Landscape Architect Articles

Color Hardening Concrete08-01-04 | News

Color Hardening Concrete

HOW THE EXPERTS* DO IT

L.M. Scofield Company
Swackhammer Concrete Contractors,
Indianapolis, Ind.

Color hardener in pecan tan and Steadman buff was worked into the surface of freshly placed concrete. Walnut antiquing release remains in the indentations adding depth to the imprinted pattern.

Why CH?

Concrete is hardened to protect the surface and reduce damage from wear, then cured and sealed. Hardener is a powdered, cementitious material with hard aggregates produced by intergrinding.

All fine and coarse aggregates in the concrete substrate must be nonreactive. Fluosilicate-type chemical hardeners are not used unless the color hardener leaves a nonuniform, whitish residue. Color hardener (CH) is streak-free and formulated as a high-opacity material for the top surface of freshly placed concrete substrates. It is also limeproof and has maximum resistance to the effects of UV sunlight. For optimum surface hardening, the aggregates are selected for hardness and purity, then carefully graded through a wide particle-size range to produce dense, wear-resistant surfaces.

Make sure the bleed water is gone from the concrete substrate before applying the stain. Color hardener is never used to dry bleed-water remaining on the surface. And don?EUR??,,????'???t sprinkle water on the surface during application, as it will discolor and weaken the surface. The necessary moisture for strengthening and bonding the color hardener comes from the concrete substrate.

Because of differences of temperature, mix and water content, every concrete surface is a bit different. It?EUR??,,????'???s necessary to identify the substrate and test its suitability for stain before you begin. For instance, if the surface is too porous or too dense, the stain may not penetrate properly.

Only uniformly slip-resistant concrete surfaces, such as broomed, swirl, lightly sandblasted or most imprinted concrete, should be considered for installation of a color hardener. A slip-resistant, flat-troweled finish is suggested. If additional slip resistance is required, special floor hardeners are available.

Coverage

CH is never applied at a coverage rate of less than 60 pounds (one bag) per 100 square feet. Light colors and whites require minimum coverage rates of 90-120 pounds (1.5-2.0 bags) per 100 square feet. The minimum coverage for each standard and designer color is available from the manufacturer?EUR??,,????'???s color chart. Because of the difference in their relative densities, the above mentioned coverage rates result in approximately 75 percent greater thicknesses than if a metallic hardener were used at the same coverage rate. Shelf life of CH is at least six months.

Safety

Do not breathe CH dust, as it contains silica (quartz) which can cause cancer. Avoid contact with eyes, skin and clothing. Wear a dust (particulate) respirator (NIOSH TC-84A approved), safety goggles, and gloves. Wet cement may cause alkali burns. Wash thoroughly immediately after handling, and store CH in a cool, dry, well-ventilated area.

Factors

Timing, application and texturing techniques, imprinting patterns, experience in use of the material, antiquing, curing, finish coating and other factors will affect the final appearance and performance of color-hardened hardscapes. Representative jobsite samples must be produced and approved prior to installation. Samples must be produced for each color and texture and be of adequate size to be representative. The samples should be produced by the same workers who will install the color-hardened flatwork using the contemplated job materials, construction tools, and techniques.

Subgrade Preparation

The subgrade must be well-drained and have adequate and uniform load-bearing characteristics. To reduce cracking, it should be graded so the thickness of the concrete is uniform. At the time of concreting, it must be moist, completely consolidated and free of frost. If necessary, the subgrade is dampened with water in advance of concreting. Concrete is never placed over freestanding water, muddy and frozen ground, or soft spots.

Concrete Mix Design

The concrete mix should contain a minimum of five and one-half sacks (517 lbs.) of cement per cubic yard of concrete. All fine and coarse aggregates must be nonreactive (free of deleterious particles). The water content should be the minimum amount practical, and the slump should not exceed four inches. A normal or retarded-set, water-reducing admixture may be used. An air-entraining admixture complying with ASTM C 260 is used when specified or required by the engineer for freeze/thaw durability and finishing procedures are adjusted or delayed to minimize formation of blisters. The concrete mix must not contain any admixture or additive that contains calcium chloride. During cold-weather concreting, a nonchloride accelerator may be used. No high-range water-reducing admixtures are added. Mixes containing high levels of fly ash as a cement replacement may be more difficult to incorporate the hardener into and to finish properly.

Concrete Installation

Prior to placing the concrete substrate, representative jobsite samples must be produced and approved. Surrounding areas, landscaping and adjacent surfaces are protected. Prior to concrete installation the required number of bags of color hardener are distributed outside the formwork proportional to the area to be covered. Quantities for the first shake, second shake and so on are separated. When concrete is placed and finished in hot, windy weather, wind breaks may be needed. Precautions are taken to prevent plastic shrinkage cracking resulting from excessively rapid drying at the surface.

Ready-to-use, powdered dry-shake hardener is being applied to the surface of the concrete by Brad of Swackhammer Concrete, also the contractor for the graded walkway on page 54. CH is hand-broadcast across the surface of the ?EUR??,,????'??plastic?EUR??,,????'?? concrete in two separate ?EUR??,,????'??shakes?EUR??,,????'??: two-thirds applied in the first shake; the last third for the second shake and final touch-up. Since concrete sets more rapidly along the edges of the slab, CH is applied to the edges first. CH is never applied at a coverage rate of less than 60 pounds (one bag) per 100 square feet, but light colors and whites require minimum coverage rates of 90-120 pounds per 100 square feet. This coverage rate results in approximately 75 percent greater thicknesses than if an equal amount of metallic hardener were used. Consult the manufacturer?EUR??,,????'???s color chart for the minimum coverage for any color.

The concrete mix is controlled to provide good batch-to-batch uniformity, and the slump is consistent throughout the project at four inches or less.

The concrete is placed and spread so that it completely fills all space inside the forms, then consolidated by vibrating to provide a suitable surface for finishing. If tamping is used for consolidation it is kept to a minimum, and concrete adjacent to the forms is spaded.

Before the appearance of excess moisture or bleed water, the surface is screeded to the finished grade specified and wood-floated to the required flatness and levelness. Suggested tolerances are given in ACI 302.1R Guide for Concrete Floor and Slab Construction. Exterior concrete is sloped adequately for proper drainage, normally a minimum of one-fourth inch per foot. When the concrete is air-entrained, floating is delayed to minimize stickiness.

The pattern has been applied and the color is setting. CH is streak-free. After each application, the dry CH is completely worked into the surface via wooden hand floats or a power troweling machine equipped with float blades (shoes). Don?EUR??,,????'???t use long-handled fresnos, and don?EUR??,,????'???t trowel the concrete until after the final application. To prevent burning or darkening of the surface, hard steel troweling (burnishing) is minimized, especially at the edges and joint lines.

Floating to embed the coarse aggregate and bring the mortar to the surface is done with wood floats so that the surface remains open. Troweling closes the surface, making it more difficult to work the color hardener into the plastic concrete. It is done only after the final color-hardener application.

Application

The color hardener is applied after the concrete substrate has reached the point where the bleed water disappears and the floating process will not disrupt the level of the surface. Earlier application may reduce durability and require a higher coverage rate. Color hardener is never be used to dry bleed water remaining on the surface.

CH is hand-broadcast uniformly across the surface of the plastic concrete by the dry-shake method. Wide areas may require the use of a bridge. Mechanical spreaders are effective on larger installations. The color hardener is normally applied in two separate broadcasting operations (shakes). Two-thirds of the material applied in the first shake, the other third withheld for the second shake and final touch-up as necessary. For applications of over 100 pounds per 100 square feet, three shakes should be considered, with one-half of the material being applied during the first shake.

Geometric patterning can be added to the color-hardened concrete by scoring or saw-cutting, giving the look of individually laid units. CH is regularly used when imprinting or stenciling concrete to produce the look of handset brick, tile or aged stone.

To prevent segregation of the aggregates, the color hardener is not broadcast from a point above knee level or thrown more than a few feet, less under windy conditions. Since concrete sets more rapidly along the edges of the slab, apply to the edges first and work it the rest of the surface. A small quantity of material is withheld from the final shake to touch up nonuniform or weak-toned areas.

The moisture for wetting the color hardener comes from the concrete substrate and develops the proper strength and monolithic bonding. Water is not sprinkled or added to the surface during application, which could discolor, weaken and reduce its durability.

After each application, the dry color hardener is thoroughly and completely worked into the surface via wooden hand floats or a power troweling machine equipped with float blades (shoes). Long-handled fresnos are not used. The concrete is not troweled until after the final application.

After the final shake, the surface is floated, then hand- or machine-troweled. To prevent burning or darkening of the surface, hard steel troweling (burnishing) is minimized, especially at the edges and joint lines.

The specified texture is applied uniformly using consistent finishing practices. All surfaces are finished within reasonably the same time after placing.

Until completely cured, the color of concrete is normally less uniform and appears darker than the final color. Color-matched curing materials will properly cure color-hardened concrete, enhance the depth of color, improve color uniformity, and provide surface protection. Curing materials are specially formulated for use with colored concrete.

For curing freshly placed concrete, sealer is applied by airless sprayer when the concrete is hard enough to be walked on gently without marring, surface moisture has evaporated and no condensation can occur.

The concrete is not covered with plastic sheeting. The surfaces remain uncovered for a minimum of four days, after which it may be covered with new and unwrinkled, nonstaining, reinforced kraft curing paper.

All surfaces are thoroughly inspected to verify and approve installation and safety, including wet and dry slip resistance, prior to opening the cured or sealed color-hardened surface to traffic.