Methodology for Designing Structures to Withstand Extreme Environments: Performance Based Specifications

Existing guidelines in BS 8500 allow the selection of concrete mix based on variables such as compressive strength, maximum water to binder ratio, minimum cement content and minimum cover thickness. This approach does not guarantee the durability and expected performance of the concrete structure in a given environment. One alternative is to develop performance- based specifications that supplement the existing guidelines in BS 8500, by specifying the required performance of concrete in terms of measurable properties such as resistance to environmental penetrations. This paper demonstrates one of such methodology for developing performance-based specifications for concretes exposed to marine environments. Chloride ingress related durability problem being critical in a marine environment, the reliability and repeatability of the different test methods for assessing the rate of chloride ingress is discussed first. Furthermore, a numerical simulation model is used to explore the test data to obtain long-term chloride ingress trends. Based on this, guidelines for selecting appropriate concrete mixes for a marine exposure is presented and discussed

The performance of concrete exposed to marine environments: predictive modelling and use of laboratory/on site test method

This paper reports an approach by which laboratory based testing and numerical modelling can be combined to predict the long term performance of a range of concretes exposed to marine environments. Firstly, a critical review of the test methods for assessing the chloride penetration resistance of concrete is given. The repeatability of the different test results is also included. In addition to the test methods, a numerical simulation model is used to explore the test data further to obtain long-term chloride ingress trends. The combined use of testing and modelling is validated with the help of long-term chloride ingress data from a North Sea exposure site. In summary, the paper outlines a methodology for determining the long term performance of concrete in marine environments

Investigation of moisture condition and Autoclam sensitivity on air permeability measurements for both normal concrete and high performance concrete

While on site measurement of air permeability provides a useful approach for assessing the likely long term durability of concrete structures, no existing test method is capable of effectively determining the relative permeability of high performance concrete (HPC). Lack of instrument sensitivity and the influence of concrete moisture are proposed as two key reasons for this phenomenon. With limited systematic research carried out in this area to date, the aim if this study was to investigate the influence of instrument sensitivity and moisture condition on air permeability measurements for both normal concrete and HPC. To achieve a range of moisture conditions, samples were dried initially for between one and 5 weeks and then sealed in polythene sheeting and stored in an oven at 50 °C to internally distribute moisture evenly. Moisture distribution was determined throughout using relative humidity probe and electrical resistance measurements. Concrete air permeability was subsequently measured using standardised air permeability (Autoclam) and water penetration (BS EN: 12390-8) tests to assess differences between the HPCs tested in this study. It was found that for both normal and high performance concrete, the influence of moisture on Autoclam air permeability results could be eliminated by pre-drying (50 ± 1 °C, RH 35%) specimens for 3 weeks. While drying for 5 weeks alone was found not to result in uniform internal moisture distributions, this state was achieved by exposing specimens to a further 3 weeks of sealed pre-conditioning at 50 ± 1 °C. While the Autoclam test was not able to accurately identify relative HPC quality due to low sensitivity at associated performance levels, an effective preconditioning procedure to obtain reliable air permeability of HPC concretes was identified

Alkali-activated Fly Ash Manufactured with Multi-stage Microwave Curing

Alkali-activated fly ash (AAFA) manufactured with conventional thermal curing (85oC) shows comparative mechanical properties to Portland cement (PC) based system, demonstrating its potential as a future low-carbon and environmentally friendly alternative cementitious material. However, the environmental benefit which could have been achieved from the low carbon nature of AAFA is often offset by the higher energy consumption from thermal curing. Since microwave heating is usually considered as a low-energy and low-carbon heating method, some pilot attempts have been made in the literature to cure AAFA with domestic microwave ovens. However, due to the lack of control on the temperature and humidity in domestic microwave ovens, splashing of fresh AAFA pastes and microcracks of hardened AAFA are two main reasons causing the unsatisfactory performance of microwave cured AAFA. In this study, a custom-made microwave oven equipped with proportional-integral-derivative (PID) control was employed to cure AAFA samples. The microwave power was automatically regulated through the real-time temperature feedback from the fibre Bragg grating (FBG) sensor embedded in the AAFA sample. The microwave curing regime consisted of four curing stages at four different temperatures, namely 65oC, 85oC, 105oC and 125oC. The similar temperature profile was applied on the AAFA cured in a conventional thermal oven. Compressive strength, reaction products and microstructure of AAFA samples with two curing methods were examined by the analytical techniques of XRD, FTIR, NMR, MIP and SEM respectively. The results showed that AAFA under microwave curing exhibited higher compressive strength with less energy consumption than that under the conventional thermal curing. The microwave curing favoured the formation of Al-rich N-A-S-H gel in AAFA and a more compacted microstructur

Microwave Curing Techniques for Manufacturing Alkali-activated Fly Ash

Alkali-activated fly ash (AAFA) has been identified as a low-carbon alternative to Portland cement (PC). However, conventional thermal oven curing, typically at 85oC, is commonly required to firstly initiate and then accelerate the chemical reaction, which may result in more energy consumption, offsetting the environmental benefits which could be obtained from AAFA. In this study, the potential of using microwave as an alternative low-energy thermal curing method for manufacturing AAFA has been explored. A microwave curing regime based on a control strategy using temperature feedback data was developed and evaluated in the manufacture of AAFA. The temperature profile within the AAFA sample produced under microwave curing was obtained by using an embedded optical fibre temperature sensor, which was thus used to adjust the microwave power in real-time in order to achieve a desired internal temperature. The AAFA samples manufactured by microwave curing were characterized using compressive strength, XRD, NMR, MIP and SEM, which were also compared with the samples from thermal oven curing. The results indicated that the AAFA manufactured with microwave curing not only showed an equivalent performance, similar reaction products and microstructure, but also showed dramatic reduction inenergy consumption

Chloride ingress into marine exposed concrete: A comparison of empirical- and physically- based models

In establishing the reliability of performance-related design methods for concrete – which are relevant for resistance against chloride-induced corrosion - long-term experience of local materials and practices and detailed knowledge of the ambient and local micro-climate are critical. Furthermore, in the development of analytical models for performance-based design, calibration against test data representative of actual conditions in practice is required. To this end, the current study presents results from full-scale, concrete pier-stems under long-term exposure to a marine environment with work focussing on XS2 (below mid-tide level) in which the concrete is regarded as fully saturated and XS3 (tidal, splash and spray) in which the concrete is in an unsaturated condition. These exposures represent zones where concrete structures are most susceptible to ionic ingress and deterioration. Chloride profiles and chloride transport behaviour are studied using both an empirical model (erfc function) and a physical model (ClinConc). The time dependency of surface chloride concentration (Cs) and apparent diffusivity (Da) were established for the empirical model whereas, in the ClinConc model (originally based on saturated concrete), two new environmental factors were introduced for the XS3 environmental exposure zone. Although the XS3 is considered as one environmental exposure zone according to BS EN 206-1:2013, the work has highlighted that even within this zone, significant changes in chloride ingress are evident. This study aims to update the parameters of both models for predicting the long term transport behaviour of concrete subjected to environmental exposure classes XS2 and XS3

`Clam' permeability tests for assessing the durability of concrete

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Evaluation of different laboratory methods for testing resistance of concrete to chloride ingress

In the past years a European cooperative project called ChlorTest was carried out with theintention to arrive at laboratory test methods applicable to EU states based on non-biasedapproaches and evaluations, and to recommend the practical use of the laboratoryperformance tests based on the verification of the in-field performance of concrete underdifferent chloride exposure environments. This paper presents the results from the evaluationof different laboratory methods for testing resistance of concrete to chloride ingress. In theevaluation two inter-laboratory tests were carried out: one for pre-evaluation and another forfinal evaluation. In the pre-evaluation six different test methods including prEN-13396, anatural immersion test NT BUILD 443, a rapid migration test NT BUILD 492, two steady-statemigration tests and a resistivity test were evaluated by eight laboratories. Four types ofconcrete manufactured with Portland cement (PC) and PC blended with silica fume, fly ashand blast furnace slag, were used in the inter-laboratory test. Based on the pre-evaluationresults and taking other factors into account, four methods, that is, NT BUILD 443, NT BUILD492, a steady-state migration test based on the conductivity measurement, and an indirectmethod (resistivity test), were selected for the final evaluation, in which a total of fifteenlaboratories participated in order to produce adequate precision data. Six types of concretemanufactured with the combination of four types of binder (PC, and PC blended with silicafume, slag and fly ash) were used in the final evaluation. Based on the evaluation results, three methods were recommended for further standardisation, i.e. a natural immersion test (NT BUILD 443), a rapid migration test (NT BUILD 492), and an indirect method (resistivity test), all of which have the precision in an acceptable range, i.e. repeatability coeffifient of variation (COV) ≤20% (11% to 20%) and reproducibility COV ≤30% (24% to 28%). Therefore, they are considered to be suitable for data exchanges and industrial applications

Evaluation of different laboratory methods for testing resistance of concrete to chloride ingress

In the past years a European cooperative project called ChlorTest was carried out with theintention to arrive at laboratory test methods applicable to EU states based on non-biasedapproaches and evaluations, and to recommend the practical use of the laboratoryperformance tests based on the verification of the in-field performance of concrete underdifferent chloride exposure environments. This paper presents the results from the evaluationof different laboratory methods for testing resistance of concrete to chloride ingress. In theevaluation two inter-laboratory tests were carried out: one for pre-evaluation and another forfinal evaluation. In the pre-evaluation six different test methods including prEN-13396, anatural immersion test NT BUILD 443, a rapid migration test NT BUILD 492, two steady-statemigration tests and a resistivity test were evaluated by eight laboratories. Four types ofconcrete manufactured with Portland cement (PC) and PC blended with silica fume, fly ashand blast furnace slag, were used in the inter-laboratory test. Based on the pre-evaluationresults and taking other factors into account, four methods, that is, NT BUILD 443, NT BUILD492, a steady-state migration test based on the conductivity measurement, and an indirectmethod (resistivity test), were selected for the final evaluation, in which a total of fifteenlaboratories participated in order to produce adequate precision data. Six types of concretemanufactured with the combination of four types of binder (PC, and PC blended with silicafume, slag and fly ash) were used in the final evaluation. Based on the evaluation results, three methods were recommended for further standardisation, i.e. a natural immersion test (NT BUILD 443), a rapid migration test (NT BUILD 492), and an indirect method (resistivity test), all of which have the precision in an acceptable range, i.e. repeatability coeffifient of variation (COV) ≤20% (11% to 20%) and reproducibility COV ≤30% (24% to 28%). Therefore, they are considered to be suitable for data exchanges and industrial applications