Student: Trevor Looney, University of Oklahoma
Trevor was presented with a 2018 Slag Cement Project of the Year Award in the Category of Research. More information about the ceremony can be found here.
Presentation Slides Below:
Ultra-high performance concrete (UHPC) is a relatively recent advancement in cementitious composite materials with mechanical and durability properties which far exceed those of conventional concrete. UHPC has been successfully used in a number of applications related to connection of precast concrete bridge components due to its superior bond development characteristics with steel reinforcement, ease of placement, and long-term durability compared to conventional concrete. Joints replaced or connections made using this material will have better durability, better resistance to impacts and abrasion, and will allow for a smaller quantity of material to be used while still obtaining adequate load transfer between connected components. Using UHPC allows for small, simple connections without the need for post-tensioning (when connecting precast elements) or large amounts of field-cast concrete (Graybeal 2010). Joints cast using UHPC also tend to behave more like monolithic construction than typical field-cast connections. The long-term benefits of using UHPC are evident, but commercially available proprietary mixture formulations are very expensive and mix design using local materials is much more complicated than for conventional concrete. The material characteristics, complicated mix design, and need for specialized mixing procedures require detailed specifications and quality control testing currently not included in the Oklahoma Department of Transportation (ODOT) Standard Specifications (2009). For this reason, ODOT commissioned the University of Oklahoma to develop a mix design using materials easily obtained in the state of Oklahoma that can be used as an additional option for joint material in existing and newly constructed bridges. The performance of the developed mix designs were then compared the commercially available UHPC, Ductal®, which is a very effective and highly tested product.
The first step of this research project was the development of the UHPC mix designs. The UHPC mix designs developed in this study use a combination of cement and supplementary cementitious materials (SCM) with differing particle sizes to create the binder paste since the water-cementitious materials ratio (w/cm) is so low, a portion of the cementitious material is not hydrated and therefore acts as an aggregate. Two different cement types, Type I and Type III, were used from two different manufacturers. The Type I cement was produced by Ash Grove. The Type III cement was produced by Buzzi Unicem. The Class C fly ash used was produced by Headwaters Resources. The slag cement was produced by LafargeHolcim. The silica fume used in this study was undensified and was produced by Norchem. The product VCASTM 140 White Pozzolans was produced by Vitro Materials. The w/cm for each mix evaluated was set to 0.2. With such a low water content, a high-range water reducer (HRWR) was used to improve workability. The HRWR chosen for this study was Glenium 7920, produced by BASF. As with most UHPC, no coarse aggregate was used in these mix designs. The fine aggregate used in this study was masonry sand meeting ASTM C33 and provided by Metro Materials. Lastly, UHPC requires the use of steel fibers to counter the brittle behavior of the concrete. Stainless steel, Grade 430, Flex-Ten® steel fibers produced by D&C Supply Co., Inc were used in this study. The steel fiber dosage was 2% by volume.
The mix development process was conducted in steps to evaluate specific alterations to mix designs separately. In each series, the mix proportions were altered based on which material the series was evaluating. Each mix design was evaluated based on flow and compressive strength. The flow was tested using ASTM C1437, and the compressive strength was tested using cubes following ASTM C109. The initial series set a starting point by evaluating two mix designs, Q and NE, from the study conducted by Graybeal evaluating local materials in various northern regions of the United States (2013), along with a third unpublished mix design developed by Dr. Royce Floyd. The remaining mix designs were created by adjusting w/cm, the aggregate- cementitious material ratio (agg./cm), and adjusting relative proportions of the cement and SCM. Several iterations were conducted altering the amount and proportion of each cementitious material, and the Modified Andersean and Anderson particle packing model to develop mix designs (Funk and Dinger, 1994). The particle packing study showed that the chemical composition of the mix designs was the main driver behind strength development. With this knowledge, the focus shifted to looking to various proportions of cement and different supplementary cementitious materials to determine the most effective combination. The mix design process went through iterations labeled A through J, with the final mixes developed in iteration J. The three best mixes were J3, J8, and J13, with J3 and J13 containing slag cement at weight proportions of 30 and 40% of total cementitious content, respectively. Incorporation of the slag cement helped improve not only the strength of the mix designs, but also improved workability and reduced the typical “stickiness” of the mixes commonly associated with UHPC containing silica fume. The highest strength mix of the three, mix J3, contained 30% slag cement.
Once the mixes were narrowed down to three, full property characterization was conducted on each mix. The compressive strength with and without fibers was determined following a ASTM C1856, using 3 in. x 6 in. cylinders and a loading rate of 150 psi/s. The effect of heat curing on compressive strength was also determined. The modulus of rupture of 3 in. x 3 in. x 9 in. prisms with and without fibers was determined following ASTM C1609 for the fiber reinforced specimens and ASTM C78 for the unreinforced specimens. The modulus of elasticity of the mixes without fibers was determined using 4 in. x 8 in. cylinders following ASTM C1856. Lastly, abrasion tests were conducted following ASTM C944 on the developed mix designs and Ductal® subjected to heat curing. The results showed that heat curing at 180°F for 36 hours allowed for the developed UHPC mix designs to reach its 28-day compressive strength and heat curing was more effective when the concrete was reinforced with steel fibers. Mix J3 had the highest compressive strength of the whole study. The modulus of rupture was found to exceed 2000 psi for all mixes, with the two mixes containing slag cement (mixes J3 and J13) obtaining the highest strengths. The modulus of elasticity was similar for all three mix designs and compared well to equations developed based on testing of Ductal® (Russel and Graybeal, 2013). Lastly, the abrasion resistance of Ductal® was higher than the developed mix designs, but Ductal® requires that the top portion of pours be ground off, which is not required for the mixes developed.
Next, the bond strength developed between UHPC cast against a Class AA standard ODOT concrete mix design was evaluated. Concrete bond testing was conducted with Ductal® and the best performing mix design developed, mix J3. Modulus of rupture style specimens with dimensions based on ASTM C78 (6 in. x 6 in. x 20 in.) were cast for evaluating the effect of different concrete surface preparations and interface orientations on bond between UHPC and the base concrete. Twelve full-length Class AA specimens were cast, which were cut in half at angles of 90°, 60°, and 30° from the horizontal after 28 days of curing. The cut surfaces of the half specimens were then prepared using two different surface preparations, wire brushing and sandblasting. The remaining twelve specimens were cast as half-specimens to allow sugar (a natural set retarder for concrete curing) to be placed on the mold prior to casting to create an exposed aggregate surface. These were simply power washed to remove the loose paste on the exterior of the specimen. A set of exposed aggregate specimens was cast for each angle, and one set was cast with a shear key. The UHPC was cast and cured at room temperature for 28 days prior to testing. Test results showed that the bond between Ductal® and the Class AA concrete was strong enough to cause failure in the base concrete in all angles and surface preparations except for one specimen. The failure of the J3 specimens was predominantly at the interface, but the stress levels were near to the flexural strength of the base concrete in most tests except the exposed aggregate surface preparation. The exposed aggregate specimen results were much lower than the other surface preparations due to the base concrete pulling water from the UHPC due to capillary action and creating a weakened surface in the UHPC directly beside the bond interface. This was due to the base concrete being completely dry prior to placing the UHPC, which is the worst-case surface prep. The removal of water from the concrete is more of an issue with UHPC due to its substantially low w/cm.
Lastly, the UHPC mix designs were tested as a joint between two conventional concrete panels. Six 4 ft x 4 ft by 8 in. thick slab specimens were cast using a reinforcing bar arrangement based on the slab reinforcement in the SH-3 bridge over the N. Canadian River in Pottawatomie Co., Oklahoma (NBI No. 19276). The slab specimens were reinforced to allow the testing to examine development length, bond strength, and flexural capacity. Two panels were connected by a 12 in. wide UHPC joint with approximately 5 in. of each bar left exposed to provide dowels into the joint. This bar extension is based on the shorter anticipated development length of 8db (5 in. for this case) embedment recommended by FHWA for mild steel reinforcing bars (Graybeal 2014). This arrangement was intended to represent the steel that will be exposed in the field after an existing joint is sawn out to be replaced. Each joint was heat cured at approximately 180°F. Two of the composite slab joint specimens were tested in static flexure and the third was set up to be loaded cyclically for 5,000,000 cycles. All tests were conducted using an 8 ft span with neoprene bearing supports and a single point load placed immediately next to the slab joint. Slab joint testing was conducted with Ductal® and the best performing mix design developed, mix J3. Each slab specimen performed as a monolithic structure, with he failure of the static-loaded slabs caused rupture of the reinforcing bars at the interface between the UHPC and base concrete. This indicates the shortened development length is adequate to develop the full strength of the bar. Also, the J3 mix performed as well as Ductal® in this test in terms of ultimate moment capacity and overall stiffness.
Accompanying this application is a series of slides outlining the three developed mix designs and the results of each test series conducted. The optimistic results obtained from this study show that UHPC with comparable performance to proprietary blends can be developed using materials readily available in Oklahoma. Slag cement played a large role in the development of these mix designs. Even with large replacement levels up to 40% of total cementitious content, the developed UHPC mix designs performed as well as Ductal® in most tested applications. Furthermore, the slag cement actually improved the workability and ease of placement over conventional UHPC while reducing the overall cost of the mixes. ODOT has also commissioned the University of Oklahoma to continue testing on mix J3 (which contains 30% slag cement) in a second phase of research to further prove its effectiveness as a repair material for concrete bridges.
- 2009 Standard Specifications Book, Construction Engineering Standards, Specifications, Materials and Testing http://www.okladot.state.ok.us/c_manuals/specbook/oe_ss_2009.pdf, Oklahoma Department of Transportation, 2009.
- Funk, J.E., Dinger, D.R., “Predictive Process Control of Crowded Particulate Suspensions,” Applied to Ceramic Manufacturing, 1994.
- Graybeal, B., “Behavior of Field-Cast Ultra-High Performance Concrete Bridge Deck Connections Under Cyclic and Static Structural Loading,” FHWA-HRT-11-023, 2010, Federal Highway Administration, McLean, VA.
- Graybeal, Ben, “Development of Non-Proprietary Ultra-High Performance Concrete for Use in the Highway Bridge Sector,” FHWA-HRT-13-100, Oct. 2013, pp. 1-8.
- Graybeal, B., “Design and Construction of Field-Cast UHPC Connections,” FHWA-HRT-14-084, 2014, Federal Highway Administration, McLean, VA. Russel, Henry G., Graybeal, Ben, “Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community,” FHW