Research is ongoing at the National Concrete Pavement Technology Center (CP Tech Center), Iowa State University (ISU), in collaboration with the Center for Advanced Cement-Based Materials (ACBM), Northwestern University, to use self-consolidating concrete (SCC) for slipform paving. Use of SCC is beneficial because it can diminish the honeycombs or vibrator trails occurring in the concrete subjected to under- or overvibration. It reduces the energy required for and noise generated by consolidation. The pavement made with SCC also can be more uniform and have longer serviceability than that constructed with conventional concrete.
Conventional concrete mixtures that are used for slipform paving generally have a low slump and very little slump spread. They exhibit good shape-holding ability but little flowability and therefore require intensive vibration. On the other hand, conventional SCC mixtures commonly have a very high slump and large slump spread. They display very good flowability but little shape-holding ability and therefore require special formwork to accommodate the hydraulic pressure induced by the concrete in construction. The behavior of the SCC mixtures developed for slipform paving is between that of the conventional pavement concrete and the conventional SCC mixtures. This SCC, used for slipform paving, should have both sufficient flowability to achieve self-consolidation and timely shape stability to assure extrudability.
Over the past two years, the researchers at the CP Tech Center and ACBM have made great efforts on investigating effects of different concrete materials and mixture proportions on concrete flowability, shape stability and self-consolidating ability. They have shown that a good balance between concrete flowability and shape stability can be achieved through engineering available concrete materials and tailoring the mixture proportions.
A performance-based mix design procedure has been developed. Lab simulations of slipform paving have been conducted, and the results have proven that the designed concrete is suitable for slipform paving without mechanical consolidation. The general properties of the concrete, such as set time, strength and rapid chloride permeability, are comparable to those of conventional pavement concrete. Recently, the research is focused on refining the developed SCC mix-design procedure by using local materials from potential paving test sites and on examining freeze-thaw durability of the concrete.
How it shapes up
The concrete mix designs were developed by adapting mixture proportions of conventional SCC. It was then evaluated how different cementitious materials (portland cement, slag, fly ash), admixtures (water reducer and viscosity modifier), additives (limestone dust, clay and fiber) and aggregate properties (gradation and void ratio) affect both flowability and shape-holding ability of the fresh concrete. Finally, it was examined how the flowability of this concrete could be manipulated to combine the good flow properties with the shape stability required for concretes used for slipform paving. The ultimate goal of the concrete mix design is to find the optimum combination to achieve a self-consolidating and at the same time shape-stable material. Table 1 gives the comparison between the SCC mix proportion recently used for the slipform paving field trial at Ames, Iowa, and the conventional concrete mix proportion for slipform paving commonly used in Iowa.
The SCC mixture used for slipform paving generally has high cementitious content, low water-to-cementitious material ratio and a lesser amount of aggregate than conventional concrete pavement. Use of water reducer, viscosity modifier and clay is not required, although it significantly helps modify concrete flow and shape-holding behavior.
Flowability has been one of the main concerns in the development of SCC. For the purpose of slipform paving, the unrodded slump test is used to indicate the flowability of fresh concrete. The concrete flowability is expressed by both the concrete slump and spread values.
The early rheology tests have indicated that use of slag to replace cement tends to increase both yield stress and viscosity of concrete, while use of fly-ash replacement generally decreases yield stress but increases the viscosity of concrete, thus providing the concrete with a better flow. Incorporating fine and clay materials, such as ActiGel, metakaolinite and kaolinite, into concrete mixtures also affects concrete flow. Acti-Gel and kaolinite tend to decrease concrete flow, while metakaolinite slightly increases concrete flow. Use of water reducer generally increases concrete flow; however, as reported by many other researchers, the effectiveness of the increase in concrete flow depends on not only the amount but also the type of the water reducers used.
It is believed that certain strength in fresh concrete, called “green” strength, might exist to provide the concrete with a given shape-holding ability. To test the green strength, fresh concrete is placed into a 4-in.-diam. cylindrical mold without the bottom. The height of the cylinder varies from 4 to 8 in. During the placement, the concrete mixture is dropped from a height of 12 in. through an inverted slump cone in the mold. Right after the placement is finished, the cylindrical mold is removed, and a small weight is gradually added to the top of the green concrete. The weight that causes the concrete to collapse divided by the cross-section area of the cylinder is recorded as the measure of its green strength.
The research results have shown that green strength of the slipform SCC is much lower than that of conventional pavement concrete. Despite such low green strength, the tested slipform SCC cylinders with a certain height could still hold their shape. The addition of clay materials into a mixture could significantly increase the concrete green strength. However, as concrete green strength increases, the concrete flowability often decreases correspondingly. Therefore, a slipform SCC should have an optimal green strength for a given flowability.
Although proper flowability can assist concrete to consolidate, it is still not a direct measurement for the self-consolidating ability of slipform SCC. The research has revealed that if a concrete mixture can consolidate well under its weight, the shape of the mixture after a slump test will show a symmetrical cone. An unsymmetrical shape of the concrete after a slump test indicates a nonuniform material distribution in the concrete. Some mixtures may have similar slump and spread, but a different shape after the slump cone is removed. A potential SCC for slipform paving generally has a slump of 6-8 in. and a spread of about 12 in.
The self-consolidating ability of a concrete mixture also can be assessed from the ratio of the unit weight of the concrete dropped from a given height (12 in.) without additional consolidation and the unit weight of the concrete rodded thoroughly. This ratio also is called a compaction factor. For a slipform SCC, the compaction factor should be greater than 98%. Mortar rheology and quantity as well as coarse aggregate gradation all influence concrete self-consolidating ability.
Miniature version
A miniature paver (mini-paver) was developed to simulate slipform paving in small-scale laboratory testing. The paver can produce slipform SCC slabs with a width of 18 in. and thickness of 3 to 6 in.
This mini-paver is designed based on an L-box concept. The vertical part of the paver is 18 in. in height and 6 in. in width. This produces pressure to the concrete to aid consolidation. The horizontal leg is the rectangular form for the slab. A platform is provided at the top of the paver for additional concrete that can be stored and continuously pushed into the vertical part. During paving, the paver is pulled forward by a crank system at a speed of 3-5 ft/min.
Observations that can be made on the slab are extrudability, surface quality and side slump. Once the concrete has hardened, it is cored and tested for density, strength and chloride permeability. These test results are generally comparable to conventional pavement concrete. Once a concrete mixture is tested successfully with the mini-paver, it can be recommended for the field application.
The field trials are being conducted with the Ames City Public Works Department and Manatts Ready Mixed Concrete Plant, Ames, Iowa. The first test was carried out in August 2006. The test pavement was 4 in. thick, 8 ft wide and about 40 ft long. It was done to test an SCC mixture developed in the laboratory and to determine the feasibility of the slipform SCC application. The paver used was a modified asphalt paver.
A second field test was conducted on July 25, 2008. The PCC test pavement is a bike path, 5 in. thick and 8 ft wide. Different from the first test, the pavement finishing, texturing, jointing and curing were performed and the long-term performance of the pavement will be observed.
Cementitious bug
Concerns raised by researchers at the CP Tech Center and other engineers for using slipform SCC are related to the relatively high cementitious content of the concrete, leading to a high material cost and a higher risk of uncontrolled cracking. The challenge is that this approach needs the fresh concrete to have self-consolidating ability and to prevent edge slump at the same time during a slipform paving operation, thus requiring good control of the mixture from batch to batch.
Work is ongoing to address these challenges. It is believed that the relatively higher cost of the concrete mixture might be offset by the increased construction speed, improved concrete pavement quality and reduced negative environmental impact of concrete construction. Currently, the research team is making efforts on reducing the cementitious content of the concrete mixture. At the same time, use of cellulous fiber and special attention on the concrete curing are suggested to reduce the concrete cracking potential.
For more information on the presented research, please contact Kejin Wang, Peter Taylor or Tom Cackler at the National Concrete Pavement Technology Center at 515/294-5798; www.cptechcenter.org.
