“Crushable” concrete for better protection against terrorists … improved designs for precast rockfall barriers … corrosion of precast concrete barrier connections … lightweight, high-performance, self-consolidating concrete for bulb-T beams. They’re all part of new research presented earlier this year during the 94th annual Transportation Research Board annual meeting, which drew 12,000-plus design, engineering and allied professionals to Washington, D.C.
Concrete Products has reviewed some of the most pertinent peer-reviewed technical papers in precast/prestressed concrete from the meeting, summaries of which are presented here. Last month we looked at new research in ready mixed and cast-in-place concrete; see “Glow in the Dark Concrete?,” March 2015. For more information about the 2016 meeting, or to obtain files of full papers from this year, visit www.trb.org.
‘CRUSHABLE’ CONCRETE CAN SERVE AS DEFENSE AGAINST TERROR
It sounds counterintuitive, but so-called “crushable” concrete has a place in barriers and safety systems designed to protect structures against terror attacks. That’s among the suggestions of Keith Doyle, Aleksandra Radlińska, Ph.D., and Tong Qiu, Ph.D., all of The Pennsylvania State University, in their 2015 TRB paper, Material Properties of Crushable Concrete for use in Vehicle Anti-Ram Barriers.
Perimeter security systems comprise a first line of defense against terror attacks, and rely on protective barriers to guard buildings and structures, they write, noting, “The primary goal of these blockades is to stop a vehicle from ramming through the barrier and towards the building.”
Barriers are typically assessed with a full-scale crash test in accordance with ASTM F2656-07 (4) and must withstand a collision without allowing more than 1 meter of penetration. “One potential method of dissipating some of the vehicle’s kinetic energy is to design an obstacle that utilizes a crushable concrete material,” Doyle, Radlińska, Qiu say. “This material will deform during the impact, dissipating some of the vehicle’s energy and reducing the load transmitted to the other parts of the barrier. A crushable concrete that is capable of dissipating large amounts of energy could potentially reduce the size and cost of a barrier without any loss of performance in stopping moving objects.”
Crushable concrete has been used for various purposes, including aircraft arrestor beds at the end of runways, which fail under a plane’s load, allowing tires to sink. “[It] has also been employed for absorbing blast energy from explosions outside of structures,” the authors note. “In this application, the crushable concrete is placed between the structure and an exterior plate. Under a blast loading from the outside of the structure, the exterior plate can move through the crushable concrete layer, reducing the load transmitted to the structure.”
Ideal characteristics of a crushable material include high deformation potential, so the reduced load can be transmitted to the structure for a long period of time, allowing large energy dissipation, Doyle, Radlińska, Qiu observe, adding, “In case of cement-based matrices, collapsible structure can be achieved via foaming agents or crushable inclusions, for example, using expanded polystyrene spheres as a partial or full aggregate replacement.”
When the polystyrene concrete is loaded, the material initially exhibits a linear elastic phase that is similar to normal concrete. After the polystyrene concrete reaches its relatively low peak compressive strength, the matrix behaves much more plastically. The relatively weak cells begin to progressively collapse inwardly, during which energy is dissipated from two mechanisms: brittle fracture of the cement matrix, and plastic deformation of the polystyrene cells.
The Penn State team investigated the energy dissipation properties of crushable, precast concrete barriers containing polystyrene spheres. Two different mix designs were tested under unconfined conditions, and four different mix designs were tested under confined conditions. The effects of confinement, water to cement ratio, air entrainment, and strain rate of loading on energy dissipation in concrete were examined.
In the first series of tests, a concrete mixture designed at 0.6 w/c and 40 percent of expanded polystyrene by volume was tested in unconfined setting. The polystyrene spheres were an average of 5mm in diameter. The results were compared to a reference concrete with the same 0.6 w/c, but regular aggregate occupying a corresponding 40 percent of volume, as shown in the Concrete Mixture Proportions table. The aggregate had specific gravity of 2.6 and absorption of 0.96 percent, and was sieved to obtain particles similar to polystyrene in size. Aggregate that passed through the ¼-in. sieve, but remained on the No. 6 sieve was used, resulting in final particle sizes between 3.35 mm and 6.3 mm.
In the second series of tests, concrete samples were tested in a confined setting. Four different mixes were developed, with w/c of 0.4 and 0.5 and varying dosage of air entraining admixture, as shown in the table. The mixture with w/c of 0.4 had high range water reducer added at the dosage of 4.5 oz. per 100 lbs. of cement. Mixtures were named according to their air entrainer dosage and water to cement ratio. For example, LAE-w/c=0.5 denotes lower air entrainer dosage and a water-to-cement ratio of 0.5. For all mixtures, concrete was mixed and cast following ASTM C192-07 (19). Cylindrical 4- x 8-in. samples were cast and stored in a moist room until the time of the test, i.e., 7, 14 or 28 days, depending on the mixture.
“Unconfined tests showed that polystyrene concrete has potential to resist a high portion of its peak strength at relatively high strains (compared to normal aggregate concrete),” Doyle, Radlińska, Qiu report. “This suggests that crushable concrete is capable of dissipating significant amounts of energy in vehicle anti-ram barriers.”
CONCRETE MIXTURE PROPORTIONS
(per cubic foot)
In unconfined loading, however, the samples failed in shear manner (similar to ordinary concrete), and many polystyrene cells were left in an uncompacted state, which does not utilize the inclusions in a desired manner, the authors note, adding: “In confined setting, substantially more energy was dissipated than in unconfined conditions. This was due to the full compaction of the sample achieved due to steel tube confining the testing material.”
They recommend that crushable concrete be designed with adequate confinement— for example, an encapsulated element—to maximize energy dissipation in vehicle anti-ram barriers. “Additionally, the tests showed that lower water to cement ratio generally was characterized by higher strain energy,” they conclude. “Tests were performed at several quasi-static strain rates. However, no clear trend was observed between strain rate and energy dissipation in the range of strain rates that were investigated.”
Future work is planned to further optimize mixture proportions for crushable concrete capable of high-energy dissipation. Also, dynamic tests will be performed to evaluate crushable concrete performance under high strain rates, corresponding to vehicle collision into a barrier.
|Unconfined uniaxial compression test performed on polystyrene concrete sample (left) and cross-section after unconfined compression test (right).|
FIBERS BOOST ROCKFALL BARRIER PERFORMANCE
Design modifications, including steel or polypropylene fibers dosing, will enhance the long-term performance of safety-geared precast structures, note the University of Akron’s Dr. Anil Patnaik, Abdisa Musa, Srikanth Marchetty and Dr. Robert Liang in Full-Scale Testing and Performance Evaluation of Rockfall Concrete Barriers.
Rockfall hazards are present throughout the Buckeye State, and the Ohio Department of Transportation uses TL-3 standard concrete barriers along the edges of roadways in high-risk areas. Yet the performance of these barriers under impact from rocks on the ditch side—and their effectiveness for rockfall catchment—is relatively unknown, the authors state. Their research saw full-scale impact tests on precast concrete barriers to simulate the effects of impacting rocks of various sizes and shapes. Numerous impacts were made at different sections and levels of the barriers to gauge structural integrity and energy absorption capacity.
“The results from this study revealed that 32-in.-high precast concrete barriers with current Ohio DOT details have an impact energy absorption capacity of up to 24 kJ under single impact,” Patnaik, Musa, Marchetty and Liang write. “The corresponding energy absorption capacity of 42-in. high cast-in-place concrete barriers is about 56 kJ under single impact. Moreover, these barriers experience severe cracking and spalling of concrete under impact loading.”
|Performance of rockfall barriers under impact testing: top, left and right, typical failure modes of precast concrete barriers; bottom, precast concrete barriers with polypropylene fibers, impact side (left) and pavement side (right).|
Several design modifications were studied in the test program, including the use of polypropylene or steel fibers for concrete used to cast existing barrier profiles. A new precast barrier design with a foam board core—fabricated with and without steel fiber—was also developed and evaluated. For cast-in-place barrier specimens, the use of smaller diameter reinforcing bars at closer spacing was evaluated, as well as the use of welded wire fabric with and without the addition of polypropylene or steel fiber to the concrete mixes.
The authors conclude the use of both fiber types significantly increased the energy absorption capacity of the barriers with revised details, while also controlling the extent of cracking, spalling and splashing of concrete debris. Epoxy coating on rebar or welded wire fabric tends to weaken the bond between the reinforcement and the surrounding concrete, they say. “Weakened bond resulted in excessive concrete cracking and spalling, as well as reduced energy absorption capacity for the barriers,” they observe. “Adding 7.5 lbs. per cubic yard of polypropylene fiber, or 40 lbs. per cubic yard of steel fiber improved the performance. This enhanced performance suggests that the use of fiber would increase safety of roadway users.
“The tests conducted on the modified concrete barriers showed an impact energy increase of more than 100 percent with the modifications suggested in this study,” Patnaik, Musa, Marchetty and Liang conclude. “Barriers made from the modified designs also experienced significantly reduced extent and severity of cracking and a reduction in spalling and splashing of concrete.”
LIGHTWEIGT HPC WELL SUITED TO BEAMS, DECK
Lightweight, high-performance, self-consolidating concrete is a good choice for precast/prestressed bulb-T beams when used with a lightweight, high-performance, conventional-slump cast-in-place bridge deck in Virginia. That’s according to Celik Ozyildirim, Ph.D., P.E., and Gail M. Moruza, Virginia Center for Transportation Innovation and Research, Charlottesville, in their 2015 TRB paper, Lightweight High-Performance Concrete in Beams and Deck in Opal, Va.
“Lightweight high-performance concrete (LWHPC) with a pozzolan (fly ash or silica fume) or slag cement is expected to provide high workability, high strength, and high durability with reduced dead load,” the authors note. “High workability is achieved by self-consolidating concrete. High strength and durability is possible through proper selection of ingredients and proportions.”
The Virginia Department of Transportation widely uses normal weight, high-performance concrete (HPC) with pozzolans (Class F fly ash or silica fume) and slag cement for cost-effective structures, they add. Specifically, normal weight HPC beam concretes have high workability, and at the hardened state, have high strength, high durability, or both. Deck concretes have high durability, which leads to extended service life, and high compressive strength allows for a reduction in the number of beams per span, a reduction in the beam cross section, and longer spans, leading to cost savings.
“Beyond the economics, additional benefits may be realized by reducing the dead load weight of the structures through incorporating the use of lightweight concrete,” Ozyildirim and Moruza observe. “For example, many bridge structures have been posted with reduced load-carrying capacities, making them functionally obsolete. However, lightweight concrete (LWC) can be used on superstructures while the existing substructure is retained because of reduced dead load.”
Yet LWC prestressed beams have been used in various states on a limited basis, they add, noting: “One of the advantages of LWC over normal weight concrete is the existence of a more continuous contact zone between the aggregate and the paste, enabling better bonding in LWC. In addition, the presence of water in the pre-wetted lightweight aggregate voids contributes to internal curing.”
Water supplied by internal curing maximizes hydration and minimizes self-desiccation and its accompanying stresses that may produce early-age cracking. Another advantage of LWC is its low modulus of elasticity, which minimizes cracking as lower stresses occur for a given deformation when the modulus of elasticity is reduced.
“Concerns about cracking apply more to decks than to beams, which are usually in compression because of prestressing,” affirm Ozyildirim and Moruza. “For prestressed girders, the reduced modulus of elasticity must be considered in the design because it results in greater cambers and increased elastic shortening losses.”
The purpose of this study was to evaluate the prestressed LWHPC in bulb-T beams, and the LWHPC in deck in the bridge structure on Route 17 in Opal. Oriented at a 27-degree skew, the bridge has two 128 ft. spans, made continuous for live load with a cast-in-place pier diaphragm.
In each span, there are four 61-in.-deep bulb-T beams with a length of 127 ft., 6 in. The LWHPC for the beams had a target unit weight of 120 lb. per cubic foot, with a maximum acceptable value of 123.4 lb. per cubic foot. “It was designed to yield a slump flow of 25 ± 3 in.,” report Ozyildirim and Moruza. “The required air content was 5.5 percent ± 1.5 percent. The 28-day minimum specified design compressive strength for the beams was 8,000 psi, with a release strength of 6,000 psi. The diameter of the strands was 0.6 in. The specified maximum permeability was 1500 C.”
The poured-in-place deck consisted of LWHPC with a specified maximum unit weight of 120 lb. cubic foot, a specified minimum compressive strength of 4,000 psi, and a specified maximum permeability of 2,500 C (coulombs). The required air content was 6.5 percent ± 1.5 percent. The concrete was designed with a conventional slump (i.e., it was not SCC).
The study was conducted in two phases involving separate precast/prestressed producers. In the first phase, a test beam was fabricated and tested to failure. In the second phase, the actual bridge was constructed. Initially, trial batches were made to develop the lightweight HPC with high workability for beams at a precast/prestressed plant. Then, a test beam similar in cross section to the actual beams in the structure was cast at Plant 1 to determine if lightweight HPC with high workability could be successfully placed, and if the beam could meet the design criteria. The beam was loaded to failure at Virginia Tech and the successful results allowed the casting of the actual bridge beams.
The contractor selected a different precast/prestressed plant to fabricate the eight girders for the Route 17/Opal bridge. The crossing was opened to traffic in November 2013; three months later, a second condition survey was conducted. The air temperature was 46° F and there were still no cracks on the LWC deck, Ozyildirim and Moruza note. “One other observation was the apparent scaling at a portion of the closure pour that had LWC,” they say. “Since the scaling was restricted to a small area along the edge, it was attributed to poor finishing practices. The rest of the deck, including the closure pour, was in good condition.”
The authors conclude:
- Lightweight HPC with a high workability and high compressive strength exceeding an average value of 10,000 psi and a low permeability of less than 1,000 C can be produced at a precast plant using quality lightweight coarse aggregates and slag cement.
- Even in the presence of conductivity-prone calcium nitrite, permeability test values were low or very low. For permeability testing of LWC with slag cement, accelerated curing where specimens are kept at 100° F for three weeks is needed, as with conventional weight concrete, to observe the low permeability at the early age of 28 days.
- LWC mixes with slag cement for bridge decks can have compressive strengths exceeding 5,000 psi and very low permeability (< 1000 C).
- A decrease in the compressive strength values in the bridge deck concrete placements on successive days draws attention to the uniformity of the mixture and the need to control water content throughout the project.
- Shrinkage values were within the expected range for conventional concretes. In general, conventional concretes exhibit transverse cracks, especially over piers. However, there were no cracks on the deck after two winters, indicating the benefits of the lower elastic modulus and internal curing of LWCs.
CONNECTION CORROSION COMPROMISES BARRIER PERFORMANCE
Performance of precast concrete median or shoulder barriers is put at risk by neglected corrosion, especially with their connections, say David Veneziano Ph.D., and Yongxin Li, Ph.D., Western Transportation Institute, Bozeman, Mont., in their technical paper, Concrete Median Barrier Connection Corrosion in the United States: Experience and Future Directions.
“Precast concrete barrier (PCB) connections are important in providing strength and rigidity to counteract crash forces,” the authors report. “Different connection systems are susceptible to corrosion, leading to a weakening of the connection itself, potentially having an impact on the overall functionality of the barrier system during a crash.”
The problem can be a surprise to owners, Veneziano and Li affirm: “Corrosion is not necessarily expected. Consequently a synthesis on this issue was undertaken to determine the state of knowledge regarding PCB connection corrosion, the maintenance of connection systems, potential approaches to address corrosion on existing and future installations, and approaches to replacement of barrier segments on an agency-side scale when necessary.”
A review of the general literature on PCBs found that a number of different designs are in use and have been shown to meet existing crashworthiness criteria. But only a limited portion of the available literature is focused on PCB connection systems.
“Information tended to focus on designs, materials and crash testing performance, rather than corrosion or replacement of connections,” the authors note. “In only one instance was the potential for metal connection systems to corrode mentioned. This absence of discussion of connection corrosion may be indicative of a lack of awareness of the potential for this problem.”
Veneziano and Li found approaches to identifying corrosion varied, and included inspections, random observation and identification during other efforts (e.g., reconstruction). When corrosion occurred, it was primarily thought to be caused by the use of winter maintenance materials, specifically salt. Smooth steel bar, wire rope and rebar were all identified as having experienced corrosion. “Interestingly, few agencies treated connections in any way to prevent corrosion, using zinc or galvanizing to do so,” the authors note. “Most agencies employed no maintenance practices, either specific to connection systems or PCB in general, to address corrosion. Those which did mainly employed simple actions such as washing and painting.”