January 25, 2008

Although the mix design team had some trouble at the beginning of the year, they are finally back on track.  One incovenience for the team was that the moist room was shut down for repairs at the end of last semester and there was no place to cure the concrete specimens.  In light of this, the construction team helped us make our own cure tent from a plastic shelving rack, some plexiglass, a humidifier, and a whole lot of visquine.  It atually works quite well.  See the picutres below.

The structural concrete mix is finished and comes in with the follwing properties: unit weight of 57.78 pcf, 28-day compressive strength of 1500 psi, modulus of rupture of 325 psf, 0.75 percent PVA fibers (by volume), and an air content of 10 percent.  In retrospect, the team probably should have tried latex admixtures to increase the air content and decrease the unit weight.  Regardless, the mix met all of the requirements set by the analysis and will be acceptable in this year's canoe.

Last week, the team finished developing the patch mix that will coat the exterior of the canoe.  To develop this mix, we increased the cement content, removed the large aggregates, and used no fibers.  Admixture dosages were much lower in the patch mix, although the dosages still exceed the manufacturer's recommended amount.

The final step for the mix team will be to develop a colored concrete inlay mix to use for aesthetic purposes.  We have never done concrete inlays before, so we'll se how it turns out.

All you need is a humidifier and plastic tubing

Attaching the plexiglass door to the front of the shelving

Viola...a new cure tent!

Structural concrete with 1.5% fibers

Making a composite beam with the final mix

The underside of the composite beam after curing seven days

Testing the patch mix on the beam

November 23, 2007

Mixing Daravair-M (AEA) with the cementitious materials

The aggregates, fibers, and fiber disperser

Adding the aggregates to the cementitious paste

Measuring the unit weight of the concrete

The cure room

November 19, 2007

The mix design team has been hard at work trying to get the structural concrete mix dialed in.  They finally got a concrete mixture that doesn't resemble sand when fully mixed, so they feel that they are finally making progress.  This year's limited water-cement ratio has made it very difficult to obtain a workable mix (and we thought it was bad last year).  The only way that the team was able to obtain a reasonable consistency was to use astronomically high dosages of high range water reducers and viscosity modifiers.  This year, the team is using Grace admixtures, namley Adva 170, AdvaCast 575, and V-Mar 3.  The combination of these three admixtures is giving the concrete decent workability with a very "sticky" consistency that should work well when applying the concrete to the form.

In addition, the team has been using an air entrainer caller Daravair AT60 that has not yielded very high air contents to date.  In fact, the air contents in the concrete have been extremely variable over the past few weeks (ranging between three and eight percent).  The large variablility indicates that the mixing is inconsistent and that it probably isn't being mixed at high enough speeds.  Since air entrainment is purely mechanical (it does not occur from a chemical reaction between molecules), the team has decided to use a mechanical mixer and to increase mixing times.  Also, there are two other air entrainers that will be explored to see whether or not a more potent air entrainer will help our cause.

Today, the team tried using an AEA called Daravair-M and increased mix times as previously stated.  In addition, they took some advice from the SJSU concrete canoe team and mixed the cementitious materials with the water before adding the aggregates.  This yielded a unit weight of 58.08 pcf and an air content of 14%.  Although the mix may still need some minor adjustments, the team is pretty happy with the overall results of the structural concrete mix.

November 3, 2007

Over the past month, we have been diligently working on our mix design, trying to get the proportions of each constituent just right.  We have been batching two to three concrete batches each week in hopes of finding our final mix design.  So far, we have gotten several parameters set, but have not been able to get the proper workability for adequate placement and consolidation on the canoe.

Our first task was to determine the optimum blend of cementitious materials.  To do so, we blended several different proportions of cementitious materials with a set amount of water and performed a flowability test on each blend to determine which mix would give us the best workability.  However, we didn't add high range water reducer to the blends and don't know whether or not the results of our tests will be accurate since HRWRA will be added to our final concrete mix.  The reason HRWRA was not added to the mixture is because each time we added it, we obtained a viscoelastic fluid that flowed extremely well but became plastic under loading.  You should try this...it's actually kind of cool to see.

Next, we performed consolidation tests on our aggregates to determine the best blend.  We took about twenty different aggregate blends of a constant volume and mixed them in a container.  Then, we put the mixture into a graduated cylider and compacted it to determine which blend consolidated the best (into the lowest volume).  We performed this test with our large aggregates only (we are using Siscor glass spheres this year); when we performed the test with the addition of microspheres, we saw that the small glass bubbles would not mix with the rest of the aggregates and had a tendency to float to the top of the cylider.

After determining the above properties, we bagan mixing concrete and have yet to determine the amount of admixtures we will use in the final concrete mix.  Thus far, it seems that extremely high dosages of water reducing agents, viscosity modifiers, and air entraining agents will be necessary to obtain the workabibility needed for concrete placement.  The limitation of the water-cement ratio to 0.4 has made it difficult to have any extra water in the mix.  We wish the other teams the best of luck in their endeavors and hope that the mix design process is going well.