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Colored Concrete Does a Fast Fade

Thursday, September 11, 2014

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Beauty is backfiring in Minnesota, where the popular colored concrete look is running aground on reports of early cracking and fading.

Within the last five or six years, cities around the state have been opting for colored (and more expensive) concrete to spruce up sidewalks, crossroads and medians.

But these projects are now exhibiting significant early distresses, the Minnesota Department of Transportation says.

The most common issue is accelerated microcracking, particularly near contraction joints, according to a 368-page report, "Investigation and Assessment of Colored Concrete Pavement," published at the end of August by the MnDOT Research & Services Library.

colored concrete
Images: MnDOT Research Project

Colored concrete projects throughout Minnesota are experiencing early deterioration, prompting transportation officials to investigate the cause.

The research, funded by the Local Road Research Board, investigated the causes for early distress in colored concrete and offered repair recommendations.

Documenting Deterioration

An estimated 45 colored concrete projects have experienced early deterioration—some within only five years—causing some neighborhoods to tear out the expensive material, Tom Burnham, MnDOT's Senior Road Research Engineer, told Crossroads, a joint blog by MnDOT and the University of Minnesota's Center for Transportation Studies.

"You go to almost any community and they're installing it—on their sidewalk and medians and also crosswalks," said Burnham, the study's primary author.

Burnham and his team visited and documented colored concrete projects in dozens of locations. The most frequently noted issue was joint distress, with microcracking reaching from top to bottom of the pavement.

Four projects that exhibited "significant early or unusual distress" were selected for an in-depth forensic examination. Core samples were extracted and examined using petrography, scanning electron microscopy (SEM) and energy dispersive x-ray spectrometry (EDX).

Problem Pores

The investigation showed most mixes were produced with a high water-to-cementitious ratio that exceeded the maximum value recommended for concrete in freeze-thaw climates.

Faced with high porosity, the concrete's small air voids were filling up with ettringite crystals, which reduces the concrete's freeze-thaw resistance, and magnesium chloride, which can cause accelerated deterioration, the researchers discovered.

Since deicers often contain magnesium chloride, snow and ice removal could be contributing to the deterioration, according to the report.

MnDOT

"Understanding and accounting for the thermal characteristics of a material is one of the primary keys to successful engineering design. This is particularly true with concrete pavements, where the forces involved can be quite tremendous," the researchers said.

Test samples also showed evidence of chemical attack, as well as an alkali-silica reaction, but Burnham said more testing would be necessary to determine if anything unique in the coloring accelerates observed chemical reactions.

In all of the tested samples, the paste-to-aggregate bond was classified as poor to fair and pigment particles were bonded to the surface of the aggregates, often non-uniformly—a factor likely to affect the paste-to-aggregate bond strength, the authors said.

"While this loss of bond strength may not be enough to affect the load capacity of the concrete, perhaps it provides opportunities for freeze-thaw forces to develop the microcracking that is observed," the report stated.

Additionally, many of the colored concrete surfaces had a smooth texture, which is "perhaps not the most ideal for crosswalks and sidewalks in a cold climate," the authors noted.

Thermal Forces

Because Minnesota often experiences freeze-thaw cycles, the researchers wanted to test the thermal characteristics of colored concrete.

"Understanding and accounting for the thermal characteristics of a material is one of the primary keys to successful engineering design. This is particularly true with concrete pavements, where the forces involved can be quite tremendous," the researchers said.

Three small test slabs with embedded sensors were constructed side by side at the MnROAD facility in Albertville, MN. Pigments were added to two slabs to produce one light panel and one dark panel; the third was produced without color.

While testing thermal expansion, the researchers discovered that the colored concrete slabs expand at a greater rate than standard non-colored slabs.

MnROADS

Researchers built three test slabs to determine how thermal forces affect colored concrete.

"[D]esigners must carefully consider thermal expansion response when laying out a project with different colored concretes next to each other," the researchers determined.

Figuring Out a Fix

Another burden of the distressed colored concrete is figuring out how to fix it. While some projects would benefit from standard repair techniques used for non-colored concrete, that won't work for all situations.

The report recommended ways to improve construction methods and mix design specifications, including:

  • Produce colored concrete with water-to-cementitious ratios no greater than 0.43 and use appropriate admixtures to acheive workability during placement;
  • Improve placement methods, especially the consolidation of the mix using vibration;
  • Ensure an adequate and durable surface is constructed during the finishing process; and
  • Use proper and adequate curing techniques, or devise new ones if necessary.

Conclusively pinpointing issues with the colored concrete material and construction techniques will require more research, the report noted. In the meantime, Burnham hopes this report at least gets the industry thinking about it.

"We hope this research is a wake-up call for the colored concrete industry too because we don't want the industry to die in Minnesota," Burnham said. "If it can work, we want cities and counties to be able to use it."

   

Tagged categories: Concrete; Concrete defects; Cracking; Decorative concrete; Department of Transportation (DOT); Failure analysis; Research

Comment from Chander Patil, (9/11/2014, 8:29 AM)

What control samples were tested to specifically pin down the defects caused by addition of color to the concrete mix? It is not clear how adding color to the concrete would lead to cracking. Wouldn't it be easier to paint the pavements instead ....Chander


Comment from Tom Schwerdt, (9/11/2014, 8:51 AM)

Concrete mixes with durable pigments (iron oxide would be a good one here) are a permanent solution which will last as long as the concrete. Paint is not - even with durable pigments, paint will eventually wear away or peel off. Wear away a few thousandths of an inch of paint, and the color is gone. Wear away a few thousandths of an inch of colored concrete, and you still have colored concrete. I'm not sure why these particular mixes are having issues - integral color in concrete has been used for a long time with success, something like 100 years of history.


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