**TECHNICAL NOTES**

**USDA-Natural Resources Conservation Service**

**Boise, Idaho**

TN ENGINEERING NO. 11 January 24, 2000

## CONCRETE ADMIXTURES

This technical note provides a discussion on some of the more common
concrete admixtures that are used in concrete applicable to conservation
practices.

This note provides information on the following five categories of
admixtures:

1.  Air-entraining Agents

2.  Water-reducing Agents and High Range Water Reducers
    (Superplasticizers)

3.  Retarding Agents

4.  Pozzolans

5.  Accelerators

The NRCS-Idaho Construction Specifications require the use of some
admixtures such as air-entrainment, permit the use of others such as
pozzolans and water-reducing agents and prohibit the use of others such
as calcium chloride.

# AIR-ENTRAINING AGENTS

Air-entrainment agents introduce tiny air bubbles into the concrete mix.
They are used in concrete primarily to improve weathering resistance to
freezing and thawing. The proper amount of air in a concrete mortar will
increase this resistance about 10 times. Air-entrainment also increases
workability, increases cohesiveness and reduces segregation and
bleeding. The smaller the aggregates in a mixture the more
air-entrainment that is required to provide the same degree of
resistance to freezing and thawing. The recommended percentage of air by
volume in concrete is:

> [Aggregate Size (Inch)]{.underline} [Percent by Volume]{.underline}
>
> 3 3.5 +/- 1
>
> 1.5 to 2.5 5.0 +/- 1
>
> 3/4 6.0 +/- 1
>
> 3/8 7.5 +/- 1

There are a number of commercially available air-entrainment products.
Most of the air-entraining admixtures are supplied in ready to use
liquid form. These products are commonly applied to the concrete mix
when it is being batched. All concrete used in the NRCS-Idaho
conservation programs will have air-entrainment. Products used shall
meet the requirements of ASTM C 260 "Air-Entraining Admixtures for
Concrete" and generally meet the percentages shown above.

# WATER-REDUCING AGENTS AND HIGH RANGE WATER REDUCERS (SUPERPLASTICIZERS)

Water-reducing agents are chemical additives that reduce the quantity of
mixing water required to produce concrete of a given consistency.
Water-reducing admixtures that reduce the water required for mixing by
12 percent or greater are classified as high-range water-reducing
additives. These products are often referred to as superplasticizers.

In the Redi-Mix designs submitted for certification the use of a
water-reducing agent that reduces the mixing water requirement by 5 to
10 percent is the most common. The use of superplasticizers requires
close quality control and their use should be limited to larger
structures where conditions warrant their cost. Superplasticizers will
typically cost about \$10/cu yd of concrete.

A reduction of water in a concrete mix results in higher concrete
strength, increases resistance of concrete to sulfate attack and
decreases the permeability of the concrete. Workability can be decreased
and may be an issue in getting concrete properly placed around corners
and tightly spaced reinforcement. These admixtures shall meet the
requirements of ASTM C 494 "Chemical Admixtures for Concrete" or C 1017
"Chemical Admixtures for Use in Producing Flowing Concrete".

# SET-RETARDING AGENTS

Set-retarding admixtures are chemical additives that retard or slow down
the setting of concrete. These admixtures allow additional time between
mixing and the final placement of the concrete. Due to the long
distances from concrete batch plants to many field sites, set-retarding
agents are used in site specific concrete designs. These admixtures
shall meet the requirements of ASTM C 494 "Chemical Admixtures for
Concrete".

# WATER-REDUCING AND RETARDING ADMIXTURE

Water-reducing and retarding admixtures are chemical additives that
provide for both a reduction in mixing water and also slows or delays
the setting of the concrete. These admixtures shall meet the
requirements of ASTM C 494 "Chemical Admixtures for Concrete".

# POZZOLANS

Pozzolans are siliceous and aluminous materials, which in themselves
possess little or no cementitious properties, but will chemically react
with the calcium hydroxide in Portland Cement at ordinary temperatures
to form compounds possessing cementitious properties. Pozzolans can be
classified petrographically as follows:

1.  Calcined clays and shales.

2.  Opaline materials such as diatomaceous earth and opaline cherts and
    shales.

3.  Volcanic tuffs and pumicites of rhyolitic, phonolitic or andisitic
    composition.

4.  Industrial by-products such as fly ash, ground brick or silica fume.

Except for rare occurrences, natural pozzolans must be ground before
use. The clayey materials including alterated volcanic ash and tuffs as
well as shales must be calcined at temperatures between 1200 degrees and
1800 degrees.

Pozzolans are not normally required in our construction specifications.
However a few Redi-Mix suppliers have mixes that commonly include
fly-ash as a substitute for a portion of the cement. The NRCS-Idaho
Construction Specifications allow up to a 20 percent substitution of
fly-ash for cement. Because pozzolan properties vary greatly, a cylinder
test break should be required to determine how the pozzolan reacts with
the cement and aggregate to be used before accepting pozzolan into a
design mix.

Advantages of using pozzolans include improved workability of the
concrete, reduced heat of hydration, reduced thermal column change,
reduced bleeding and lower permeability of the concrete. Some pozzolans
control or reduce the alkali-aggregate reaction where reactive
aggregates and high-alkali cement are used.

Disadvantages include reduced resistance to freezing and thawing until
the concrete is thoroughly cured, possible excessive drying and
shrinking of concrete, reduced concrete strength and lower durability.

Pozzolans should be used with caution. Their properties may vary widely
and some may add adverse qualities into the concrete. Before accepting
the use of pozzolans other than fly ash in a concrete design you should
evaluate the advantages/disadvantages for the proposed structure.
Concrete cylinder test breaks need to be supplied by the concrete
supplier using the cement and aggregate for the site specific concrete
mix prior to acceptance. Pozzolans shall conform to the requirements of
either ASTM C 618, "Fly Ash and Raw or Calcined Natural Pozzolan for Use
as a Mineral Admixture in Portland Cement Concrete" or C 989, Ground
Grandulated Blast-Furnace Slag for Use in Concrete and Mortars".

[ACCELERATING AGENTS]{.underline}

Accelerators are chemical additives, which decreases the time for the
setting of the concrete. Accelerators can be used to increase early
strength in concrete but they can cause a reduction in strength of the
cured concrete and increased shrinkage of the concrete.

Calcium chloride is a commonly used accelerating admixture. Most
requests are from contractors wanting to use calcium chloride in lieu of
properly heating and curing concrete under cold-weather conditions.
However, calcium chloride is not an anti-freeze for concrete and it does
not materially lower the freezing point of concrete. Calcium chloride
can also cause corrosion of reinforcement steel.

The use of accelerators is not allowed in the NRCS-Idaho construction
specifications or the NEH-20, Concrete Construction Specifications.

The use of Type III, high early strength cement is recommended when
early concrete strength is desired or needed.

# REFERENCES

-   ACI 301, Field Reference Manual, Publication SP-15-89

-   Concrete Manual, 8th Edition, USDI, Bureau of Reclamation

# 

Prepared by G. Arthur Shoemaker, State Conservation Engineer, January
2000.

File this in the Engineering section of the Technical Notes, Section VI
of the Technical Guide.