Now what is meant by an inadequate mix? Inadequate mixes are those that have reduced strength and improper water to cementitious ratios. Right now, most codes specify either a 4000 or 5000 psi concrete compressive strength and a water to cementitious ratio no greater than 0.45. That being said, why are we still finding concrete with water to cementitious ratios in the 0.50's and strengths less than 4000 psi? It is my opinion that this is largely due to inadequate eduction, lack of care in what we produce, and given the times a reduction in product costs to keep prices down. I will say, though, that now is not the time to save pennies by making less quality products. Now is the time to get educated and make the best possible product for customers!!!
Cement requires very little water to become hydrated enough to start the process of forming paste. The rest of the water goes toward aiding in workabilty and making the concrete easy to form. However most precasters should have access to admixtures that can aid in workability without the addition of adding water. Using modern superplastizers there is no reason why a precaster cannot get his/her water to cementitious ratio down around the .40 region or lower. And the lower the water to cementitious ratio, the lower the amount of pores in concrete, thus making a very durable product. Modern self compacting concrete admixtures can make concrete almost like water while still maintaining high strength and low porosity.
In addition strength is something we have great control over. Cement is typically the number cost in a concrete mix design. The more cement the greater the cost in your mix. However, the more cement the higher the strength. Given the recent economic scale, it is easy to try and cut cost by making a less cement rich mix. This, however, is not the right move!! I would be the first to caution, cutting cost by making a more inferior product is not good practice. Most codes specify a 4000 psi concrete for tanks. Recent studies have found that concrete around 5000 psi or higher will stand up to more adverse conditions than a 4000 psi mix. Some municipalities even spec a 6000-7000 psi mix for wastewater applications. Again, given the advances in concrete admixtures a 5000 psi batch is not hard to attain by today's standards. Setting the bar at a 5000 psi minimum is a great starting point.
In addition to mixing adequate concrete, checking and testing is also just as important. One should keep constant monitor on what is coming out of the mixer. Check your air, check your strengths, check your yields. Keep tabs on everything. That is the only way to learn! The days of mixing on the fly are over. Engineers and GC's are demanding more and more of us everyday, keep yourself in check. Also, use your resources. Talk with your cement salesman, talk with your admix salesman find out what is going on in the industry. Put them to the test, tell them what you want out of your concrete. Keep yourself educated!!!
To summarize this I'm going keep this short and sweet.....THERE IS NO SUBSTITUTE FOR GOOD QUALITY CONCRETE...Learn it, live it!