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. © 2013 The Authors

The influence of reusing ‘Formtex’ controlled permeability

The effects of the reuse of ‘Formtex’ Controlled Permeability Formwork (CPF) liner on strength and durability properties of concrete were investigated at two different water-cement ratios and the results are reported in this paper. Test blocks were cast using the CPF on one side and impermeable formwork (IF) on the opposite side of the mould so that direct comparisons could be made between the two. The strength was assessed using the Limpet pulloff tester and both the air permeability and the water absorption (sorptivity) were measured using the Autoclam Permeability System. Both these instruments measured the ‘covercrete’ properties. In addition, cores cut from the test specimens were subjected to an accelerated carbonation test and a chloride exposure test. The results showed that the ‘Formtex’ CPF increases the surface strength and the durability of concrete compared to the IF. There was an almost complete elimination of blowholes. The permeability of concrete decreased and its resistance to the ingress of both carbon dioxide and chlorides increased when CPF was used. The beneficial effects of the Formtex CPF were most evident in concrete of higher water-cement ratio. With the reuse of the Formtex liner twice, that is a total of three uses, the performance of the CPF to improve the properties of concrete remained almost the same. In this research the CPF liner was cleaned thoroughly between each use, which must be adhered to for site applications for reproducing the beneficial effects observed in the laboratory.

Durability and water absorption properties of surface treated concretes

Durability of concrete can be improved by applying surface treatments. Pore-lining treatments prevent or delay the ingress of water-borne salts while allowing vapour transfer across the concrete surface. The most common pore-liners are silanes and siloxanes; both reported to give good results. One area of concern, however, is variability in effectiveness of the treatment. This variability may be due to inconsistent coverage or extreme drying conditions. With care these can be controlled but another source of variability which is difficult to control is the moisture profile within the concrete at the time of application of the treatment. This paper describes a test programme to assess the sensitivity of three different surface treatments to moisture gradient in the concrete at the time of application of treatment. The test programme included durability parameters such as chloride ingress, corrosion due to chloride ingress, freeze-thaw salt scaling resistance. Water absorption (sorptivity) of treated and untreated concretes was also measured with a non-distructive test technique called Autoclam with the aim of determining if the Autoclam sorptivity test can be used to assess the effectiveness of surface treatments. Using these results it is possible to avoid, or allow for, moisture conditions which would adversely affect the success of a pore-liner. However there are advantages in specifying an expected performance of the surface treatment rather than specifying the conditions in which it must be placed. By this method a treatment would have to achieve a specified value of sorptivity or a specified reduction in sorptivity. Failure to do so would be an objective basis on which to make a decision of whether or not to reject the treatment. The Autoclam is a device capable of measuring sorptivity values down to the range typical of surface treated concrete. The paper assesses if the device can be used to discriminate between acceptable treatment and unsatisfactory treatments.

Monitoring the development of microcracks in reinforced concrete caused by sustained loading and chloride induced corrosion

Chloride-induced corrosion of steel in reinforced concrete structures is one of the main problems affecting their durability and it has been studied for decades, but most of them have focused on concrete without cracking or not subjected to any structural load. In fact, concrete structures are subjected to various types of loads, which lead to cracking when the tensile stress in concrete exceeds its tensile strength. Cracking could increase transport properties of concrete and accelerate the ingress of harmful substances (Cl ^-, O₂, H₂ O, CO₂). This could initiate and accelerate different types of deterioration processes in concrete, including corrosion of steel reinforcement. The expansive products generated by the deterioration processes themselves can initiate cracking. The success of concrete patch repairs can also influence microcracking at the interface as well as the patch repair itself. Therefore, monitoring the development of microcracking in reinforced concrete members is extremely useful to assess the defects and deterioration in concrete structures. In this paper, concrete beams made using 4 different mixes were subjected to three levels of sustained lateral loading (0%, 50% and 100% of the load that can induce a crack with width of 0.1mmon the tension surface of beams - F _0.1) and weekly cycles of wetting (1 day)/drying (6 days) with chloride solution. The development of microcracking on the surface of concrete was monitored using the Autoclam Permeability System at every two weeks for 60 weeks. The ultrasonic pulse velocity of the concrete was also measured along the beam by using the indirect method during the test period. The results indicated that the Autoclam Permeability System was able to detect the development of microcracks caused by both sustained loading and chloride induced corrosion of steel in concrete. However, this was not the case with the ultrasonic method used in the work (indirect method applied along the beam); it was sensitive to microcracking caused by sustained loading but not due to corrosion. © 2014 Taylor & Francis Group.

A research-based approach to exploring the adoption of novel UK-based concrete test methods in Chinese construction sector

As part of a UK-China science bridge project - a UK government funded initiative linking leading universities and businesses in selective partnering countries in 2009 a collaborative research programme was initiated between Queen's University and the Research Institute of High Performance Concrete (part of the Central Research Institute of Building and Construction) in Beijing.

For further details email b.magee@ulster.ac.uk