Blue Circle Cement (now owned by Lafarge North America) faced a dilemma in the spring of 1999. The company had constructed two cement silo clusters in the Boston area in 1970. A total of six, 150-ft-high, 23-ft-diam silos — a cluster of four and another of two — were constructed of reinforced concrete. Over the past 30 years, structural cracks had developed, caused by an insufficient amount of vertical and circumferential hoop steel. In fact, structural analysis based on current codes revealed that more than 30% of the required steel was missing, because of oversights in the original design and construction of the silos.

The short-term course of action for Blue Circle was to limit filling of the silos to approximately half of their height to avoid a structural failure. This was hardly a satisfactory solution, particularly with lucrative market conditions and the rising demand for cement product. Instead, the company was forced to decide if it would be economically feasible to attempt to strengthen the silo clusters or to demolish and replace them altogether. The decision was complicated by the prevailing market conditions, which dictated an immediate need for storage, hence requiring quick turnaround time to implement whatever decision was made.

Strengthening options

Demolition and replacement of the Blue Circle cement silos, though probably the simplest solution from a design standpoint, was quickly rejected on the basis of cost, turnaround time, and storage logistics. However, while it was believed that finding a repair and strengthening solution would be more cost-effective than demolishing and replacing the silos, the upgrade approach introduced a series of special challenges resulting from the cluster orientation of the structures.

With a silo cluster, full access around the outside perimeter is not achieved easily because of the common intersecting walls. Bearing this hurdle in mind, a number of strengthening options were considered. One was to strengthen the interior silo walls with a reinforced concrete liner. Another possible approach was to apply external post-tensioning. Other options included the use of a variety of interior and exterior fiber-reinforced plastic (FRP) techniques.

In addition to cost considerations, many of these options had potential drawbacks. The interior liner approach posed the issues of loss of storage volume, as well as considerable additional weight bearing down on the foundation. The external post-tensioning option introduced the problem of passing tendons through and around the common walls.

Likewise, interior FRP repairs would not be a practical option, because of the typical temperatures of the cement materials contained in the silos. However, exterior FRP options that were considered included the sheet material form of FRP. While FRP sheet material has been used on many infrastructure upgrades (such as beams, slabs, and columns), it turned out to be a weak solution in this case, as it would be impossible for the material to completely confine each silo in a continuous manner. (Terminating the FRP sheet material at the common wall would be the equivalent of using a discontinuous rebar for circumferential steel in a column.)

FRP rods to the rescue

The best strengthening solution turned out to be incorporating FRP rods. This method of FRP strengthening provides the durability benefits of FRP sheet material, while offering near-surface mounted (NSM) reinforcement that could restore the missing horizontal and vertical steel, as well as address the challenge of the common walls in a constructable, cost-efficient manner.

The NSM FRP rod is a new version of rebar slotting or “stitching dogs.” With rebar, a groove is cut at the proper width and depth to insure bonding and corrosion protection. Rebar is then inserted and grouted in with an adhesive. The major benefits of the FRP rod versus rebar pertain to the grooving process, as well as the noncorrosive nature of FRP materials. The groove width is minimized based on to the strength-to-diameter ratio of the FRP rod, while the groove depth is minimized because FRP does not corrode, hence minimizing cover requirements.

Another benefit of FRP rods for this cluster silo application was their ability to be anchored into the common wall to a depth that ensures the complete development of the rod capacity; with complete development of the rod, acceptable strengthening could be achieved at the wall intersection.

One hurdle stood in the way of implementing this solution: this type of FRP strengthening technology had never before been used in an application of this magnitude. To ensure the viability of the application, certain technical details needed to be worked out and field-verified. Among them were the optimum properties of the FRP rods, the ideal size of the grooves to ensure a proper bond, the optimal groove depth and installation technique for a vertical and horizontal grid pattern on a round structure, the best adhesive for bond and placeability, plus the proper size and depth of holes at the common walls for doweling/anchoring.

To ensure the successful application of the FRP rod technology, a design-build relationship was formed between Blue Circle; the project engineering firm CoForce America of Rolla, Mo.; and the repair contractor, Baltimore, Md.-based Structural Preservation Systems, Inc. Part of the team included academia, via research and development support provided by the University of Missouri at Rolla's Center for Infrastructure and Experimental Study (CIES). This combination of applied research and practical experience resulted in the successful implementation of a commercially viable technology.

Let the repairs begin

Access to the cement silos was gained using customized, suspended swing stages. A comprehensive interior and exterior inspection of the silos identified and mapped out all cracks and spalls. The initial repairs consisted of epoxy crack-injection and concrete repairs using a form- and cast-in-place application technique. The repair procedure removed only one silo at a time from service for inspection, repair, and strengthening. This way, upon completion of each silo, Blue Circle had full capacity for inventory and minimized the amount of silos out of service at any given time. Adopting this procedure proved to be a major benefit from an economic standpoint.

Installation of the rods began with the grooving operation. Customized grooving tools allowed Structural Preservation Systems' technicians to cut all of the appropriate grooves in one pass, as opposed to multiple passes when steel reinforcement is utilized. Wherever a groove intersected common wall locations, an anchoring hole was drilled that was tangent to the curve of the silo, to the depth determined by earlier pullout test results.

First, a two-component, high-viscosity epoxy adhesive was gunned into the deeper vertical grooves. The vertical rods were inserted into the grooves and embedded into the adhesive, followed by a second layer of adhesive applied on top of the rods. Since the FRP rods were so lightweight, they could be handled in single pieces, even those as long as 150 ft.

Once the vertical installation was completed, the circumferential FRP rods were installed. Adhesive was applied into the anchor holes in the two common wall intersections, as well as into the horizontal grooves. Starting at the first dowel hole, a single-length rod about 60 ft long was inserted, then placed into the horizontal groove around the circumference until it met the other common wall intersection. The remaining length was inserted into the second anchor hole. Next, a second layer of adhesive was applied on top of the horizontal rod, and the material was then tooled to create an aesthetically pleasing surface appearance.

One silo at a time

Because of the pathfinding nature of this project, it was decided to monitor the success of the strengthening effort one silo at a time. This consisted of fully loading the first silo with cement while the existing reinforcement and FRP reinforcement were monitored for performance. Instead of using the conventional strain monitors, a new fiber-optic system was installed into the FRP groove and concrete to measure the desired strains. Since these types of fiber optic monitors do not deteriorate over time, the information could be monitored continuously from a junction box at the base of the silo.

After the load test results on the first strengthened silo were deemed successful by the project team, work proceeded on the remaining five silos. In all, more than 15 miles of FRP rod material were installed. Throughout the work, ongoing quality control procedures included pullout tests and cores for visual inspection.

The final step in the repair process was to coat each silo with a breathable acrylic topcoat, to meet both durability and aesthetic requirements. Each silo cluster was adorned with Blue Circle's logo and company colors, completely hiding the new reinforcement. As aesthetics are usually more noticeable than technology, in the end this may have meant as much to the surrounding businesses as the fact that the silos had been strengthened and made safe and fully functional.

Bottom-line benefits

The special challenges posed by this silo cluster project fostered an open, goal-oriented relationship that resulted in the development of a strengthening approach that could not have been successfully implemented with a business-as-usual attitude to the bidding process and work procedures.

The Blue Circle silo strengthening project saved the owners significant dollars compared to the alternative of demolition and replacement, not to mention 18 months of construction time saved in the process. To top off the achievement, in 2000 the International Concrete Repair Institute awarded the project an Award of Excellence in recognition of the success of this pathfinding repair project.

Jay Thomas is vice president of sales and Abhi Tipnis is project manager, both for Structural Preservation Systems, Inc., (410) 850-7000.