Specifications and resources to get the job done right.
By Phillip Cutler, P.E.
As I travel the country performing consultations at precast concrete plants entering the NPCA Plant Certification Program, I often find facilities that use ready-mixed concrete. Would you be surprised to know that at least half of the plants I have visited in the past two years use ready-mixed concrete?
You may be thinking these must be really small production facilities. Well, think again! These plants range in size from consuming just a few cubic yards of concrete per day to facilities requiring 100 yards per day. At 100 yards of ready-mixed concrete per day, can you imagine what it must be like to have a dozen ready-mixed trucks delivering concrete to your plant each day? Let’s face it: On any given day, situations occur at your plant that create changes in your daily production schedule. Plants that use ready-mixed concrete are no different. When the truck arrives, the plant must have its pre-pour inspections complete and be ready to start pouring.
So many questions come to mind that there is no hope of anticipating all of them or attempting to answer them all in just one article. So let’s hit some of the high points relating to the raw materials, batching and mixing, and concrete testing from a quality-control perspective.
The raw materials that make a yard of ready-mix are no different from those used by a facility that batches and mixes its own concrete in a centralized mixer. All of the raw material certifications and requirements are identical. The cement, supplementary materials, aggregates, admixtures and mixing water must conform to the relevant ASTM, AASHTO and/or specific project requirements. In addition, plants must obtain these certificates of conformance and keep them in the plant records for a period of three years to meet NPCA Plant Certification requirements. Sometimes special arrangements are made with the supplier of ready-mix in order to get the plant exactly what is required from a records standpoint. Good examples of this are cement, cementitious supplements and aggregate certifications. A typical ready-mixed concrete plant may consume as many as 10 truckloads of cement and three loads of fly ash per day in addition to huge quantities of coarse and fine aggregate each month. Is the precast plant using ready-mix required to obtain a certification on each and every truckload of cementitious material and the multitudes of gradations and moisture tests for the aggregates? Absolutely not. This is just not practical, and the plant will need to work with its supplier to obtain adequate records for program compliance. I usually suggest the precast plant obtain a certificate of conformance for each unique lot of cement or cementitious materials supplied, and monthly aggregate test results for gradations, unless project specifications stipulate otherwise.
Comparing ready-mixed concrete to plant-produced concrete, the mechanics of batching and mixing are not really all that different. The obvious exceptions are the mixing vessel and tools; the time and amount of material being batched at one time for each load; and supplementary items that need to be added at the plant location or additions that might not be available at the ready-mixed plant (retarders, water reducers or specific supplementary materials). If we neglect the mixing vessel, mixing tools and quantities of materials for now, the big questions are how much time is available once the truck arrives and what else needs to be accomplished before the pour can begin. The resources (ASTM C94, ACI 318, AASHTO M157, PCA Manual – Design and Control of Concrete Mixtures, and the NRMCA) all agree that the time limit between batching and complete discharge of the concrete at the job site is a period of 90 minutes or before the drum has rotated 300 times after mixing water is added. As with everything in life, there are always exceptions to the rules (Mother Nature, Murphy’s Law and “Acts of God”). And don’t forget that the purchaser has the right to waive any and all of the project requirements as each case may dictate. I don’t know about you, but I like things a little more controlled than this. Some flexibility can be used to your advantage if it is based on facts, plant records and data backing your expertise. Otherwise, you may be left with a very unpleasant and potentially costly decision.
Here is a real case to consider: The ready-mixed truck arrives an hour after batching takes place at the ready-mixed plant, because it was stuck behind a school bus. It’s 84 F (29 C); the sun has been beating down on your forms all morning (you are in luck this time, because your setup technicians decided to put the reflective blankets over the forms to keep the dust out following their completed post-pour inspections); and the contractor called this morning to say he needs the structures delivered to the job site in seven days to meet the DOT inspector’s schedule. What do you do? You can throw up your hands, point the driver to the gate and call it a day. But, being a “never say die” NPCA certified precaster, you wipe the sweat from your brow and politely ask the driver for the delivery ticket. You then casually ask him if he has made any water additions to the mix along his delayed journey to your plant. Upon confirming that the mix has not been doctored with additional water, you jump on the truck, climb the ladder and look down in the drum at the mix; not too bad, maybe a little stiff, you say to yourself, and down you go. You yank out your calculator, punch in the delivery ticket cement and water content data, and reveal that your delivered water-cement ratio is 0.39. The target ratio for the project is 0.4 maximum, there are 9 yards on the truck, and you are dying to add some water to be able to place the load before it locks up and save the day. What would you do next? Is there really enough information to make this decision?
In this case, I recommend the plant obtain a few more tidbits of information to increase the probability of success. These nuggets of wisdom should come directly from the plant files that have been generated over time from concrete testing performed at the plant. In the absence of this information, any decision will be a like a trip to Vegas: a roll of the dice. NPCA certified plants, whether using ready-mixed concrete or a centralized batch plant, are required to have the data necessary to make an informed decision in this case and be successful when conditions are not optimal. The data we seek is taken directly from the complete batch description printed on the delivery ticket and recorded along with the ambient temperature, the results of slump (slump flow and vsi), concrete temperature, air content, unit weight and compressive strengths for each load of concrete delivered to the plant over time.
You need nothing more than to calculate the maximum amount of water you could theoretically add to the entire load and not exceed the required water-cement ratio, right? Not so fast! This, too, is a little risky, and you must be cautious when taking only this single, additional step. You should look further into the history on batches of similar makeup and conditions in addition to the level of their cylinder breaks and past experiences prior to making a calculated addition to place the concrete successfully. Remember: For each gallon of mixing water added per yard, the compressive strength reduction is approximately 150 to 200 psi.
Now back to the truck and driver patiently waiting for your answer. With renewed confidence, you ask the driver to add 4.5 gallons of water knowing you have not exceeded the design water-cement ratio and that the concrete strength will be well above the design limits. You start the pour and wait patiently for your ACI certified technician to bring you the results of plastic concrete tests confirming your genius. What would you have done without the data?
The simple fact that historical data was available in the plant records led to an informed decision and avoided a potentially costly and risky move. This is just a single testimony to the value of NPCA plant certification. This same bank of historical data allows the plant to communicate effectively with the ready-mixed concrete supplier when considering refinements to the mix design and managing expectations of the ready-mixed plant. In turn, it leads to additional internal efficiency.
At the end of the job, you look back and reflect that the load was poured successfully; overnight and seven-day breaks were within strength parameters; and the contractor received his product on time and to specification. You just gotta love it when a plan comes together.
The specific requirements pertaining to raw materials can be found in Chapter 2; Batching and Mixing requirements in Chapter 3; Ready-Mixed Concrete in Chapter 3; and Concrete Testing in Chapter 5 of the NPCA Quality Control Manual.
The NPCA Quality Control Manual for Precast and Prestressed Concrete Plants is a free download from www.precast.org/npca-plant-certification-qc-manual.
For additional information on the NPCA Plant Certification Program, visit www.precast.org/npca-producer-plant-certification or e-mail [email protected].
Phillip Cutler, P.E., is the director of Technical Services at NPCA.
Make Sure to Check Out
For ready-mix resources, such as the batch ticket spreadsheet and calculator specifically made for plants that use ready-mixed concrete.