CONCRETE GUIDELINES FOR LABYRINTHS
Most of this page is directly inspired by Robert Ferre at Labyrinth Enterprises.
Executive Summary:
- The concrete contractors are creating a canvas on which we will apply our art. We expect this installation to last for at least 300 years.
- Concrete should be even without dips or bumps.
- Slab should have a medium broom finishto prevent slipping by labyrinth walkers as well as give our color extra surface area on which to grip. Please extend medium broom finish all the way across the slab. Do not drag the broom from the center out to the edge. Drag the broom from one side of the labyrinth all the way to the opposite side of the labyrinth. Drag the broom parallel to one of the axes of the labyrinth. I suggest the brooming texture be in line with the concrete walk that leads to the entrance of the labyrinth.
- Slab needs to be slightly domed from center to edge to prevent puddling of any water. 1/4 inch in 12 inch slope has worked well for us in the past.
- We prefer to place the edge of the labyrinth 6 inches inside the expansion joint at the edge of the slab. Our circle will vary about 1/4 inch radius over a 40 foot diameter. When we have 6 inches of space between the round labyrinth and the expansion joint at the edge of the slab, variations in slab dimensions are not so obvious. (This makes it much easier (faster) to place the concrete forms) See red lines in attachment(Ventura LC-4 labyrinth.pdf)
- Expansion or control joints at the center of the slab should create a square about 10 feet 6 inches square. This will keep your expansion cuts out of the center design of the labyrinth. Please see red lines on plan for placement of expansion joints.
- ABSOLUTELY NO surface treatment, chemical, or curing agent on the concrete. If we have to strip the surface of the concrete, you will be liable for the extra cost involved. The surface of the concrete must be bare. See last the very last paragraph of this file.
In the event that Golden Spirit is engaged to install an all-concrete labyrinth, the concrete work is completed in advance by a local contractor. Precise requirements for installation of concrete slab will be supplied.The labyrinth requires nothing exotic. It requires normal concrete which meets industry standards, such as those provided by the American Concrete Institute orthe Portland Cement Association. Local contractors should be aware of the regional soil and weather conditions and how to address them. It is necessary to impress upon the contractor and crew that this is an art installation. They are creating a canvas upon which we will place art. The quality of their work is very important. Making a labyrinth is different from anything they may have done before and is more exacting than sidewalks or patios or driveways, even though the concrete itself is the same.Attention to detail is much more critical. In that regard, these guidelines should be helpful. The installation by Golden Spirit assumes that the concrete pad will be in place upon our arrival, properly finished and cured for a minimum of 28 days.
SIZE AND SHAPE OF CONCRETE PAD
We suggest that at least 6 inches of space be left between the edge of the design and the edge of the concrete slab. A circle is the most efficient geometric shape, enclosing the greatest amount of area with the smallest amount of framing (perimeter). Making an accurate circle requires skill. Leaving a metal frame around the edge is unnecessary. Where two framing boards butt end to end, there can be an unwanted flat space. This should be avoided. An octagon can be attractive, especially if there are similar geometries in the adjacent settings. If the concrete pad is minimal in size, benches around the labyrinth (which we strongly advise) would be in the unpaved area. This could be good, given a surrounding band of mulch or some other material, as it would help frame, enclose, and set off the labyrinth. It would be less desirable, however, for wheelchair accessibility or if the ground gets soft when wet. In the latter case, a square or rectangular pad may be efficient, which is easy for the contractor to frame. The labyrinth is circular, leaving generous corner spaces which can contain benches, planters, or other features.
COLOR CONSIDERATIONS AND DECORATIVE CONCRETE METHODS
In a concrete labyrinth, the background (path color) can be determined by adding integral color to the concrete. Buff is quite common, as is rose. Integral color is made from oxides which are restricted to a range of earth tones. Or, the concrete can have a mix design to assure a very light gray color, suited for a darker gray line color. This gives it somewhat of a terrazzo look.Concrete can be shaped and stamped and formed and sand blasted, not to mention coloring and staining. Powdered color hardener can be broadcast across the surface of the wet concrete and troweled in. This is more labor intensive than just adding the color to the concrete itself, and the color does not go very far into the concrete. On the other hand, it hardens the surface (hence, its name), which is an advantage. Decorative concrete methods, such as stamping and staining, can be used for labyrinths, but with mixed success. It is for that reason that we developed the polymer concrete method that is more dense, opaque, and long-lasting than most alternatives. We strongly recommend it over sandblasting and staining. Texturing the concrete to make it look like stone makes it more difficult to walk on, as the surface is uneven and holds water after it rains. This makes it hard for the elderly or infirm to walk the labyrinth. It also adds complexity to an area that is already quite complex. It can become confusing or disorienting. We normally prefer and recommend a simple broomed finish, which gives the concrete a good slip-resistant surface.
CONSTRUCTION DETAILS
The concrete pad should have at least a six-inch sub-base of compacted stone (over compacted earth), topped with a minimum of four inches of concrete (more if there will be vehicular traffic). For stability, a deeper perimeter footing (eight to 12 inches) is suggested. Piers may be added for unstable soils and to resist movement due to the freeze/thaw cycle. Four-inch wire mesh and perimeter rebar are other elements that help to make the concrete strong. Both mesh and rebar should be cut to form a gap directly beneath the control joints, thereby directing any movement to be expressed in the designated joints rather than as random cracking. Drainage is accomplished by having the center slightly crowned, with a fall of approximately 1/4 inch per foot. It is preferable for the entire pad to be poured in one campaign, to avoid construction (“cold”) joints. The contractor’s crew should be of sufficient size to keep up with all aspects of the placing, finishing, and curing.A number of labyrinths installed by Golden Spirit have had a pipe in the center, with a removable screw-off cap. This gives a place to put a pole in the center of the labyrinth, for a May Day dance or to hold a lantern at night, or for a banner or other item. Generally the pipe has an inside diameter of one to one and one-half inches.Excess water rising through the concrete can be damaging to the polymer concrete color and the sealer. In most cases, a vapor barrier is recommended to avoid any such difficulty.
CONCRETE MIX DESIGN
The concrete mix design will be determined by the weather and local circumstances.Placing concrete in cold weather or hot weather results in special designs to assure the proper setting and curing. Concrete is a bit of a paradox in that water (hydration) is needed for the concrete to harden but too much water makes it weak and causes problems. Then, once set, water is used to cure it. A low slump (drier mixture) is best.Nonporous aggregates should be used, not to exceed one inch in size. Coarse sand is better in most instances than fine mortar sand. In the event the concrete is being pumped or if for some other reason a looser mix is required, the use of plasticisers will give the intended result without weakening the concrete with excess water. Air entrainment is almost always appropriate, as it protects against freeze/thaw cycles.Pozzoanic materials such as fly ash and silica would not be used under ordinary circumstances. Fiber, on the other hand, can add strength and help prevent cracking.Accelerants or retarders may be necessary in hot or cold weather. For an accelerant, add liquid chlorine to the mix water, rather than using calcium chloride, which can discolor the concrete. The more cement added to the concrete, the darker gray will be the color. If an integral color is to be added to the concrete, this could affect the final result. One solution is to replace part of the cement with a substitute such as Newcem.This results in a lighter gray. One can also ask for the sand and aggregate to be light colored. If necessary, one may have to use white architectural cement to achieve a good color. Since more than one concrete delivery truck will be used, it is imperative to know exactly how much concrete is in the truck so that the color can be added in the correct amounts to achieve a consistent shade. Some concrete suppliers now have computerized systems with liquid pigment which assures a uniform mix and color.Please note that Golden Spirit uses a solvent-based sealer, which slightly darkens the color of the concrete. Our sealer will make the concrete look wet. This should be taken into consideration when determining the desired color.
PLACING THE CONCRETE
Be sure to have a large crew for placing and finishing so that the concrete does not begin to harden before it has been properly installed. The surface quality and color of the concrete are very important for a labyrinth. It is the nature of concrete to crack, as hydration causes shrinkage. Prevent needless cracking by avoiding thermal stress and uneven drying. Concrete literature, such as Design and Control of Concrete Mixtures(by the Portland Cement Association) should be consulted and followed. Wetting the sub-base will prevent it from drawing out the water from the concrete, making the bottom part of the concrete too dry. In warm temperatures, the slump of the concrete is reduced, which means it gets thicker. Adding water seems like a simple solution, both when placing and finishing. However, doing so will seriously weaken the concrete and result in other difficulties. In hot weather, the water and mix materials can be cooled. The aggregate can be dampened, so that it doesn’t draw water away from the cement.Transportation and placing should be done as quickly as possible. Prolonged mixing in transit or on site should be avoided. The best suggestion we can make is to use an experienced contractor with an observable track record of doing things properly.
FINISHING THE CONCRETE
Wind and direct sun cause evaporation to exceed the rate of natural bleeding (water coming to the surface when drying). As a result, the surface gets too dry, developing plastic shrinkage cracks. This can be controlled to some extend by erecting wind breaks and shading the concrete. As soon as the concrete has been screed, a polymer-based evaporation retardant can be sprayed onto the surface. It forms a membrane that is harmless to the concrete but slows down evaporation. Immediately after leveling off the concrete, before the bleedingbegins, the surface is bull floated. Finishing should not be done before the bleeding has stopped. The bleed water should not be finished into thesurface of the concrete, as that would weaken it. A second bull floating should be performed at right angles to the first floating. Hand troweling should be done in a way to avoid over-working the concrete lest the steel trowels burn and discolor the concrete.The final step is to place a medium broom finish across the surface. This is done in a single direction, parallel to either the vertical or horizontal axis. The axis is determined by the orientation of the labyrinth pattern and also the control joints (see below). Finishing is of critical importance. Most concrete is utilitarian, for sidewalks or parking lots, and therefore does not require the attention to detail that we require for the labyrinth pad. Special care should be taken to assure maximum quality. For example, we request that the concrete have a medium broomed texture. In many instances the contractor uses a long handle to extend the broom to the center of the labyrinth, dragging it outward. The same procedure is repeated from the other side of the pad. Unfortunately, in the very center, the most noticeable part of the labyrinth, the overlap of the two brooming runs is often irregular. In such a case, the brooming detracts from the coherent look of the final design. It would be better to use a rope arrangement or other technology to pass the broom across the entire pad in a single run. By “medium broom finish” we mean a distinct and regular broom pattern. Don’t be conservative; smooth is NOT good.
CONTROL JOINTS
Control joints give a channel for any future cracking, thereby controlling cracks and keeping them from being random. Control joints are cut one-fourth of the depth of the concrete. We prefer that the joints be cut by saws, as finishing the joints by hand disrupts the brooming pattern. Generally, control joints should be no more than 20 feet apart. The important details of the labyrinth pattern are in the center and on the vertical and horizontal axes. By using a tic-tac-toe pattern for the control joints, these areas are not affected. The same pattern can also be used for octagonal pads. There are now types of metal fiber which can be used in such a way that control joints are not needed.Once we know the actual measurements of the slab, we are happy to supply precise dimensions for placement of the control joints.
CURING THE CONCRETE
Especially in warm or windy weather, the curing must start immediately after finishing is complete. The first few hours are the most important time to avoid shrinkage cracking. The surface must be kept damp, through a light fogging or misting. Covering the surface with plastic or Kraft paper is a common procedure, but can discolor the concrete. More preferable would be burlap, kept wet by placing sprinklers or soaker hoses in appropriate places. Another possibility is to wet the burlap and then cover it with plastic, to help retain the moisture and protect from the wind. Before the moisture evaporates, the plastic must be removed and the burlap re- dampened. If the weather is favorable, no covering may be required. Perhaps the best curing system that we have seen is to make a perimeter dam around the labyrinth (attached to the framing) and then flooding the labyrinth with a covering of water. Wet curing should continue for at least several days. The concrete must cure for at least 28 days before the labyrinth is installed. With care and attention, the surface should have a uniform texture and color, free from excessive cracking, ready for a labyrinth.
Note: The concrete MUST NOT be sealed or have any kind of surface chemical applied which will subsequently interfere with the bonding of our polymer concrete. This is imperative. A contractor ignored this instruction in California. It cost the Client $2000 for a sand blaster to come and remove the surface treatment before we could proceed.