In an effort to encourage students to become more aware of the American Concrete Institute (ACI) and someday become members, the ACI Construction Liaison Committee, along with the Kansas and Missouri Chapters of ACI, sponsored a competition to solve a construction problem. Groups from colleges and universities within a hundred miles of Kansas City (location of the 2005 ACI fall conference), eighty-five students in all, entered the competition. The problem states that the contract has already been signed, implying that there isn't an opportunity to ask for more money, but there was no requirement that the solution be cost-effective.

The Problem

Your company has received a contract to construct a 12-inch-thick, 12 x16-foot reinforced concrete mat foundation for a masonry telecommunications building (the mat will also serve as the floor slab). Unfortunately, the construction site is a three-hour drive from the nearest ready-mixed concrete supplier. Further, the average air temperature is expected to be above 90° F during construction.

The engineer's specification requires that the concrete reach a strength of 3000 psi at 28 days, have an entrained air content of 6%, and be produced and handled in compliance with ASTM C 94-04 and ACI 301-05. Although Section 11.7 of ASTM C 94 requires that concrete be discharged from a truck mixer within 90 minutes after the mixer is charged, Section of ACI 301 allows submitting a request for a waiver, along with a description of the precautions to be taken.

In the pre-construction meeting, the engineer agreed to work with you to find a solution. She will waive the 90-minute time limit on the fresh concrete but only if you provide a satisfactory submittal showing how you will handle and place the concrete over the necessary time and under the expected hot weather conditions.

Prepare a submittal showing how your company plans to handle this situation.

The winning submittal

The judges selected a team from Kansas State University as the best submittal. The team members were Charlie Robben (project manager), Sam Eak, Pete Kelley, and Brian Maximuk. Their faculty advisor was Professor Kimberly Kramer. Here is their solution:

We have encountered a problem with the distance between our site and the nearest concrete batch plant. The travel time it will take for the mixing truck to arrive at our site is three hours, which is well over the 90-minute maximum stated in section 11.7 of ASTM C 94. Also, the temperature is expected to be above 90° F on the day we plan to pour our 12x16-foot mat foundation. To solve this problem we have thought of many different approaches:

  • Mixing the concrete onsite with a ¼-yard mixer. This would require an unreasonably large number of bags of cement and is not economical for our situation.
  • Loading a mixer truck with dry aggregate, transporting the dry batch to the site, then adding water from a water truck. This causes a problem with the concrete balling up during the mixing process.
  • Precasting the mat foundation and transporting the sections to the site. This is time-consuming and difficult because of the weight.
  • Using admixtures and changing the design of the specified concrete. Although this approach is possible, many batch plants advise against this option due to the risk of a rejected load and cost of re-engineering.
  • Using a volumetric mobile mixer that transports all raw materials separately and mixes onsite at our desired time.

We are proposing to use the volumetric mixer. The volumetric mixer can provide any slump and is capable of adding air entrainment. The volumetric mixer comes in ranges of 5 to 12 cubic yards, which will satisfy our need for 8 cubic yards for our foundation. Volumetric mixers contain all the materials needed to produce concrete—coarse aggregates, sand, water, cement, pigments, and admixtures—in a mobile, self-contained package. There are no time restrictions using this mixer because none of the raw materials are mixed until the truck has arrived at the site. The mixer is loaded and driven to the site, ready to mix at any time. Therefore, there will be no problems with early hydration. The volumetric mixer conforms to both ASTM C 685 and ACI 304. Therefore, the strength of the required concrete is in compliance with all applicable codes. This method of mixing concrete is our best option to complete this project with our current situation. It eliminates the threat of high temperatures and overmixing.

We have found a volumetric mixer near Jefferson City, Mo. The total cost to provide, transport, and place to all design specifications is $1475. We have contacted Barry Zimmerman of Clearview Construction, and he said that hot weather is not a factor with the volumetric mixer, and he is willing to let us rent his machine.

The use of a volumetric mixer is the best solution for the problem we have encountered. It is in compliance with all relevant codes, and is our best option.

Other suggested solutions

The parts of the problem that most student teams focused on were the 90-minute time limit restriction, high ambient temperatures, and the required air-entrainment of 6%. Other student solutions included using admixtures like Delvo by Master Builders to put the concrete to sleep during transit and reactivate it when it reached the jobsite, shipping the aggregates and water to the jobsite and then adding cement and admixtures, premixing all the dry materials (drying the sand and aggregate beforehand) and adding water at the jobsite, precasting the slab and trucking it to the site, adding liquid nitrogen or ice to keep the mix cool during transit, and placing the concrete at night at cooler temperatures. Some entries mentioned proper curing as part of the process to achieve the design strength.

What would you do?