High Performance Concrete Pavements
Project Summary

CHAPTER 10. IOWA 1 (a and b) (Highway 5, Carlisle, and US 30, Carroll)

Introduction

The Iowa Department of Transportation's (DOT's) first TE-30 project consists of an evaluation of the effect of concrete mixing time on several critical plastic and hardened concrete properties. This investigation was conducted using several different mix designs for pavements constructed at two different sites: Iowa Highway 5 near Carlisle and US 30 near Carroll (Cable and McDaniel 1998a). The locations of these sites are shown in Figure 31.

Figure 31. Location of IA 1 projects.

Study Objectives

The issue of mixing time has become a concern to the Iowa DOT as several plant manufacturers have claimed consistent and sufficient mixing in as short as 30 seconds (Cable and McDaniel 1998a). Therefore, the primary goal of this research project is to investigate the effect of concrete mixing time on the resultant air content, air distribution, consolidation, and workability of concrete used for pavement construction (Cable 1998). Secondary objectives include the evaluation of a contractor-designed concrete mix and the evaluation of an alternative mixer.

Project Design and Layout

The investigation of mixing time on concrete properties was conducted at both the Carlisle and Carroll test sites. Nominal mixing times of 30, 45, 60, and 90 seconds were selected for evaluation, and two different mix designs were used: a standard mix developed by the Iowa DOT and a contractor-developed mix (Cable and McDaniel 1998a). No information on the exact composition of the two mixes is available, but the contractor mix reportedly is a "Shilstone" mix containing a uniformly graded aggregate.

Two different mixers were also employed in the study. A standard 7.65 m³ (10 yd³) drum mixer was used at the Carlisle project, whereas a modified drum mixer employing rotation of blades within the drum was used at the Carroll project (Cable and McDaniel 1998a). The same contractor was employed for the construction of each paving project. The experimental design matrix for this study is shown in Table 14.

State Monitoring Activities

To achieve the objectives of this project, the Iowa DOT and the Iowa State University jointly participated in the testing and monitoring of the concrete mixing and paving activities. The paving was performed in the summer of 1996.

The testing methods in ASTM C 94 were used in this study to determine the significance of the mixing time on the consistency of the concrete mix delivered and placed on grade (Cable and McDaniel 1998a). ASTM C 94 is designed to check the consistency of the material at the beginning and near the end of the truck discharge. Using this standard, measurements of slump, unit weight, air content, retained coarse aggregate, and compressive strength were obtained and used to compare the consistency of the mix at different points of delivery (Cable and McDaniel 1998a).

At both the Carlisle and Carroll sites, the tests listed in Table 15 were conducted for each combination of mixing time and mix design included in the investigation. For the slump, unit weight, plastic air content, and wash tests, samples were obtained from three different locations for the same load of material: the center of the haul truck, the side of the haul truck, and on grade in front of the paver (Cable and McDaniel 1998a). Compressive testing of cylinders and cores was performed on concrete retrieved from the same batch as for the slump, unit weight, air content, and wash tests (Cable and McDaniel 1998a). Air void distribution testing was conducted on hardened concrete cores also taken from the same batch (Cable and McDaniel 1998a).

The haul trucks to be tested were selected at random at approximately ½- to 1-hour increments (Cable and McDaniel 1998a). Trucks were selected for testing only when the paver and plant were in continuous operation to ensure representative samples. Sufficient concrete was obtained from each truck to provide for tests of slump, unit weight, air content, and for the preparation of cylinders for compressive testing (Cable and McDaniel 1998a). Upon completion of the unit weight test, the material in the unit weight bucket was washed through a sieve to remove all fines and cement, and then the retained coarse aggregate was weighed and compared to the unit weight and the expected weight of coarse aggregate in the unit weight bucket (Cable and McDaniel 1998a).

The approximate location of a sampled batch of concrete in the pavement was recorded during the paving operation for later coring operations (Cable and McDaniel 1998a). Core sampling was conducted at the noted locations after the concrete had reached a strength of 3.4 MPa (500 lbf/in²).

The instruments used for the air void analysis of hardened concrete cores were a Hitachi 2460 N low-vacuum scanning electron microscope, a Tetra back-scattered electron detector, Deben stage automation, and an Oxford Instrument ISIS x-ray analysis system (Cable and McDaniel 1998a). Samples were prepared from the cores, and special software was used to determine the area and size of the air voids in each image (Cable and McDaniel 1998a).

No further monitoring or reporting is anticipated. This project has now been completed.

Results/Findings

Extensive statistical analyses were conducted on the data collected from each test site. Comparisons of key concrete properties (slump, unit weight, air content, and compressive strength) were made between testing location (side, center, or on grade) and between the various mixing times (30, 45, 60, and 90 seconds). Based on these analyses, the following general conclusions were drawn (Cable 1998; Cable and McDaniel 1998a):

• Dump-truck-type hauling units do not significantly change the quality of the material being delivered to the paver and should continue to be allowed in addition to agitator-type hauling vehicles.
• Mixing times of 60 seconds or greater do have a positive influence on the physical characteristics of the concrete product and should be retained as the minimum mixing time for all mixer types.
• Mixing times did not significantly affect the hardened air content or distribution for the Iowa DOT mix designs, but the data showed conflicting results for the contractor-designed mix. This may be the result of a different matrix of coarse and fine aggregates in the contractor mix. Therefore, it is recommended that contractor mix designs should be thoroughly laboratory tested prior to use in the field to determine the impact of admixtures and differences in aggregate/cement matrix on the physical performance characteristics of the mix.
• Mixing times of less than 60 seconds should be allowed only when steps have been taken to change the mixing process to ensure coating of all aggregates prior to mixer discharge into the hauling unit.
• Visual examination of the mix at the Carlisle site (30- and 45-second mixing times) indicated visible sand seams (uncoated sand particles) in the discharged material. The concrete produced under this set of mixing conditions was also noted to be very difficult to place and finish.

Points of Contact

Jim Cable
Iowa State University
Department of Civil and
Construction Engineering
378 Town Engineering Building
Ames, IA 50011
(515) 294-2862

Mark Dunn
Iowa Department of Transportation
800 Lincoln Way
Ames, IA 50011
(515) 239-1111

References

Cable, J. K. 1998. "Evaluation of Mix Time on Concrete Consistency and Consolidation." Proceedings, Crossroads 2000 Conference, Ames, IA.

Cable, J. K., and L. L. McDaniel. 1998a. Effect of Mix Times on PCC Properties. Iowa DOT Project HR-1066. Iowa Department of Transportation, Ames.

More Information

• High Performance Concrete Pavements: Technical Summary
• Pavement Publications

Contact

Sam Tyson
Office of Asset Management, Pavement, and Construction
202-366-1326
E-mail Sam