Concrete Deterioration in Mining Structures

Transcription

1 Concrete Deterioration in Mining Structures A. Masarira & T. L. Bopape Anglo Operations Limited, Johannesburg, South Africa ABSTRACT: Concrete structures form a large section of mining infrastructure. They find use both in opencast as well as mine shafts, containment structures as well as material processing plant structures. Given the huge investments that accompany the design and construction of these structures as well as their criticality in the production line of a mine, it is important that these concrete structures do not experience premature failure or deterioration that would have an impact on production. The reality however is that these structures experience regular failure which requires remedial action. Some of the causes of such failures are due to poor design practices, poor construction and workmanship, failure to fully understand the environmental conditions in which these structures will operate and inaccurate assumptions made during the design process on the loads (especially dynamic loads) which the structures will be expected to carry during their lifetime. Another cause is the operational changes made resulting in these structures being required to carry loads they were not originally designed for. This paper presents some of the common forms of concrete deterioration in mining structures, their possible causes and some of the remedial action taken to restore the integrity of theses concrete structures. 1 INTRODUCTION Concrete structures form a large component of mining infrastructure. Common concrete application in mining operations is in areas such as the shaft lining, material containment structures (silos and bunkers), liquid containment structures (water reservoirs), foundations of material processing equipment such as rock breakers, crushers and mills as well as in building structures such as scrubbing and screening or crushing buildings. The conditions under which these structures operate in the mining, in terms of loading and environment are unique and the structural design needs to consider these. The type and level of concrete deterioration in mining structures is a function of these operating conditions and it is therefore critical that any repair and rehabilitation methods developed take cognizant of these factors. 2 OPERATING CONDITIONS Concrete mining structures are often exposed to cor rosive environments as a result of the wet processes that often takes place in these operations. The washing and cleaning of mineral stock which is an integral part of mineral processing is not always done with fresh water, but rather with recycled water. This water permeates through the cracks that usually exist on concrete structures and result in the corrosion induced damage of these structures. Sometimes chemicals are used in mineral processing water, which result in corrosive conditions. There are some mining operations that are located in coastal regions and the chemical composition of sea water makes that environment highly aggressive. Abrasion Physical Causes of Concrete Deterioration Surface Wear Cavitation Erosion Volume Changes Cracking Temperature Extremes Structural Loading Figure 1. Physical Causes of Concrete Deterioration

2 Mining structures are often expected to carry large material loads (rocks and ore) and a number of the processes generate dynamic loads e.g. through crushing, scrubbing, screening or milling processes. The support of heavy equipment and machinery as well as the generation of dynamic loading condition result in the concrete structures being susceptible to an increased rate of deterioration. In shaft mining, depths to levels such as 2 000m are easily reached and the concrete structures in such shafts are often under extreme environmental and loading conditions. Mining structures, in general, can be exposed to large temperature fluctuations with differences between the highest and lowest temperatures as high as 40 0 C. These and other factors result in general concrete deterioration (Fig.1) Figure 4. Concrete Support of Rock Breaker 3 CONCRETE STRUCTURES IN MINING The various concrete structures in mining are exposed to a wide range of loading and operating conditions. A number of such structures are indicated in the photos below. Figure 5. Concrete Plinth of Scrubber Figure 2. Concrete Barrier at Primary Crusher Tip Figure 6. Concrete Plinths of Scrubbers Figure 3. Concrete Structures at Primary Crusher Concrete structures supporting mineral processing equipment are susceptible to dynamic as well as impact loads. Structures in the crushing areas (e.g. Figures 1 3) are exposed to impact loads (reversing trucks) or rock-breaker forces.

3 Figure 7. Scrubbing and Screening Plant Building Figure 10. Coal Bunker Solid material (ore) and liquid containment structures such as silos, bunkers or thickeners (Figures 7-11) are often built in concrete. This has to do with ease of construction, costs and the fact that concrete offers a higher degree of resistance to abrasion and corrosion than metal materials. Figure 8. Wearing and Abrasion inside Silo Figure 11. Thickener/Clarifier Figure 9. Silo on Platform Concrete deterioration on silos often occurs on the inside wall due to abrasion. Some highly abrasive ore like Platinum ore leads to a high rate of wearing inside the silos (Figure 8). In coal mines, there are relatively frequent cases of spontaneous combustion of coal inside the silos leading to the burning coal causing damage to the concrete. Liquid containment structures such as thickeners or clarifiers could experience leaking of the water through the walls. Leaking might be a result of cracks forming in the wall or through constructions joints that were not sealed adequately. The passage of water though the

4 wall can result in the corrosion of the reinforcing steel and further cracking of the concrete. Figure 12. Wearing of Cover In order to protect them from corrosion and other forms of deterioration due to the wet process that are common in mineral processing, structural steel columns are often placed on concrete plinths (Figures 12-14). However the concrete plinths themselves experience levels of deterioration which result in surface wear or cracking. Figure 15. Complete Loss of Cover The deterioration of concrete in such structural members could result in the penetration of water into the concrete and the corrosion of the reinforcement steel. In some case this deterioration leads to loss of cover and thus expose the rebar steel completely (Figure 15). 4 CONCRETE DETERIORATION Figure 13. Cracking of Concrete Plinth Figure 14. Cracking of Concrete Column The level of deterioration of concrete is indicative of its durability, which is its ability to resist weathering action, chemical attack, abrasion or other processes of deterioration. Concrete structures in mining experience both chemical and physical degradation processes. There are many stages in the processing of mineral ore which require the use of water. The water mostly used in these processes is recycled and therefore not as clean as fresh drinking water. This water often gets into contact with the concrete structural elements of the processing plants and encourages concrete corrosion. Very often this water is used to clean concrete floors, slabs and other structures and this water often collects on various surfaces of the structures. Therefore the leaching (and even acid attack) of concrete is relatively common in such mining operations (Figures 12, 13). Sulfate attack is also

5 possible, especially in mineral processing where effluents that are sulfate-containing are present. With time the water finds its way through cracks in the concrete (Figures 5, 13, 14) resulting in the corrosion of the reinforcement. This leads to further cracking and spalling of the concrete. In some mining operations the temperature difference changes in a single day might be such that a freeze-thaw effect is experienced by concrete structures. This can lead to cracking of the concrete. The deterioration of concrete structures in mining and other industrial operations can lead to structural failures. In order to avoid structural failures and operational downtime, it is imperative that concrete repair and rehabilitation procedures be developed and implemented. 5 REPAIR AND REHABILITATION 5.1 General The basic stages in the process of concrete repair and rehabilitation are: Investigation and analysis of the concrete Diagnosis of deterioration of the concrete Assessment of maintenance priority Develop concrete repair solutions and remedial strategies. The main objective of the repair procedure and methodology is to stop the progress of any corrosion process that might be taking place in the structure as well as to reduce the probability of corrosion occurring in the future. A clear indication of corrosion damage is the presence of cracks on the concrete. These cracks allow the permeation of moisture into the structure and leading to damage of the steel reinforcement and further concrete damage. Cracks are generally classified by width ranging from about 0.05mm to 1 mm. It is therefore critical that the presence of cracks be taken seriously and the width of the cracks be considered in determining which repair procedure to use. 5.2 Surface Preparation The repair procedure requires a surface preparation i.e. conditioning of the existing concrete to receive repair materials. Conditioning can involve the removal of deteriorated, contaminated or damaged concrete in order to provide surfaces that will promote bonding of the repair material. Remove damaged or deteriorated concrete using methods such as chipping and drilling. When embedded reinforcement steel is encountered, special care must be taken not to damage it. Prepare surface boundaries and the geometry of boundaries should minimize edge lengths. This helps avoid stress concentration points where debonding may occur. Clean concrete surface using high pressure water jetting or other methods such as sand blasting. Surface cleaning is necessary to achieve adequate bonding between the repair and existing concrete. 5.3 Cleaning of Reinforcement If corroded reinforcement steel is encountered under the concrete cover, the area around the corroded steel must be uncovered. Depending on the degree of corrosion and the extent to which the steel has lost its cross-sectional area, a different method of intervention is required. For minimal corrosion of steel, cleaning of the steel and the application of corrosion inhibiting material might be adequate. For significant corrosion of steel (steel has lost significant cross-sectional area), the steel reinforcement would need to be replaced with equivalent reinforcement steel. Extensive corrosion of the steel reduces the load carrying capacity of the reinforcement and hence the integrity of the structure. The new reinforcement steel must be installed in such a way that it overlaps the rest of the steel. The length of the overlap on either side must be the larger of the two values; 15 times the steel diameter or 300mm. The replacement steel could also be welded to the old steel. But in the case of welding, it is recommended that a competent engineer approves the welding procedure. 5.4 Patch Repair After the reinforcement steel has either been cleaned (for minimal corrosion) or replaced with new steel, the concrete cover must be reinstated. The most common and efficient method is patch repair i.e. the application of a cementitous repair mortar. This is a multi-purpose patching and repair mortar and can be used on interior or exterior horizontal and vertical surfaces. In cases where the crack width is less than 1mm, this can be considered to be minor cracks and the concrete would not require extensive repair methods. However these cracks need to be sealed, in order to stop the penetration of moisture

6 into the concrete. First of all the substrate (concrete surface) must be cleaned with high pressure water jet to expose the cracks. Polyurethane or epoxy material can then be injected or applied to fill the cracks. 5.5 Specific Repairs In the areas where the reinforcing steel is so close to the surface that it is exposed (visible), it is recommended that the steel be protected from corrosion damage through a coating system. The concrete surface as well as the exposed steel be thoroughly cleaned. If the steel reinforcement has not lost any significant cross-section, it must simply be cleaned (but not replaced). An appropriate maintenance procedure on the steel needs to be applied. The visible cracks at the lower portions of the columns, for example, are very likely a sign of deterioration due to the frequent presence of process water around the columns. The following is therefore highly recommended: Water should not be left standing for long periods of time on the concrete surfaces and around the columns. The cracks should be sealed as recommended in section 3. The concrete must be observed and monitored regularly in order to determine if the cracks reappear. Structural components (both steel and concrete) constantly exposed to water and processing chemicals. The following is therefore recommended: Open up and remove concrete cover in all areas where cracking is evident. All the corroded portions of the reinforcing steel be exposed and removed. New steel reinforcing be installed according to the procedure outlined in section 2.2 New concrete cover be applied Apply protective coating on the new surface in order to reduce the corrosion process in the future. It is possible that the corrosion deterioration on the structure has progressed such that it would be recommended to conduct substantial demolition on the structure and construct a new one. If in the course of removing the concrete cover over 20% of the reinforcement steel has to be replaced (due to corrosion damage), it is recommended that the entire structure be demolished and be replaced with a new one. 5.6 Liquid Containment Structures The deterioration on liquid containment concrete structures (e.g. thickeners) could be a result of some chemical attack on the concrete. Sometimes there is evidence of moisture seeping through the walls from the inside to the outside. This often leads to the deterioration and damage of some portions of the concrete cover. It is recommended that At the next scheduled emptying of the tanks, a lining (e.g. bitumen) be applied on the in order to seal the inside of the tanks. Corrosion inhibitors and protective coating be applied on the surface. 5.7 Replacement of Structure or Members In cases where any form of repair is either not possible or economically viable, the entire structure or structural member would have to be replaced. 6 CONCLUSION The repairing or rehabilitation of concrete structures need to be carried out by competent contractor. The construction repair material and corrosion inhibitors can be sourced from reputable suppliers. The concrete structures and the repairs done should be regularly monitored for corrosion damage. The initial signs of corrosion damage are wetness, brownish stains or flaking of concrete skin. Procedures for repair must be implemented as soon as these signs appear on the concrete. REFERENCES Bohni, H Corrosion in Reinforced Concrete Structures, Woodhead Publishing Limited (CRC Press). Bissschop, J 2011, Concrete Durability II, Institute of Building Materials, Swiss Federal Institute of Technology, Zurich Mackechnie, J.R., Alexander, M.G Repair Principles for Corrosion Damaged Reinforced Concrete Structures (Research Monograph No.5), University of Cape Town. Mehta, P.K Concrete in the Marine Environment, Elsevier Applied Science.