Nov 13

Fly Ash Concrete

Fly ash in concrete

FROM GREY TO GREENISH …

If you consider yourself a green person, you may think using fly ash, a by-product of burning coal, in your concrete mix rather sacro-saint! After all, coal plants are some of the biggest polluters and CO2 emitters.  In recent decades, in an attempt to minimize its environmental impact, the EPA forced coal-powered plants to trap particulate matter before it is released in the smokestacks, to properly capture and dispose coal ash, a.k.a. as fly ash, the residue found after smokestacks are cleaned. Disposal of fly ash is usually done by mixing it with water and placing it into open impoundments (that could be damaged by storms), storage ponds or buried in landfills that have been known to occasionally breach or leak as in the case of the December 2008 collapse at the Tennessee Valley Authority’s Kingston Fossil Plant, releasing 5.4 million cubic yards of coal fly ash into the Emory River! So how much are we reusing? Approximately 43-45% of all fly ash is being recycled today.

Electrical PoleWith 50% of all electricity in the US being generated by coal plants, one couldn’t begin to imagine the huge amount of fly ash produced by coal-powered plants daily. Capturing it and reusing it in a safe way would in fact prevent the inevitable spread of some of the heavy metals  it contains (depending on the coal bed makeup: arsenic, lead, chromium, mercury, dioxins etc.) in the atmosphere we breathe, the water we drink… So when the first attempts at substituting fly ash to Portland cement resulted in a concrete with greater strength and durability, what was once a worrisome problem looked like it could be turned into a solution for the environment and the construction industry. Recycling fly ash into concrete would not only reduce CO2 emissions but also neutralize the harmful effects of the known carcinogens it contains and the need to mine for new raw materials. Also called geopolymer concrete, this “green” concrete is cheaper to produce and is being widely adopted for all types of concrete applications.

2 TYPES OF FLY ASH
There are 2 classes and both can be used as a replacement for Portland cement or hydrated lime. They act as a filler providing contact points between larger aggregate particles in asphalt concrete mixes.

Class F: is made from burning anthracite and/or bituminous coal
Class C: is made from lignite or subbituminous coal

BENEFITS OF USING FLY ASH IN CONCRETE

Earth-Friendly-IconECO FRIENDLY?
The process of making cement is quite energy-intensive and could account for 5-8% of greenhouse gas emissions around the world. Factor in steady population growth and the number will continue to increase unless a massive adoption of greener alternatives happens. Fly ash concrete is one of them because of its abundant availability, cheap cost and the fact that it could reduce as much as 90% of CO2 emissions when considering the end to end process.

Green builders can gain points towards their LEED goals (Leadership in Energy and Environmental Design) if they can replace at least 40% of Portland cement with it. Bricks that are made with fly ash can even store CO2 from the atmosphere.

ALL AROUND BETTER CONCRETE
Fly ash is known to improve concrete’s workability, pumpability, chemical resistance, finish, strength, and durability. Fly ash particles are similar in size with Portland cement and as a result requires less water during the mixing stage. This means better surface finish, sharper edges in precast concrete. Its fine particles reduce bleeding and segregation and improve overall workability. This is especially helpful in extending working time in hot weather.

DECREASED PERMEABILITY:
Fly ash produces a concrete that is less porous than Portland cement. The pozzolanic effect of fly ash creates a denser product due to smaller pore sizes. It also reduces bleed channels and permeability in concrete.

Widely used in road projects, fly ash increases the stiffness of the asphalt, improving rutting resistance and mix durability.

EXAMPLES OF FLY ASH REUSE

In addition to replacing Portland cement, fly ash is being used in the following areas and the list keeps expanding:

  • Road construction: ideal self-compacting backfill material (as a replacement for compacted earth) for backfill, embankments, road sub base construction, mineral filler in asphaltic concrete, as a loose application on roads for ice control, in highway sound barriers…
  • Agriculture: fertilizer, soil amendment and stabilization.
  • Aggregate material substitute: for brick production replacing clay.
  • Dam construction: in roller compacted concrete dams.
  • Waste management: waste solidification and stabilization, conversion of sewage sludge into organic fertilizer or bio fuel.
  • Roofing material: tiles, granules.
  • Marine: pilings, artificial reefs
  • Binding agent: paints, undercoating.
    And much more

IN CONCLUSION

Only time will tell whether or not small amounts of heavy metals could leak from the concrete over time and if this new found durability is truly long lasting. Recent spills have prompted environmental groups to call for tougher regulations, so whether or not the EPA decide to classify it as hazardous  material remains to be seen. But until then and until coal powered plants are being phased out for greener alternatives, and the huge reserves of fly ash remains an environmental issue, the safe use of fly ash in concrete makes sense and should continue. 

United Equipment Sales

Sources:
Marc Boyer: How fly ash concrete works
Fly Ash on Wikipedia
Fly ash products
Fly ash suppliers

 

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Jul 17

TESTING FRESH CONCRETE – PART 3: Ball Penetration Test

In our previous article, we looked into Flow test as another common test method to ensure that fresh poured concrete has the right consistency and workability. As in all testing methods, always keep in mind that some limitationsoften apply. Today we will focus on a third method called Ball Penetration Test (ASTM C360) and better understand main causes of segregation and bleeding in concrete.

Kelly Ball

THE BALL PENETRATION TEST (a.k.a Kelly Ball Test) can be conducted on site by measuring the indentation made by a Kelly Ball into plastic concrete. The device consists of a 6 inches steel cylinder (15cm) in diameter with a semi spherical shaped bottom and 4 5/8 inches in height. It also comes with a frame that guides a vertical handle marked in ¼” increments on one side and half inches on the other.

Advantages: this test can be conducted directly on the concrete in site (in a form, a hopper, wheelbarrow), there is no need for filling and rodding a container. This means that the test can be completed faster than with a slump test and will yield more precise results.

California Case Study:
The California Division of Highways used to perform a slump cone method but in 1953, they adopted the Kelly Ball Test. In addition to the ASTM and AASHO requirements, they looked for a minimum depth of 6 inches on concrete and a minimum clearance of 9 inches. If the test fails to show 1 inch of slump (1/2 inch of penetration), more reading must be taken until 3 successive readings within 1 inch are obtained. They average the first three valid penetration readings. ASTM and AASHO test methods are to the nearest ¼ inch while with the California method each inch of penetration.is read as 2 inches on the ball shaft.

WORKABILITY
DEGREE

SLUMP/PENETRATION (mm)

USES

Very low

0-25

Roads with power operated machines

Low

25-50

Road with hand operated machines

Medium

50-100

Flat slabs with crushed aggregates

High

100-175

For congested reinforcement (not for vibration)

Limitations:
There are several known sources of variances of the Kelly Ball Test, namely: sampling, testing, differences in moisture content, inadequate mixing of materials, variation in mixing time and types of aggregates.

To better master concrete workability, one must study and understand  what causes segregation and bleeding in fresh concrete. Below is some useful information that should help concrete operators on the job.

CONCRETE SEGREGATION
This phenomenon occurs when the elements of the cement paste separate creating an uneven distribution. It can happen when the concrete is mixed, transported, placed or compacted.

Types of segregation
When coarse particles are mixed with finer particles they can settle more because they are heavier. This is the first form of segregation also called coarse segregation.  The mix will have a low asphalt content, low density, lots of air voids. It is the main cause for segregation.
The second type of segregation can happen in wet mixes, when cement and water separate from the mix. It is called Fine segregation and produces cement with high asphalt content with low density, concrete deformation and more.

Factors causing segregation:

  • Too much coarse aggregate in the concrete mix (in coarse segregation)
  • Too much fine aggregate in the mix (in fine segregation)
  • Excessive vibration
  • Overworking and flowing of concrete along the form through a chute
  • Pouring concrete from considerable height
  • Sudden change of direction through a chute

CONCRETE BLEEDING

This happens when the solid elements of the mix do not properly hold to the water when they are poured and some of the water rises to the surface of the fresh concrete. This being said, most freshly placed concrete will result in some level of bleeding. The amount of bleeding is proportional to the depth of concrete poured. Bleeding can create pores or “wormhole channels” in the interior and weaken the concrete. Do not attempt to remix the bleeding water during the finishing steps as this will result in a weak surface, non-durable concrete vulnerable to freezing and thawing and rebar corrosion.

What can cause bleeding
Too much water in the mix. Whatever time you thing you may have saved during placement will be lost waiting for the water to evaporate! Excessive compaction can also push up water to the surface of the fresh concrete.

Solution:

  • Wait for the bleeding water to evaporate before completing the finishing of the top surface.
  • Never trowel concrete while bleedwater is still on the surface!
  • Use more finely ground cement.
  • Add calcium chloride to cement and increase the fineness of cement.
  • Add fly ash, pozzolans or aluminium powder in the concrete.
  • Select a proper finishing method to protect the slab surface.
  • For air-entrained concrete, use an air-entraining agent to lower the amount of water needed to achieve the desired slump.

Having the right concrete pump for the job is also important to minimize risks of segregation or bleeding. Not sure about which pump to buy? Why guess when you can just give us a call us at (503)281-2105? We always love a challenge and are ready to help you save money on your next job.

SUGGESTED FURTHER READING

Testing Fresh Concrete Part 1: Slump Test
Testing Fresh Concrete Part 2: Flow Test
Report from the California Division of Highways
More info on Concrete workability

 

Feb 16

Recycled Glass Use in Concrete

 The concrete industry is constantly looking for innovations, better performance and ways to lower its carbon footprint. Surprisingly, cement production accounts for about 5% of the total CO2 emitted in the atmosphere around the world.

CO2 Emissions

Another way to look at it is that each ton of cement produced outputs a ton of CO2. Much of it comes from the high melting temperature (2500°F) and from the process of decarbonation of limestone. So to lower the carbon footprint, the industry would have to either change the cement production process or to use less cement in concrete mixtures. Much research has been done around the world in finding ways to dispose of industrial byproducts like fly ash or silica fume and use them in making concrete. So much that these are now commonly found in most concrete mixtures.  What other material can be recycled into cement?

Recycled glass

Meet recycled glass
In the US, only 33% of the glass products is being recycled and only 40% of the glass that is collected is actually recycled. Part of the reason may be that glass recycling is still costly due to transportation and color sorting.  Crushed glass in large particles (larger than 75µm) produces an expansive gel that results in concrete cracking, so the particle size must be finer than 75µm to not create an alkali silicate reaction.  When this happens, mortar durability is increased. The only exception to this is the use of large glass particles in making concrete kitchen counter tops. These are becoming very popular as they combine cost saving, green practices, beautiful and unique results.

A better solution is glass powder as it has been found to improve concrete’s compressive strength and durability. Another great discovery was the water reducing effect of glass powder, leaving more water to improve the cement’s workability.

Michigan State University has been conducting extensive research in their labs on the use of glass in concrete on their campus.  They have managed to replace about 20 percent of the cement used to produce concrete by milled, or finely ground glass. The resulting concrete becomes lighter but has the same appearance with regular concrete, maybe a little lighter in color. More research is underway to test all variables needed to make this new element a standard in the industry.  To read more on this, two papers were published on the durability of the mixture, one in the Journal of Solid Waste Technology and Management, and the other in the Journal of Construction and Building Materials of Michigan State University.

The American Journal of Engineering Research (AJER) conducted workability test, density test and compressive strength test on waste glass powder in concrete. They also found that glass powder does increase the workability, density and compressive strength of concrete.  Read the complete report here.

Europe and India have also successfully spearheaded research in this emerging field. The study conducted by Dr. G.Vijayakumar, Ms H. Vishaliny and Dr. D. Govindarajulu at  Pondicherry Engineering College, in Pondicherry India, had the following very positive conclusions:

“Replacement of glass powder in cement by 20%, 30% and 40% increases:

  1.  the flexural strength by 83.07%, 99.07% and 100% respectively.
  2. the compressive strength by 19.6%, 25.3% and 33.7% respectively.
    Glass powder concrete increases the compressive, tensile and flexural strength effectively, when compared with conventional concrete. Very finely ground glass has been shown to be excellent filler.”  The complete report can be read here*

The Centre of Sustainable Development in Quebec, Canada was an early adopter of powder glass in concrete, which allowed them to reach a LEED Platinum certification. They simultaneously managed to reduce the amount of concrete used and the amount of glass dumped in landfills.

It is still too early to know whether or not glass can be used for shotcrete applications. Not enough testing and research has been done on wet or dry mix shotcrete. However, promising results were discovered in early findings for dry mix on rebound and reinforcement of bar encapsulation in the placement phase.

Stay abreast on new technologies and the latest research with United Equipment Sales. With over 40 years in the industry and all things concrete, we can help you on your next concrete job, solve challenges and find you a great deal on concrete pumps.

ADDITIONAL READING

Recipe for success: Recycled glass and cement 
Glass Powder Utilization in Concrete Production from the European Journal of Applied Sciences
Studies on Glass Powder as Partial Replacement of Cement in Concrete Production
Performance of Using Waste Glass Powder In Concrete As Replacement Of Cement from the American Journal of Engineering Research