The Life-Cost Cycle of Hardscapes

By: Ray Clark and Alan Starling, Paveston Company

Life-cycle cost of pavement is an analysis that owners of pavement projects tend to attribute the most value.

A life-cycle cost analysis evaluates the cost associated with maintaining a pavement over a specified term or life of the pavement. In cost comparisons, a common assumption is to see the overall cost and the initial cost of an item to be the same. When initial cost is the only factor considered, sometimes the good buy is nothing more than a short-term solution.

Initially, an asphalt road appears more economical than interlocking pavestones. However, asphalt requires more maintenance and on average, an overlay every 10 years. Factor in the required costs to maintain the project and it becomes much less cost effective. Interlocking concrete pavement costs are normally lower than asphalt when cumulative annual costs are evaluated (Rollings, R.S., Precast Concrete Paving Block Pavement, Geotechnical Lab report, U.S. Army Corps of Engineering, Waterways Experimental Station, Vicksburg, April 1979). Research has shown that the costs for interlocking concrete pavements can be five to six percent lower at the end of 10 years, and 17 to 28 percent lower at the end of 30 years compared to other forms of flexible pavement such as asphalt (Rollings, R.S., Precast Concrete Paving Block Pavement, Geotechnical Lab repot, U.S. Army Corps of Engineering, Waterways Experimental Station, Vicksburg, April 1979).

Concrete pavestones are a superior cost and performance pavement alternative, especially in the following circumstances: 1) areas subject to heavy and concentrated wheel loads; 2) repetitive traffic; 3) areas subject to varying temperatures, frequent fuel or oil spills; 4). when access to underground utilities is crucial.

An example of these areas might be public streetscapes, bus terminals, industrial ports or airport runway aprons. Life cycle costs play a major consideration for the owner and design team in these applications. Lower maintenance costs in the life of the pavement is an important factor to an owner who is charged with the responsibility to maintain the pavement. Costs associated with maintaining interlocking pavestone surfaces are estimated to range between 12.5 percent and 35 percent of those costs associated with maintaining asphalt pavements (Shackel, B., Design and Construction of Interlocking Concrete Block Pavements, Sydney, 1990).

When an owner or engineer is evaluating a pavement, the present worth of costs (PWOC) is widely used and accepted as a method to evaluate and compare costs of alternative pavements. Typically, the pavement that tenders the lowest PWOC is selected. Engineers use a common formula for calculating the PWOC.

To assure maximum investment is achieved, it is important to include life-cycle costs when evaluating the initial project cost. An owner who is interested in getting a good buy will appreciate the investment value of concrete pavestones.

Articulating Concrete Blocks (ACBs) for Erosion Control

Articulating concrete blocks, like interlocking concrete pavestones, date back to the ancient Romans and Mayans. The ACB systems are comprised of precast, positive interlocking articulating blocks used in conjunction with a geotextile fabric underlayment. This composite structure allows them to provide working solutions to erosion control problems. To combat erosion control due to hydraulic flow, ACBs provide a cost effective solution. The level of understanding, acceptance and use of ACBs has grown significantly in recent years. Structural revetment systems exhibiting vegetative growth and habitat for animal life are now ecologically favorable. The environmental-friendly nature of ACBs is one of the most attractive benefits they offer. The open voids provide permeability and native grasses are allowed to grow through the concrete block providing a natural landscape. To facilitate vegetative growth, hydroseeding is the most common application. Use of a tackifier in the application minimizes loss of seeds prior to germination. Other methods, such as the placement of organic or bio-degradable mats (with or without seeds) on top of the ACB revetment can substantially accelerate and increase the likelihood of vegetative cover.

"The life cycle cost in most markets confirms ACBs competitive to alternate erosion control methods, such as RipRap and gabion mattreses, and they are typically cheaper than cast-in-place concrete," explained Richard Bodie, of the Pavestone Company. "Rate of installation and job completion is another factor in determining cost effectiveness," Bodie added. "Production rates vary depending on the type of system being installed. These factors include accessibility of the project, geometries of the bed slope, and side slope and overall configuration of the placement. In general, four inches and six inches thick hand placed blocks can be installed at a rate of 40 to 90-square-feet-per-man-hour. Prefabricated cabled mattresses can be installed at a rate of three or four mats per-crane-hour. And like pavestones and segmental retaining walls, the interlock of the stones allows them to shift with the earth movement requiring little-to no maintenance. ACBs are precast, requiring no on-site time for curing which increases installation efficiencies."

Segmental Retaining Walls for Retaining Structures

Cost-effective, aesthetically pleasing segmental retaining wall units (SRWs) are rapidly becoming the design professional and contractors best option for taming transitions in grade and enhancing the overall beauty of landscaping projects. Equally at home in residential, commercial and heavy highway applications, SRWs are backed by proven design standards and accepted by the engineering community as viable options to traditional costly poured concrete retaining walls, masonry walls and crib or railroad tie walls. SRWs have the advantage of offering multiple shapes, sizes, colors and textures as well as proven economics, constructability and a proven design methodology.

"The cost comparison of SRWs compares favorably to other type retaining walls," according to Sam Miller, P.E., president of Scott Miller Consulting Engineer, Little Rock, Ark. "For walls over 5 to 8 feet in height, SRWs typically come in 25 to 40 percent cheaper than properly engineered concrete cast-in-place, natural stone and masonry walls. Fill sights offer the greatest advantage to SRWs and difficult transitions in grade greater than eight feet in height are almost always more economical with an SRW. Often, CIP retaining walls are value engineered on over budget projects by savvy wall contractors resulting in a cost savings to the owner."

While timber and railroad tie walls are less expensive than SRWs initially, timber walls typically have a much shorter design life on the order of eight to 15 years when compared to the 75 to 100 year design life for SRWs. So while timber walls are only moderately cheaper in the short term, the life cycle costs and replacement costs of timber walls (typically three to four times over the life of a comparable SRW) are much higher. Additionally, SRWs are much better for the environment and do not contain chemical additives such as creosote, arsenic and nickel like timber walls. Across the country treated timbers are increasingly being classified as hazardous waste.

Many municipalities are banning timber walls and many landfills will no longer accept them or do so at premium cost.

The strength of SRWs is the fact that being a dry stacked, mortar-less wall system the individual units naturally flex and move with shrink/swell cycles of the soil and can tolerate this movement without affecting either the structural performance or the aesthetics of the wall. This same movement can detrimentally crack and damage concrete and masonry systems, shortening their life expectancy and necessitating repairs. SRWs, by their virtue of being mortar-less systems, are permeable and readily allow water through the wall face; in combination with a drainage tile, this helps to alleviate the build up of hydrostatic pressure. Other wall systems depend primarily upon weep holes and are typically more susceptible to failure due to hydrostatic pressure.

Unlike timber walls, SRW blocks are relatively inert and not affected by wet soils and humidity, and will not rot and degrade as natural wood products. If damaged, SRWs are normally easy to repair. Individual blocks that are damaged can be singularly removed and a new face inserted on a block-by-block basis. Projects have been designed with this ease of repair in mind. By constructing an SRW over buried utilities, the wall can be dismantled block-by-block in a V pattern over the buried utility, the blocks repalletized, the utility accessed and repaired and the wall reconstructed with the original blocks leaving no visible change to the aesthetics of the wall. This can be very difficult to virtually impossible to accomplish with other types of retaining wall systems.

SRWs can also be installed much quicker than other types of walls including CIP walls, timber walls, stonewalls and masonry walls. An experienced SRW installation crew will lay between 750 and 1,500 square feet of wall-per-day on larger projects. Which means that a 5,000 square foot wall can be completed in less than one week. An equivalent concrete or masonry wall would require approximately twice as much time considering the form work, rebar placement, concrete placement and curing time, and backfill placement. A significant amount of the savings in time is the fact with an SRW the backfill is placed at the same time the wall is built instead of waiting until after the wall is completed as with other type walls.

About the authors: Ray Clark is a regional sales manager and Alan Starling is a district manager and wall specialist for Pavestone; both are graduates of ICPI certification programs.