Long before the gray “pea soup” is ladled onto the ground, the exact details of each ingredient are measured, recorded, tested, approved, and tested again.  The effects of the differences in the recipes are as vast and as subtle as the flavor fest featured at any chili cook-off.

           The core ingredients (Cement, Fly ash, Rock, Pea Gravel, Sand, & Water) are nearly always the same but the exact proportions and the testing methods will be the difference between a highway and home—literally.  Specifications on strength, durability, and workability are outlined by the type and scope of the job.  The recipe is then dictated by the attributes of each of the ingredients and the effects on the characteristics of the concrete. concrete.

  Once the rough draft of the recipe is on paper, each item is then procured and the Quality Assurance phase begins.  Prior to combining the ingredients, Gradation, Moisture & SPG (Specific Gravity) testing is done on each component individually and must fall within the acceptable range of the overall specifications. 

  Gradation testing is the process of separating all the aggregates and sorting by size through a set of sieves.  Once this is done, the total cumulative sizes are recorded and then added together.  There must be an acceptable distribution of the aggregate sizes as well as overall percentages of “coarse” and “fines.”  These figures have a direct correlation to the overall strength and workability of the concrete.  Experience and science have determined that the correct proportion will deem the concrete “well graded” meaning that the aggregates fit well together and compacts tightly around any reinforcement material required for the job.  “Gap graded” concrete has more gaps between aggregates, more air pockets, and higher water ratio requirements.  ts. 

  Moisture & SPG tests take a reading on the amount of moisture in the aggregates and the weight of the aggregate in water, respectively.  Based on the moisture content, the water ratio may need to be adjusted to meet the strength requirement; the less water, the stronger the mix.  Material weights are used to determine the absolute

volume of material, which translates to a batch and unit weight.  The unit weight serves as a benchmark for the Slump Test to make sure that the correct amount of material is used and that mixer trucks are not overloaded so that the concrete is delivered safely to the job site. 

 On occasion, all invested parties will collaborate, supervise, and verify stages in the mix testing process.  In January, John Hart & Brian Burr from the Iowa DOT and Matt Proctor & Jimmy Hammond from Cedar Valley met with our Quality Control team at the Brooklyn Lab to test a mix for the I-235 Downtown Des Moines project. 

 Each ingredient was carefully measured and then poured into the portable mixer.  Once mixed, a concrete temperature reading is taken.  Ideally, the concrete temperature will simulate what will happen in the field.  The higher the temperature the quicker the concrete will cure and harden. 

  Next, a Slump measurement is taken.  Slump is the distance that cone flattens immediately after the Slump Cone is removed.  The more dense or consistent a mix is the less it will slump indicating the overall moisture content.  The mix must be within the specified range to pass this test and is a good indicator that the strength will meet specs as well.

Since this project involves paving, the remaining concrete is poured into beams that will be used for Flexural Strength (pressure to bend) testing. If this were a structural project, such as a building foundation or Insulated Concrete Form job, cylinders, instead of beams, would be cast and Compressive Strength would be tested.  After approximately 20 hours, the concrete has set and is removed from the beam forms.   Since concrete must be kept moist during the curing process, the beams are then placed in a curing tank (right) containing a water/lime solution.

 At specified intervals, the beams are removed from the curing tank, dried and placed in the Strength Testing Machine (left).  Pressure is applied until the concrete fractures.  The exact amount of pressure is recorded.  After all the tests are complete and all specs are met, the mix is approved and plans move forward. 

This type of extensive testing process continues throughout the duration of the project.  It is this attention to detail that makes a quality product, a job well done, and a satisfied customer.