Deal yourself a hand of repair material properties and you will be ready to make the best selection based on an objective and logical approach. In December 2012, Concrete Construction published an article titled “Flashcards for Engineers” by P.J. Morrissey. Recently, Morrissey developed decks of the actual flashcards to use in this approach. Here we present a step-by-step process for using the flashcards and accompanying spreadsheet.
As noted previously, the ConSpec Associates Material Selection Criteria (CSA-MSC) provides three benefits in the selection process:
1. A thoughtful and tactile process to review all of the properties of repair materials being considered and then to produce an objective ranking of the products for a specific application.
2. If a substitute is proposed by the contractor, an objective comparison of the submitted material can be made to the specified materials.
3. In case of a failure, there will be documentation of the objective process used to make the selection—the specifier and contractor will be able to show that they used due diligence in their decisions.
In the past there has been no objective numbers-based way to select repair materials. Choices were made based on the specifier’s relationship with the manufacturer or on properties reported which could not be compared since they were often based on internal tests, not standard ASTM tests. Recently the International Concrete Repair Institute has developed data sheet protocols which define the properties of various repair materials and indicate which standard test method would have to be used. This allows one to compare different repair materials on a apples-to-apples basis; if a manufacturer proposes a material that does not have the necessary data, it could be rejected outright.
So when trying to select the best material for a repair application, get out your CSA-MSC card deck and deal yourself a hand. Choose the properties that are most important for the specific application and then lay them out on the desk. Now, arrange the properties according to which is most important (weighting). Move the cards around, toss some out, and add some to get the ideal properties for your project. “It is up to the specifier to lay down his test criteria in the order that he thinks is most important. Then he fills in the blanks with the materials he is considering, and thereby accepts or rejects certain materials and then any submissions that are provided by the low bidder,” Morrissey says.
How it works
Once you have selected the properties, here’s how the process works to fill in the spreadsheet and come up with a ranking of the materials under consideration. See the order of importance table
1. List the properties and the required test method for each in the order of importance. Also, indicate if there is an acceptable minimum or maximum value for each property.
2. Assign a weighting to each property according to the most important properties for the specific repair application. Start at 10 (most important) and work down. One could attempt to assign higher or lower weighting to specific properties, but it’s simpler to just go down from 10 to 1 (if there are that many properties under consideration) and it’s also more defensible.
3. Fill in the actual property values for each property provided by the repair material manufacturer on its Data Sheet Protocol. If a manufacturer does not provide a specified value, then what? “If it is critical, then the product is eliminated,” says Morrissey. “If it is just important, then it gets a zero. The higher up the system this missing property is, the more it affects the results. Purpose accomplished!”
4. Now compare the property values for each of the various materials and rank them against one another for the specific property. If there are four materials, then rank them 1 to 4, with 4 being the best. (If you are considering six materials then rank starting at 6.) If a value is not provided, then the material gets a zero ranking for that property. So, for example, in the CSA Repair Material Selection Ranking spreadsheet shown on page 30, since lower is better for shrinkage, Repair Materials B and C are tied at 4 while Material A gets a 3. Material D gets a zero since that value is not reported. If a property is over or under the minimum or maximum value, then it also gets a zero.
5. Let the spreadsheet then multiply the Weighting times the Rating to get the Property Score for each repair material. Add up the scores and get a final score that leads to a ranking of the products under consideration.
This simple but elegant method is something you can rely on and fine-tune with experience. “Whether you are a specifier, contractor, or owner, consider this system for your next repair or restoration project,” says Morrissey. Flashcard decks are available from ConSpec at www.conspec-rep.com.