March 2010 Vol. 237 No. 3

Features

Improving Concrete For Enhanced Pipeline Protection

Different concrete mixes are used in various applications in the oil and gas industry, from oil well cementing to providing stability to offshore platforms. Concrete is also used in protecting oil and gas pipelines, most of them made of carbon steel, or providing them negative buoyancy in offshore applications and other wet environments.

In onshore pipeline projects that cross challenging terrains – rocky areas, urban, densely populated areas, etc. – concrete is used in mechanical protection coatings (usually wire or fiber-reinforced concrete), in concrete casings and tunnels (for road or utility crossings), as well as in steel rebar-reinforced concrete slabs that deny access to the pipeline trench in regions where gasoline and product theft from the pipelines is still a major issue.

Current concrete materials, when applied as a pipe coating, can offer a significant amount of mechanical protection; now a new class of cement-based materials has been developed as a replacement for traditional concrete coatings. Engineered cementitious composites (ECC) are a type of high-performance fiber-reinforced cementitious composite (HPFRCC) that offers several advantages in pipeline protection applications, such as improved durability, damage resistance, and flexibility. While research thus far has been centered on protecting pipelines by applying ECC coatings, the results are open to potential extrapolation to the other mechanical protection concrete applications mentioned for onshore pipelines.

Controlling Cracking In New Ways
ECC materials are unique among all concrete- and fiber-reinforced concrete materials being used in pipeline applications. Applying fracture mechanics in addition to conventional strength-based theory, ECC materials are designed to control the formation of cracks rather than prevent them. This begins with controlling the types of cracks that form. All existing concrete and fiber-reinforced concrete materials form Griffith-type cracks under tension. These cracks are defined by their length and width. The hallmark of a Griffith-type crack is that as it grows longer, it must grow wider (Broek, 1986). While these large crack widths accommodate large amounts of deformation, they significantly degrade the protective properties of the coating layers. Most importantly, they reduce the load-carrying capacity across the crack as reinforcing bars, wire, or fibers are pulled out and lose load capacity.

Steady-state Flat Cracks
Rather than forming long, wide Griffith-type cracks, ECC is designed to form steady-state flat cracks that can grow long but do not widen (Yang and Li, 2007). This is accomplished by balancing the energy required to propagate cracks within the ECC cement matrix against the energy needed to pull ECC reinforcing fibers out of the cement matrix. The benefits of steady-state flat cracks are two-fold. First, since these cracks do not widen as they grow longer, the load-carrying capacity of reinforcement crossing the cracks does not drop, allowing the formation of many micro-cracks. Second, the crack width of steady-state flat cracks in ECC is designed to stay below 100 micrometers to retain durability, penetration resistance, and impact resistance even in a cracked and flexible state. The formation of multiple thin micro-cracks (about 60 micrometers average width) in ECC material under tension is shown in Figure 1.

Figure 1: Closely spaced steady-state flat cracks in ECC material under tension loads (Fischer and Li, 2007).

In the case of pipeline coatings, the difference between Griffith-type and steady-state flat cracking is manifested as quasi-brittle vs. ductile material performance in the field. Under increasing load, concrete- and fiber-reinforced concrete materials form ever-widening Griffith-type cracks that carry lower loads as crack widths grow. This is the quasi-brittle behavior known as tension-softening shown in the stress-strain curve in Figure 2. Due to the formation of flat steady-state cracks in ECC, crack widths do not grow and load capacity does not fall (and rises in most cases) as many cracks form, resulting in a strain-hardening behavior shown in Figure 3.

As a result of the tensile strain-hardening response, ECC is highly damage-tolerant and can absorb a substantial amount of energy before fracture localization. These are material characteristics commonly associated with ductile metals like mild steel and are the central difference between ECC materials designed for tensile ductility and tight crack widths and traditional concrete materials which are designed for compressive strength.

(See Figure 2: Material performance of conventional concrete, fiber-reinforced concrete, and ECC under tension loads. Adapted from Fischer and Li, 2003.)

Micro-cracking To Resist Damage
As a result of this uniquely ductile, strain-hardening behavior and tight crack-width control, ECC protective pipeline coatings exhibit great potential to resist impacts, penetration and environmental deterioration while maintaining high flexibility in field applications.

To verify the ability of ECC materials to resist common damage mechanisms through distributed micro-cracking, a series of experimental tests were carried out comparing traditional fiber-reinforced concrete coatings (8-10 mm thick) and wire-mesh reinforced concrete coatings (25 mm thick) to thin ECC pipeline coatings (8-12 mm). As most fiber-reinforced concrete coatings are used in the European markets, impact resistance was tested according to the prevalent European standard, the German standard DVGW GW-340 (Figure 3). The testing method used to compare the impact resistance of the ECC to traditional wire-mesh reinforced concrete coatings, mostly used in North America, Australia and Southeast Asia, was also very similar to the DVGW GW-340 specified method.

Figure 3: ECC coating testing for impact resistance (according to DVGW GW-340).

Impact resistance was determined by drop tower tests using the same 25-mm diameter load point dropped onto steel pipes with FBE or 3LPE corrosion protection. The 10-mm ECC coating sustained multiple impacts up to 1,000 J along with formation of radial micro-cracking as shown in Figure 4 and no significant formation of holidays in the anti-corrosion coating. Sustaining such large impact loads with comparatively little damage is possible due to the high energy absorption associated with multiple micro-crack formation in very tough ECC materials. For comparison, the impact resistance levels for currently used concrete coatings were only 25-45% of the impact resistance of the ECC coating. In separate studies, ECC has demonstrated fracture toughness similar to that of aluminum. (Maalej et al, 1995).

Figure 4: Multiple impact resistance of ECC protective coatings.

ECC also demonstrates significant ductility through the formation of closely spaced, flat, steady-state microcracks. In a pipeline-coating system, this has resulted in a minimum deflection capacity of 1.4° per pipe diameter with a testing mandrel radius of 490 mm. Higher values have been observed in previous laboratory demonstrations at small scale. While this flexural capacity does meet industry requirements, it is accompanied by distributed micro-cracking, which might cause concern for material durability under harsh environmental conditions and mechanical loads.

However, the ECC environmental durability under fatigue, alkali-silica reaction, freeze-thaw cycles, water absorption, chloride-induced steel corrosion, and abrasion have been studied and summarized in Lepech and Li (2006), Sahmaran et al. (2009, 2008, 2007), and other studies. These studies demonstrate the combined benefits of micro-crack deformation modes to dissipate damage in ductile composites, tight crack width control for reduced corrosive transport properties, and material ductility in mitigating structural damage from steel corrosion embedded in ECC materials.

Designing For Ductility
The material behavior of ECC under tension is distinctly different than that of traditional steel-reinforced concretes or even fiber-reinforced concretes using steel, carbon, or polymeric fibers (Figure 2). In most cases, ECC can offer ductility between 300-400 times that of regular concrete in tension while maintaining the characteristic tight crack widths associated with steady-state flat micro-cracks. This design for ductility can be leveraged as an effective pipeline-coating material, but also in a number of other applications.

The high ductility of ECC is particularly beneficial in steel-concrete interaction zones. In members where steel and concrete meet, such as anchor zones, bearing plates, or composite structures, traditional concrete and fiber-reinforced concretes often fail. This is not due to a lack of compressive or tensile strength in the concrete, but rather a vast material incompatibility between steel and concrete materials. With ultimate strain capacities on the order of several percent, steel is widely used in construction due to its good ductility in tension.

Yet, when paired with brittle concrete materials, which have an ultimate tensile strain several hundred times smaller, incompatibility with large steel tensile deformations results in cracking, mechanical deterioration, and lower load transfer within steel-concrete hybrid elements. Qian and Li (2008) demonstrated this phenomenon in steel anchor zones within both concrete and ECC materials and found that due to material ductility, steel anchors in ECC material had a 53% increase in load capacity and 220% increase in slip capacity as compared to steel anchors in concrete.

While concrete specimens failed by brittle fracture failure associated with a lower ultimate strength, the ECC specimens were gradually damaged by ductile yielding of ECC materials and plastic deformation of steel. This is just one example of the benefits of ECC material ductility in pipeline construction apart from coating materials.

A number of field applications have been suggested for ECC materials in the oil and gas industry based on this unique material behavior. These include oil platform applications that require extreme durability in harsh environmental conditions, foundation elements in which steel anchors and concrete footings are in direct connection, the fabrication of foundation pilings as a result of high resistance to impact loads and environmental deterioration, and seismic applications which require significant ductility in pipeline response to large earthquake events.

Conclusions
The engineered cementitious composite (ECC) materials have proven a viable and mechanically superior pipeline protection material when compared to existing concrete coatings. This is due to the intentional formation of a different class of cracks within ECC material as compared to concrete or fiber-reinforced concrete under load. Steady-state flat cracks in ECC do not grow wider as they grow longer, thereby allowing ECC material to deform in a ductile fashion under tension similar to mild steel.

This ductile behavior through micro-cracking provides several improved mechanical properties to pipeline coatings as compared to conventional technologies including penetration resistance, impact protection, and flexibility. The radical improvement in the technical performance of the concrete can be potentially used to design other new applications for the oil and gas industry and further research will be done to develop ECC solutions for unstable ground conditions such as seismically active regions and permafrost, as well as for offshore environments.

Authors
Michael D. Lepech, Ph.D., is an assistant professor of Civil and Environmental Engineering and fellow at the Center for Sustainable Development and Global Competitiveness at Stanford University. His research interests lie in the development of ductile cementitious composites and other composite materials, in particular for applications in the sustainable built environment. He is also an expert in the application of life cycle assessment techniques for design of sustainable infrastructure materials and systems. He received his Ph.D. (2006) and MBA (2009) from the University of Michigan, Ann Arbor. He can be contacted at mlepech@stanford.edu.

Vlad Popovici is a marketing manager at Bredero Shaw. He manages multiple economic and strategic assessments of pipeline projects worldwide and is in charge of identifying and developing new pipeline protection concepts. Popovici holds an MBA (2005) from McGill University in Montreal and has been publishing technical articles and economic analyses since 1996. He can be reached at VPopovici@BrederoShaw.com.

Gregor Fischer, Ph.D., is an associate professor of Civil Engineering at the Technical University of Denmark. His research interests center around the development of new ductile cement-based composite for infrastructure applications, most recently in the development of ductile precast commercial and residential building elements. He received his Ph.D. (2002) from the University of Michigan, Ann Arbor.

Victor C. Li, Ph.D., is the E.B. Wylie Collegiate Chair, professor of Civil and Environmental Engineering, and professor of Material Science and Engineering at the University of Michigan, Ann Arbor. As director of the Advanced Civil Engineering Materials Research Laboratory (ACE-MRL) his research interests include the development of Engineered Cementitious Composite materials, most recently with specialized functionalities of sustainability, self-healing, and self-sensing capabilities. He is also a fellow of the American Society of Civil Engineers, American Society of Mechanical Engineers and the World Innovation Foundation.

Richard Du, Ph.D., is a director in charge of bendable, impact-resistant ECC pipeline coating business with Mecc Technologies, Inc. in Asia. He received his Ph.D. (1991) in engineering from Zhejiang University. He is also acting founding director of the Laser Research Center, Guangzhou Institute of Industrial Tech in China.

References
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Fischer, G., and V.C. Li, (2003) “Deformation Behavior of Fiber-Reinforced Polymer Reinforced Engineered Cementitious Composite (ECC) Flexural Members under Reversed Cyclic Loading Conditions, ” ACI Structural Journal, 100(1):25-35.

Fischer, G., and V.C. Li, (2007) “Effect of Fiber Reinforcement on the Response of Structural Members”, Journal of Engineering Fracture Mechanics, 74(1-2):258-272.

Lepech, M.D. and V.C. Li (2006) “Long Term Durability Performance of Engineered Cementitious Composites,” International Journal for Restoration of Buildings and Monuments, 12(2):119-132.

Maalej, M., Hashida, T. and Li, V.C. (1995) “Effect of Fiber Volume Fraction on the Off-Crack-Plane Fracture Energy in Strain-Hardening Engineered Cementitious Composites,” Journal of the American Ceramic Society, 78(12):3369-3375.

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Sahmaran, M., and V.C. Li, (2007) “De-icing Salt Scaling Resistance of Mechanically Loaded Engineered Cementitious Composites,” Journal of Cement and Concrete Research, 37:1035-1046.

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