NON DESTRUCTIVE EVALUATION FOR ASSESSMENT OF CHEMICAL EFFECTS IN CONCRETE

OPENING… Chemical effects on concrete- where

concrete has been attacked by chemicals, the nature of attack and chemicals need to be identified.

Measures are taken to reduce their effects

Some of the evaluations in this process are…

CHLORIDE CONTENT

The main deicer is NaCl

This effects concrete in two forms; degradation of concrete, corrosion of embedded steel

1)Attack of deicers

EVALUATION

The tests for this chemical effect are rapid chloride penetration test and, Standard Test Method for Scaling Resistance of Concrete Surfaces Exposed to Deicing Chemicals.

DETECTION The degree of

change in color of the tip is proportional to the chloride content.

2) INFRARED THERMOGRAPHY Capable of

detecting delaminating

Sensitive detection of temperature difference is the main principle

Also detects water leakages, moisture blocks, carbonation depth.

3) QUANTAB TEST Measures the

chloride content in concrete

A solution of 5 gm of powdered concrete is prepare first

It is then tested for extent of chloride content in chlorimeter

CARBONATION TEST

  Carbonation of concrete occurs when the carbon dioxide, in the atmosphere in the presence of moisture, reacts with hydrated cement minerals to produce carbonates, e.g. calcium carbonate.

Acid base indicators Petro graphic analysis X-rays Infrared thermography

ACID BASE INDICATORS Phenolphthalein

solution is sprayed on freshly exposed concrete surface.

Pink colour indicates highly alkaline good concrete, while no change in colour indicates the carbonated portion.

PETRO-GRAPHIC ANALYSIS Petrography is a

term known from geology meaning, "the systematic characterization of rocks in hand specimen and thin section".

The hardened sample of concrete is cut, ground and polished.  The polished surface is then examined with a microscope.

Fully carbonated paste in the concrete surface. Carbonated paste appears orange-brown in crossed polarized light.

X-RAY FLUORESCENCE ANALYSIS X-ray fluorescence is the

emission of characteristic X-rays from a material that has been excited by bombarding with high-energy X-rays. 

Under radiation, the sample will emit characteristic X-ray intensities depending on characteristics of the beam, powder particle

size distribution, degree of compaction, and the compounds in the matrix.

ALKALI AGGREGATE REACTION• In most concrete, aggregates are more

or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of many years. • This alkali-aggregate reaction has two

forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).

ALKALI SILICA REACTION

Alkali-silica reaction (ASR): is of more concern because aggregates containing reactive silica materials are more common.

In ASR, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment.

These gels can swell and induce enough expansive pressure to damage concrete 

Reactive fine aggregate particle with alkali-silica gel filled air voids as viewed in thin section.

ALKALI SILICA REACTION.ALKALI SILICA REACTION

PETROGRAPHIC ANALYSIS The best technique for the

identification of ASR is the examination of concrete in thin section, using a petrographic microscope.

 Alternatively, polished sections of concrete can be examined by scanning electron microscopy (SEM);

this has the advantage that the gel can be analysed using X-ray microanalysis in order to confirm the identification 

CONTD..

Alkali Silica Reactions (ASR)

Concrete thin-section, viewed with a petrographic microscope, showing a aggregate particle (at the right of the image) from which alkali-silica gel has extruded into adjacent cracks. 

ALKALI CARBONATE REACTION

Alkali-carbonate reactions (ACR) observed with certain dolomitic rocks.

• Dedolomitization, the breaking down of dolomite, is normally associated with expansion.

• This reaction and subsequent crystallization of brucite may cause considerable expansion.

• The deterioration caused by ACR is similar to that caused by ASR.

UAF METHOD

The current method for identifying ASR gel uses ultraviolet fluorescence to image the distribution of a uranyl acetate stain that has a high affinity for the silica gel.

A suitable source of ultraviolet light is used. Apply uranyl acetate solution to newly

exposed surface of concrete. Allow the solution to react for 3-5 minutes with

anyreactive silica gel that may be present on the surface.

CONTD.. Position the sample

under the UV light and observe through the viewer

Record the presence of any yellowish-green fluorescence indicating the presence of silica gel.