P.A.M. Basheer

Surface treatments for concrete: assessmentmethods and reported performance

Abstract Several products for surface treatment are available on the market to enhance durability characteristicsof concrete. For each of these materials a certain level of protection is claimed. However, there is no commonly accepted procedure to assess the effectiveness of these treatments. The inherent generic properties may be of use to the manufacturers and those responsible for specifications, however, practising engineers are interested in knowing how they improve the performance of their structures. Thus in this review an attempt is made to assess the engineering aspects of the various surface treatments so that a procedure for their selection can be proposed.

PREDICTIVE MODELS FOR DETERIORATION OF CONCRETE STRUCTURES

Abstract Permeation characteristics and fracture strength are the fundamental properties of concrete that influence the initiation and extent of damage and can form the basis by which deterioration can be predicted. The relationship between these properties and deterioration mechanisms is discussed along with the different models representing their interaction with the environment. Mehta presented a holistic model of the deterioration of concrete based on the environmental action on the microstructure of concrete. Using a similar approach, a detailed investigation on the causes of concrete deterioration is used to develop a macro-model for each mechanism relating to the physical properties of concrete. A single interaction model is then presented for all types of deterioration, emphasizing the permeation properties of concrete. Data from an in situ investigation of concrete bridges in Northern Ireland is used to validate this model. This is followed by a micro-predictive model which includes an ionic transport sub-model, a deterioration sub-model and a structural sub-model and affords quantitative prediction of the deterioration of concrete structures. The quantitative predictive capabilities of the micro-model are demonstrated with the use of reported experimental data.

Monitoring electrical resistance of concretes containing alternative cementitious materials to assess their resistance to chloride penetration

Chloride ion penetration into concrete and the resulting deterioration (cracking and spalling due to the corrosion of reinforcement) is a major concern of engineers and owners of bridges and marine structures. Several publications have reported the excellent performance of concrete containing alternative cementitious materials (ACMs), such as pulverised fuel ash (PFA), ground granulated blast furnace slag (GGBS), microsilica (MS) and metakaolin (MK) in marine environment and highway structures. The resistance offered by these concretes has been related to the low mobility of chloride ions due to either the reduction in the number of interconnected pores as a result of the pozzolanic reaction of the ACMs or the chemical binding with the cement hydrates. However, the secondary reaction products are formed slowly in Portland cement concrete containing ACMs and as a result it is likely that the resistance offered to the penetration of chloride ions also increases slowly with time. In order to monitor the continuous behaviour of concretes containing these ACMs in a chloride exposure regime, an investigation was carried out, the results of which are reported in this paper. Ten different concrete mixes were subjected to a cyclic ponding regime with 0.55 M sodium chloride solution and the changes in concrete were monitored by measuring the changes in resistance between pairs of stainless steel electrodes embedded in the concrete at different depths from the exposed surface. The test was continued for nearly one year. The results indicated that, although the resistance of concrete decreased initially due to the penetration of chlorides, in the longer term the resistance of concretes containing ACMs outperformed the control concrete made with ordinary Portland cement (OPC). Drilled dust samples extracted after different durations of ponding were tested for the chloride content, which confirmed that the increase in resistance of the ACMs was due to the combined effects of the reduction in the penetration of chlorides and the continuous hydration activity of the ACMs.

Effects of three durability enhancing products on some physical properties of near surface concrete

Abstract This paper investigates the influence of three fundamentally different durability enhancing products, viz. microsilica, controlled permeability formwork and silane, on some of the physical properties of near surface concrete. Microsilica (silica fume) is a pozzolan, controlled permeability formwork ( cpf ) is used to provide a free draining surface to a concrete form, while silane is a surface treatment applied to hardened concrete to reduce the ingress of water. Comparisons are made between the products when used individually and used in conjunction with each other, with a view to assessing whether the use of combinations of products may be desirable to improve the durability of concrete in certain circumstances. The effect of these materials on various durability parameters, such as freeze-thaw deterioration, carbonation resistance and chloride ingress, is considered in terms of their effect on permeation properties and surface strength. The results indicated that a combination of silane and cpf produces concrete with very low air permeability and sorptivity values. The influence of microsilica was more pronounced in increasing the surface strength of concrete.

Near surface moisture gradients and in-situ permeation tests

Abstract Both the durability of concrete and permeation test results are influenced by the degree of saturation of capillary pores in concrete. In a laboratory, the moisture distribution of a test sample can be standardised by oven drying, however, this is not feasible for tests carried out in situ. Therefore, an investigation was carried out to monitor the moisture distribution in the near surface concrete over an extended period and study its effects on in situ air permeability and sorptivity test results. The results indicated that moisture distributions in the near surface concrete within 10 mm of the exposed face are highly susceptible to changes in exposure conditions. However, when the internal relative humidity of the near-surface concrete layer was less than 80%, the results of the in situ permeation tests could be used to identify the variation in quality of concrete.

Modelling the rapid retreat of building sandstones: a case study from a polluted maritime environment

Abstract Sandstones are widely used as building stones throughout NW Europe. Unlike limestone, sandstones tend to experience episodic and sometimes rapid surface retreat associated with the action of salts and often leading to the development of hollows/caverns in the stone. The unpredictability of these decay dynamics can present significant problems when planning conservation strategies. Consequently, successful conservation requires a better understanding of the factors that trigger decay and determine the subsequent decay pathway. An overview of results from previous studies provided the basis for simulation experiments aimed at identifying the factors that (a) initiate decay and (b) permit the continuance of salt weathering despite rapid loss of surface material. These simulation studies involve investigation of changes in micro-environmental conditions as surface hollows develop and examination of salt weathering dynamics within such hollows. These data combined with knowledge gained from previous work have allowed the refinement of a conceptual model of rapid sandstone retreat. In this model decay is linked to the establishment of positive feedback conditions through interactions between factors such as porosity, permeability, mineralogy and their effect on salt penetration.

NEAR-SURFACE SENSORS FOR CONDITION MONITORING OF COVER-ZONE CONCRETE

Abstract With more demands being made on reinforced concrete, 100-year guarantees of durability will become a necessity. Lifetime calculations, and prediction of the residual service-life of structures, require quantitative information on cover-zone properties and threshold values for corrosion initiation. It is clear that there exists a need to determine quantitatively those near-surface characteristics of concrete which promote the ingress of gases and/or liquids containing dissolved contaminants. In addition, in-situ monitoring of the temporal change in such properties could assist in making realistic predictions as to the in-service performance of the structure; likely deterioration rates for a particular exposure condition or compliance with the specified design life. This paper details covercrete sensor arrangements; format of data presentation and information that can be obtained from embedded sensors. Such sensors could, ultimately, form part of a high-level monitoring strategy and should be considered at the design stage.

Developments in Performance Monitoring of Concrete Exposed to Extreme Environments

AbstractThe performance of the surface zone of concrete is acknowledged as a major factor governing the rate of deterioration of reinforced concrete structures because it provides the only barrier to the ingress of water containing dissolved ionic species such as chlorides, which ultimately initiate corrosion of the reinforcement. In situ monitoring of cover-zone concrete is therefore critical in attempting to make realistic predictions as to the in-service performance of the structure. To this end, this paper presents developments in a remote interrogation system to allow for continuous, real-time monitoring of the cover-zone concrete from an office setting. Use is made of a multi electrode array embedded within cover-zone concrete to acquire discretized electrical resistivity and temperature measurements, with both parameters monitored spatially and temporally. On-site instrumentation, which allows for the remote interrogation of concrete samples placed at a marine exposure site, is detailed together wi...

Depth-related variation in conductivity to study cover-zone concrete during wetting and drying

In situ electrical property measurements are used to study cover-zone concrete under a cyclic wetting and drying regime. OPC concrete and OPC with partial replacement with GGBS and PFA were used in the experimental programme with samples exposed to water and chloride solution. Monitoring the variation of electrical properties at discrete points within the surface 50 mm, during absorption and drying, provides information on water and ionic penetration into the cover zone, drying response, and evidence of continuing hydration and pozzolanic reaction.

Monitoring of Corrosion in Structural Reinforcing Bars: Performance Comparison Using In Situ Fiber-Optic and Electric Wire Strain Gauge Systems

This work addresses the problems of effective in situ measurement of the initiation or the rate of steel corrosion in reinforced concrete structures through the use of optical fiber sensor systems. By undertaking a series of tests over prolonged periods, coupled with acceleration of corrosion, the performance of fiber Bragg grating-based sensor systems attached to high-tensile steel reinforcement bars (ldquorebarsrdquo), and cast into concrete blocks was determined, and the results compared with those from conventional strain gauges where appropriate. The results show the benefits in the use of optical fiber networks under these circumstances and their ability to deliver data when conventional sensors failed.