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Ishita Manjrekar |
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Sourabh Manjrekar |
Non-prestressed reinforced concrete liquid containment structures—in particular, non-circular tanks, often exhibit vertical and diagonal cracks that are aesthetically objectionable. More importantly, cracks could result in loss of stored liquids, leakage of hazardous materials, concrete deterioration, and corrosion of reinforcing bars causing serviceability failures. Concrete liquid retaining tanks have a stringent serviceability requirement in terms of limiting crack widths. It is important to note that such cracks are seldom indicative of structural failure and for the purpose of this article we will be limiting the discussion to non-structural and non-corrosion cracks
Shrinkage and differential thermal expansion and contraction due to temperature and other environmental gradients in the exposed surface usually result in restrained movement of the RCC and tends to cause vertical and diagonal cracks in liquid containment structures. It is important to appreciate that not all cracks require repair and reader may refer to ACI 224R, Table 4.1, for crack widths that require repair or remediation.
Liquid containment structures, such as large rectangular tanks, often exhibit vertical and diagonal cracks that are usually the result of restrained concrete shrinkage and thermal contraction, typically spaced 1.2 to 3 m apart. Additionally, such structures may have concrete roof slabs that keep the structure liquid-tight to prevent contamination of the contents by exterior exposure. In these cases, differential shrinkage and thermal deformation of the concrete could result in significant wall and roof cracking if the appropriate expansion joints are not provided.
| Structures with movement joints in the walls and without matching joints in the base slab are prone to crack development not only in the walls adjacent to the joint, but also in the base slab below the joint. Due to the restraint of the base slab, the cracks typically extend diagonally, vertically, or both, and occur on both sides of the movement joints. The width and spacing of cracks depends on concrete shrinkage and creep, the size and spacing of horizontal reinforcement, wall thickness, height and length of each placement (distance between vertical construction joints), and length between movement joints, member restraints, and the concrete mixture. Crack widths can be controlled with appropriate reinforcement and detailing that result in tight cracks that do not leak. |  |
Recommended Best Practices For Repair Of Vertical And Diagonal Cracks In Liquid Containment Structures
Prior to selecting a repair methodology, the cause of the cracks is required to be analysed. It is important to classify cracks as active or dormant and determine if corrosion is active in cracked areas. Shrinkage of concrete continues over an extended period of time and the resulting shrinkage cracks should be considered active, especially if the structure is subjected to cycles of wet and dry periods. Dormant cracks usually result from an event of limited duration, such as temporary overload conditions during construction. It is important to verify that the cracks are non-structural and unrelated to corrosion of steel.
A. Waterproofing And Repair Treatment To Dormant Cracks
A dormant crack is one whose width does not change with time. In the absence of corrosion, dormant but leaking cracks can be repaired by pressure injection of SUNEPOXY 368TM low viscosity epoxy grout or POLYALK 2M1PTM ultra low viscosity polymeric chemical grout by vacuum injection, or routing and sealing on the interior or exterior wall surfaces, or both.
B. Waterproofing And Repair Treatment To Active Cracks
Active cracks are repaired by pressure injection with chemical grouts or by routing and sealing with a flexible sealant on the interior or exterior wall surfaces, or both. Further it is most important to apply a joint free, liquid applied flexible barrier membrane on the liquid retention side of the wall (positive side) like POLYALK WPTM. Active cracks can be repaired by: 1) pressure injecting of chemical grouts; 2) routing and sealing of cracks; and 3) installing a flexible waterproofing and barrier system. These methods are considered serviceability repairs and not structural repairs.
Active cracks are repaired by pressure injection with chemical grouts or by routing and sealing with a flexible sealant on the interior or exterior wall surfaces, or both. Further it is most important to apply a joint free, liquid applied flexible barrier membrane on the liquid retention side of the wall (positive side) like POLYALK WPTM. Active cracks can be repaired by: 1) pressure injecting of chemical grouts; 2) routing and sealing of cracks; and 3) installing a flexible waterproofing and barrier system. These methods are considered serviceability repairs and not structural repairs.
i) Chemical Grout Injection — Injection of flexible hydrophobic polyurethane foam grout material SUNAQUASEALTM is often used for the crack repair in containment structures. SUNAQUASEALTM retains most of its volume after curing, even if the surrounding concrete should become dry, which is advantageous for repairing active cracks. Typical hydrophilic grouts tend to shrink when allowed to dry out and lose volume, resulting in active leaking when the liquid is reintroduced at a later time.
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SUNAQUASEALTM can be used to mitigate leaking cracks with injection performed from the exterior side of a liquid containment structure so the tanks need not be emptied. Interior injection can also be accomplished without draining the tank by experienced divers performing the work underwater. Some excavation may be required to access cracks below grade. When leakage is present, injection using a water-activated resin like SUNAQUASEALTM is recommended. The leakage of water will be slowed down, and possibly stopped, during the injection process. |
For tanks containing potable water, chemical grouts and other repair products directly exposed to the water must comply with CFTRI requirements for use in potable water.
For extensive cracking below grade, the application of a waterproofing system might be necessary. There are conditions, however, where injection from the inside wall face is especially recommended to prevent liquid exfiltration, which could require the tank be emptied. Injection from the inside, also provides access for crack repair below grade for buried or partially buried structures without excavation.
The proper climatic condition is crucial for successful crack injection, especially if SUNAQUASEALTM polyurethane chemical grout is used. In cold climates, it is best to complete work at moderate temperatures. Additionally injection in the summer when cracks are the narrowest should be avoided. Repairing cracks in the winter, when they are the most open, is beneficial but costly in case of freezing temperatures. Special heated enclosures could be required to facilitate proper injection and setting of the injection material in freezing conditions.
Crack injection should not be used to repair cracks caused by corrosion of steel reinforcement unless supplemental means are used to mitigate the cause of the cracks and corrosion. If corrosion is present, it should be evaluated before making repairs. This article does not cover repair of cracks resulting from steel corrosion. There are various methods to mitigate, prevent, and control corrosion of reinforcing steel in concrete (ACI 222R) which may be referred to as required.
Injection penetration can be assessed by extracting core samples that intercept the repaired cracks. Usually, one or two cores taken at random locations for every 30 m of injection is adequate as per ICRI guidelines. Typically, penetration is considered adequate if 90 percent of the crack is filled with injection grout. Although, some non-destructive acoustic test methods may be used in some circumstances for testing epoxy adhesive injection repairs, it is not recommended to use these methods for flexible injection materials because the presence of low-modulus materials in cracks and voids do not significantly change the acoustic response from the structure.
ii) Routing And Sealing — with a flexible sealant, incorporating details that permit some movement. Because routing and sealing are performed on the liquid side of the containment structure for tank leakage, the structure should be emptied. In some cases, routing and sealing cracks on the exterior side can be used to reduce the potential for contaminants penetrating the containment structure.
C. Waterproofing Of Liquid Retaining Tank By Installation Of A Liquid- Applied Joint Free Flexible Membrane (Barrier) System
ELASTOROOF PUTM is the most effective and durable treatment to repair and prevent containment structures from leaking. This method is preferred for large tanks which need a long term and durable solution. The tank should be completely empty, and allowed to dry (surface moisture < 4%) before application of the barrier liquid membrane system. ELASTOROOF PUTM is applied in two coats over a suitable primer, and allowed to cure for 48 hours. The resultant barrier membrane is strong, joint free, highly flexible and elastic and extremely durable.
ELASTOROOF PUTM manufactured by Sunanda Global is a single component, high build, liquid applied polyurethane chemistry was adopted as the technology of choice for the waterproofing coating system. This material chemistry offers significant benefits and is able to waterproof 100% in the most trying conditions.
Due to their high mechanical properties, elongation and flexibility, particularly at lower temperatures, as well their capability to cure under a wide range of conditions, ELASTOROOF PUTM polyurethane based liquid applied membranes can also be used in a diverse range of climatically challenging environments. Unlike other waterproofing systems like sheet membranes and acrylic/cementitious liquid applied membranes, this ELASTOROOF PUTM polyurethane application requires comparatively less skill and supervision. As being a liquid applied membranes, ELASTOROOF PUTM cures to a joint free barrier membrane which is especially important in case of large spans/slabs/areas and pre-cast construction projects. Application is fast and curing of the ELASTOROOF PUTM membrane is also quite rapid making the system rain resistant almost immediately after application. Further, the rapidity of the curing reaction enables for speedy over coating and further laying of the protective course. Single component chemistry saves time and eliminates the risk of mixing errors and make the product easy to use.
Considerations For Tanks Containing Aggressive Materials
When chemicals such as acids, alkalis, or process contaminants are present in the liquid contained by the structure, the materials used to inject the cracks should be carefully selected for compatibility and chemical resistance. The sensitivity of materials to acid and alkali-driven chemical attack depends on their composition, the containment chemistry, and the severity of exposure conditions, such as concentration and temperature.
Repair materials are prone to deterioration by permeation if solvents in the tank are close to the solubility of the repair material. The lower the molecular weight of the solvent, the more rapidly it diffuses into the repair material. Crack repair material should be resistant to chemical attacks and other detrimental effects to avoid corrosion. Testing, consultation with the material supplier, or both, is recommended to address chemical compatibility and chemical resistance.
When liquids being contained are corrosive and chemical deterioration of the crack repair materials is expected, additional barrier linings may be required to assure long-term performance of the repair.
References:
- ACI PRC 364.12T-15: Repair of Leaking Cracks in the Walls of Liquid Containment Structures
- ACI 224.1R-07 – Causes, Evaluation and Repair of Cracks in Concrete Structures
- ACI 222 R – Guide to Protection of Reinforcing Steel in Concrete against Corrosion
- ACI 515.2R – Guide to Selecting Protective Treatments for Concrete