What is Portland cement concrete?

PCC Concrete is one of the most ubiquitous construction materials in the world, and is widely used in transportation and infrastructure projects, as well as in industrial, commercial and residential buildings.  According to the Portland Cement Association, more 340 million cubic yards of concrete are produced each year in the United States.  Concrete structures range from large dams, highway overpasses and airport runways to the sidewalks on which children walk to school.  At the end of its useful lifespan, concrete can be crushed into Recycled Concrete Aggregate (RCA) for recycling into new concrete products or reused in other ways, such as flood control riprap or a replacement for sand and gravel in construction projects.

Many people confuse the terms “cement” and “concrete.”  Cement is a fine powder-like material that is manufactured from virgin and recycled materials in a large industrial cement kiln.  When mixed with water, cement becomes a gel that binds together coarse and fine aggregates – also virgin or recycled -- to produce a slurry. There are many types of concrete, with mix designs usually engineered to produce a cured strength that is defined in pounds per square inch (PSI).  Most forms of concrete do not harden completely for many days, and so are tested according to their compressive strengths at differing time intervals.

The most visible form of concrete is Ready Mixed Concrete.  Ready-mix concrete is produced in a local ready-mix concrete plant and delivered to a job site in slurry form in a distinctive rotating concrete truck.  At the job site, the concrete mix is poured or pumped into forms that determine the final shape of the hardened material.

Portland cement concrete is also commonly used to produce precast concrete in a variety of shapes and sizes.  Precast concrete is produced in a factory and then shipped as finished product.  Examples of precast components include bridge girders, concrete blocks, concrete pipe, concrete vaults, and jersey barriers, which are temporary separators used on many roadway projects.

How is Foundry Sand used in manufacturing Portland cement concrete?

Concrete is a mixture comprised of cement (10-15 percent), coarse and fine aggregates (60-75 percent) and water (15-20 percent) by volume.   Foundry sand can be used as a fine aggregate substitute in PCC concrete.  Fine aggregates are generally 3/8 inch or smaller, while coarse aggregates are 3/8 to 2 inches in diameter.  Foundry sand meets two of the critical requirements for concrete aggregates:  it is uniformly graded, and is strong, hard, and durable.

Concrete mix designs require a range of aggregate sizes, with fine aggregate making up between 20 and 35 percent of the total volume of the mix.  Particle size distribution or gradation is critical to the performance of the resulting concrete.  In most instances, foundry sands are too fine to allow for total substitution for the fine aggregate in a concrete mix.  Foundry resin sands, lacking the finer clay particles, are often a better substitution for virgin concrete sands than are green sands. 

How does Portland cement concrete perform using Foundry Sand?

Commercial experience using foundry sand in ready mixed or precast concrete is somewhat limited due to the inability to completely substitute foundry sand for concrete fine aggregate.  Removing the fines or blending foundry sands with other fine aggregates to achieve the desired gradation curve adds costs to the process.  However, there has been commercial-scale use in states such as New York, Ohio and California, generally in instances with where resin sands can be sourced in sufficient quantities to meet the volume requirements of a ready-mix or precast concrete producer. 

Mixes containing foundry sand have shown improved resistance to freeze-thaw damage to those without foundry sand.  This is consistent with superior freeze-thaw performance where foundry sands are used in subbase and structural fill applications.  Additionally, some producers have noted improved resistance to salt scaling when foundry sand is introduced to the mix.

Concrete mixes have been successfully proportioned with fine aggregate replacement ranges from 25 to 45 percent foundry sand.  Some concrete mixes exhibit lower initial compressive strengths when manufactured with foundry sand, so it is important to batch test sands from individual foundry sources before scaling up to full project use.  Foundry sand mixes may also require higher water ratios, due to the presence of clay and other binder materials.  In general, resin sands have fewer performance limitations than foundry green sands and should demonstrate compressive strengths, tensile strengths and modulus of elasticity values similar to those of concrete produced with virgin sands.

Foundry sand is compatible with the air entrainment and chemical admixture systems used in modern concrete production.  Although not itself a cementitious material, it can be used in mix designs with other recycled or reused materials such as fly ash, steel slag, and silica fume.

What are the technical issues associated with Foundry Sand in Portland cement concrete manufacturing?

Particle size distribution is a critical element of concrete mix design, as it will affect cement and water requirements, porosity, shrinkage and durability.  Foundry sands must generally meet the gradation requirements of ASTM C33 in order to be considered alternative fine aggregates for Portland cement concrete production.   Because foundry sands are usually finer than the ASTM C33 aggregate fineness index, fines may need to be removed or the foundry sand blended with other fine aggregates to achieve the necessary gradation.

ASTM C33 has other technical guidelines that require thorough testing of foundry sands sources on a recurring basis.  For example, no more than 5 percent of the fine aggregate can pass the No. 200 sieve and the total aggregate material must contain less than 3 percent clay lumps and friable material. 

Although not a technical issue, aesthetics and perception also play a role in inhibiting the broader use of foundry sand as a concrete aggregate.  Foundry green sand is generally black in color, giving the resulting concrete a grayish cast.  In most applications, there is no technical reason why ready mixed concrete or concrete precast products need to be light in color; however, the darker color of foundry green sand mixes is often considered a detriment.  Using 15 percent foundry sand will produce almost imperceptible color change.  Resin sands usually resemble virgin sands in color and are therefore not subject to the same perceptual barrier.

Are there any specific QA/QC issues that suppliers and/or end users need to be aware of?

Physical testing of foundry sand is critical prior to developing a concrete mix design.  The sands should be tested for specific gravity and absorption; unit weight; fineness; and the percentage of material passing the No. 200 sieve.

Foundry sand used as a concrete fine aggregate must be processed prior to use.  That generally involves crushing to break down remnants of molds and cores, as well as screening to remove all extraneous materials such as tramp metals and large core pieces.  Generally speaking, this step needs to take place before the sand is delivered to the ready mix plant or precast concrete manufacturer.

On the part of the foundry sand supplier, foundry sand for concrete use should be segregated, screened and stored under cover to avoid excess moisture.  At the concrete plant, a separate aggregate bin or silo should be used for foundry sand storage.  Foundry sand needs to be kept dry in order to avoid bulking, which would make it difficult to proportion the mix. 

Are there any specific environmental issues associated with the use of Foundry Sand in Portland cement concrete?

No.  Foundry sands from iron, steel and aluminum foundries have been extensively studied and declared by U.S. EPA to be safe in construction applications. Most foundry sands have environmental profiles comparable to natural sands and aggregates, with the exception of some sands from brass and bronze foundries.  Because the aggregates in concrete are bound into the hardened concrete matrix, there is little to no likelihood of environmental degradation from the use of foundry sand in concrete. 

The concrete industry, along with the Portland cement industry, has a substantial program to promote concrete as a “green” and sustainable product.  Where foundry sand can be substituted for virgin aggregates in concrete production, there will be significant savings in energy and water usage and the other environmental costs of virgin mineral extraction. 

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