Concrete petrography applications – an overview

Concrete petrography applications – an overview

Concrete petrography is the description of the physical and chemical composition of concrete and is vital in all phases of construction and especially post-construction in order to determine the causes of concrete structure failures, writes Dr Nicolaas C Steenkamp.

Concrete petrography takes into consideration not only the primary mineralogy of the aggregates and the matrix materials, but also the secondary mineralogy as a result of the concrete production and mixing process, the proportions of materials and external factors such as the environment it is exposed to and intense single events like fire.

Petrographic examinations of concrete are typically performed due to the premature failure of an existing concrete structure or a problem during a construction project. A concrete petrographer is retained to determine the cause of the problem and often to determine responsibility.

Petrographic examinations are often performed as part of a condition study to determine if a concrete structure can be rehabilitated or needs to be replaced. Concrete petrography also helps to determine the nature of deterioration or defects, to determine the degree of damage and to evaluate whether the damage will continue. Concrete petrography results always need to be done in context of the larger setting it was collected from.

Reasons for concrete petrography investigations

The root cause of concrete deterioration can often be attributed to quality issues seen at the microscopic level of the material. Common problems requiring petrographic examinations are: surface scaling, delamination, aggregate pop-outs, plastic shrinkage cracking, corrosion of reinforcing steel, low strength test results, blistering, dusting, drying shrinkage cracking, structural cracking, D-cracking, cement-aggregate reactions, discoloration, sulfate or acid attack and pyrrhotite oxidation.

It can also be used to determine depth of carbonation, the bond between the aggregate and the paste and physical influences such as frost damage and depth of fire damage and determination of temperature of the fire.

During the course of a concrete petrographic investigation, various factors are considered. This ranges from the aggregate to the cement paste that constitutes the matrix and the included air voids. In terms of the fine and coarse aggregates, descriptions are done on: aggregate type and composition, the quality of the aggregates, the presence of deleterious aggregates and identification of cement aggregate reactions and the presence of dirty aggregates. When considering the matrix material, descriptions is done on: the cement paste content, water-cement ratio, degree of cement hydration, the presence and relative abundance of fly ash and of slag cement. The final element described is the air void system and entails a description of total air void content, size and spacing of air voids, distribution of air voids and relative amount of entrapped air.

The cause of concrete degradation or failure can generally be attributed to one of three sources. The first is the supplier of the ready mix and could be as a result of inadequate air void system, elevated water-cement ratio, improper setting time or improper mixture proportions. The second most likely cause is the concrete contractor or finisher and may relate to addition of water on site, improper finishing techniques, improper curing or improper joint placement, spacing or timing. The third is the aggregate supplier relates to high proportions of physically or chemically deleterious constituents or poor aggregate gradation.

Concrete Petrography: Basic Method Description

Petrographic testing is mainly focused on the use of optic microscopes to examine samples of aggregate or concrete to determine its mineralogical and inferred chemical characteristics. The general sequence of a concrete petrography investigation usually entails the collection of samples for the petrographic examination from lump samples or cores and a visual inspection. This is followed by the application of a pH indicator.

The samples are impregnated with resin and suitable surfaces, both polished and thin sections are prepared by sawing, lapping and polishing. The samples are examined through a petrological, such as a geological polarising microscope, using either reflected or transmitted light. The light sources range from normal to ultra-violet or polarised light. More detailed examination can then be considered, especially in the case of secondary mineralision and could include, but not limited to, scanning electron microscopy with energy dispersive X-ray spectroscopy. The final findings are outlined in a comprehensive report.

Description types

Aggregate information

Aggregate generally makes up 70% of concrete composite. A petrographic exam of the aggregate in hardened concrete identifies the aggregate type, size, shape and amount to determine if it is within the design specifications. The bond to the cement paste is also evaluated which often correlates with the final concrete mix strength.

Cracking

Cracks are measured, and patterns and sources are identified through a visual and microscopic inspection. The characteristics of the cracks are then compared with typical causes such as drying shrinkage, thermal contraction, plastic shrinkage, settlement, applied loads, chemical reactions and so on.

Secondary deposits

Chemical reactions of the concrete components are detrimental to the integrity of the structure. These reactions result in formation of minerals or deposits which result in expansion and cracking within the concrete. Internal and external sulfate attack, alkali aggregate reactions and chloride ingress can be identified and evaluated best by using scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM/EDS).

Water /cement ratio and porosity distribution

Correlations between the density of the concrete cement paste and the water / cement ratio can be made using fluorescent microscopy and backscattered electron microscopy. These analyses aid in determining whether the material is appropriately dense for its application and specifications, and whether or not it is well mixed. Evaluation of the porosity distribution can also uncover finishing issues at the surface.

Binder type and paste content

The type of binder or cementitious material used in concrete is specifically designed for the specified performance and application it was placed. The paste content is correlated to the cement content in the mix. The area volume of cement paste is estimated or calculated and compared to the typical range for good performing concretes and / or to the design specifications.

Depth of carbonation

Carbonation occurs when calcium in the material reacts with carbon dioxide from the air. By examining how deep the carbonation has penetrated, impacts to steel passivity or surface durability can be determined.


 

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