Although considered a “castable stone”, unprotected concrete can be attacked and degraded while exposed to harsh environmental service conditions in operating Fertilizer Production Plants. Means & methods are available to understand, prepare and provide long lasting durable repair solutions to address deterioration of these critical Civil Infrastructure Assets.
As the most used building construction material in the world, concrete can trace its roots back before the Roman era (300 BC to 476 AD) with much of the Roman Empire’s concrete infrastructure still visible today in the form of the Coliseum, Pantheon, aqueducts and cisterns spotting the Italian and to a lesser degree, the European landscape1. Although differing chemically from today’s concrete (hydraulic cement based versus Portland cement derivatives), overall concrete has proved to be an incredibly durable and resilient building construction material capable of withstanding the test of time. Having the capability to exploit local mineral resources, historically concrete consists of proportioned fine & coarse aggregates, cement and local potable water. Modern concrete mix designs also incorporate various chemical and cement replacement mineral admixtures to enhance the place-ability and long term durability of the concrete while in service. These mix design enhancements also assist in providing a lower carbon footprint to assure continued sustainability of concrete as a building construction product world-wide. In addition to the concrete mixture material enhancements noted, modern concrete structures also incorporate embedded reinforcing systems to address tensile stresses. Concrete as a construction material, is strong in compression but weak in tension. Recognizing this deficiency, in the late 19th Century, design practices began to incorporate structural section detailing employing steel bars to address and carry tensile stresses when a structural element or frame is subjected to an imposed load.
When reviewing Industrial Plant operations, concrete as a material, doesn’t on its own generate business revenue. However, in most cases reinforced concrete foundations, bases and frame structures support the electromechanical processes that do generate saleable product produced by the Plant. Fertilizer Manufacturing Facilities are no exception and in fact have a substantial inventory of Civil Infrastructure Assets constructed of reinforced concrete. Understanding that equipment and critical systems require proactive maintenance and repair strategies such as Risk-Based Inspection (RBI), Facility Owners have been developing similar approaches addressing their Civil Infrastructure Assets. One such successful program developed is the Plant Condition Management System (PCMS®) 2 which evaluates the current condition of Civil Infrastructure Assets (i.e., reinforced concrete, structural steel, fireproofing, secondary containments, etc.), prioritizes condition/repair severity and provides scheduled repair costs for phased financial allocation of maintenance budgets.
Although believed to be an impermeable material, concrete is actually a “hard sponge” that consists of capillaries, entrapped/entrained air voids and micro-cracks. Identified as a “heterogeneous” composite consisting of aggregate bound together in an inorganic paste hydrate (C-S-H) matrix, concrete performs well in the natural environment but can degrade when exposed to chemical environments that alter/change the concrete mix design constituents3. Exposure to acidic or caustic solutions/compounds over time place the concrete cement paste at-risk if not initially designed for severe chemical exposure or inadequately protected by chemical barrier coatings. Aggressive chemical exposures to concrete support structures in a Fertilizer Manufacturing Facility can include:
- Nitric Acid – HNO3
- Phosphoric Acid – H3PO4
- Sulfuric Acid – H2SO4
- Ammonium Nitrate – NH4NO3
- Potassium Chloride – KCl
- Potassium Sulfate – K2SO4
- Ammonium Sulfate – (NH4)2SO4
- Urea – CO(NH2)2
Each of the chemical compounds described above are aggressive “disintegrators” when exposed to unprotected concrete surfaces4. These mechanisms of degradation vary involving either the dissolution of the cement paste hydrate (i.e., effectively the “glue” that holds the concrete components together), constituent expansion mechanisms or thermal shock due to low/high temperature exposure upon chemical release from closed loop systems. Additionally, change of use, overloading, mechanical impacts from equipment, rapid pressure release in close proximity to concrete surfaces (i.e., blasts or jet streams), pyrogenic events (i.e., fires) and freeze-thaw cycling from cryogenic service/environmental exposures also prematurely shorten the service life of these critical support assets.
Concrete Asset Evaluation
Understanding the baseline condition of existing structures is a critical feature during the development of any strategic maintenance program not unlike when evaluating existing equipment such as pumps, compressors and process vessels. However, for Civil Infrastructure, much of the construction/fabrication is performed on-site and the construction followed a detailed set of Construction Documents (i.e., plans & specifications). Using a systematic asset management program approach like PCMS®, helps establish and understand deterioration trends and sets a baseline for a Facility Owner and assists Plant Maintenance personnel develop a schedule and financial budgets for phased yearly repairs. Performing a Field Reconnaissance Visit begins the process when a Field Investigator walks down the Facility with an Owner discussing the pertinent details of the investigative process including areas of focus as well as abandoned or decommissioned portions of the Plant that won’t require scrutiny. Once the regions are identified for evaluation, all available archival documentation (i.e., Equipment ID’s, Plant Layout and Construction Drawings/Details) is requested, reviewed and electronically inputted (if available) into a tablet platform at the Office prior to performing the next phase of field work. The Field Investigation Phase is really where the “rubber-meets-the-road” and inclusive structures (i.e., reinforced concrete, structural steel, fireproofing, secondary containments, etc.) are evaluated, rated, prioritized and repair cost estimated for inclusion into a Comprehensive Report. By implementing limited Non-Destructive and Semi-Destructive Testing (NDT & SDT respectively) during an evaluation, deterioration trends can be identified to validate proposed repair scenario recommendations. Frequently, multiple repair option scenarios are provided as well as Service Life Modeling (Stadium®) for Owners who require information regarding service life extensions or a “time-to-failure” understanding should a Civil Infrastructure Maintenance Program be implemented or delayed, respectively.
Formulating a Repair Strategy
As stated earlier, numerous degradation mechanisms can occur and sometimes in combination that require a collaborative effort between engineering and contracting entities to ensure that repair programs address the root cause of the deterioration as well as potentially secondary/opportunistic deterioration mechanisms. Without adequate pre-planning, maintenance activities can be put into a “repair-and-repair-again” cycle addressing only the symptoms of deterioration and not the root cause. Once adequately identified, typically the repair process should include:
- Identification of the extent and depth of removal of distressed concrete
- Selection of the means & methods of repair and subsequent repair implementation
- Selection of repair material type and/or protective barrier system
- Installation of temporary support/shoring of affected equipment or processes prior to repair activities
- Demolition and resultant concrete surface preparation in accordance with International Concrete Repair Institute (ICRI) and American Concrete Institute (ACI) Guidelines and Specifications including exposed steel reinforcement preparation and augmentation when necessary
- Conveyance and installation of concrete repair materials onto prepared cavity surfaces and curing repair materials in accordance with Repair Material Manufacturer guidelines and ICRI/ACI Best Practices
- Installation of Protective Barrier Treatments designed to counter aggressive environmental process service conditions and applied/cured in accordance with Coating/Lining Manufacturer’s guidelines
- Project Type: Reinforced Concrete Urea Prill Tower
- Affected Elements: Reinforced Concrete Tower Shell
- Dimensional Info.: 140 feet (42.67 Meters) Tall by 18 feet (5.49 Meters) Diameter Vertical Tower
- Age: 1965 Vintage
- Location: Western USA
- Owner Concern: Owner’s concern was falling concrete along exterior and within the Prill Tower
An evaluation was arranged that put Field Investigators into an articulated, extended reach man-lift to review vertical concrete surfaces both internally and externally. Performing NDT & SDT follow-up testing, it was determined that localized deterioration in the form of cracks, delamination, open spalls, chemical attack, surface erosion and original construction defects combined to place the Prill Tower in “fair” condition. The “fair” condition designation prompted the Owner to acquire repair/maintenance funding from corporate yearly maintenance budgets in order to maintain operating serviceability and assure Urea product quality.
Reviewing investigative Condition Survey Drawings as well as collected field and laboratory testing data established a repair approach known as “repair-in-kind” following the original designer’s intent. Essentially, deteriorated and chemically attack concrete materials were removed and replaced after exposed corroded steel was repaired/augmented to maintained existing reinforcing continuity. Pneumatically placed concrete (i.e., shotcrete) was placed within prepared cavities and cured in accordance with ICRI/ACI Best Practices. A protective barrier system was recommended for interior and exterior repair surfaces however the Owner opted for repair only; citing the 50-plus year service history of the existing Prill Tower without a protective barrier system. However, the Owner has placed the entire structure on a regularly scheduled inspection program every 5-years following process outage opportunities and a protective barrier system is still an option should service conditions warrant in the future.
- Project Type: Multi-Level Open Frame Structure
- Affected Elements: Elevated Reinforced Concrete Floor Slab
- Dimensional Info.: 70 feet (21.34 Meters) Long by 120 feet (36.58 Meters) Wide in Horizontal “footprint”, Multiple Levels
- Age: 1975 Vintage
- Location: Southeastern USA
- Owner Concern: Owner’s concern was falling concrete along elevated floor slab soffits as well as cracking, open spalls and surface erosion
A team of Field Investigators was assembled to address various forms of concrete distress being experienced at the multi-level open frame structure. A wide range of chemical processes take place within the structure and process changes occur periodically. Fertilizer finishing products such as Ammonium Sulfate, Potassium Chloride and various acidic compounds regularly contact unprotected concrete surfaces requiring frequent washdowns. Review of original construction documents revealed that the Floor Slabs were placed on metal pan decks however, the metal decking was not integrated into the reinforced concrete slab design. Visual observations revealed only remnants of competent cross-section with most steel deck regions extensively corroded or lost revealing only concrete deck impression of the original metal deck formwork along the Floor Slab soffits. Performing a series of NDT & SDT testing protocols, it was determined that the chemical characteristics of the concrete were “very poor” although the physical properties (i.e., compressive strength) of intact concrete revealed compliance with original construction documents. However, extensive corrosion was observed surrounding through-Floor Slab penetrations as well as exposed corroding reinforcing steel bars at the base of open spalls.
Owing to the relative thinness of the Floor Slab, a partial depth repair was not an option and a complete removal and replacement of the Floor Slab was necessary to provide an enduring repair. Wanting to lower the overall weight of the new structural cross-section, the Owner decided to replace the Floor Slab “in-kind” using metal deck forms and replacing the Floor Slabs in a phased approach that allowed continued process operations yet protected products from contamination via work space confinement shielding/protection. Repairs were performed under “hot work” conditions and “fire watch” protocols to assure safe working conditions. Corrosion detailing included hot-dipped galvanized metal decking, corrosion resistant through-floor slab embedments, dense proprietary concrete repair materials incorporating shrinkage-compensating & corrosion-inhibiting admixtures as well as discreet anode placement along repair perimeter regions. Subsequent to concrete repair material placement and curing, the Owner requested that a high quality reinforced chemically resistant lining system be applied to repaired concrete top surfaces. This protective lining system provides a barrier to address an inconsistent washdown schedule of spilled aggressive chemicals.
As illustrated above, Fertilizer Production Facilities subject reinforced concrete Civil Infrastructure to a harsh aggressive service environment. By understanding the true breadth & extent of the condition of their existing Civil Infrastructure Assets via a comprehensive assessment, it’s now possible for Owners to wrap their arms around Maintenance Strategies and develop effective Repair Programs. Implementing systems that allow an Owner to stay ahead of the curve prior to an “unscheduled” maintenance event is a valuable tool that will substantially reduce preventable failures of these critical Civil Infrastructure Assets.