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					Section 2.11

1

Describe a programme of investigation to determine the condition of a concrete bridge
carrying a motorway over a railway. (20 marks)
..................................................................................
Desk top study - try to obtain the drawings and history of the bridge - determine loadings and
codes used in design.

Visual survey - look for rust staining, cracking etc.

Planning main survey - Calculate what can be done with available budget, posession of rail
tracks and road. If possible establish criteria for results for different outcomes.

Survey methods:
Cover meter
UPV
Crack width
Rebound hammer
Cores
Pull out
Loading
ISAT, FIGG
Potential mapping, resistivity
Phenolphthalein
Chloride contents

2,3 No solutions

4

(a) Describe the procedure which should be followed to ensure that good quality concrete test
cubes are produced on site (8 marks)

(b) If 3 cubes are tested and the percentage defectives is 5% what is the probability of one
cube failing ? (6 marks)

(c) Describe a method which may be used to check the strength of the concrete in a structure
if cube failures have occurred (6 marks)
..............................................................................................................................
(a)
Use steel moulds
Use mineral shutter oil
Use a steel tamping bar




                                                              1
Obtain sample from discharge from mixer, not first or last fraction.
Compact in layers
Cure with hessian and polythene, protect from frost.
Use geated curing tank
Keep accurate records.

(b)
5%  95%  95%  3 = .135

(c)
Methods described could be:
Core tests - describe choice of location etc.
Ultrasonic - describe transducer arrangements
Rebound hammer - describe grid
etc.

5

You are considering carrying out a major refurbishment to the Sir John Laing building (the
University Civil Engineering building). Describe the methods you would use to determine the
condition of the concrete frame. (15 marks)
Along the back of the building where the cars are parked by it you find significant variations in
the rest potential of the reinforcement near ground level. Describe the different possible
causes for this and how you would investigate them. (5 marks)
........................................................................................................
Methods:

Desk study. Find drawings and records. Assess likelihood of HAC or Calcium Chloride in
concrete.

Visual survey. Use binoculars. Check for rust staining etc.

Rebound hammer
Large number of readings on grid. Gives an indication of strength but mainly used to identify
areas for further investigation.

PUNDIT
Gives a better indication of strength. Best used in direct mode but needs accurate
measurement of member thickness (difficult on walls).

Potential survey
Have to cut into concrete to expose steel for electrical connection. Then gives large amount of
data fairly quickly. Will indicate areas where corrosion is possible but not the actual condition
of the steel.




                                                   2
ISAT & Figg
Fairly non-destructive but Figg requires a 12mm drilled hole and ISAT normally needs some
fixing holes and leaves a greasy mark. Thus positions for these tests must be chosen carefully.
They will indicate the likely durability.

Carbonation depth with phenolphthalein. Normally easy to find a spot where the concrete can
be broken off without showing too much. Will give a good clear picture of the progress of
carbonation.

cover meter. This must be used extensively if the structural drawings have been lost. Also very
important to give overall picture of accuracy of cover.

Cores. Essential to take one or two. Positions must be selected (and checked with cover
meter) to do minimum damage. Results are then extrapolated to other area with PUNDIT and
rebound hammer.

Chemical analysis.
May use cores or bits broken off for carbonation tests. Might be used to look for HAC or
chlorides.

(b)
Causes:
Probable: regions of salt contamination from car park
Others: variations in mix design, variations in carbonation due to different exposure, previous
repairs, e.g. surface coatings.

Investigation:
Plot contour diagrams to determine low potential areas. Cut away cover and have a look. Look
for correlation between potential and condition of steel.

6

A concrete structure has been built and a few months after the concrete has been cast ten
cores are taken at randomly distributed locations on it. The cores are tested in compression
and they show four results below the characteristic strength for the concrete.

a. Describe how a decision should be made about the benefit of further testing. [8 marks]

b. If further testing is required describe how it should be carried out. [12 marks]

............................................................................
Candidates are expected to address most of the following points or adopt a valid alternative
approach.
Marking scheme: one mark for each correct point made




                                                3
a.
The following questions should be answered before a decision is made:
1. Is the stated characteristic strength required or has it been specified in order to achieve
durability (or neither) ?
2. Were the bad cores taken from near the top of wall/column pours ? this would give low
results.
3. Were any unusual failures reported ?
4. Did any of the cores have steel or debris in them ?
5. What was the spread of the results ? Are there figures available to show the expected s.d.
for sets of cores taken in this way ?
6. Are the areas from which the bad cores came on the critical path ? Is there time for further
tests ? What could/would be done if low strengths were proven ?
7. Were all of the failed cores from the same part of the structure or were they randomly
distributed ?
8. Does the organization which took and tested the cores have a good reputation / NATLAS
accreditation ?

b.
First the objective of the investigation should be determined, what are the possible outcomes
and what type of results would be required in order to decide on one of them ?
All tests should be carried out at positions on an accurately measured grid in order to get
good statistical results.
If the strength is required as an indication of durability (e.g. as in BS5328) the tests should
measure durability (e.g. ISAT)

NDT is the best way to start, some tests are: [the student should describe them]
Rebound hammer
Ultrasonic
Or, if durability of interest:
ISAT
FIGG
Cover depth
The NDT should be used to plan where to take cores, carry out pull-out tests etc.
7

Describe a programme to investigate the structural condition of an estate of houses with
precast concrete frames which are 40 years old. [20 marks]
……………………………………………………………………………………………………..
Determination of objectives, is it for mortgaging ? what will satisfy the lenders ?
Desk top study, are there any records of construction ?

Visual survey, are there any obvious signs of distress, corrosion etc.

Plan of investigation, detailed survey of one or two and random checks through the rest ? Is
there one in poor repair that could be demolished ?




                                                4
Start with chemical test for HAC (one house only) and calcium chloride.

If possible far best to take some structural members out of a house. Can be cut up for
mechanical tests in lab.

NDT on other houses with, if possible, minimum disruption to finishes. Rebound hammer and
Ultrasonic [student should describe tests].

Cover depth should be measured throughout. ISAT or FIGG in some locations.
Phenolphthalein indicator in several locations will give carbonation depth.

8

Very large variations in the results from cube and slump tests are observed from a particular
site.
a) Describe the possible causes of this (10 marks)
b) State which of these causes would indicate the presence of poor quality concrete in the
construction and outline the methods which could be used to detect it (10 marks)
………………………………………………………………………………………….
a) Variations in slump/ cubes
Variations in supply
Water added on site
Poor sampling
Poor cube manufacture/ bad moulds
Poor curing and despatch of cubes
Poor slump procedure, affected by vibration etc.

b) First two affect insitu concrete
Detect with: Rebound hammer, Ultrasonics, cores.

9

A 55mm diameter core is cut from a structure and a 40mm sample is cut from it. The sample is
tested for permeability by applying a pressure of 10MPa to one end and preventing flow from
the curved surfaces. The test is carried out with water with a viscosity of 10-3 Pas. If a flow of
5ml/hour is observed what is:

a. The coefficient of permeability. [6 marks]

b. The intrinsic permeability. [3 marks]

c. If the sample is re-tested with an oil with a viscosity of 4  10-4 what will the flow rate be ? [3
marks]




                                                  5
d. Why would these tests be carried out ? [8 marks]

..............................................................................
a
Area = 3.1416  55  55/4 = 2376 mm2
Pressure head = 10MPa / (1000  10) = 1000 m
V = (5E-6/3600)/2376E-6 = 5.8 E-7 m/s
k = 5.8 E-7  .04 /1000 = 2.3 E-11 m/s

b
K = 5.8 E-7  1E-3  .04 / 1 E7 = 2.3 E-18 m2

c
V = 2.3 E-18/ 4E-4  1 E7 /.04 = 1.4 E-6 m/s
flow = 1.4 E-6  2376 E-6  3600 = 12 ml/hour

d
Permeability is of interest for:
1. Dam construction
2. Durability

The oil might be used to make the test go faster or (more likely) because it is inert in concrete
(does not promote hydration).

10

On a construction site a significant number of concrete cube failures have occurred. Describe
how you would identify the cause of the failures. [20 marks]
…………………………………………………………

Find out whether it is from the concrete or the testing by examining testing process (casting/
storage/ test procedures) [student should describe] or by carrying out tests on the structure
(Schmidt hammer/ pundit/ cores etc.). In particular should look at spread of replicate results.

If cause is from the concrete check to see:
changes in mix design.
possible batching errors.
poor mixing or excessive times in trucks.
changes in aggregate (or cement).
etc.

11

Describe the main features of a programme of investigation to determine the requirement for
maintenance expenditure on the reinforced concrete components of the University library




                                                                             6
block during the next 10 years. (The library block has a reinforced concrete frame with precast
panels and is approximately 20 years old).
...............................................................................
Desk top study
Find drawings, reports of previous investigations.

Visual survey
Look for and obvious defects or causes for concern

Plan main investigation
Depends on budget constraints etc.
Try to envisage different maintenance scenarios (e.g. do nothing, carbonation coating etc.) and
only do the tests necessary to choose between them.
Decide on method of analysis and presentation of results.

Possible tests [student should describe with particular reference to the library]
crack width.. only possible if visible cracks can be found (probably not)
ultrasonic.. Easy on columns because there is access to both sides but probably not relevant
unless there is reason to suspect low strengths.
cover meter.. Espescially on panels where poor cover is already evident.
(rebound hammer / cores / pull out tests ... unlikely)
ISAT (surface absorption).... Must be done somewhere where the marks will not be seen.
Figg test... must be on hidden surface as for ISAT
Potential mapping... Probably not much use in carbonated areas due to increase in resistivity.
Resistivity... Probably not much use (best used with potential mapping)
Phenolphthalein (most important and relevant test)

12

A major fire has occurred in a reinforced concrete structure and you are required to
determine the extent of the loss of durability of the structure in the event of subsequent
exposure to chlorides and sulphates. Describe how an experimental programme to do this
should be designed. (25 marks)

…………………………………………………………………………………………………..

The aim of the programme will be to measure the transport properties of the concrete and
compare it with unaffected “good” concrete.

Insitu flow tests.

ISAT, FIGG, CAT etc. These can be used to measure transport of gas and water.
Preconditioning of the concrete is important. Many other limitations including depth of
measurement for ISAT and effect of drilling for Figg.




                                               7
Electrical tests.

Potential survey unlikely to work very well because the fire will have calcined the surface and
may have caused carbonation. In any event it is not a good predictor of future corrosion.

linear polarisation needs cast-in probes and there may have been de-bonded by the heat.
Measures current corrosion rate – not future trends.

Resistivity – probably best of the electrical tests but affected by surface calcination.

Lab tests

Permeability tests on cores probably the best test. Adsoroption (or even ISAT) could work
well because surface layer could be cut off.

For sulphates an expansion test would be useful.


Programme

Always compare with control area (half the tests will be control)
Plan the programme in advance.
Always use statistics (e.g. for differences between populations)

13

Discuss how destructive and non-destructive testing may be used together to maximum
advantage in the investigation of a concrete structure. Use examples in your discussion and
describe the tests that you refer to. (20 marks)
.....................................................................................
Examples:

Survey a structure using rebound hammer and then cut cores from the areas giving low results
(and some from control areas). Use a rebar locater to ensure that coring does not cut steel.

Survey using potential mapping and then break out cover to inspect low areas.

Use visual survey to find corrosion product (or cracking) and then use phenolphthalein on
broken surfaces to measure carbonation depth in worst areas.

[each test should be described] eg

MEASUREMENT OF COVER




                                                 8
There are a number of different electronic devices on the market which claim to measure
cover and bar diameter. They all work quite well at locating reinforcement, are reasonable at
cover depth and vary from moderate to useless when measuring bar diameter. They do not
work in areas of highly congested reinforcement or if there is other metal in the concrete, e.g.
conduit or excessive amounts of tying wire.

REBOUND HAMMER TESTS

The rebound hammer (often a "Schmidt" hammer) measures the rebound of a weight from the
concrete surface. If a good number of observations are made over a grid (see the BS) it will give
a reasonable indication of relative strength (it actually measures Young's modulus). A good use
for it is for deciding where to take cores and extrapolating core test results. Note that a hard
surface layer may prevent a rebound hammer from detecting the effect of High Alumina
Cement.

CORE TESTS

Cores are cut with diamond tipped core drills. BS1881 gives conversion factors for obtaining
equivalent cube strengths when the cores are crushed.

        Do not expect to get the target mean.
        The ends of the cores must be ground or capped before testing. The quality of the
result is only as good as the quality of this work.
        Always use plenty of water or the core bit will be damaged.
        If carrying out chemical tests on the core remember that the cutting water will have
washed out some chlorides etc from near the core surface. In the lab this may be avoided by
using oil as the cutting fluid.
        Use a cover meter to locate the steel and try to avoid it. Cutting the steel is bad for the
structure, is slow and wears out the coring bit.

PULL OUT TESTS

There are many of these including the Windsor Probe. In them a stud is cast or shot fired into
the surface and the force required to pull it out is measured. This can give a reasonable
indication of strength (better than rebound hammers but not nearly as good as cores)

14

Describe a programme of investigation to determine the condition of the reinforced concrete
in a road bridge. You should give brief descriptions the tests that you would plan to use. (20
marks)
..............................................................................
Marks will be given for the relevance of the discussion to a road bridge.

PRELIMINARY DESK TOP STUDY




                                                  9
Before any site work is started all available documentation should be studied. Of greatest
interest are mix designs, cube test results and sources of materials.

INITIAL VISUAL SURVEY

It is always worth spending sufficient time for a detailed record of all cracks, spalling,
segregation or movement to be made.

PLANNING THE INVESTIGATION

There are many restraints on the investigation, these include cost, time, access, safety and
damage caused by the testing (e.g. cutting cores). The following principles apply:
    Do not be over ambitious, a few tests clearly analyzed and reported on time are far
       more use than a mass of data.
    Ensure that adequate control areas are used. Almost all of the tests are comparative so
       there must be some results from sound areas to compare with the suspect areas.
    Where possible carry out the tests on a accurately measured grid.
    Try to decide on a method of analysis and presentation before work starts.
    On a road project speed is essential due to the need for lane closures etc.

TEST METHODS (should be described)

Strength tests:
Rebound hammer
Pull out tests
Cores
Load tests

Durability tests:
Cover depth
ISAT
Figg
Potential mapping
Resistivity
Phenolphthalein

15

a. Describe THREE methods for determining the strength of the concrete in an existing
structure. (12 marks)

b. Discuss the differences between the strengths obtained in part (a) and the characteristic
strength of the test cubes made at the time of construction. (8 marks)
...............................................................................................................................................




                                                                      10
a)
The student should describe three of the following:
Rebound hammer
Ultrasonic test
Load test
Pull-out test
Core testing
etc.

b)
No real reason why they should be the same
Differences:
inaccuracies in test method.
Expected strength would be target mean, not characteristic.
Statistical distribution of cube results.
Method of casting and curing cubes is not the same as the structure. - in particular there will
be major variations of strength in structure, e.g. low at top of wall pours.
Platten restraint in cube testing
Cube test is uni-axial

16

A feasibility study is to be carried out on the old university library building to determine the
cost of converting it for an alternative use. The design brief requires that it should not require
further expenditure on maintenance for the following 20 years. Formulate the main features
of a programme of investigation of the concrete components of the building. Justify your
choices. (The old library block has a reinforced concrete frame with precast panels and is
approximately 25 years old). (25 marks)
...............................................................................

Desk top study
Find drawings, reports of previous investigations.

Visual survey
Look for and obvious defects or causes for concern

Plan main investigation
Depends on budget constraints etc.
Try to envisage different maintenance scenarios (e.g. do nothing, carbonation coating etc.) and
only do the tests necessary to choose between them.
Decide on method of analysis and presentation of results.

Possible tests [student should describe with particular reference to the library]
crack width.. only possible if visible cracks can be found (probably not)




                                                11
ultrasonic.. Easy on columns because there is access to both sides but probably not relevant
unless there is reason to suspect low strengths.
cover meter.. Espescially on panels where poor cover is already evident.
(rebound hammer / cores / pull out tests ... unlikely)
ISAT (surface absorption).... Must be done somewhere where the marks will not be seen.
Figg test... must be on hidden surface as for ISAT
Potential mapping... Probably not much use in carbonated areas due to increase in resistivity.
Resistivity... Probably not much use (best used with potential mapping)
Phenolphthalein (most important and relevant test)

17

Describe four methods which are used to measure strength of concrete either in the laboratory
or on an existing structure. For each method give typical uses and discuss the limitations of the
method. (5 marks for each method described.)

..........................................................................

CUBE STRENGTH
(student to describe standard procedure)

Used for all routine quality control on site

Does not give the actual strength of the concrete under load in the structure


TENSILE STRENGTH
(student to describe standard procedure)

This can be measured (with difficulty) directly on "dog bone" samples but is normally
measured indirectly by cylinder splitting

Used occasionally in labs

Limited use because tensile strength not used much in design


FLEXURAL STRENGTH (MODULUS OF RUPTURE)
(student to describe standard procedure)

This is normally measured on 500mm by 100mm by 100mm prisms .

The ends of the failed prism may be tested as equivalent cubes.

Used occasionally in labs




                                                                             12
Limited use because tensile strength not used much in design


ULTRASONIC PULSE VELOCITY TESTING

The general procedure for use is to establish acoustic contact between the concrete and the
emitter and receiver and record the transit time for the pulses. The distance is then measured
and the velocity calculated.

Used on site either when cubes fail or on old structures.

Very sensitive to distance measurement, cracks and moisture in concrete etc.

PULL OUT TESTS

There are many of these including the Windsor Probe. In them a stud is cast, drilled or shot
fired into the surface and the force required to pull it out is measured. This can give a
reasonable indication of strength (better than rebound hammers but not nearly as good as
cores)

LOAD TESTS

These are very expensive but can yield a lot of information. The load applied should be the
working load plus a suitable safety factor. A number of different methods may be used to load
structures, heavy vehicles may be driven onto bridges, buildings may have temporary tanks of
water on the floors or stressed cables through to ground level.
In a load test the deflection is measured throughout loading and unloading. One of the most
important measurements is to check whether the loading results in any permanent
deformation, i.e. whether it has been loaded beyond its elastic limit.


CORE TESTS

Cores are cut with diamond tipped core drills. BS1881 gives conversion factors for obtaining
equivalent cube strengths when the cores are crushed.

The ends of the cores must be ground or capped before testing. The quality of the result is
only as good as the quality of this work.
Always use plenty of water or the core bit will be damaged.
If carrying out chemical tests on the core remember that the cutting water will have washed
out some chlorides etc from near the core surface. In the lab this may be avoided by using oil
as the cutting fluid.
Use a cover meter to locate the steel and try to avoid it. Cutting the steel is bad for the
structure, slow, and wears out the coring bit.




                                              13
REBOUND HAMMER TESTS

The rebound hammer (often a "Schmidt" hammer) measures the rebound of a weight from
the concrete surface. If a good number of observations are made over a grid (see the BS) it
will give a reasonable indication of relative strength (it actually measures Young's modulus). A
good use for it is for deciding where to take cores and extrapolating core test results. Note
that a hard surface layer (e.g. from carbonation) may prevent a rebound hammer from
detecting the effect of High Alumina Cement.

18

Describe methods to measure the following properties of concrete (use diagrams where
appropriate):

a
The compressive strength, using a sample cast for the test. (5 marks)
b
The compressive strength, using a sample obtained from a structure. (5 marks)
c.
The tensile strength (5 marks)
d.
The flexural strength. (5 marks)

..........................................................................................................................
a
A cube of concrete (normally 150mm on site but 100mm in labs) is cast and tested by crushing
on two opposite faces. This test does not pretend to give an accurate simulation of loading in
a structure (for example it is "uniaxial" compression rather than confined "triaxial"
compression) but has been found over 100 years of use to give a good idea of the "quality" of
concrete. The accuracy with which the cube represents the concrete structure will also
depend on the curing of the cube and the structure.

Cubes are the only samples normally made on sites. They are generally struck from their
moulds the day after casting and sent off to a testing lab within a few days.
b
Cores are cut with diamond tipped core drills. BS1881 gives conversion factors for obtaining
equivalent cube strengths when the cores are crushed.

The ends of the cores must be ground or capped before testing. The quality of the result is
only as good as the quality of this work.
Always use plenty of water or the core bit will be damaged.
If carrying out chemical tests on the core remember that the cutting water will have washed
out some chlorides etc from near the core surface. In the lab this may be avoided by using oil
as the cutting fluid.




                                                           14
Use a cover meter to locate the steel and try to avoid it. Cutting the steel is bad for the
structure, slow, and wears out the coring bit.


c
Tensile strength
                   Load P
                                       Hardboard pads

                    Stress                  Concrete test
                                            cylinder
                    =2P                     Length L
 Diameter D
                      lD
                                           Crack
                                           forms here



fig. 2.6.9

This can be measured (with difficulty) directly on "dog bone" samples but is normally
measured indirectly by cylinder splitting (see Fig 2.6.9). BS1881 specifies a rate of stress
increase in the range 0.02 - 0.04 N/mm2/s.

d
Flexural strength (modulus of rupture)


                                  Load P

            L/           L/                L/3
            3            3
                                                      depth d
                                                   width b

                     Span L
 Crack forms in this region
 Flexural stress =      PL
                              2
                        bd
 Modulus of rupture = flexural stress at failure




                                                             15
This is normally measured on 500mm by 100mm by 100mm prisms (see Fig 2.6.10). BS1881
specifies a rate of stress increase in the range 0.06 ± 0.04 N/mm2/s.




                                          16

				
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