At 40, Architectural Precast No Worse For The Weather

Panels tested after prolonged, continuous exposure to harsh Chicago climate have answered the question of architectural concrete’s durability. Performance

Panels tested after prolonged, continuous exposure to harsh Chicago climate have answered the question of architectural concrete’s durability. Performance of Architectural Concrete Panels in the PCA Outdoor Display, a Portland Cement Association (PCA) study examining the impact of four decades of Midwest weather exposure on test panels, largely acquits architectural concrete of succumbing to the elements.

In the early 1960s, PCA’s Research and Development Laboratories erected 60 concrete wall panels illustrating a wide variety of decorative surfaces. To make way for property development, PCA dismantled the panels in 2001, but not before subjecting them to close visual inspection, a thorough power-wash cleaning, photographic documentation before and after the treatment, and a pre- and post-cleaning performance evaluation. Panels were rated excellent, good, fair, or poor.

Examination of the panels revealed little alteration in color, minimal loss of aggregate particles, and little or no noticeable loss of surface paste. A slight etching or softening of sharper edges was noticeable on some surfaces. Though cleaning improved the appearance of some panels, many exhibited only minor staining from environmental sources (dirt, grime, air pollution, acid rain, etc.); and, some panels looked nearly identical to their condition of 40 years ago. Other than trimming vegetation around the display, little maintenance occurred during the four decades of the study. While most surfaces were unimpaired and relatively unchanged in appearance, all 60 panels were found to be durable, that is, all remained structurally sound. Overall, 93 percent received an excellent or good rating.

The collection of 60 panels, each measuring 910 mm (3 ft.) wide by 1520 mm (5 ft.) high by 100 mm (4 in.) thick, was assembled to demonstrate seven production procedures popular at the time of their erection. Study results by panel type were noted as follows:

  1. Horizontally precast exposed-aggregate panels Û This widely used precast technique involves coating the bottom of forms with a chemical retarder to expose colorful aggregate in the concrete cast against that surface. Two-layered construction is common, employing white portland cement and special aggregates only in a thin, face mixture. Of the 18 panels in this category, 12 were rated excellent, five good, and one fair. The latter specimen exhibited fading of a pink marble and dirt streaks still evident after cleaning. Another panel showed fading of a blue pigment in the face mixture.

  2. Horizontally precast panels using rubber, plastic, and wood form liners Û A wide selection of textures and patterns, now more than ever, is available through the use of form liners for precast as well as cast-in-place concrete walls. Of 13 panels in this section, three were rated excellent and 10 were rated good. Textures were sharp, although some paste erosion was evident on surfaces originally cast smooth. Minor spalling on a few panels was attributed to the power washing process.

  3. Horizontally precast exposed-aggregate panels using sand-bedding techniques Û For bold and massive design effects in exposed-aggregate walls, stones ranging in size from 25 mm (1 in.) to 200 mm (8 in.) in diameter can be used with this technique, as well as flagstones to produce stone-faced wall panels. Two panels of this type were rated excellent, and two were rated good. Debonding of some aggregate was evident on two panels due to lack of sufficient embedding in the sand before casting. To guard against debonding, recommendations specify embedding two-thirds of the diameter of aggregate particles in the concrete.

  4. Integrally colored precast panels Û A wide range of lime-resistant coloring agents is available today for producing colored concrete panels. White cement is recommended for pastel colors and for deeper hues where true color is required. Of the four specimens in this category, one was rated fair, while the other three were rated poor. The ÎfairÌ panel, retaining some of its yellow color, demonstrated the relative durability of iron oxide pigments; the other three (noniron-oxide) black, green, and blue panels faded to tones of gray. Generally, colored concrete in exterior applications shows fading over time.

  5. Other horizontally precast panels using sandblasting, bushhammering, grinding & polishing, and surface-bonded sheeting Û These production and surface-finishing techniques suit commercial precast applications and some vertically cast-in-place architectural concrete walls. Of eight panels, five were rated excellent, and three were rated good. The four sandblasted panels all received an ÎexcellentÌ rating. Weathering of the bushhammered panel, in spite of a ÎgoodÌ rating, indicated that a softer aggregate is required for this technique. For finishes involving grinding and polishing, results demonstrated that a well cured, high-strength concrete is essential to achieving a high polish on the surface matrix.

  6. Vertically cast-in-place exposed-aggregate panels using prepacked-aggregate concrete Û Because aggregate can be precisely placed against the sides of forms, this technique allows pre-positioning the stone to create a uniform, dense, exposed-aggregate finished wall surface. After water and external vibration are used to densely pack aggregate in the form, the panel is grouted from the bottom up with a cement-sand-water slurry mixture that replaces water in the aggregate voids. Over 40 years, the paste turned dark on all four panels, but cleaned up well.

  7. Vertically cast-in-place exposed-aggregate panels using gap-graded aggregate concrete Û A gap in the gradation of the aggregate creates a surface texture showing predominately one size of coarse aggregate. Cast-in-place walls produced by this means with naturally rounded stone (gravel), crushed limestone, and crushed granite have exhibited outstanding decorative characteristics. Four panels were rated excellent and one good. Despite a ÎgoodÌ rating, one panel was considered unattractive due to excess space between coarse aggregate particles, a function of mix design procedures.

In sum, PCA’s Stonehenge demonstrated the lasting aesthetic value and structural durability of architectural concrete. Although concrete construction has experienced ongoing change during the 40 years since the panels were built, many techniques used to create the panel surfaces are still in use because they remain highly effective. Additionally, technical advancements including high-strength/high performance concrete, tilt-up construction, form liners, color additives, stains and coatings make architectural concrete even more viable today.

Information was adapted from Aging Gracefully: Architectural Concrete Panels Turn 40 Years Old, William Panarese, former manager, Construction Information Services, Portland Cement Association, Skokie, Ill.