A Long

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.

WHY ASSESS THE PROPERTIES OF NEAR-SURFACE CONCRETE?

Abstract The deterioration of concrete structures is a major problem in many countries throughout the world. This has prompted the search for methods of predicting the service life of both existing and new structures. Current prediction methods are still in their infancy and, before they can be used with confidence, more reliable information on the properties of the concrete in these structures will be required. In this paper a range of test methods for determining the strength and transport properties and the extent of corrosion are critically reviewed. Whilst all provide useful information the ability to measure the transport properties has been shown by many researchers to give the most reliable indication of the likely durability. In spite of this it is recommended that an holistic overview is adopted as no single test is sufficiently reliable at present. Using such an approach, durability-based design criteria can be developed for concrete and the remaining life of existing structures estimated.

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.

Development and Longer Term In Situ Evaluation of Fiber-Optic Sensors for Monitoring of Structural Concrete

In this paper, new solutions to the problem of making measurements, of carbonation and chloride ingress, in particular, in concrete structures are considered. The approach has focused on the design, development, and use of fiber-optic sensors (FOSs), recognizing the need in that conventional devices are often either inaccurate, expensive, or unsuitable for encapsulation in the material. The sensors have been designed to monitor, in situ and nondestructively, relevant physical, and chemical changes in cementitious materials. Three different types of FOS were constructed, tested, and evaluated specifically for this application, these being a temperature sensor (based on the fluorescence decay) and pH and chloride sensors, based on sol-gel (solidified gel) technology with appropriate impregnated indicators. The sensors were all designed to be inserted into the structures and evaluated under the harshest conditions, i.e., being mounted when the mortar is poured and thus tested in situ, with the temperature and pH sensors successfully embedded in mortar. The outcomes of these tests have shown that both the temperature sensor and the pH sensor were able to function correctly for the duration of the work - for over 18 months after placement. The laboratory tests on the chloride sensor showed it was able to make measurements but was not reversible, limiting its potential utility for in situ environments. Research is ongoing to refine the sensor performance and extend the testing.

Corrosion of reinforcement in concrete repair

Abstract For a satisfactory patch repair to a concrete structural element the prevention of reinforcement against further corrosion is an important consideration. Therefore, the performance of a repair mortar and reinforcement coating in protecting the reinforcement against further corrosion may need to be considered in the selection of a repair system. An experimental investigation was designed to provide comparative data on the performance of five repair materials in preventing corrosion of both coated and uncoated reinforcing bars. A normal outdoor environment in the British Isles and an environment exposed to de-icing salt were considered. Corrosion was assessed by the half-cell potential method and visual examination. The paper discusses how the repair material influence the resistance to corrosion of reinforcement at the interface between the repair material and the surrounding concrete is affected by shrinkage, adhesion and compaction of the repair material. Conclusions and recommendations for corrosion protection are made. These can be used in assessing the suitability of repair materials for any given application.

State-of-the-Art Applications of the Pull-off Test in Civil Engineering

In 1974, the concept of using pull-off test as a means of predicting the compressive strength of concrete was patented in the UK by Professor Long at Queens University Belfast. Initially, the pull-off test was developed primarily aiming at unplanned in-situ strength determination. It was found later also particularly suited for assessment of bond strength of repairs. This test has now been accepted in British Standards, BS 1881-207:1992 for assessing the in-situ concrete strength in structures and BS EN 1542:1999 for assessing the bond strength between overlays and substrate concrete in patch repairs. In addition, BS 1881-201:1986 recommends it for quality control and long-term monitoring of hardened concretes. It has also been accepted in North America (ASTM C1583-04), Holland and some other countries for similar applications. This paper reviews its development history, principle, factors affecting the test results as well as its current applications in civil engineering fields.

In-situ Assessment of Durability of Concrete Motorway Bridges

Assessment of durability of concrete motorway bridges in Northern Ireland is part of a programme of collaborative research of the Civil Engineering Department with the Roads Services of the Department of Environment (Northern Ireland). This involves testing bridges on site in order to develop methods which can be used to investigate the condition of approximately 1300 reinforced concrete bridges in the province so that a planned maintenance scheme can be proposed. This paper describes results of comprehensive tests which were carried out on a severely delaminated motorway bridge both prior to and after repair. It has been observed that the strength of concrete is not a reliable index of proneness to deterioration, but permeability results allied to the knowledge of exposure to salt are more likely to provide a useful guide to the durability. The in-situ tests for sorptivity indicated that with the two silane treated bridges, one keeps the moisture out whereas the other fails to do so. High permeability concrete combined with leaking expansion joints have been found to be the main causes of absorption of chloride and the corrosion of steel.

Repeatability and reliability of new air and water permeability tests for assessing the durability of high-performance concretes

This paper reports on the accuracy of new test methods developed to measure the air and water permeability of high-performance concretes (HPCs). Five representative HPC and one normal concrete (NC) mixtures were tested to estimate both repeatability and reliability of the proposed methods. Repeatability acceptance was adjudged using values of signal-noise ratio (SNR) and discrimination ratio (DR), and reliability was investigated by comparing against standard laboratory-based test methods (i.e., the RILEM gas permeability test and BS EN water penetration test). With SNR and DR values satisfying recommended criteria, it was concluded that test repeatability error has no significant influence on results. In addition, the research confirmed strong positive relationships between the proposed test methods and existing standard permeability assessment techniques. Based on these findings, the proposed test methods show strong potential to become recognized as international methods for determining the permeability of HPCs.