January 28, 2009

The mix design team has selected a final concrete mix!  After 27 different batches, they have arrived at a mix that has sufficient workability to place on the male mold.  Although they have yet to test the concrete strengths, all of their designs have been strong enough to meet the demands of the analysis.  The mix they selected has a unit weight of 55.20 pcf.  It is designated as Batch NCC0921K.  We will update the strengths as we obtain them. One thing to note about the following data is that many of the batches have a reported strength at 14 days that exceeds the strength at 28 days.  This is because only one cylider is tested at 14 days and two are tested (and their values averaged) at 28 days.  The trend seems to be that the cylinders gain most of their strength in the first 14 days.  Thus, the strength at 28 days is very similar, and the average of the two values often decreases the reported strength of the batch.

2009 Concrete Mix Design Data

January 14, 2009

After 24 different batches, the team is well on their way to having a final mix for the canoe.  As always, the team is having trouble obtaining concrete with reasonable workability.  The addition of fibers decreases the malleability of the concrete and small increases in admixtures never seem to help.  The team found one mix design that yielded a cohesive mixture with sufficient workability to construct a canoe.  But the unit weight of that mixture was around 56 pcf, and the team hoped to keep the unit weight below 55 pcf.  They are still working on the design, but the previously mentioned mix may be used if they aren't able to reduce the weight by the end of the month.  Below is an updated table specifying the properties of each mix design.

December 13, 2008

Although this is the first posted update for the concrete mix design process, the team has been hard at work for the past several months.  This year, there is a larger interest in mix design from our newer members (which is always exciting for the main mix design engineers).  The team mixes concrete twice each week, and they have  batched 17 mixes to date.

There have been some delays with obtaining materials (aggregates, fibers, a fiber dispersing admixture), but the team continues to dial in parameters that they can control (w/cm ratio, cemetitious materials content, aggregate gradation, admixture dosages, etc.).  This year, the team is using Siscor aggregates, which are made from recycled glass (satisfying the required recycled aggregate content).  The 3M K1 microspheres were elimintated this year because they historically reduce the workability of the mix; our theory is that the particles have a very high surface area and very low density, putting a large demand on the cement paste.  The cementitious materials for this year's mix consisit of white portland cement, metakaolin, and fly ash.  Several admixtures will be used, including air entrainer, high range water reducer, a fiber disperser, and a viscosity modifying admixture.  So far, the team has obtained excellent unit weights and compressive strengths for the concrete.  The properties of each concrete mix are shown in the table below.

Today, the team added fibers to the mix for the first time, and the workability of the concrete decreased dramatically.  Even though the mix is extremely cohesive, the dosages of the admixtures will need adjusting in the future.  Overall, the mix design is well underway, and they should have a final mix sometime in January.

Weighing out cement

Mixing concrete...it's a little dry

Making a beam to test the flexural strength of the concrete

Katie breaking concrete cylinders