R. R. Rakesh and A. Shirole

BARC- IAEA Regional Training Course

on

Development of a Near Surface Disposal Facility

Mumbai, India

February 15-19, 2010

Condition Monitoring of Near Surface Disposal Module Structures

L-12

Condition Monitoring

Condition monitoring or health monitoring of existing structure is evaluation of its integrity with respect to available strength and serviceability.

Condition Monitoring for RCC Structures

FOR BUILDINGS

• Residential

• Industrial

FOR WASTE DISPOSAL

AND STORAGE

STRUCTURES

• RCC Trench

• Tile Holes

NSDF Disposal Modules

Earth Trench

RCC Trench

Tile Holes

Types of monitoring

Defect oriented - To identify the cause of unhealthy symptoms in structure or failure in serviceability

Routine - As recommended by DSRC for every 10 years

Type of disposal modules

Earth Trench RCC Trench Tile Holes

Condition Monitoring Steps

Documentation (Step – 1)

•Identify the history of structure i.e. codes, standards and design criteria, drawings, etc.

•Static and dynamic loadings

•Identify the type or grade of materials used that time

•If defect oriented, root-cause analysis

•If routine, identify susceptible locations of defects

Visual inspection (Step – 2)

Non destructive and partially destructive testing (Step -3)

•USPV test and Rebound hammer test

•Checking Possibility of corrosion of reinforcement

•Core tests

•Carbonation test, etc

Laboratory tests (Step -4)

•Soil - (Grain size distribution, Atterberg Limits, Soluble salts and pH, etc.)

•groundwater – pH, major ions present in the groundwater

•Concrete core – UCS, pH, soluble salts, and mineralogical cement hardening products using XRD and SEM)

Analysis of results and defect diagnosis, if necessary (Step -5)

Decisions and recommendations for (Step -6)

•Repair and strengthening

•Preventive methods for future

•Quality assurance for new structures

NDT Tests – Rebound Hammer Test

• Rebound Hammer Principle: It works on the principle of dynamic impact in which when a body is hit by a moving object, the rebound energy is proportional to the surface stiffness, strength and static inertia of the body.

In this the rebound hammer is triggered to hit the RCC concrete surface

through a steel plunger in contact. The rebound energy is proportional to

strength of concrete.

This test give fair idea of strength of concrete near surface.

Sometimes, the test reading may be misleading because of the presence

of a harder stone aggregate just at the face of concrete and hence number

of readings should be taken at and around a particular location to get the

average strength at the location of the member.

Site RC Trench Range of compressive

strength (N/mm2)

Average Compressive

strength (N/mm2)

Site -1

Old (No. 1)

New (No. 11)

18.26 to 36.32

9.0 to 42.21

26.19

30.10

Site -2 Old (No. 19A)

New (No. 38P)

18.68 to 25.2

18.68 to 27.86

20.54

22.85

Site -3

Old (No. E1/E2)

New (No.

GG5/HH5)

21.35 to 29.46

20.9 to 36.49

26.47

25.58

Rebound Hammer Test - Results

Tests were performed as per IS: 13311 (Part-2), ASTM C805

NDT Tests - Ultrasonic Pulse Velocity Test

• Principle: The velocity of ultrasonic pulses travelling through a solid material depends on the elastic properties of the material.

• The elastic properties are an indication of the strength of concrete.

• Thus the pulse velocity gives an indirect indication of strength and elastic properties of concrete which can be used to assess its quality.

• Since there is no unique correlation between the velocity and strength of the concrete of different mix proportions, this technique is used more as a qualitative test than as a quantitative test.

density

ratio spoison' dynamic

elasticity of modulus dynamic E

where

211

1Ev

IS 13311 (Part 1): 1992 velocity

criteria for concrete quality grading

Pulse velocity by

cross probing (km/s)

Concrete quality

grading

> 4.5

3.5 – 4.5

3.0 – 3.5

< 3.0

Excellent

Good

Medium

Doubtful

Ultrasonic Pulse Velocity Test - Results

Site RC Trench Quality of concrete

Site 1 Old (No. 1)

New (No. 11)

Good to excellent

Good

Site 2 Old (No. 19A/20A)

New (No. 38P)

Good to excellent

Good

Site 3 Old (No. E1/E2)

New (No.

GG5/HH5)

Excellent

Fair

Different methods of Ultrasonic testing

NDT Tests – Half-cell Potential Method

Instrument used: Multi-cell Surveyor

It is a quick, automated instrument

for carrying out potential surveys to

locate regions of corroding

reinforcement non-destructively.

The potential difference is

measured by comparison with a

stable potential source, called a

reference electrode placed on the

concrete surface.

By this method, it is possible to locate

regions of corroded steel, but rate

and/or extent of corrosion can’t be

quantified.

Multi-cell surveyor

Half-cell Potential Method Test results

Potential (E) Probability of

Corrosion

< -350 mv > 95%

-350mv < E < -200mv ≈ 50%

> -200 mv < 50%

Correlation between the

probabilities of corrosion and the

measured half cell potential (E)

•Empirical correlation

•derived from a large number of

measurements at actual sites and

laboratory

•presence of any foreign material (dust,

grease, plastic etc.) on the

reinforcement surface, may affect the

results

Location Half cell potential (mV) Prob.

corrosion Individual value Range

0.83 m

below roof

top

210, 205, 195, 200, 180, 195,

200, 185

180 -

210

05 %

1.16 m

below roof

top

215, 210, 200, 210, 190, 195,

185, 160

160-

215

05 %

1.68 m

below roof

top

250, 232, 250, 235, 207, 215,

232, 257

207-

257

50 %

1.8m

below roof

top

277, 270, 290, 295, 265, 322,

280, 260

260-

322

50 %

Core Test

Instruments for core drill

Core Test – Performed to determine the actual strength of concrete.

Core is extracted from concrete structure and Unconfined Compressive Strength (UCS) is find out by direct compression

Outcome: actual strength of concrete but can not perform more in number

Used to verify the values obtained from other tests, if necessary

Limitations: Only limited number of tests

Carbonation Test

• Carbonation - Reaction between carbon dioxide in air and the alkalis (lime) in concrete.

• Concrete is a highly alkalinity material (pH >= 12) carbonation reduces alkalinity of concrete (to ~ pH9), leading to depassivation of steel reinforcement and consequently enhances the probability of corrosion to take place

• Highest rate at relative humidity of around 70%

• Cores were subjected to depth of carbonation (Phenolphathalene test) using 5% solution of Phenolphathalene in ethyal alcohal. Solution was brushed along length of core. Transparent solution turns to pine wherever there is carbonation.

Laboratory Tests

Soil

• Grain size distribution

• Atterberg limits (Plasticity)

• pH

• Soluble salts

Concrete

• pH

• Soluble salts

• alklanity

Groundwater

• pH

• Soluble salt concentration

Analysis of Results

NDT tests (rebound hammer, ultra sonic pulse

velocity, corrosion probability)

Laboratory tests (on soil,

groundwater, construction materials)

Partially destructive tests

(core carbonation)

Conclusion

strength of concrete of various RCTs at three NSDFs is well above the design strength and the quality of concrete in trench is still very good after 35-45 years

Concrete samples from sites are still having moderate level of alkalinity, indicating resistance to corrosion.

Probability of corrosion of reinforcement was found to be negligible in new RCTs. In old trenches at few sites increase in probability of corrosion with depth was noticed. Physical examination of re-bar has shown that the extent of corrosion is negligible

Repairing of the RCC Trench Repair of Minor/Macro Crack

Materials:

1. Crack bridging/filling materials based on polymeric (epoxy and epoxy phenolic (IPN)) systems : • crack width is ranging from 1 mm to 10 mm then IPN based repair

mortars can be used as crack bridging materials.

• If the cracks are on the surface then also this can be used as crack filling material.

• On badly eroded surface of concrete a primer coat of IPN system is to be given and after curing of the primer for 5 to 6 hours crack filling material should be applied

2. Crack bridging/filling materials based on polymer modified cementations

systems:

• repairing small cavities, cracks and restoration of deteriorated concrete

structures

• If these are to be used for structural repair then as per condition and

requirement structures, some reinforcing materials are to be used with

these repair materials. Reinforcing materials may be glass fiber cloth or

steel sections/rods as anchoring material etc.

Repairing of the RCC Trench

Application of Crack Bridging/Filling Materials Surface preparation (Step – 1)

must be free from all the bond inhibiting materials (loose concrete, dirt, oil, grease etc.)

remove the grease or oil with the help of solvent and then clean the surface either by sand blasting or water blasting.

If these are not possible then remove the loose material with the help of wire brushes

Before the applications of repair materials, substrate must be either dry or damp but not dripping wet

Application (Step -2)

After surface preparation cracks/cavity should be repaired properly by skilled person. The surface should be made smooth by drawing a flat over the surface. This breaks and removes the air bubbles and promoting the proper adhesion to the joint site.

Repair material should be protected from rain and other adverse conditions up to 16 hours after the application.

Final treatment (coating of water proofing/sealants) should be given after 16 hours curing of the repair mortars.

Only small batches should be made on the site which can be consumed within 2 hours

Protective coating(step -3)

For getting the best results of the repaired structure, a protective coating of IPN primer should be applied on the leveled surface. After curing it for 7-8 hours, intermediate coat should be applied. After curing the intermediate coat for 24 hours, a coating based on aliphatic polyurethane should be applied.

• Rigorous work on latest and oldest RCTs have been

done for condition monitoring of RCTs. Results

showed that even 40 year old trench is performing

well presently.

• This results gives confidence that these structure

will see its designed service life with periodic

maintenance.

Conclusions of Condition Monitoring

Closure of Near Surface Disposal Facility

Closure: Systematic action that is conducted after the receipt of waste ceases and waste emplacement operations have been completed with the intention of providing a final configuration for the disposal system.

Aim: disposal system should isolate harmful constituents for a sufficiently long

period so that risk to future generation and ecosystem become acceptable.

Concepts and Role of Closure

After closure, the site is put under supervision for an institutional control period to:

•Prevent intrusion into the repository;

•Prevent removal of, or interface with the radioactive waste;

•Confirm the satisfactory performance of the repository by monitoring; and

•Perform remedial actions, if necessary.

Closure Plan

•Roles and responsibilities for organizations involved with closure and post closure care of the facility;

•A detailed description of the closure method;

•An updated safety assessment for the facility;

•Monitoring and surveillance plan;

•Description of record keeping and record prevention system;

•Long term controls that will be implemented during the post closure phase

Factors for Closure Plan

Regulatory requirements: demonstrate that after the

closure the system will provide the adequate safety for radiation

protection workers, the public and protection to the environment.

Scientific and technical considerations: • quality assurance,

• waste inventories and characteristics,

• site characteristics etc.

Socio-economic considerations • Health, safety and environmental impact assessments

• Contamination control measures,

• Control of access to the site,

• Impact on local employment,

• Visual appearance of the site after closure,

• Post-closure monitoring surveillance and land use restriction

Performance Requirement and Criteria

• permeability of the top of the disposal modules (cap) should be as low as possible

Impermeability

• the waste form, cap and the disposal modules should be designed to retain the structural integrity

Integrity

• waste form, cap and disposal structures are designed to prevent degradation due to external forces

Resistance to degradation

• designed to quickly locate where a cover has failed or any subsidence has occurred Reparability

Caps

It is the minimum requirement for the closure of the disposal facilities

Purpose

to prevent rainwater infiltration

to prevent exposure of the

waste by erosion

to prevent waste-human

interaction

Types of Caps

Resistive barrier

The resistive barrier: It is based on low permeability material such as compacted clay or geosynthetics clay linear that is designed to reduce / stop ingress of rainwater into the repository.

Conductive barrier

It diverts the water away from the waste. The barrier consist of a highly permeable material such as course gravel, followed by fine-grained material and waste emplaced in disposal modules.

Types of Caps

Infiltration control barrier

based on the principle of enhanced run-off as a means of controlling the infiltration and subsequent water percolation to the waste. By using these infiltration can be reduced through enhanced run-off followed by impermeable cover with drainage channels and little amount of vegetation the probability of water percolating to waste is minimized.

Vegetated soil cover

Vegetative soil cover play an important role in removable soil moisture by evapo-transpiration along with protection of soil erosion. In arid and semi-arid environments such covers may remove up to 100% of the moisture that infiltrate into the cap soils.

Types of Caps

Multi-layer cap design consists of a rock or

vegetation top layer followed by capillary

barrier, drainage layer, and compacted low

permeability soil layer followed by gas vent

layer .

Parameters required to design the

Multilayer cap:

i. effective permeability of the multilayer

system

ii. durability study of the materials used in

multi layer (geotextiles, geosynthetics

etc.)

iii. slope stability analysis

iv. erosion studies

v. subsidence study

Multi-layer cap

Cut-off Walls

Purpose: provided to prevent lateral ingress of groundwater into a disposal facility or egress of leachate •These walls may be placed during construction of the facility or constructed at a later stage to upgrade the existing facility

•They are generally constructed of either reinforced concrete or soil and bentonite or cement and bentonite

Drainage System

Approaches • drainage system for surface run-off water

• drainage system to remove water away from the disposal units using sloped cap and drainage channels

• drainage system to takeout the leachate away from the disposal facility

Markers

• permanent markers should be used to indicate the presence and characteristic of disposal facility

• markers are provided to protect the future population form the risks associated with inadvertent intrusion into disposal facilities

Materials used in Closure System

Vegetation

Clay

Cement Concrete

Cement based grouts

Institutional Control