“Et tu, Brute.”
“I only regret that I have but one life to lose for my country.”
“We are not responsible for the contents of articles that we publish.”
Some words leave indelible marks in our history.
There also have been final words that capture the essence of reports. They can reduce 5 to 80 report pages into a synopsis or an executive summary that enhances understanding. Following are examples that seem too unreasonable to be true, but indeed they are, and were undoubtedly written to capture the reader's attention.
This report was related to cracked 6-inch-thick slabs with saw-cut joints spaced at 24 feet. The cracks formed polygonal patterns on 2- to 4-foot spacings because of unaccommodated drying shrinkage. Cracks occurred below each saw-cut joint so the joints had functioned as designed, but still the polygonal cracking was to be expected. Joint spacings should have been at 12 to 18 feet. The 24-foot spacing was too large to accommodate normal drying shrinkage.
The maximum-sized, 1-inch crushed limestone coarse aggregate and natural siliceous sand fine aggregate were well graded and uniformly dispersed—Jim Shilstone would have been proud. The cement content was 5½ bags per cubic yard, the water-cement ratio was 0.48, the concrete was well finished and well cured; carbonation extended about ¼ inch into the concrete, based on petrographic examinations.
A completed investigative report reviewed the concrete design, the properties of the individual components, the installed concrete, and the inappropriate joint spacings. Without mentioning unaccommodated drying shrinkage, the report concluded that “the cracks are due to stress relief.” That certainly is a perfectly acceptable, simple, and most obvious statement—all cracks are due to stress relief.
A concrete canopy suffering old age
In a snow-belt environment, a decades' old concrete canopy over a walkway showed deterioration—cracking, scaling, and corrosion of reinforcing steel. Questions arose about remediation—if it was needed, technically possible, and economically feasible.
Studies revealed that the concrete was non-air-entrained. As a result it had scaled and contained extensive microcracks. The reinforcing steel was lightly corroded—carbonation had reached the steel. Obviously, the concrete's structural capability and the future durability were in question.
A detailed engineering field survey, additional petrographic examinations, compressive strength tests, a number of additional laboratory studies, and an engineering evaluation were done to answer the remediation question. The report, a hefty 80 pages, detailed all the work done and proclaimed that the canopy was structurally sound, that it had a possible future. That is, until the last sentence, which almost seemed like a hastily contrived afterthought, despite the snow-belt location—”except that it will not sustain a several-foot snow load.”
The segmented bridge conundrum
During the early days of segmental bridge design and construction, the construction of one was begun during winter. Why not? The precast and match-mated individual concrete segments were completed much earlier in the year when there was no need for special winter concreting procedures. Only the mechanical installation had to be done. The work proceeded nicely into the summer. Ambient temperatures rose to the high 80s and even higher in the concrete segments. When construction was essentially completed, one segment (installed during winter) displaced downward an inch or so—a sure sign that something was amiss.
The design called for epoxy-buttered ends where segments abutted. The epoxy served three purposes: (1) as lubricant to facilitate the final segment-mating process, (2) as a joint seal, and (3) to assist in load transfer across the joints. The epoxy was thus engineered to be part of the structural system.
During the survey immediately following the failure, volcanic-like eruptions of small, pliable material were found on joints between winter-placed segments. They felt and acted like plumber's putty when poked. Laboratory analyses (using infrared methods) revealed that the volcanoes were unhardened epoxy and that the activator that was supposed to make the resin harden was not uniformly distributed. Due to the cold winter temperatures, its viscosity was like that of molasses. The soft epoxy failed to establish structural continuity between the segments.
Several investigations ensued. One primary report ended by stating that the bridge was structurally sound “except during summer months.”
Our famous last words this month are, “Keep the faith, hang on, read our column, and let us know what you think.”
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 47 years.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.