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.