An Opportunity for Ready Mixers and Concrete Contractors
March 17, 2011
Right now I believe the ready mix producers and the concrete contractors have an excellent opportunity to enhance their financial gains simply by taking the opportunity presented and promote Fiber Reinforced Concrete. The engineering community and the code writers finally have realized there is an advantage to using a 3-dimensional secondary/temperature-shrinkage reinforcement versus a single plane of wire mesh, which more often than not ends up residing on the subgrade.
First, I believe the engineers are seeing that it is not technically prudent to use one of the five (5) empirical formulae/equations…..yes I said 5 to determine the quantity of wire mesh to incorporate in the concrete as temperature-shrinkage reinforcement when they can access real quantifiable data for Fiber Reinforced Concrete. Not only are there five different formulae available for use to calculate the amount of wire mesh to specify, none of the formulae produce the same numbers. The range is staggering (see below). Of importance in this discussion is the fact that the drag coefficient formula found in ACI 318 yields the lowest cross-sectional area required. Does not surprise me. Remember, the cost is a factor.
These data were taken from Table 1 in Wire Reinforcing Institute’s TF705 “Innovative Ways to Reinforce Slabs-on-Ground”, authored by Robert B. Anderson, PE.
As we say above the range of results is staggering and none of the formulae and thus the resulting answers are based on actual engineering data……they are empirical formulae………amazing!
So now with Fiber Reinforced Concrete the engineer can look at real plastic shrinkage and drying shrinkage data generated by standard consensus test methods. We can show the engineer that the fibers can reduce plastic shrinkage cracking while increasing the fatigue strength and impact resistance in addition to holding the concrete together after it cracks, which is the sole purpose of temperature-shrinkage reinforcement.
Simply stated the engineer and thus the owner of the concrete element is getting fully more bang for their buck. Expect longer concrete life expectancy with lower long-term maintenance costs.
Before venturing further let us take a closer look at the full menu of empirical formulae available for calculating the amount of wire mesh required to perform as secondary reinforcement. The formulae are identified as:
Procedure Steel Area Required WWF Designation
Subgrade Drag 0.021 sq in 6×6 W1.4 x W1.4
Confirmed Capacity 0.070 sq in 12×12 W7 x W7
Temperature 0.135 sq in 12×12 D13.5 x D13.5
Equivalent Strength 0.228 sq in 6 x 6 D11.4 x D11.4
Crack Restraint 0.704 sq in 3×3 D17.6 x D17.6
Let’s explore another area where engineers are focusing their concrete technical skills and that is the actual performance of the temperature-shrinkage reinforcement. The code bodies finally are recognizing that just specifying wire mesh as secondary reinforcement does not mean that what is envisioned becomes the as-built product. Typically, since year one specifications called for wire mesh as temperature-shrinkage reinforcement but never specified how it was to be incorporated into the concrete cross-section. In essence, the contractor was left to decide how the wire mesh would be placed in the concrete. Although in their documents the Wire Reinforcing Institute relates their recommendations as to where the wire mesh needs to reside in the cross-section to achieve optimum performance, WRI never demanded nor did they even suggest that specifications be written stipulating that the wire mesh is placed on permanent supports at the proper height in the concrete cross-section.
The International Code Council (ICC) has taken the lead in adopting a meaningful specification for the proper placement of the wire mesh. ICC in their 2009 International Residential Code (IRC) now stipulate that when wire mesh is used in floor slabs the wire mesh must be placed on permanent supports and at the mid-height of the slab. This is truly a landmark change in thinking. We are now seeing state code agencies, with Florida being the first, to call for either wire mesh or synthetic fibers to be used in floor slabs as secondary reinforcement. Obviously, the code also stipulated the wire mesh must be placed at the proper height.
Enter the Steel Deck Institute (SDI). In their new Manual #31, they have established a minimum requirement for temperature-shrinkage reinforcement in composite steel deck systems. In an elevated deck cross-section using composite steel decking the composite steel decking is the tensile component in the cross-section. This means the specification cannot be used with steel form deck or steel roof deck. The specification states that the minimum temperature-shrinkage reinforcement shall be either 6×6 W1.4 x W1.4 wire mesh or 25.0 lbs/cy of steel fiber or 4.0 lbs/cy of macrosynthetic fiber. This was a very significant milestone for SDI to recognize the value of the 3-dimensional fiber reinforcement system.
Currently, ICC is considering adopting SDI’s Manual #31. The single portion of the document that has not been accepted at this point in time is the secondary reinforcement verbiage. ICC would like to see technical documentation of the equivalent values shown in the secondary reinforcement verbiage. The Fiber Reinforced Concrete Association has been working with SDI and the Structural Engineers Association in developing a test method that will accommodate all three reinforcement systems and provide the required comparison of performance as secondary reinforcement.
TO BE CONTINUED…
-R.C. Zellers, PE/PLS, Director, Engineering Services