ConcreteScience© , Technical Article # 2

What is Petrography?

The definition of petrography is the branch of science concerned with the description of rocks, especially by microscopic study. We, at Concrete Science, use petrography as a baseline for examining the concrete or construction material and / or the sand and gravel. Different materials that we look at, but not limited to, are:

• Concrete

• Stucco

• Mortar/Grout

• Aggregates (coarse and fine particles)

• Toppings/overlays/underlayments (paint, gypsum, plaster, epoxy, etc.)

• Fire damaged construction material

• Asphalt

Interested in Petrography services? Contact us.

Why Petrographic Examinations are Important?

To examine the concrete and construction material, a petrographic examination will be needed.  A petrographic examination will determine what is “going on” with the construction material.  A petrographer examines the concrete and assesses volumetric proportions and characteristics of the concrete to see how compatible the products are together that create this “man-made rock”.  Also, we look at the sand and gravel used in the concrete to make sure it is compatible to the concrete matrix.  So, if the concrete or construction material performance is either failing, a problem has occurred, or just comparing the mix design to the sample, a petrographic examination is the best way to assess this.  Our Petrographer uses various types of microscopes to evaluate the microstructural quality and the optical properties of the materials.  The Petrographic examination consists of a detailed report and identify the failure or deterioration of the material.  The petrographer will make recommendations on repairing or replacing the material.  

During the examination, the Petrographer identifies:

• Concrete quality evaluations: air void analysis, mix design, fly ash, slag, cement, low and high compression strengths, improper finishing, steel reinforcement quality and placement, presence of curing compounds and carbonation.

• Concrete deterioration: flooring failures, fire damage, cracking, delamination, scaling, spalling, crazing, pop outs, discoloration, blistering, dusting, efflorescence and curling.

• Chemical attacks: alkali silica reaction (ASR), alkali carbonate reaction (ACR), sulfate, chloride, seawater attack and steel corrosion.

• Physical attacks: freezing and thawing, wetting and drying, and high temperatures.

• Aggregate evaluation: type and composition, quality/compatibility, gradation, flat and elongated particles, and quantify amounts of deleterious/reactive particles.

• Mortar/grout evaluation: matching historic mortar, troubleshoot problems, and tuck point compatibility.

• Asphalt: crack evaluation and proper repair methods

Common Concrete Problems

Concrete is a very common construction material used in numerous ways.  Concrete is used in roads, bridges, buildings, parking structures etc.  Concrete is made up by mixing together Portland cement, sand, small stones or gravel, and water with sometimes other admixtures added to it.  The admixtures can add durability, strength, and workability to the concrete. 

Listed below are the most common problems that can occur to concrete:

• Finishing

• Aggregate Capability

• Air Content

• Water Cementitious Material Ratio

• Cracking

Other Materials

Topping/overlays/underlayment

This is a very broad area and it refers to anything placed on top of the concrete or construction material.  Most of the time it is a cement based or gypsum based product or it could be a material like an epoxy, tile, or a membrane.  It can, but not always, have some kind of fine aggregate particles imbedded within.  We will look at how well bonded the layer(s) are, if there is an adhesive or bonding agent in between the layer(s), how many layers and what they all consist of.  We can assess the compatibility of the layers to each other and to the base concrete.

Fire Damage

Concrete and stucco can hold up well when in a fire and does not penetrate very deep.  However, the material can become weakened and cracked.  The paste in the concrete has some available water that helps strengthen the concrete and hydrate the cement particles within it.  Once a fire occurs, the concrete paste will dehydrate and can change colors when heated to high enough temperatures.  The aggregates within the paste can also go through a change in color depending on the available iron content within them.  We take a look at the quality of the non-damaged area of the concrete and the aggregates used in the concrete.  We then compare it to the fire damaged area and see if the quality of the concrete was compromised.  An assessment on the depth of the fire can inform you of the condition of the material and if a repair will be needed.  

Standards Used for Testing

ASTM standards used for testing the construction material are:

• ASTM C295 - Petrographic Examination of Aggregates in Concrete

• ASTM C457 - Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete

• ASTM C856 - Petrographic Examination of Hardened Concrete

• ASTM C1324 - Examination and Analysis of Hardened Masonry Mortar

We provide services to owners, contractors, engineers, architects, property managers, material suppliers, lawyers, City and County, and more.  Call or email us. 510-581-2342, www.concretescience.com