Court Construction and Maintenance: Concrete Thinking
More facility owners are considering tennis courts built on post-tensioned concrete slabs because they’re proving to be longer lasting and more resistant to cracks.
By David LaSota, with Mary Helen Sprecher
The game of tennis is played on more surface types than any other traditional sport. From natural and artificial grass to clay, asphalt, concrete, wood, acrylics, rubber, carpets and tiles, the playing surface lends a unique dimension to the game, which can be challenging and enjoyable. Like a new can of balls, a newly constructed or recently resurfaced court allows the player to focus more on technique and strategy and increases the desire to play more often.
Annually, the USTA Community Tennis Development group works with hundreds of tennis facility owners that manage thousands of tennis courts nationwide, providing consultation on planning, design and construction of tennis court surfaces and supporting infrastructure. A large percentage of those facilities seeking assistance from the USTA are addressing cracks that form in traditional asphalt tennis court pavements.
While most hard courts in the U.S. are asphalt, more facility owners are considering post-tensioned concrete for a durable, long-lasting tennis court pavement. When designed and constructed properly, post-tensioned concrete slabs can remain crack-free for many years after they are completed.
PT concrete has been a common tennis-court construction technique in Southern California for many years. “Post-tension reinforced tennis-court slabs have been a standard for our court building since 1985,” says Richard Zaino, president of Zaino Tennis Courts in Orange, CA. “We discovered in our earliest experience with engineered post-tension courts that were designed and built due to poor soil conditions, the post-tensioned concrete courts were performing much better then steel rebar-reinforced slabs on good soil conditions. So we took what was given to us by the post-tensioned concrete engineers for specific sites and made this a standard in all our court building.
“Reinforced slabs with thicker concrete, larger size rebar and/or thicker sections of base were still not reliable and would develop cracks,” Zaino continues. “We were seeing such a great success with the ‘cable’ courts that we decided in the mid-80s to make this our standard and promote and build all courts with post-tensioned cables. Over the years, we’ve developed and refined this design so we can provide owners with none to very few cracks. The combination of the properly prepared subgrade, vapor barrier, concrete mix design and cables will provide the longest lasting controlled crack slab possible. The post-tension courts built in 1985 are surpassing all other types of hard courts for crack control.”
“In the past,” adds Darrel Snyder of Mid-America Courtworks of Wichita, KS, “asphalt courts have been less costly to build. Now with the increasing price of oil, asphalt prices have risen to the point that concrete prices are fairly equal.”
Many areas of the country are experiencing an increase in post-tensioned concrete court construction, due to the durability and long life of PT concrete slabs. Among the USTA sections that have seen more PT courts are Intermountain, Texas, Missouri Valley and Mid-Atlantic.
“Asphalt tennis courts in the Northeast will crack, there’s no getting around it,” says Michael Fortuna of Classic Turf Co. in Woodbury, CT. “The environment is too extreme. Winters are cold, summers are hot, and there is a lot of precipitation to make the soils active and unstable. Post-tensioned concrete slabs are specifically designed to handle these extreme conditions.”
With this growth in post-tensioned concrete slab development, the industry also has seen an increase in slab failures and court cracking as a result of the designer or court contractor not understanding the fundamentals of PT concrete slab construction. Once the slab fails or significant cracks form, it is nearly impossible to fix the problem areas of the court without significant investment in replacement or overlays.
“It is vitally important to get the right design from the designer and/or builder and hire a builder who has a thorough understanding and experience with post-tension cables, concrete mix design, vapor barrier and site conditions for tennis courts,” Zaino says. “Tennis courts are not building foundations and crossover ‘designs’ can be totally wrong. An improper or inadequate vapor barrier could be detrimental to the concrete curing and surface coatings. The wrong concrete mix design and placement will also have a detrimental effect to the foundation and cable reinforcement.”
Remember that even among concrete contractors, there will be significant variations. A local concrete company with no skill or experience in tennis-court construction will not be suitable for the highly specialized work demanded for a successful PT concrete court installation. Problems ranging from merely aesthetic (spotting of the surface due to inadequate sheathing on the cables) to total catastrophic failure can result.
The standard PT concrete tennis-court slab consists of well-compacted subgrade soils, stone subbase layer, a fine aggregate “cushion” layer, polyethylene-sheeting layers (vapor barrier), Portland cement concrete and greased steel cables. “We like placing a very tight aggregate base, compacted to 95 percent with a smooth drum vibratory roller, then install two 10-mil sheets of vapor barrier,” says Zaino.
The greased steel cables set in plastic sheathing are spaced evenly across the length and width of the tennis-court pavement. After the concrete is poured, the cables are stressed, which places the concrete under permanent compression. Like a rubber band, steel cables when stretched want to rebound to their original length. It is the force exerted on the cables and transferred to the concrete that keeps the slab from forming cracks during the curing process for the life of the concrete.
Concrete, when curing, loses water while the concrete strengthens. This loss of water will shrink the concrete slab, and if not properly addressed with reinforcing, joints or post-tensioned cables, will result in cracks forming throughout the slab. According to the Post-Tensioning Institute’s publication, Design and Construction of Post-Tensioned Sport Courts, a “typical post-tensioned concrete court will shorten by approximately ¼ inch per 40 feet of length over the long term.”
For a 120-foot-long tennis court, this would equate to ¾ of an inch. In order to allow the slab to move during the curing process, the contractor should minimize the friction on the underside of the slab so that when the slab concrete shrinks and cracks form, the cables pull the slab sections together, thereby minimizing the impact any crack that may form would have on play. This minimization of friction is accomplished with fine stone aggregate and polyethylene sheeting, typically in two sheeting layers just below the concrete.
Cable Tension and Spacing
Most tennis courts include net posts, nets, center strap anchors, center straps and fencing as part of the court. Net posts, center strap anchors and fencing typically require foundations below the court surface for structural support. Posts and anchors that penetrate the post-tensioned concrete slab can, if not properly installed, impede the ability of the slab to shorten during curing, thereby defeating the purpose of the post-tensioning process. A properly designed and constructed PT concrete tennis court will include foam isolation material where a post or anchor must penetrate the slab. The perimeter fencing should, when possible, be constructed independent of the slab and outside the slab footprint.
As mentioned, the steel cables, when stressed, provide the compressive forces to hold the court together. The cables must be spaced close enough so that the cables can pull the weight of the slab and overcome the friction of the underside of the slab. Design and Construction of Post-Tensioned Sport Courts is an excellent resource that provides tendon spacing formulas to ensure the cables can properly move the slab during the curing process.
When we’re asked to review a facility that has had slab failures, we often find the tendon spacing calculations have not been completed, or the contractor used spacing that may have worked for them in the past without considering the total length of the court or slab thickness.
“We have found success placing cables closer than required and using the higher standard formulas,” Zaino says. “The other practical benefit for closer spaced cables are the additional cable intersections for chair supports, providing better support of the cables in the middle of the slab, particularly during the concrete placement.”
The major benefit of post-tensioned concrete construction over the old standard reinforced concrete is PT concrete slabs do not need the same amount of pavement joints to control cracking. A reinforced concrete pavement will require joints to control cracking as close as 12 feet in all directions, which result in joints being located within the playing lines of the court. While a PT concrete slab does not require the same joint spacing as a reinforced slab, metal key joints should be installed at the net line and between courts of a post-tensioned concrete slab to allow for horizontal movement.
One misunderstanding we often find of post-tensioned concrete construction is that many think PT concrete tennis courts can bridge over poor soils and therefore proper preparation and compaction of the soils under the court is not necessary. We strongly recommend all poor and expansive soils beneath a PT concrete court be removed and the soils be properly compacted before the court is built. As with all pavements, efforts must be made to direct water away from the court pavement so that the pavement sub-base and subgrade remain dry.
Many contractors in the industry install a 2-inch-thick layer of sand under the post-tensioned slab to provide a base for the slab and reduce friction on the underside of the slab. We have found that stone fines provide a firmer base than sand, because the sand tends to deform when construction personnel walk on it to set the cables and pour and finish the concrete.
While asphalt tennis courts will most likely remain the predominant court pavement throughout the U.S. for the next several years, a properly designed and constructed post-tensioned concrete court will emerge as the most cost-effective long-term life-cycle alternative.
The American Sports Builders Association (ASBA) is a nonprofit association helping designers, builders, owners, operators and users understand quality sports facility construction. The ASBA sponsors informative meetings and publishes newsletters, books and technical construction guidelines for athletic facilities including tennis courts, running tracks, athletic fields and indoor structures. Available at no charge is a listing of all publications offered by the Association, as well as the ASBA’s Membership Directory. Info: 866-501-ASBA (2722) or sportsbuilders.org.
The ASBA and USTA publication “Tennis Courts: A Construction & Maintenance Manual, 2012” includes a chapter on post-tensioned concrete court construction. To obtain a copy of the manual, visit sportsbuilders.org.