Low Strength/ Mix Design Determinations
Petrographic and chemical analysis may be used in conjunction to reverse engineer original concrete mix designs. Cement content, presence of supplementary cementitious materials, aggregate type and property, water to cementitious materials ratio, and air content and distribution may all be estimated through a combination of petrographic and chemical analyses. Other features such as degree of curing, excessive bleeding, or paste to aggregate bond quality are also addressed in the examination. Petrography is generally used in quality control investigations where other methods fail to identify the cause of deficient strength results.
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The cement paste (CP) adhered to the coarse aggregate grain (CA) is darker-colored with good hydration characteristics. The surrounding lighter-colored paste is highly porous. At higher magnification, the portland cement in this paste exhibits extreme hydration and sparse cementitious binder. Companion core samples broke at 40% of the specified strength. The dark-colored paste is a relict of the originally intended mix with the surrounding matrix representing a severely overwatered cement paste. It was determined that the concrete had been retempered to improve the workability thus compromising the strength and durability. |
A high performance concrete designed for 10,000 psi at 28 days exhibited strengths between 7,000 and 8,000 psi. Chemical analysis revealed no differences in mix proportions between these samples and cores that had met specification. However, the silica fume (labeled SF) was observed petrographically as copious undispersed lumps. The benefit of the microsilica addition was not completely achieved as a relatively large quantity remained unreacted due to poor dispersal during mixing. |