The use of Segmental Retaining Walls (SRWs) is a modern technique but concrete as a building material goes back to the Egyptians. Dry-cast or zero-slump concrete block products began in the United States in 1876, making one block unit at a time. A hundred years later, zero-slump concrete SRW units were created and marketed nationally and internationally as an economical solution for earth retaining walls. About the same time, synthetic geogrid soil reinforcing was introduced. By combining the technologies of SRW’s and geosynthetic soil reinforcement, structurally strong and economical retaining walls were developed.
Development of Zero Slump aka Dry-Cast Concrete
The trend to develop hollow core building products began in 1850 with a British inventor named Joseph Gibbs. His method of building solid concrete walls by means of timber forms or lattices on each side included the block could be hollow cast, with the hollows to be filled with concrete. Several systems followed. These methods of construction produced products with sufficient strength and durability for building construction. In 1897, Harmon S. Palmer was credited with the development of the first commercial process for the manufacture of concrete blocks in the United States. A three-man crew could produce 200 blocks in the 10-hour day with these early machines.
In the period from 1905 to the 1970’s production equipment was developed to be bigger, faster, and produce more economical products. Significant increases in production demand and quality control requirements when the 1980’s brought zero-slump products to the landscape markets with SRWs and interlocking concrete pavers. With the introduction of computers to the assembly process, a three man crew can now produce over 6,000 units in one 8 hour shift on one machine.
SRW units are produced on standard concrete masonry production equipment. The standard masonry unit used for building construction is eight inches tall, eight inches wide, and 16 inches long (8 x 8 x 16 in.) (203 x 203 x 406 mm). As the basic product for early production needs, block manufacturing equipment was designed around this shape of unit. Maximum height of units may be up to 12 in. (305 mm) on some machines and minimum height of three inches. Block curing systems were developed for 8 in. (203 mm) high products, so the taller units are possible but very limited. The typical depth (face to tail) of the units is 11 to 12 in. (279 – 305 mm). This depth provides a unit that is easy to construct with, provides good structural stability and is economical. There are deeper units available (20 to 24 in. [508 – 610 mm]); however the cost to produce the larger units is more.
Expanding Choice and Competition
To be competitive, contractors switched switched from 20 to 24 in (508-610 mm) units to the smaller depth units (12 in. 305 mm) since both units have the same exposed face area. With soil reinforcement both are structurally sound. Modular systems with attractive face panels and attachable tail sections for flexibility in design, as well as units using recycled content added to the available options. Market demand grew quickly for SRW products and producers developed more facing options for the consumer. These included the Hard-Split and Soft-Split for a rustic look, Stamped-Face, and Tumbled-Face for a rock look.
When SRW’s first entered the market they were the standard concrete grey, tan or brown color that were typical for concrete masonry products. As demand continued to develop manufactures developed capacity to produce colored units and many added systems to produce variegated colors combining two or three different colors to give the units a more natural colored appearance.
Product capacities were increased significantly to meet the market demands, chemicals and admixes were developed to enhance manufacturing and product performance and specifications were developed for using SRW products in more critical applications. Introduction of the products into the highway markets required higher quality control standards and product checks to ensure quality products were used in manufacturing met the department of transportation specifications. Most manufacturers of SRW products have a quality control program in place and published QA/QC procedures. Units are tested on a routine bases for strength, absorption, density and in northern climates, or durability.
In the years since their development, concrete and segmental retaining walls are grown with the needs of society. Quality control and efficiency have joined with flexibility and choices to make SRWs a leading choice in building and earth retention.
This article is an excerpt from SRW Marketing History.