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