PETROGRAPHY AND CHEMICAL INVESTIGATION OF CONCRETE STRUCTURES

 

What is Petrography?

“Petrography” is a branch of geological science, and it refers to the “study of rocks” as carried out by microscopic techniques. Having been found to be very effective in the diagnosis of rocks, these techniques have been extended to various man-made materials including “concrete”.

In advanced countries in particular, “Concrete Petrography” is regarded as a very powerful investigative tool in evaluating and ascertaining the performance of concrete. In many countries, it is extensively used to diagnose concrete deterioration.

 

What is meant by “Chemical Investigation of Concrete”?

It is widely known that due to environmental exposure concrete is amenable to multifarious chemical, degradation such as carbonation, chloride attack, sulphate penetration, acid attack, alkali-silica or alkali-carbonate reaction, etc.

The specific causes of such chemical degradation of concrete can be identified only with the help of

systematic chemical investigation of judiciously taken core samples from concrete structures, preparation of micro samples from such large core samples, and adopting highly skilled analytical techniques. Such chemical investigations are often supplementary to the petrographic studies of concrete structures.

 

When do you perform such specialised investigations?

Petrographic examinations are typically performed where there are some problems during construction, or on the premature failure of an existing structure. Petrographic examinations are also performed as part of a condition survey to determine if a concrete structure needs to be repaired.

 
Microscope photo of air entrained, homogeneous concrete   Alkali Aggregate Reaction: Amorphous ASR gel which has
cracked due to expansion

What are the common problems requiring petrographic examination and chemical investigation?

Petrographic examination and chemical investigation are required to determine the:

  • Difference between the types and qualities of fine and coarse aggregate
  • Presence of deleterious aggregates
  • Aggregate reactivity of cement
  • Total air-void count
  • Bonding quality of coarse aggregates and cement paste
  • Condition of wearing surface concrete
  • Causes of low concrete strength (test cylinders not meeting the strength)
  • Extent of corrosion of reinforcement steel.
  • Causes of cracking distresses
  • Presence of discolouration
  • Sulphate attack effects
  • Extent of carbonation
  • Depths of chloride diffusion
  • Extent of fire damage in concrete structures

Concrete Microstructure with large crystals of
Calcium Hydroxide and large pores

What are the broad outcomes of such investigations?

Through microstructural and chemical examination,
you can get the following perspectives, which no other
techniques can reveal:

  • The current condition of concrete
  • The causes of distress in concrete
  • Whether the deterioration will continue
  • The amount of damage to be expected in continuing deterioration.

Compact microstructure of concrete with no gaps
between the aggregates and the cement paste

What kind of laboratory do you need to consult for petrographic and chemical investigations?

In addition to possessing the commonly used non destructive and partially destructive investigational facilities such as rebound hammers, ultrasonic pulse velocity metres, corrosion analysers, Infra red Thermography, core-cutting machines, etc. the laboratory must be equipped with Optical / Electron Microscopes, specialised sample preparation facilities including concrete sawing, grinding and polishing, chemical extraction and analytical facilities like Specific lon electrodes, Rapid chloride penetration test, (RCPT) etc. Needless to say, the equipment alone will not yield good results if the commensurate expertise is not available in the investigative laboratory.

 

 

Dr. Fixit Institute of Structural
Protection and Rehabilitation,

C/o Pidilite Industries Ltd.,
Ramkrishna Mandir Road, Andheri (E),
P.O. Box No. 17411, Mumbai 400 059.
Tel : 022-2835 7666 / 7822 / 7149
Email : info@drfixitinstitute.com
Visit us at : www.drfixitinstitute.com
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