Breathing Life into Aging Infrastructure
Daily, infrastructure ages throughout the world, regardless of the means and methods provided to protect it against deterioration while in service. In light of shrinking industrial maintenance budgets in a suffering economy, Owner/Operators frequently make “hard” decisions that in their own personal life they wouldn’t think twice about….whether to repair, patch or delay actionable maintenance. In one’s own personal life, prior to taking a long trip on vacation, an individual routinely tune-ups and services their vehicle, making sure critical features such as oil, fuel, brakes and tires are all operating at top efficiency, providing insurance against premature “breakdown” while traveling. Unfortunately, many maintenance budgets only activate subsequent to a significant event caused by unscheduled outages (i.e., fires, explosions, cessation of process, etc.). As such, Owners/Operators are looking for “out-of-the-box” solutions to their maintenance challenges.
Innovative approaches to these challenges have emerged more frequently when the stakes are the highest, as in a stalled economy. Transferring technological advances in infrastructure rehabilitation from alternate market-places to the Petrochemical sector has provided an edge in many situations to Restoration Professionals – even in a down-side economy. Many times, especially in Petrochemical Facilities, plant processes must remain on-line; making asset repairs only during planned outages a wistful dream. Commanding knowledge of available technologies to answer the call of demanding Owner/Operator requirements requires a large cache of cross-industry experience in restoration.
A Repair Case History is presented to illustrate the implementation of a repair cost alternative that employed technological advancements in rehabilitation, yet provided a long-term, enduring restoration. The repair approach/alternative comes from the Power Generation and Marine sectors of the economy and was developed based on the need in those sectors for excellent Return-on-Investment (ROI) strategies due to the massiveness of the structures and the sheer number requiring repair. As with structures in the Power Generation and Marine industries, many similar massive equipment support structures exist within Petrochemical Facilities. These structures require restoration normally allocated for conventional “means and methods” of repair during short duration outages when maintenance budgets are fully-funded.
Repair Case History
A massive 50 year-old conventionally reinforced concrete structure supporting a critical Crude Unit Vacuum Tower within a Petroleum Refinery along the Gulf Coast in the Southwest USA, was significantly distressed and perceived by plant reliability personnel to have reduced operating serviceability; leaving the Owner/Operator in a quandary as to whether they should repair or replace the structural support system. Supporting well over 600,000 lbs of process equipment weight while in service, the eight (8) Column support structure was experiencing on-going embedded metal corrosion activity resulting in extensively deteriorated support Columns and Beams. Significant cross-sectional losses in reinforced concrete members were noted, as well as falling concrete hazards. The Owner/Operator had to barricade the areas surrounding the structure from plant personnel as a hazard mitigation measure. Recognizing the need to address the conditions of this critical support structure, the Owner/Operator contracted to have a comprehensive Condition Assessment and Structural Analysis be performed on the subject structure. The evaluative effort, it was hoped, would identify “Root-Cause” deterioration mechanisms as well as provide recommendations that would restore the support structure to as-designed serviceability.
Conventionally Reinforced Concrete Structure Metrics:
- 8 Columns – 30” diameter circular in cross-section by 33’-0” in height
- 8 Beams – 2’-6” x 3’-0” x 7’-6” rectangular cross-section
- 16 Anchor Bolts – 1-1/4” in diameter by 3’-3” in length (ASTM A-7)
- Minimum Concrete Compressive Strength (f’c) 3,000 PSI
- Minimum Specified Concrete Cover – 2-1/2”
Determining the physical and chemical characteristics of the materials-of-construction is an important facet of a Condition Assessment as it sets the stage for a more complete understanding of the alterations and modifications to the reinforced concrete in aggressive service environmental conditions. Especially significant in 1960’s construction in the USA was the practice of incorporating Calcium Chloride admixtures into Ready-Mix Concrete to accelerate the rate of strength gain without the incorporation of additional Portland Cement. This practice, at the time, was thought to lower the cost of the delivered product. Unfortunately, although well-intentioned, this practice introduced an aggressive electrolyte (i.e., chloride ions) into the concrete mixture that initiated and exacerbated the process of embedded reinforcing steel bar corrosion. In the case of the subject support structure, not only were chlorides introduced via admixtures, but being located close to the Gulf Coast region of the USA, provisions regarding the washing of local aggregates were not routinely practiced. Consequently, brackish mixing water conditions existed at the time of original Ready-Mix Concrete batching and placement.
During the Condition Assessment, a series of Non-Destructive and Semi-Destructive Testing (NDT & SDT) protocols were employed to determine both the chemical and physical characteristics of the concrete materials-of-construction. Existing concrete materials were determined to be carbonated (cement paste matrix at a pH <9.8), extensively delaminated (internal separations within the concrete caused by the expansion of corroding embedded reinforcing steel bars) and thoroughly contaminated with elevated levels of chloride (2 to 5 times the threshold value established by the American Concrete Institute). Although poor chemically, once the delaminated concrete was removed, the physical characteristics of the concrete were good with corrected existing compressive strength values ranging 4,500 to 7,500 PSI on a structure originally specified to be a minimum of 3,000 PSI.
Because certain features of the Support Structure required further and more sophisticated scrutiny, a Finite Element Analysis (FEA) was authorized by the Owner/Operator as standard repair “means and methods” would undermine the structural integrity, without incorporating a “sequential” repair program. The Owner/Operator, during this phase of the assessment, imposed an additional requirement due to economic necessity. The Support Structure’s process equipment would have to remain functional – uninterrupted and “On-Line”. This requirement further “loaded” the FEA Model with not only Dead Loads, but also Live Loads, Seismic Loads and Wind Loads up to hurricane force due to it’s proximity to the Gulf of Mexico. Based on the results of the structural analysis, a sequential process of opposing structural members of equivalent vertical measure could be repaired simultaneously without the need to shore the overlying process equipment. However, the repair as specified was cost prohibitive and involved a lengthy repair construction schedule.
By reviewing various structural repair alternatives with passive and active repair features, an innovative hybrid repair program was developed that would both lower the repair construction costs, as well as decrease the duration of the project. The program included the “active” shoring and support of existing Process Equipment, as well as repair technology borrowed from the Power Generation & Marine Facilities markets. The main feature of the borrowed technology involved incorporating Lifejackets® into the repair both at Column and Beam locations.
Employing a comprehensive Condition Assessment prior to developing a Repair Strategy is important because the root-cause analysis can determine the actual deterioration mechanisms. In this case, understanding the plant environment to be essentially “coastal” and the reinforced concrete to be laden with elevated chloride ion contents, either the structure had to be removed in its entirety or “active” means and methods were needed to mitigate the embedded metal corrosion processes. Active techniques at mitigating ongoing corrosion processes typically means the application of cathodic protection methods to embedded reinforcing steel portions of the structural concrete members. Specifically, Lifejackets® provide sacrificial zinc-alloy technology – allowing the anode to be preferentially consumed rather than the embedded reinforcing steel bars. This action significantly extends the service life of the reinforced concrete structural members.
The repair process began with the removal of delaminated (i.e., partially detached/unsound) concrete from specific support Columns – essentially repairing opposing Support Column Legs, two at a time in phases in a manner similar to tightening lugs bolts when changing a car tire. The repair required that the two phased Columns be completed prior to proceeding to the next set of opposing Support Column Legs with the repair of the elevated supporting Beams following a similar sequential repair format. Work took place even with all the process equipment supported as an additional safety precaution.
With the delaminated/unsound concrete removed, the exposed corroded reinforcing steel was cleaned and new reinforcing steel installed to augment existing reinforcement containing significant cross-sectional losses. The exposed reinforcement was then tested for continuity and electrically connected where discontinuous. Then, the two-piece Lifejacket® assembly was placed overtop of the prepared surfaces, strapped into place and the annulus cavity grouted with prebagged cementitious grout, mixed, batched and placed on-site. The installed cementitious grout not only provides a structural composite repair to the existing substrate, but also encapsulates and promotes electrical connectivity between the embedded reinforcing steel and the Zinc-anode attached to the external Fiberglass Form.
The added value of the prebagged cementitious grout materials was rapid strength gain which allowed relatively quick cycling between phased repair operations and the flexibility to adjust the construction schedule should the Owner/Operator require modifications due to process demands.
An unexpected feature of the project, discovered during the course of the Work, was that of Corrosion-Under-Insulation (CUI) involving the Vessel Skirt Base Ring and Anchor Bolts. Essentially, once cementitious fireproofing materials were removed from overtop the structural steel vessel support elements, significant laminar corrosion deposits were observed. These observations required timely evaluative efforts as well as a strategic repair approach that wouldn’t affect the Project Repair schedule. Repairs were made to the Vessel Skirt that included repair/strengthening of the Vessel Skirt Base Ring, Base Ring Stiffeners, Anchor Bolts, and Anchor Bolt Chairs. Due to the severity of the corrosion and the associated Work Scope changes, a more substantial Shoring Support Structure had to be designed and installed. The enhanced support structure contacted additional process equipment members while the Vacuum Tower was in operation.
Although challenging, this project provided the Owner/Operator with an innovative and proven repair concept integrated from other industries that not only was less expensive but also offered a shorter project duration. Initially designed to be performed during a short duration outage (T/A), the repair design was flexible enough to be undertaken even though the Vacuum Tower came back On-Line due to Owner/Operator Process requirements.