Innovations in precast concrete materials and impact resistance mean more design alternatives for urban security barriers.
By Marc Caspe , P.E., S.E.; Jun Ji, Ph.D., P.E.; Lin Shen, Ph.D., P.E.; Qian Wang, Ph.D., P.E., LEED AP; and Yanzhi Zhai, P.E.
As a worldwide threat, terrorism is among the top concerns in infrastructure protection design. Vehiclular bombing has proved to be one of the most disastrous of terrorist attacks, especially in densely populated urban areas. Functional and efficient perimeter security barriers have become extremely important for shielding critical facilities and, more importantly, safeguarding human life.
To maintain the minimum blast standoff distance after a vehicle crash, perimeter barriers must fully stop any impacting vehicles and keep explosives out of the secured area. Barrier specifications and design must comply with crash test standards such as the recent ASTM F 2656-07, “Perimeter Barrier Vehicle Crash Test Standard.” When specifying, designing and installing urban security barriers, some major steps should be followed, including:
• initial investigation of the site;
• identification of potential terrorist attack vulnerabilities;
• determination of blast standoff distance and level of protection;
• selection of anti-crash barrier design criteria;
• evaluation of cost and constructability;
• selection of aesthetic design (including landscaping) and function; and
• installation of the selected barrier.
Among the hundreds of security barrier types available in the marketplace, precast concrete barriers have increasingly become the preferred solution. Compared with metal alternatives, concrete is a superior barrier material because it offers higher design compatibility with the urban environment and architecture, high corrosion resistance, low maintenance and measurable environmental benefits.
Along with technical developments and material advancements, many innovations in reinforced concrete design have taken place, particularly for precast barriers. These technological advancements include high-performance concrete; green technology/LEED compliance; energy-absorbing mechanisms; hybrid systems; and urban beautification concepts. In this article, the authors survey and evaluate the innovations in precast concrete barriers from a technical view and provide security design data for owners, engineers and architects.
Conventional perimeter barriers
Fixed or anchored concrete traffic barriers. Median barriers, also known as Jersey barriers, are commonly used precast concrete structures for anti-crash systems and typically comply with low-impact specifications such as the M30 or M40 rating defined by ASTM. Jersey barriers were originally developed by the New Jersey State Highway Department to divide highway lanes. A precast Jersey barrier stands 2 to 3 ft (0.6 to 0.9 m) high to minimize damage and reduce the probability of vehicles crossing into oncoming lanes in collisions. When used to resist right-angle (direct) impacts, median barriers are designed 3 ft above grade; these precast barriers are meant to engage a 15,000-lb (6,800-kg) truck
with 40-in. to 44-in. (1,020-mm to 1,120-mm) tires and a main frame with longitudinal chassis rails 2 ft (0.6 m) above the road. Variations from the original shape include:
the constant slope barrier; the F-shape barrier; and the California K-rail.
Fixed or anchored concrete planters. These planters are either precast, cast-in-place or concrete masonry units (CMUs). Typically designed to serve as urban architectural elements, precast planters function as street furniture and as impact-energy absorbers when filled with soil and vegetation. To adequately resist highspeed vehicle intrusion, planter barriers must either be embedded in the ground or anchored into a base like a concrete sidewalk diaphragm.
Concrete or CMU walls. Short retaining walls and tall free-standing walls are usually constructed of precast concrete, CMUs or other masonry materials, and typically reinforced with steel. Walls may be perforated or discontinuous to enhance visual appearance or allow pedestrian passage while still satisfying security requirements. Similar to fixed planters, security walls must be either embedded in the ground or anchored to an expanded base slab to sufficiently resist the significant momentum of vehicular impact. The short
retaining walls are normally intensively reinforced and maintain a height at least 3 ft (0.9 m) above grade to block the impacting truck tires and main frame. The heavy reinforcing in short concrete walls can resist large shear impact forces and maintain stability due to the mass of passive soil pressure.
Other heavy objects as security barriers. Heavy objects can be large sculpted pieces or massive boulders and concrete forms used like bollards to prevent vehicle ingress while allowing pedestrian and bicycle passage. Engineering design and evaluation are required to ensure that these barriers can effectively reduce the anticipated terrorist threat level for a given site.
Evaluation of barrier materials
Compared with metal fixtures, concrete is a superior barrier material due to its greater design flexibility (color, shape, texture, finish) and aesthetic compatibility with urban settings. Precast barriers offer high corrosion resistance, low maintenance and a typically lower cost. On the other hand, concrete barriers may require more production effort. In response to growing environmental and economic forces, designers are seeking innovative and efficient solutions that conserve non-renewable resources. Increasingly, concrete is being recognized for its many environmental benefits in supporting creative and effective sustainable development.
Innovations in high-performance concrete
Self-Consolidating Concrete (SCC). SCC is a highly flowable, non-segregating concrete that easily spreads into place, completely fills formwork voids and encapsulates even the most congested reinforcement – all without any mechanical vibration. As a high-performance concrete, SCC delivers these attractive placement benefits while maintaining all of concrete’s inherent mechanical and durability characteristics.
Since its inception in the 1980s, the use of SCC has grown quickly. The development of polycarboxylate polymers and viscosity modifiers have made it possible to create “flowing” concrete without compromising durability, cohesiveness or compressive strength. Flowability of SCC is measured in terms of spread using a modified version of the slump test (ASTM C 143). Viscosity, visually observed by the rate at which concrete spreads, is an important characteristic of plastic SCC and can be controlled through mix design to suit the type of application being constructed.
SCC’s unique properties give it significant economic, constructability, aesthetic and engineering advantages. SCC is an increasingly attractive option for optimizing site manpower, lowering noise levels, allowing for a safer work site, minimizing equipment wear and overall project cost. Very high early stripping strengths can be achieved to yield a quicker turnaround on forms. And SCC’s smooth surface finish minimizes or eliminates the need for time-consuming cosmetic repairs.
SCC allows easier pumping flows (even from the bottom up) into complex shapes, transitions through nearly inaccessible spots and minimizes voids around embedded items to produce a high degree of product homogeneity and uniformity. SCC can accommodate greater steel reinforcement, optimize concrete sections and shapes, offer greater freedom in design, and produce superior surface finishes and textures.
Figure 1 shows an example of SCC application in precast security barriers. This design shows high rebar density, size and complexity/congestion. It would be a very challenging task to fill this form with conventional concrete. Despite the dense reinforcement, the final glossy surface finish was excellent (see Figure 2).
Fiber-Reinforced Concrete. FRC is concrete containing fibrous material made of steel, glass, synthetic or natural fibers. There have been significant developments in FRC with respect to the concrete paste matrix, fiber design, fiber-matrix interface and composite production process. Research has led to a better understanding of the fundamental mechanisms controlling FRC material behavior, and this work has resulted in a continually improving cost-to-performance ratio (Naaman 2007).
Substantial progress has been made in concrete material research, including:
• A new generation of additives like superplasticizers;
• Viscosity agents;
• Active or inactive micro-fillers;
• Enhancement of different fiber properties adding significantly to the strength, ductility and toughness of the resulting composite;
• Increase in bond between fibers and concrete matrix;
• New additives to improve concrete shrinkage and corrosion-reducing properties; and
• SCC mix designs that improve the uniform mixing of high-fiber volumes and decrease matrix porosity.
With major advantages over conventional steel-reinforced concrete and availability at a reasonable cost, FRC is increasingly preferred by engineers and architects for applications with special needs such as high-strength, lightweight members; heavily loaded deck diaphragms; structural retrofitting; and particularly for security barriers that successfully resist large impact or blast forces.
Global warming from greenhouse gases is considered to be one of the most serious sustainability issues. The current atmospheric concentration of CO2 that makes up 85 percent of all greenhouse gases is the highest in recorded history. An important national sustainability initiative to reduce greenhouse gases is the Leadership in Energy and Environmental Design rating system. Developed by the U.S. Green Building Council, LEED has become a powerful tool for conservation of energy and resources in sustainable construction.
LEED concepts encourage the use of regional building materials like concrete that is produced locally to reduce transportation and fuel costs. Cement production accounts for 5 to 8% of total man-made CO2 emissions. There is increasing pressure, therefore, on specifiers and producers to improve the sustainability of cementitious materials through reductions in total cement content. Options available for reducing cement use are:
1. Recycled concrete: Use concrete with reclaimed industrial materials or recycled concrete.
2. Optimized design: Minimize concrete consumption through innovative architecture, structural and material design. For example, improving concrete durability can significantly enhance service life of civil infrastructure. Durable concrete can be achieved through improving particle packing density by the introduction of silica fume and graded aggregates; reduction in water/cement ratio (without the expense of early-age cracking) with the help of superplasticizers; and careful selection of cement, aggregate and admixtures for environments characterized by certain types of chemical and physical attack.
3. Admixtures: Reduce the unit cement content by using superplasticizers and/or optimizing aggregate size and gradation to obtain the required concrete workability and consistency (see Figure 3).
4. Supplementary Cementitious Materials: Increase the percentage of conventional SCMs (fly ash, slag, silica fume and rice-husk ash) without impairing concrete’s mechanical and durability properties. Concrete mixtures made with high-volume fly ash can be more resistant to physical and chemical attacks (shrinkage thermal cracking, alkali-silica reaction and sulfate attack) (See the new Engineer’s Forum column on fly ash in this issue).
To reduce cost and increase sustainability, the mix design of the precast security barrier shown in Figure 2 incorporates the following strategies:
1. Use of superplasticizer and optimizing aggregate size and gradation to retain workability and reduce cement content;
2. Incorporation of fly ash to replace conventional portland cement and thereby reduce thermal cracking and increase material durability;
3. Use of Type V cement to resist potential environmental sulfate attack and to enhance durability.
Energy dissipation concept
Deep foundations are almost always required for conventional metal or concrete barriers. To avoid extensive excavations in urban areas with underground utilities, some new design concepts have been proposed. One innovative idea is the use of a relatively constant “calibrated force” resistance provided by cushioning energy dissipaters incorporated into the design. Such energy dissipaters allow the barrier to stop an explosive-laden truck with a relatively low and constant deceleration force that is exerted for a sufficient duration.
Reinforced precast concrete barriers containing energy absorbers make ideal security barriers because of the flexibility of both structural and geometric/architectural design; large stiffness and strength; compatibility to connections; and the inherent mass to resist destructive impact. A barrier’s decelerating energy dissipaters can be accurately set at any force and stroke required by advanced numerical studies and crash validations. Calibrated deceleration forces can be validated in real crash tests to ensure the deceleration of a vehicle to zero velocity outside the secured perimeter while simultaneously controlling the forces imposed on the foundation. This kind of precast barrier technology, therefore, eliminates the need for a deep foundation that may not be feasible in highly developed areas.
In recent years, there have been new barrier system developments comprised of different materials or functional components. Such systems can be defined as hybrid systems that contain either different materials (like steel and concrete) or offer multiple functions (like stationary and operable barriers).
Typical hybrid systems are made of concrete and steel. Combinations include precast planters and steel bollards or guardrails; precast concrete posts and steel cables; or a combination of concrete walls and steel fences (Figure 4A). Hybrid systems often employ concrete barriers as foundation or anchorage, or as supplemental or decorative features that offer the important aesthetic advantages of architectural enhancements or landscape harmonies in urban environments.
For multifunctional components, operable systems can be integrated with stationary barriers to achieve high anti-crash ratings while eliminating complex implementation efforts required for a stand-alone foundation or a base anchorage. A typical sample of a multifunction barrier is the cable/gate system anchored to a concrete barrier at both ends (Figure 4B).
A critical design issue to be addressed in security planning for densely populated areas is the achievement of the maximum possible blast standoff distance from the facility. As more and more crash barriers are necessarily installed in urban areas, it is important for these barrier elements to become an integral part of the city landscape without sacrificing performance.
Barrier beautification design should consider existing environments and future urban planning as well as landscaping needs. Security barriers should offer attractive visual aesthetics for cities. In the planning stage of any new construction project, the careful consideration of appropriate design of security protection is more important than ever.
Typical Applications of Precast Barriers
The ease of constructability and functional efficiency of the median (Jersey) barrier make it particularly applicable in antiterrorism security measures. U.S. military forces use median barriers extensively in Iraq to fortify roadblocks and install taller variants for public infrastructure. In addition, Jersey barriers have been employed domestically as perimeter security barriers following the Oklahoma City bombing and the Sept. 11, 2001, terrorist attacks to enforce standoff distances from federal buildings and monuments. The Willis (formerly Sears) Tower in Chicago and the Library Tower in downtown Los Angeles are examples.
Fixed precast planters can cover security levels from ASTM M30 to M40 with variations in height and mass. Precast planters, embedded in the earth or anchored to a base diaphragm, can effectively stop moving vehicles (for example, a 15,000 lbs (6,800 kg) truck traveling at velocity greater than 30 mph [48 km/h]) even when the planters are arranged in a scattered pattern of individual pieces. Planters are typically taller than 3 ft (0.9 m) and larger than 3 ft in plan view.
Walls with greater heights will engage more parts of the truck’s front and cause a greater overturning moment and, therefore, require a wider foundation slab than those for shorter retaining walls. To limit cost and construction site work, precast concrete wall panels have become the predominant material of choice. Precast panels are easily integrated into the entire perimeter security system for most situations.
Precast concrete is an excellent material for security barriers because of its high aesthetic compatibility and design flexibility for urban environments, high corrosion resistance, low maintenance and significant environmental benefits. Many technical and material innovations recommend reinforced concrete, particularly precast concrete, including high-performance concrete, green technology/LEED compliance, energy-absorbing mechanism, hybrid systems and architectural benefits for urban environs. Functional and efficient precast concrete perimeter security barriers have become preferred systems to protect human life and critical facilities, and moreover, precast offers a more sustainable structural solution that compliments urban architecture.
1. ASTM C143/C143M – 10 Standard Test Method for Slump of Hydraulic-Cement Concrete, 2010
2. ASTM Standard F 2656-07, Standard Test method for Vehicle Crash Testing of Perimeter Barriers, 2007
3. Federal Emergency Management Agency (FEMA) 430, “Perimeter Security Design,” 2007
4. Antoine E. Naaman, “High Performance Fiber Reinforced Cement Composites: Classification And Applications”, CBM-CI International Workshop, Karachi, Pakistan, 2007
Marc Caspe received M.S. in civil engineering from Lehigh University and is a registered civil and structural engineer with more than 45 years of professional experience. Marc has published numerous technical papers and developed several patents on technical innovations.
Jun Ji received his Ph.D. in civil engineering from University of Illinois at Urbana-Champaign and is a registered civil engineer with 15 years of professional and research experience. Dr. Ji’s research interests include risk assessment and engineering responses to terrorism and natural hazards, earthquake engineering and development of innovative security and retrofitting techniques.
Lin Shen received his Ph.D. in civil engineering from University of Illinois at Urbana-Champaign and is a registered civil engineer. Dr. Lin’s research interests include experimental research and structural retrofit, advanced cementitious materials, non-destructive tests and structural rehabilitation strategy and design.
Qian Wang received his Ph.D. in structures, mechanics and materials, from University of Iowa and is a registered civil engineer and LEED AP. Dr. Wang’s research interests include numerical optimization-based analysis and design and experimental and numerical studies with applications to structural, seismic, security and retrofitting engineering.
Yanzhi Zhai received her M.S. in civil engineering from University of Illinois at Urbana-Champaign and is a registered civil engineer with 12 years of professional and research experience. Ms. Zhai has intensive experience in highway and bridge design, advanced structural analysis, vehicle traffic analysis, concrete barrier applications and site planning.