Anchored in the Real World
An evaluation of the processes and methods when dealing with anchor bolt maintenance and ensuring continued integrity.
By definition, anchor bolts are typically a headed or threaded metal bolt or stud that is cast-in-place, grouted-in-place, or drilled into finished concrete. They are used to hold various structural members or embedments in the concrete and are able to resist shear, tension and vibration loading. Unfortunately, the condition of the ‘business-end’ of the anchor bolt is unknown until an owner/operator experiences an unfortunate event in the form of:
- Bolt tensile failure
- Pull-out (concrete tensile) failure
- Lateral bursting (blowout) failure
- Localised bearing failure
- Concrete splitting failure
However, from a maintenance standpoint it is important from that these unplanned events be avoided, as far as possible, through monitoring and mitigation.
In petrochemical facilities, the common use of anchor bolts is to fasten process vessels or machines to a foundation structure, thus allowing the loads to be transmitted to the foundation and, ultimately, to the ground below. Although there are many types of anchorages, we will focus on anchor bolt types that are cast-in-place during original construction and deteriorate in service thereby requiring evaluation prior to repair and restoration. Understanding the type of installed anchor bolt is a critical feature in providing an accurate in-service assessment. Essentially, when tell-tale signs of deterioration generate concern for an owner/operator, methods are available to accurately evaluate in-situ anchor bolt conditions.
Several types of evaluation programmes, generally a mix of non-destructive testing (NDT) and semi-destructive testing (SDT) techniques, can be assembled to quickly assess the type, quality and quantity of the tested anchor bolts. Essentially, the three most popular testing techniques available include:
- Acoustic impact – each anchor bolt is struck with a 3-pound maul hammer and the resulting audible tonal variations are noted – a sharp “metallic ring” typically identifies a sound condition – a dull “drummy” sound is indicative of an unsound anchor bolt.
- Ultrasonic metal flaw detection – the top of each anchor bolt is ground smooth and flush using an abrasive grinder, perpendicular to thread orientation. Using a piezoelectric transducer applied to the ground portion of the anchor bolt’s top surface, a mechanical stress wave is propagated (500kHz to 10MHz). These waves will travel through a given metallic medium at a specific velocity, in a predictable direction, and when they encounter a boundary with a different medium (metal/concrete interface), they will be reflected or transmitted according to predetermined wave patterns applying simple rules of wave propagation.
- Excavation and discovery – exposing sub-surface regions of embedded anchor bolts, the bolts are cleaned of laminar deposits of corrosion (i.e., rust-pack) and a vernier caliper is used to determine the existing diameter of the exposed anchor bolt. The caliper operates as a graduated rule with one sliding ‘jaw’ and one that is stationary.
Once an anchor bolt opportunity has arisen and the owner/operator has engaged the necessary expertise, several strategies are available to re-establish anchor bolt integrity. The following restoration strategies can be considered:
- Do nothing – numerically, testing may show that the equipment/vessel requires only a portion of the hold-down capacity. Monitoring the status of the remaining ‘sound’ anchor bolts is necessary until conditions change and require a corrective action program.
- Repair existing anchor bolts – separated anchor bolts, near the surface, can be repaired using mechanical couplings or welding stud extensions. A complete understanding as to the anchor bolt’s ‘root cause’ failure mechanism is required. Repairing a failed anchor bolt, without recognising the root cause, places the owner/operator in the dubious position of ‘repairing-the-repair’ should the anchor bolt fail again in a similar manner.
- Relocation of anchor bolts – when changes occur in original design and equipment/vessel loadings require new ‘fastener’ configurations, then anchor bolt chair assemblies can be designed and incorporated into existing anchorage ‘schemes’ that reposition fastening points. Should this type of placement be inapplicable, positions inward or outward may be considered as a viable alternative. Anchor bolt chairs are welded or bolted/riveted assemblies are solidly affixed to the foundation support member/skirt just above the concrete foundation interface.
- Full-depth anchor bolt extraction and reinstallation – many times the prospect of cutting embedded reinforcing steel bars is unacceptable, especially in those structures that are being upgraded by increased vessel loadings. The best alternative in many situations is the complete removal and replacement of the anchor bolts. In elevated structures, large equipment/vessels can have the anchor bolts extend through the entire reinforced concrete member thickness. The existing corroded/broken anchor bolts were forced through the grout sleeve annulus using modified jack-hammers fitted with drill-steel tips. Subsequent to removal, anchor bolt hole sidewalls were cleaned, new threaded bolt studs installed, washer/nuts assembled and the vessel was tightened to OEM (original equipment manufacturer) compliance. The resulting new anchor bolt annulus was grouted ‘tight’ for corrosion protection with a flowable, cementitious grout that typically incorporates corrosion inhibiting admixtures.
It should be noted that retrofit anchor bolt selection should be performed by engineering professionals with assistance from the manufacturer’s technical service staff working to provide the appropriate anchorage system to address the service environment and required service loading.