by Peter Shaw, Aimin Xu STK Inter Test AB, S-254 67 Helsingborg, Sweden
Acknowledgement:
NDE TECHNIQUES CAPABILITY DEMONSTRATION AND INSPECTION QUALI FICATION
Proceedings of the Joint EC OECD IAEA Specialists Meeting held at Petten on 11 - 13 March 1997
Edited by U. von ESTORFF and P. LEMAITRE
European Commission JRC, Institute for Advanced Materials PO Box 2, NL-1755 ZG Petten, The Netherlands
Directorate-General Joint Research Centre
The evaluation of the condition of some types of concrete structures is possible by following guidelines set out in procedures and manuals, such as the "Handbook for Bridge Inspection" published by the Swedish Road Directorate. It would be desirable to have similar guidelines for other categories of important structures, particularly in cases where safety aspects are dominant. A nuclear power plant (NPP) consists of a number of functionally different structural components in a variety of environments and effective inspection is only possible if we know what we are looking for and have the capability to apply reliable and relevant testing techniques.
This report briefly discusses actual and potential deterioration processes and their causes based upon several years of investigative work on concrete structures in Scandinavia, including NPP and provides some input for development of practical inspection procedures. Some common deterioration processes are highlighted to bring attention to the need for inspection. A more detailed study of chloride attack and reinforcement corrosion is presented, and finally a summary is given of state-of-the-art technologies for in-situ testing.
The humidity paradox
Concrete is a living material and in terms of durability and structural ageing is governed by moisture content. As pointed out by Mehta et al [1] that water is "at the heart of most of the physical and chemical causes underlying the deterioration of concrete structures". Among other effects, moisture levels determine the risk of corrosion attack occurring on cast-in steel and reinforcement and the rate of deleterious mechanisms such as alkali-aggregate reaction (AAR). At the same time a long-term ageing effect caused by drying-out of the cement matrix in concrete will be evident and the result will be reduced strength. A combination of dry and wet concrete may cause differential shrinkage which in turn may well lead to cracking. A balanced and stable moisture level would seem to be desirable, but cannot be achieved since the structural members are usually massive and are subject to different environments.
Moisture variations affect testing performance as the speed and penetration ability of acoustic and electromagnetic pulses used in modern techniques are strongly dependent on this factor. The criteria used in evaluating electrochemical test results are similarly affected by moisture content (oxygen availability). It may be said that any advances in non-destructive testing methods will be dependent on the ability to determine the moisture condition of massive concrete members on site and the ability to use this information in processing measurement data.
Damage in relation to the age of the structure
When the structure is newly taken into service there may occur damage which is attributable to unsatisfactory construction practice. The damage may have an immediate effect on the structural integrity, such as in the case of voids in walls of which there may be no visible evidence - concealed defects. Poor construction usually leads to reduced durability which will manifests itself in later years.
The working life of the structure may be reduced or extensive maintenance may be required as a result of deterioration of materials, usually steel subject to corrosion attack. Evidence of this type of damage may appear after 15 or 20 years and is strongly environment dependent. The damage processes may occur in ancillary structures of an NPP and are similar to many other forms of concrete structures. Corrosion may be detectable at an early stage and prior to serious damage occurring to the extent that the functionality of the structure is affected.
Should an NPP be required to maintain its capability of shielding or isolating radioactive materials over long periods of time following decommissioning then extra demands are placed on durability. At any age the quality of a structure in the sense of integrity and potential durability is almost totally dependent on the quality of construction. It could therefore be argued that condition assessment testing should be carried-out at as early an age as possible. preferably immediately following construction.
Nuclear containment buildings are designed to withstand internal pressures and to act as a barrier to the release of radioactive products in the event of an accident. The walls are typically 1 - 1.5 m nominal thickness pre-stressed monolithic structures and may have a steel liner cast in some 300 mm from the inner surface. These structures are often cast using the slip-form technique, which is briefly described below.
Construction using the slip form technique
The process of slip-forming involves using a form which moves continually upwards at a rate of up to 300 mm per hour while fresh concrete is poured in 150 mm layers into the top. As shown in Fig. I, the forms are inclined inwards some 6 mm/m to allow the concrete to detach itself from the bottom of the form. Plastic concrete placed into the top of the form should be stable by the time the formwork leaves it below (a penetration resistance of between 50 and 200 psi measured in accordance with ASTM C 403 is normally required at the trailing edge of the formwork) and should therefore not sag.
TABLE OF CONTENTS
Fig. 1. Slip form construction may cause problematic zones in concrete.
In Fig.1 it can be seen that the thickness of the concrete wall increases successively from the top of the form until the side support ends. This increase in thickness is partly hindered by the reinforcement near the surfaces. This means that the concrete outside the reinforcement can move more readily against the form than the inner concrete. The risk of cracks forming is thus dependent on the consistency of the concrete, Fig.1(a). This type of cracking is hazardous in open-air environments where aggressive elements such as Cl, CO2, SO2 and rainwater can penetrate to the reinforcement.
Another form of cracking is that caused by separation of the upper section of the plastic concrete in the form. A large horizontal separation crack develops and is masked by cement paste which is smeared by the form at the surface, Fig. 1 (b).
In Fig.2 we see the result of concrete "hang-up" in the shell wall of a concrete chimney. This particular defect went unnoticed for many years after construction. The design of the concrete mix in this case was probably unsuitable and this combined with poor compaction led to the formation of a large internal void. Internal voids adjacent to the steel liner of an NPP deny the corrosion protection normally provided by cement paste and this may have serious consequences in the warm and humid environment of a power station. Structures dating from 1940, 1970 and 1985 have been inspected by the authors and have shown similar patterns of damage as described above.Pre-stressed cables and their protection
Tensile stresses in the containment walls and roof of a NPP are intended to be restrained by pre-stressed cables. These are high-tensile steel cables or wires which are placed in ducts of approximately 70 mm diameter. After tensioning the cable ducts are injected with a cement grout to provide corrosion protection. In some cases the ducts are filled with grease or oil. After injection with cement grout it is not possible to remove the cables for inspection. Generally speaking pre-stressed cables may be prone to one or more of the damage types listed below:
corrosion of the steel cables due to external effects (intrusion of aggressive elements)
initial damage caused by heat-treatment process
handling damage during transport and construction
weather conditions during construction
injection process
injection material and its chemical composition
hydrogen embrittlement and stress corrosion
leakage currents ( hydrogen formation at the cathode or anodic corrosion effects )
In the most extreme cases damage to pre-stressed cables may lead to failure and collapse, as occurring in the case of bridges and industrial buildings in Germany and the UK.
One of the primary areas with respect to corrosion attack of the stressed cables is a void in the cable duct since the steel is not protected by the alkaline cement environment and is exposed to intrusion of aggressive elements. The testing situation is thus one of first locating the cable ducts m the concrete members (preferably by non-destructive means) and then detecting voids in the grout filling of the duct. If the physical condition of the steel cables can be detected using NDT then this is of course a bonus.
Concrete moisture content and differential shrinkage
Fig. 2. A slip-formed concrete chimney after removal of the cover concrete by hydro-demolition. The large horizontal void had existed since the time of construction
Water is one of the major components in concrete, it reacts with cement and makes the fresh concrete mixture workable. Only part of the water will be consumed by the cement hydration, and the rest will be absorbed in the capillaries in concrete, ready to evaporate when the concrete is exposed to a dry ambient condition. Drying of the concrete causes moisture to evaporate from the capillaries, and the cement paste shrinks to compensate for the surface energy change. As the drying is taking place through the concrete surfaces, it will create an uneven moisture distribution from the surface Awards and consequently a differential shrinkage for the concrete member. This may lead to tensile stresses with resulting crack formation.An example of a thick concrete element with one-sided water pressure is shown in Fig.3. The humidity profile in this case is shown for depths 150, 300 and 800 mm from the "dry" surface. The age of the structure at the time of test was approximately 25 years. Visible cracks at the surface were found to extend to approximately 300 mm depth.
Chloride induced reinforcement corrosion - Case histories
The presence of chloride (Cl) in reinforced concrete has been recognized, since the 1950s, as one of the major reasons for reinforcing steel corrosion in reinforced concrete, such as maritime structures, highway bridges and parking decks in areas where Cl-containing de-icing salts are used during the winter season. Corrosion has been attributed to the fact that cl destroys the protective passivation film which is formed on steel surfaces due to the high pH condition in concrete pore solution (pH=12-13).
In recent decades, research on concrete durability has greatly increased and has demonstrated that many of severe problems, such as cracking, delimitation and spalling of concrete, are related to cl induced steel corrosion. The corrosion induced damage is a serious problem for the infrastructure and commonly found in structural members such as pillars and beams. If sea-water is used as the coolant in power stations, it can of course affect the concrete cooling water system and could lead to stoppages in production.
Fig.3. Moisture distribution in a concrete element with one surface exposed to a dry (room) environment whilst the opposite surface was occasionally wetted by condensed water.
From the point of view of providing sufficient information to the owners of concrete structures to enable them to plan the best strategy for production, including repair and maintenance work, it is important to inspect the Cl distribution in concrete. Fig.4 demonstrates the variation of total Cl in concrete, i.e. the profile of Cl content in relation to depth from the surface which had been exposed to sea-water. The ingress of Cl depends on both the concentration of Cl in the seawater and the quality of the concrete. This is evidenced by the samples taken from Structure 2, which despite the shorter service time and less saline seawater, the depth of the affected zone is found to be greater compared with Structure 1.The surface layer of a few millimeters thickness contains less Cl . This is more obvious for the lower quality concrete (Structure 2) and for the concrete at semidry conditions, and can be mainly attributed to the deterioration of the concrete. The surface concrete in the seawater splash zone was carbonated by the CO2 m the surrounding air. It was washed by seawater and rain, which has caused decomposition of the cement paste due to leaching-out of lime, and was subjected to other physical and chemical deterioration processes such as sulphate and magnesium attacks.
The maximum Cl content is about 0.2 % by mass of concrete, occurring at about 5 mm under the surface for Structure 1. The maximum Cl content in Structure 2 is about 0.25% which seems to be quite high, considering the fact that the Cl content in the seawater in this area is only 1.8 g/l (or 0.05 mole/l). This can be explained by its lower designed strength (higher water/cement-ratio ) which corresponds to a higher porosity and water permeability. Experimental results also showed that about 70% of the total Cl in concrete could be extracted by water, which indicates that a large part of the Cl ions are only loosely adsorbed by the concrete and may participate in the steel corrosion process.
Indeed the in-situ measurement of electro chemical potential of Structure 2 showed active areas where the reinforcement may have started to corrode. According to Sagoe-Crentsil and Glasser [2], in a dilute Cl solution of 0.010-0.015 mole/l, the solubility of iron at pH appr. 13 abruptly increased from nearly zero to 0.170-180 mole/l, i.e. the corrosion is likely to occur even at the presence of such low concentration of cl. A higher threshold value proposed earlier by Hausmann [3] was mole ratio Cl/OH > 0.6 (Cl=0.06 mole/l in concrete with pH=13), which can be expected to prompt a rapid corrosion process.
Alkali-aggregate reactions
This is a form of chemical reaction which occurs between aggregates which contain alkali-reactive constituents and cement alkalis in the pore water of the concrete. The most common case is alkali-silica reaction (ASR), which is a reaction between alkali and some siliceous compounds in aggregates producing a type of gel. When in contact with water, the gels swell causing tensile stresses and ultimately cracking, which often results in a "map pattern" on the concrete surface. Of the Scandinavian countries ASR damage has so far only been a serious problem in Denmark.
The detrimental expansion takes several years to develop in field concrete structures, thus the potential risk is often evaluated in the laboratory under accelerated conditions. For example, some Norwegian rock types have been considered to be non-reactive only to be re-evaluated on the basis of experiment. In these studies it was found that for reactions to occur then at least 20 % of the total aggregate content should be reactive, and that the alkali content of the concrete should be high i.e. more than 3 kg equivalent Na2O/m3 concrete [4]. A pre-requisite for AAR is of course high moisture levels. When a structure is suspected to have AAR, both the reactivity of the aggregates and humidity levels in concrete need to be examined, and the development of cracking closely monitored.
It should be noted that the development of cracking may accelerate after some years. The consequences, apart from reduced durability due to cracking, are primarily reduced tensile strength.
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Fig.4. Cl distribution in concrete.
Structure 1: Quay at a seaside, 23 years.
Structure 2: Water channel (for sea-water), 18 years. Cl- content was determined by chemical analysis of specimens sampled from the structures.
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