In the currently popular “Crime Scene Investigation” TV shows, experts sift through evidence and examine the minutest of clues to determine cause, place blame, and see justice done. But real police, detectives, and forensic specialists will tell you that in the real world solving a crime is a lot less glamorous than it's portrayed on TV. It takes discipline, routine investigative technique, fine combing for detail, and paperwork. In the real world, solving problems usually takes more perspiration than inspiration.
The same is true on the jobsite. What if the “crime” is a sudden decrease in concrete compressive strength, an increase in air content, or the mysterious appearance of cracks that weren't there yesterday? When concrete crimes and misdemeanors are serious enough, it is only a matter of time until somebody wants to know what went wrong, why it went wrong, how to keep it from going wrong again, and how to fix what you are stuck with. And depending on the consequences, the ultimate question might be, “Who can we blame?” Getting to the bottom of any problem usually takes a disciplined examination of the evidence, regardless of whether the problem is related to strength, air, shrinkage, finish, color, or ... you name it.
When dealing with concrete problems, following the chain of clues from the unsatisfactory “effect” back to the original “cause” can be complicated and expensive. It can also involve scientific apparatus every bit as sophisticated and nerdy as that shown on CSI. Optical microscopes are often used to observe air voids,
aggregates, and other features in hardened concrete. Scanning electron microscopes can look for reaction products, estimate paste porosity and water-to-cementitious materials ratio, and search for fly ash or silica fume particles. Infrared imagers, ultraviolet light, x-rays, neutron rays, microwaves, magnetic fields, chemical tests, and electric currents all have been used to pry information out of defenseless concrete samples. Past Concrete Construction articles on such subjects include “Looking Inside a Concrete Bridge” (Nov. 2004), “Microwave Water-Content Testing” (Nov. 2000), “Fast Field Tests for Concrete” (July 1996).
All these tests have their places and are absolutely essential when the problem is complex and the stakes are high. But in many cases, less sophisticated techniques can be used, just as plain, old-fashioned pavement-pounding detective work solves more crimes than the flashier lab analyses.
When the folks directly involved in the project know what to look for, they can do a lot of the detective work themselves. At a minimum, they can have some key information ready when a consultant is brought in to help solve the problem, and in the process they may be able to save themselves some time and money.
Defining the problem
When any of us goes to a doctor for treatment, the first question is, “What are the symptoms?” In the crime show equivalent, the detective's first question is, “What have we got?” (which must be pronounced “waddawegot,” although the “Hill Street Blues” gang preferred “run it down for me.”)
As basic as this seems, my experience has been that in many cases, the person who calls or faxes or e-mails with a concrete problem does not really know what the concrete problem is. Consider these typical descriptions of the situation:
- “There's something wrong with the cylinders.”
- “The air is wacky.”
- “We are not getting the breaks.”
- “There's some sort of cracking.”
Once the nature of the problem is better established, which can take days, it's time to get more specific. The concrete detective may ask, for example, “Who says the concrete cylinder breaks are low, and what is the evidence?”
The next question is usually, “If these cylinder strengths are unacceptable, how high are they supposed to be, and who says so?” The definitive answer to that question is usually found in the project specifications, or notes on the drawings, or both, citing the authority of the contract documents. Of course, the original specifications likely included a number of industry standard documents that set requirements for the concrete, most notably ACI 318, “Building Code Requirements for Structural Concrete,” and ACI 301, “Specifications for Structural Concrete.” Numerous addenda, field orders, memos, and minutes of project meetings may also have modified the original requirements. Nevertheless, a full description of the problem will include a statement of the concrete's current behavior, how we know that to be the case, and comparison of this problematic behavior with what is required by contract. This clarity is essential because a whole lot of money and time can be wasted trying to solve the wrong problem, or trying to make the concrete do something the contract does not require it to do.