -110- EpoCem, and applied with one or two coats of 0.6 mm each. Rebar corrosion damage levels of 0%, 10% and 20% (of steel mass loss) were simulated by mechanical grinding of sound rebar samples, attempting to represent the condition of cleaned corroded rebar. Two steel bars, Y12 and Y16, of different diameters were considered. Three repair materials with CEM I 52.5N were considered in this study, which included one concrete with a w/b ratio of 0.45 and two mortars with w/b ratios of 0.47 and 0.65, referred to as C45, M45 and M65, respectively. Pull-out testing was conducted on a total of 108 specimens to assess the effect of coating thickness on the bond with respect to the different parameters of this study. The results indicate that the w/b ratio of the mortars had a significant influence on the bond and the addition of stone showed no difference on the failure loads obtained. There was no significant effect of the rebar diameter on the pull-out force for uncorroded rebar, although with cleaned corroded rebar the effect was significant. Specimens with 10% mass loss had larger reductions in pull-out force compared to those with 20% mass loss. While coatings reduced the bond of uncorroded and cleaned corroded rebar, the thickness of the coating had practically no influence on the pull-out force. Using C45-12 with two coats on 20% corroded steel, the pull-out force was 80% compared to before corrosion. Failure of uncorroded specimens was dominated by splitting, and slip was mostly seen for cleaned corroded specimens. With coatings applied to uncorroded bars, slip coating failure was evident with uncorroded steel while splitting with coating failure was evident especially with Y16. From the corroded coated specimens, there appeared to be good contact between the cleaned corroded steel and repair material.
- ItemOpen AccessEffects of structural repair and strengthening on stiffness and ultimate capacity of corrosion-damaged RC beams(2010) Tigeli, Mojabeng; Moyo, Pilate; Beushausen, HansOver the years, the need for repair of reinforced concrete (RC) structures has increased significantly, both for old and new structures. The main cause of structural damage in RC structures is corrosion of reinforcement. Therefore there is a need to understand the effects of corrosion damage on the structural properties of RC members such as stiffness and ultimate capacity in order to develop improved and effective repair strategies that will minimise the risk of further damage and structural failure both in the short and long term. Corrosion reduces the cross-sectional area of steel at the corrosion sites, leading to reduced stiffness and capacity of the structure to carry loading, which may result in structural failure if not detected and repaired or strengthened. This study presents experimental results on the effectiveness of combined repair and strengthening of corrosion-damaged RC beams. Four RC beams 154 mm wide × 254 mm deep × 5000 mm long were tested. Three of the beams were subjected to different levels of accelerated corrosion (using 1000 mA impressed anodic current and 5% NaCl solution) under sustained service loads (2 kN) while one beam acted as a control. All four beams were monitored for deflections during the accelerated corrosion period. The beam dynamic characteristics were assessed at all stages; namely the undamaged stage, 10 % corroded, 15% corroded, exposed reinforcement, patch repaired and finally the CFRP strengthened stage. The three corrosion-damaged beams were patch repaired with a cementitious grout; two of them were strengthened with carbon fibre reinforced polymer (CFRP) laminates in addition to the repair. The effect of damage on the stiffness of the beams was inferred from strain measurements, deflections and dynamic properties (natural frequencies and modal shapes). The effect of patch repair and strengthening on corrosion damaged beams was deduced from the changes in their ultimate capacities. From the deflection measurements measured during the corrosion process, stiffness decreased with the increase in corrosion level. Cracks develop and widen as the level of corrosion increases. The results obtained from dynamic testing consist of natural frequencies, mode shapes and damping ratios. The dynamic results show a drop in natural frequencies due to damage and not much change on the fundamental mode shapes is observed when comparing the modes by the Modal Assurance Criterion (MAC) values. The MAC values did not prove useful in identifying damage or changes in RC beams. The static results reveal that both the stiffness and the ultimate capacity are improved by about 25% and 50% respectively when both patch repair and FRP are applied on a damaged beam. Patch repair on its own improved the stiffness by approximately 5% but did not improve the ultimate capacity. As expected, it can be drawn from the results that the combined effect of both patch repair and CFRP strengthening improves the stiffness and ultimate capacity of the structure.
- ItemOpen AccessImprovement of the approaches to monitoring of reinforced concrete bridges subjected to chloride-induced reinforcement corrosion in bridge management systems(2022) Simon, Jaziitha; Beushausen, Hans; Otieno, MikeReinforcement corrosion in reinforced concrete (RC) bridges has undeniably become the main cause of civil infrastructure durability problems in many countries. The steel reinforcing in concrete corrode when certain conditions (e.g., carbonation, chlorides, insufficient cover and moisture) are met. Even though RC bridges are built to provide a service over a specified period, their serviceability changes over time due to gradual deterioration. In the marine environment, this deterioration is mainly caused by the ingress of chlorides in concrete, which results in chlorideinduced steel corrosion. Thus, effective maintenance and management are needed to ensure the functionality of bridges throughout their designed service life. Bridge management systems (BMSs) are effective means for managing bridges throughout their design life. BMSs require the collection of data related to the condition of the bridge for decision making. South Africa uses the Struman BMS to assess and prioritise bridges for maintenance and repair. The system solely relies on visual inspection as a basis for the identification of deterioration processes and bridge condition rating. However, visual inspections do not allow detection of rebar corrosion until damage has occurred; and once the damage has occurred, it becomes very costly to repair. Thus, this research focuses on identifying available reinforcement corrosion monitoring methods that can be used for condition assessment to supplement visual inspections in the Struman BMS. The research involved reviewing different BMSs with the emphasis on their structure, application, and assessment methodology. The review of various BMSs used in the USA, Canada, UK and Southern Africa shows that while BMSs differ, they all assess the risk of failure and prioritise bridges for repair within limited budgets. Focusing on the Struman BMS, shortcomings with respect to the monitoring of reinforcement corrosion were identified. It was found that the condition ratings of RC bridges are based on visible defects assessed during visual inspections, which is still the dominant method used for bridge inspection. There exists a lack of integration of appropriate monitoring systems in the BMS, especially with regards to the monitoring of reinforcement corrosion. Available corrosion monitoring methods were critically reviewed based on their principles, applications, and technical aspects. These corrosion monitoring methods were categorised into visual inspections, Non-Destructive Testing (NDT), and remote monitoring methods. It became evident that monitoring technologies are continuously evolving and that the progress achieved to date is promising. However, further development is needed when using these methods in the BMSs for condition assessment, particularly in the continuous monitoring of bridges. The use of these modern technologies will allow earlier detection of problems and hence support sound maintenance and damage prevention. This is expected to simultaneously reduce the costs needed for maintenance and repair. In addition, modern monitoring methods have the potential to enhance the speed and scope of condition assessments, to provide reliable and wide-ranging data, and to reduce traffic interruptions when taking measurements. An integrated approach is proposed based on applying corrosion monitoring methods as a supplement to visual inspections in the Struman BMS. Integrating these methods with the existing approach to bridge management is expected to compensate for the limitations of the Struman BMS and enhance its capability. The proposed approach involves identifying bridges that need to be assessed (new or existing bridges). Selecting monitoring methods for new bridges based on the needs and the type of data that need to be collected from bridges, and existing bridges based on the past assessment data and visual inspection. Monitoring is proposed to incorporate periodic and permanent monitoring. The former includes NDT measurements using any selected methods, and the latter includes installing sensor systems in new and existing bridges. Condition ratings are defined for monitored parameters, including corrosion risk (from corrosion potentials and electrical resistivity), corrosion rate, pulse velocity, defective areas (from the presence of cracks and delamination), moisture content and visual inspection. Other corrosion-related parameters considered include chloride content, cover depth measurement and monitoring time to corrosion onset. The obtained data from visual inspection and corrosion monitoring methods are then holistically analysed and interpreted to obtain integrated condition ratings, which are used to rate the condition of the bridge and its various RC elements. Consequently, this approach is expected to assist in prioritising critical bridges for repair and maintenance. Lastly, recommendations are provided related to a comprehensive study on the implementation of available reinforcement corrosion monitoring methods for the monitoring of in-service bridges. The findings of this research also need to be taken forward by an experienced bridge inspector to help develop practical guidelines for applying corrosion monitoring methods in the Struman BMS.
- ItemOpen AccessService life extension of reinforced concrete structures using hydrophobic impregnation(2018) Sohawon, Haris; Beushausen, HansOver the last few decades, the increasing premature deterioration of reinforced concrete structures, mainly due to rebar corrosion, has become a worldwide concern. This has been attributed to insufficient quality and quantity of cover concrete resulting from inadequate mix design and poor onsite workmanship respectively. Engineers also often lack understanding of concrete durability and prescribe insufficient cover depths relative to the exposure conditions. Concrete degradation has many financial and social implications on a larger scale. Direct costs relate to the repair and rehabilitation of existing structures to maintain serviceability while indirect costs include loss in productivity and reduced economic growth. With increasing demand for infrastructure and subsequent expansion of the built environment, there is greater need for concrete to withstand and perform in corrosive environments. Hence, designing for durability has gained significant importance amongst engineers and other stakeholders in the construction industry. Furthermore, the appearance of cracks can be considered as an inevitable phenomenon in the design life of reinforced concrete structures, due to concrete being an inherently cracked material. The presence of cracks within the cover zone changes the dynamics of transport mechanisms and corrosion development. Cracks provide preferential ingress paths for deleterious species such as chlorides and this leads to a reduction in the time taken for reinforcement corrosion initiation and thus reduces the service life of the structure. Most service life models also consider concrete only in the uncracked state, leading to an overestimation of the actual performance and design life of reinforced concrete infrastructure. Extensive research has been carried out to find means to promote the service life of reinforced concrete structures in aggressive environments. Hydrophobic (silane) impregnation represents a cost-effective way to increase the durability of concrete structures in cases where insufficient cover quality and depth have been achieved. The hydrophobic impregnation agent lines the internal capillary pore structure and provides a water-repellent surface without affecting the external appearance of the concrete. Thus, the risk of reinforcement corrosion and subsequent deterioration can be reduced as the ingress of water-dissolved aggressive species is minimised or prevented. The influence of silane impregnation on chloride ingress is well documented in literature and several experiments have been carried out over the last decades. However, there is limited work on the service life modelling of silane treated concrete. Hence, the purpose of this study was to investigate and quantify the influence of silane impregnation as a remedial measure for poor quality cover or insufficient cover depth in newly constructed structures and ultimately predict the time to corrosion initiation for specific cover depths and concrete types. The effectiveness of silane impregnation in cracked concrete was also studied. Two w/b ratios (w/b = 0.45 and w/b = 0.60) and four binder types (CEM I 52.5N, Fly-ash (FA), Ground granulated Corex slag (GGCS) and CEM III/B 42.5N) were selected. Hence, a total of 8 main (normal) concrete mixes and 4 poor quality mixes were used. Poor quality concrete was produced by exposing the concrete specimens to relatively high temperature at early age. Cracked concrete was obtained by loading notched reinforced beams until the formation of cracks. Steel spacers were then placed in the notch and the beams were unloaded to create crack widths of 0.2 mm and 0.6 mm (below and above the commonly assumed crack width threshold of 0.4 mm). Silane treatment was performed at a specimen age of 28 days by applying Sikagard®-706 Thixo at a consumption rate of 400 g/m2 . Several experimental tests were performed on untreated and silane treated concrete. Compressive strength and Durability Index (DI) tests were carried out to characterise the concrete mixes. Accelerated carbonation and moisture profiling tests were undertaken to assess the influence of silane impregnation on concrete carbonation and relative humidity. Finally, uncracked and cracked (untreated and treated) concrete mixes were ponded in sodium chloride (NaCl) solution for 80 days, followed by chloride profiling. The data for the uncracked concrete was curve fitted using a solution to Fick’s second law of diffusion. The regression parameters obtained (surface chloride concentration (Cs) and apparent chloride diffusion coefficient (Da)) were then incorporated in a mathematical solution to Fick’s second law to obtain suitable expressions that describe the penetration of chlorides with time for silane treated and untreated concrete mixes. Hence by determining the time taken for chloride concentration at the rebar level to reach the critical threshold (assumed to be 0.4% by mass of binder), the time to corrosion initiation of untreated and silane treated concrete was predicted for particular cover depths. The results indicate that the silane penetration depth is highly dependent on the quality (porosity) and moisture content of the near surface zone as deeper penetration was recorded in the higher w/b ratio and poor-quality concrete mixes. Silane impregnation also reduced the capillary absorption and chloride conductivity for all the mixes. In terms of the bulk diffusion test, chloride ingress in the treated concrete mixes was suppressed and lower chloride surface concentration (Cs) and apparent chloride diffusion coefficient (Da) were recorded. The influence of silane impregnation on carbonation was negligible in the w/b = 0.45 concrete mixes while a slight decrease in carbonation depth was observed in the w/b = 0.60 concrete mixes. The relative humidity of treated concrete (near the surface) initially increased relative to the untreated concrete. However, the difference in relative humidity between silane-treated and untreated concrete is reduced with time. Higher chloride concentrations were measured in the cracked concrete at depths of 50-60 mm compared to the uncracked concrete. Greater chloride ingress was also recorded in the 0.6 mm crack width relative to the 0.2 mm crack width. For a particular crack width, chloride ingress in cracked concrete was influenced by the type of binder; a significant reduction in chloride content was recorded in the cracked slag (GGCS and CEM III/B) concrete mixes relative to the CEM I mix. The results also suggest that silane impregnation reduces chloride ingress in cracked concrete (up to a crack with of 0.6 mm) and consequently minimises the risk of premature reinforcement corrosion initiation, especially in slag concrete. The service life prediction results emphasized the importance of adequate cover depths in the extreme marine exposure class (XS3) and highlighted the superior performance of slag concrete relative to CEM I concrete. A lower rate of chloride ingress was predicted in the silane treated concrete and consequently to achieve the same time to corrosion initiation, smaller cover depths are required. Alternatively, the results also show that the initiation period of rebar corrosion in structures with insufficient cover depth and quality can be effectively extended using silane impregnation.
- ItemOpen AccessThe influence of various factors on the results of carbonation, cover depth, half-cell potential and resistivity tests that are used in the assessment of reinforcement corrosion(2021) Akhalwaya, Mohammed Faraaz; Beushausen, HansReinforcement corrosion is the main cause of deterioration in concrete structures. Condition assessment tests help to predict whether reinforcement corrosion is occurring and aid in the prevention of its consequences. The results of these condition assessment tests can, however, be affected by certain factors. A set of experiments that included carrying out four of these tests, namely the half-cell potential (HCP), cover depth, carbonation and resistivity tests was performed on several concrete structures on upper campus at UCT. The half-cell potential and resistivity tests were carried out across a four-month period between August and November 2018 during both dry and wet periods and on days with different temperatures. This was done to assess how changing weather conditions can affect the test results. The results from these tests were also used to do a comparison between HCP and cover depth results and HCP and resistivity results. A second set of experiments involved taking cover depth measurements and cores for carbonation testing from different locations across a building and assessing how variations in measurement location and sample size can affect the results. The HCP and resistivity results showed changes due to the effects of rain and temperature. Rainy weather caused the values to become more negative, while dry weather led to more positive values. An increase in temperature showed a slight decrease in the values of both the half-cell potential and resistivity measurements. The changes suggest that using prescribed value ranges to interpret the risk of corrosion may prove to be too simplistic. Contour plots of the HCP results proved to be a more stable method for assessing reinforcement corrosion than using prescribed value ranges. The overall trend for the comparison between HCP and cover depth results showed that HCP values decrease as cover depths decrease. The comparison between HCP and resistivity results was expected to show an overall decrease in HCP values as resistivity values decrease, but this did not hold true for some of the test locations. The results of the cover depth analysis showed significant changes in calculated statistics due to both changes in location and sample size. A cover depth analysis should thus be widespread and include a large number of measurements in order to provide useful results. The variation in results for the carbonation testing was contradictory for the two buildings that were tested, with one building showing significant variations with measurement location and the other building showing very little variation.
- ItemOpen AccessThe state of Namibia's concrete infrastructure – A comparative study of Walvis Bay, Swakopmund, Arandis and Usakos towns(2020) Chirembo, Kondwanie Frank; Beushausen, HansIn Namibia, the design and construction specification of concrete (for durability) follows the traditional method of prescribing parameters which over time are believed to produce durable concrete. These parameters include concrete exposure definition, cover, concrete strength, concreting materials and concreting methods. International research has shown that some of these parameters do not have a direct relationship with the durability of “As-built” concrete structures; rather that concrete structures can be designed and constructed to meet specific performance levels of the environment they are built in. In this case, testing of the concrete during design, construction and post-construction (to set acceptable limits and confirm adherence to the limits) becomes part of the process. This project was undertaken to look at how concrete practices (design and construction specification) followed in Namibia have impacted on the durability of concrete structures. To achieve the objective, the approach followed included; first assessing the prevailing concrete conditions (in different areas across Namibia), reviewing current concrete design and construction specifications used in the industry and finally assessing practitioner knowledge on performance-based concrete design and construction specification. Concrete infrastructure in four towns (Walvis Bay, Swakopmund, Arandis and Usakos) were inspected and deteriorations recorded. From these inspections, an evaluation of the causes of the deterioration was undertaken. A comparison was undertaken on the prevalence of different deterioration mechanisms in the four areas. Design standards, for durability, followed by structural engineers in Namibia were reviewed including specifications demanded by different infrastructure developing agencies. The limitations of these were highlighted. Interviews and observations were undertaken with structural engineering practitioners on knowledge of design for concrete durability. From the information collected, the comparison and assessment it was concluded that there is a need to revise the concrete design and construction specification to ensure concrete performs better against the most significant risk to concrete service life; damage due to reinforcement corrosion. Concrete specifications need to have measurable durability parameters which can be used for acceptance of concrete works besides compressive strength. The Namibia Port Authority (NAMPORT) has taken a step in this direction with the development of concrete construction specifications which have adopted the South African Durability Index method as part of the concrete acceptance criteria. Challenges encountered (by NAMPORT) include lack of testing equipment and a lack of knowledge by material testing engineers. With Namibia having the challenge of not having a national concrete construction standard, it is further recommended that major infrastructure developers (Namibia Water Authority, NAMWATER, and Roads Authority, RA) should lead in adoption of similar specifications to the NAMPORT ones. As most engineers in Namibia undertake works for either NAMWATER or RA, the industry will be more willing to undertake the training of engineers on the South African durability index method as it will make economic sense. The training is recommended to include training of laboratories offering material testing services in Namibia.
- ItemOpen AccessTowards a triphasic Theory of Porous Media-based model for chloride-induced corrosion in reinforced concrete(2019) Ndawula, Joanitta N.; Skatulla, Sebastian; Beushausen, HansThis thesis is concerned with the initial development of a multiphase material model using the Theory of Porous Media (TPM) for the penetration of intermediary rust product into reinforced concrete subjected to chloride-induced corrosion. Research has shown that although the majority of time-to-cracking service life models for reinforced concrete structures neglect the permeation of rust into the cement paste adjacent to the reinforcement, it is this mechanism that is responsible for discrepancies between experimental data and model results. The model presented may be used to simulate the transport of water and gas through the capillary pores in concrete and the diffusion of iron III chloride within the pore solution. Iron III chloride is a soluble chloride complex formed as an intermediary product during the oxidation reaction at the anode of the corrosion cell. This solute is transported in the pore solution from low oxygen conditions and is oxidized in oxygen-rich conditions resulting in the precipitation of rust in the concrete pores. The Theory of Porous Media has proven proficient for modelling the material behaviour of porous solid bodies saturated with one or more fluids but has yet to be applied to chloride-induced reinforcement corrosion of reinforced concrete. This work outlines the initial efforts of using TPM to model the rust transport process coupled with the poro-elastic material response of reinforced concrete. The latter accounts for the stress build-up due to rust precipitation and volume expansion. The chloride complex is described by a concentration within the liquid phase and is therefore not assigned a unique volume fraction. Precipitation of the rust is not included here as it will be added at a later stage in the development of a more accurate reinforced concrete chloride induced corrosion model. It is intended that the model thus developed may be adapted for other deterioration mechanisms in concrete.