non destructive evaluation for assessment of chemical effects in concrete
DESCRIPTION
This presentation focuses on how to evaluate the cause of chemical attack on the concrete. This is a part of Non-destructive testing.TRANSCRIPT
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NON DESTRUCTIVE EVALUATION FOR ASSESSMENT OF CHEMICAL EFFECTS IN CONCRETE
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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…
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CHLORIDE CONTENT
The main deicer is NaCl
This effects concrete in two forms; degradation of concrete, corrosion of embedded steel
1)Attack of deicers
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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.
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DETECTION The degree of
change in color of the tip is proportional to the chloride content.
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2) INFRARED THERMOGRAPHY Capable of
detecting delaminating
Sensitive detection of temperature difference is the main principle
Also detects water leakages, moisture blocks, carbonation depth.
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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
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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
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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.
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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.
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Fully carbonated paste in the concrete surface. Carbonated paste appears orange-brown in crossed polarized light.
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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.
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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).
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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.
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ALKALI SILICA REACTION.ALKALI SILICA REACTION
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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
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CONTD..
Alkali Silica Reactions (ASR)
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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.
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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.
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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.
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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.
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