Precast Concrete Blocks
"Precast concrete blocks can be used over earth materials to provide a hard surface for flow to pass safely without eroding the underlying surface, and are commonly referred to as articulating concrete blocks (ACB) when used for this purpose. An ACB system is comprised of a matrix of individual concrete blocks placed together to form an erosion-resistant revetment with specific hydraulic performance characteristics. The term “articulating” implies the ability of the matrix to conform to minor changes in the subgrade while remaining interconnected with geometric interlock and/or additional system components such as cables or anchors. These systems have also been referred to as cellular concrete mats (CCM)." [1]
"There are many types of precast concrete blocks, each with its own geometry, useful application based upon hydraulic performance and erosion prevention, installation procedures, aesthetic value, and cost. Of most importance for providing overtopping protection is to select a product that has been tested under the flow conditions expected during overtopping. Applications for overtopping protection typically include high velocity flows, steep slopes, and possibly energy dissipation on the flow surface."[1]
"The four main types of articulating concrete blocks are described below.
- Cable-tied: Concrete block units that are individually formed with or without open areas, and laced together with cables into large, sometimes uniquely-shaped mattresses for installation. The blocks are normally cabled together (both longitudinally and laterally) at the manufacturing plant and are delivered to the site on flat-bed trucks for placement as a mattress with a crane.
- Articulating Block (AB): Fabric-formed, cable-reinforced, concrete mattresses that are cast in place. The AB fabric form consists of a series of compartments linked by an interwoven perimeter. Grout ducts interconnect the compartments, and high-strength revetment cables are installed between and through the compartments and grout ducts. Once filled with grout or small aggregate concrete, the AB mats become a mattress of pillow-shaped, rectangular concrete blocks. The interwoven perimeters between the blocks serve as hinges to permit articulation. The cables remain embedded in the concrete blocks to link the blocks together.
- Interlocking: Concrete block units that are individually formed with mortise and tenon-type features that are fit together on site. Some interlocking blocks have open areas, and some are solid. Some may also use cables, but this type is generally hand-placed or brought to the site on geotextile mats.
- Overlapping: Concrete block units that are tapered wedge-shaped blocks or concrete slabs that are overlapped shingle-fashion from the toe of the slope to the crest. The block units are formed at the manufacturing plant and delivered to the site on pallets. Individual units in the system are staggered and interlocked for enhanced stability. Each row of units is laterally offset by one-half of a block width from the adjacent row. When placed, the blocks form a stepped surface with slots providing open areas for venting subgrade uplift pressures. They are hand-placed, but have been cabled together on site.
- Butt-jointed: Commercially-available concrete construction blocks, also known as concrete masonry units (CMUs) or cinder blocks, that are normally used in the construction of walls for buildings. These have a large open area and are hand-placed with butt joints on the earth surface for erosion control. This system would only be applied to very low head installations due to failure under fairly low velocities not considered comparable for most overtopping situations."[1]
"All types, except ACB mats (fabric-formed concrete), are manufactured at a precast concrete facility using a high-volume steel mold that is shipped close to the project under construction for standard applications. Some may use a plastic form or mold to cast the blocks if a unique situation develops. Most ACBs are from 4 to 9 inches in thickness. Each product may or may not have an open area equal to anywhere from 2.5 percent for the wedge-shaped blocks, to 18-35 percent for other types of articulating concrete blocks. Some varieties of blocks rely on a vegetative cover grown in soil placed into open areas of the blocks or over the top of the blocks to improve performance."[1]
"All products require placement over a smooth subgrade with a geotextile and/or a bedding or drainage layer between the subgrade and the block system. Installation requirements and techniques vary with the product and affect product performance. Product performance in overtopping or high velocity flow conditions down steep slopes varies significantly. Only products that have been tested in like conditions of the design application with flume or field test conditions should be considered for use."[1]
Examples
See examples of riprap and concrete block armoring
Best Practices Resources
Technical Manual: Overtopping Protection for Dams (FEMA P-1015), FEMA, 2014
Trainings
On-Demand Webinar: Introduction to Armoring Embankment Dams and Earth-Cut Spillways with ACBs
Citations:
Revision ID: 5532
Revision Date: 12/14/2022