Testing and Monitoring of Civil Structures
This unit addresses aspects of the following syllabus outcomes: A student:
H1.2 differentiates between properties of materials and justifies the selection of
materials, components and processes in engineering
H2.1 determines suitable properties, uses and applications of materials in engineering
H4.1 investigates the extent of technological change in engineering.
Extract from Engineering Studies Stage 6 Syllabus © Board of Studies NSW 1999.
Civil structures such as dams, skyscrapers, warehouses, roadways and bridges
are common engineering and construction projects.
Because many civil structures are large, subject to high stresses, and used by
the general public their structural integrity requires testing and monitoring on a
regular basis. As well as the mass of a structure creating stresses in its footings
other stresses are set up in response to other external forces (wind pressure in
the case of high rise buildings, and water pressure in the case of dams).Testing
and monitoring can include scheduled visual inspection, monitoring devices that
are checked periodically, monitoring devices that are checked when a fault is
suspected, and wireless devices that communicate their results to remote
headquarters.
Testing during construction
Concrete slump test.
The main issue with premixed cement is the need for sufficient water to be
added to the mix to ensure that the chemical reaction within the Portland
cement can be completed. If there is too little water, the concrete slurry will not
form into the corners and around the reinforcing materials and reactions may be
retarded. If there is too much water the resulting concrete will have reduced
strength and may also flow out of any cracks in the formwork, leaving the
aggregate behind with little bonding.
To ensure an appropriate amount of water is present in the slurry, a slump
test is carried out once the concrete is on-site. The test consists of filling a
standardised conical shaped vessel in a well-disciplined manner (filling and
tamping in a certain time) and then removing the cone and measuring how
much the wet concrete collapses or “slumps”. The more water in the mix the
more the material will collapse and the greater the difference from the original
height.
Recently an Australian Standard has been developed to cover general purpose
applications. The allowed slump is 100 mm. This allows for acceptable cured
strength and pouring flow.
Go to YouTube for a detailed video on conducting a concrete slump test
.
Reference marks
Before a site is ready for construction various surveys must be carried out.
These are designed to determine the project contours, underlying rock strata,
porosity and soil chemistry. In some cases datum marks are used to record
important physical reference points.
With the introduction of satellite imaging and Global Positioning Satellite (GPS)
technology on large projects, grid references and datum points can be checked
and positioned with great accuracy over large distances without the need of
having many intermediate points. However, the theodolite, laser level and
dumpy level are still used for laying out and checking reference points within the
boundaries of a project.
Go to YouTube for a detailed video on using a dumpy level
.
A detailed video on using a laser level can be found at:
http://www.wonderhowto.com/how-to-use-laser-levels-for-concrete-forming192435/
Laser guidance with GPS technology can be used to ensure that the correct slope
of gradients is maintained. The blade of this grader is coupled to a laser detector
and this records whether the blade is too low or too high and warns the operator
of any error. The laser signal is aligned from a reference point and a computer
performs the calculations.
Testing in use
Australian Standards are used to ensure that building materials such as
reinforcing mesh, waterproof membranes, structural beams and any other
structural components perform to a certain Standard. Designs and calculations
for new structures are based upon these specifications.
However once the structure is in use, many unforseen events may impact on the
integrity of the structure. Because of this, procedures and testing devices must
be used to ensure that unexpected changes are identified and supervised.
Video cameras
Often cavities and pipe work needs to be monitored to ensure they are in good
order. Video cameras and lights are able to be lowered or guided along places
which are impossible for a person to enter to evaluate structural faults and
monitor repair procedures.
Often friction, water leaks and voids can create temperature differences in
structures. Thermal imaging cameras which can detect these differences can be
used to target areas requiring further investigation.
Reference marks/points
Once a site has been surveyed, datum points in the way of wooden pegs and
brass buttons are often left in place and used to monitor the relative positions
during the service life of the structure.
Testing Dam Structures
There are many different methods used to test dam integrity. In Australia dam
safety is reliably monitored in that we have had few dam failures, those that
were in danger of failing have been repaired before any major catastrophe.
Other places in the world have not been as attentive and loss of life and property
has been major. There is on average one major dam failure in the United States
each week. This, at the very least wastes a lot of stored water and causes major
damage to the dam, at worst it floods towns and worksites without warning and
causes loss of life and creates hardship.
Camara Dam, Brazil, June 04
Visual inspection
Seepage measurement.
This is done by measuring the seepage from a trench at the base of the dam
wall. The rate is monitored and any variation is noted and investigated.
Electronic Monitoring
MicroStrain
is a website dealing with modern bridge monitoring methods.
When further investigation and monitoring has been required, a hole can be
drilled into the dam wall or substrate and a piezometer can be introduced to
record water levels. One type is the Vibrating Wire Piezometer. As the water
pressure pushes against the diaphragm the “tone” of the wire changes much like
a guitar string. The pickup coil can send the data to a computer where remedial
action can be organised.
Drilled hole for monitoring with a piezometer.
CONCRETE AS A COMPO SITE MATERIAL
Concrete on its own is good at withstanding compressive forces, but can only
withstand about 10% of the same force in tension. In a simple system where
only compression forces act then concrete could be a satisfactory material.
Unfortunately there are no simple systems in real life and many different forces
come into play. Because of this even a load sitting on the ground may have to
deal with tensile, torsional and bending forces which would cause the concrete to
fail.
With the addition of steel as part of a composite (i.e. the concrete and steel are
bound intimately together) the reinforced concrete benefits from the tensile
strength of the steel.
Reinforced concrete
Concrete is poured around steel reinforcing mesh or rod at the construction site.
The steel is placed on the tension side of the slab so that it can resist tensile
stresses. The bulk of the concrete is on the compression side of the slab to resist
the compressive stresses
Pre-tensioned concrete (pre-stressed concrete)
Concrete is poured around reinforcing bar or mesh which is being held in elastic
tension. This process is often performed off-site with the completed components
then being transported to site for assembly.
Post tensioned concrete (segmented)
Pre-stressed concrete sections post tensioned with steel tendons
Chopped strand stainless steel
To enable complex shapes to be poured without the need for installing steel
mesh, chopped strands of stainless steel are mixed in with the concrete slurry
and once the concrete has cured the interlocking strands give the concrete mass
a high degree of tensile strength.
Many civil structures are constructed using concrete materials because of their
ability to be moulded to create complex shapes and their ability to withstand
weathering with little surface coating.
WHAT IS CONCRETE?
Concrete is a composite material made from Portland cement, sand (fine
aggregate),aggregate and water.
How concrete works
Portland Cement is a fine, manufactured powder which when added to water
forms an adhesive gel which cures and hardens as a result of an exothermic
chemical reaction.
The curing causes a marked, fast-acting shrinkage in volume. Because of this it
is desirable to have an inert, tough filler material, aggregate, to reduce this
shrinkage and add to the bulk of the concrete.
The Portland cement powder reacts with water to form tobermorite gel, and
smaller amounts of other complex compounds.
Read about the structure and components of hydrated gel
.
This gel begins to harden as the water combines with the cement powder. The
tobermorite gel makes up the greatest proportion and provides the major
cementing compound. The other compounds resulting from the reaction with
water (hydration) also play an important role.
Activity 1
Draw and label a micrograph showing the microstructure of a plain concrete
sample.
Activity 2
Have a look around your local area and on your way to school to see evidence of
faults which have developed in the various concrete structures. These can be
paths, bridges, stairs, station platforms. List and sketch some of the problems.
Do you see any evidence of repairs having been made to previous faults?
Activity 3
What is the significance of elastic tension in prestressed concrete reinforcing?
Activity 4
What is the meaning of the following terms






Exothermic chemical reaction
Endothermic chemical reaction
Elastic tension
Seepage
Slump
Composite material
TESTING AND MONITORING OF CIVIL STRUCTURES - SUGGESTED
ANSWERS
Activity 1
Draw and label a micrograph showing the structure of a plain concrete sample.
Activity 2
Have a look around your local area and on your way to school to see evidence of
faults which have developed in concrete or brick structures. These can be paths,
bridges, stairs, station platforms. List and sketch some of the problems. Can you
see any evidence of repairs having been made to previous faults?
The roots of the tree growing near the fence have expanded the soil under the fence and the
mortar has cracked apart. There is one brick cracked in the third top row. The mortar is
weaker than the fired clay bricks.
Activity 3
What is the significance of elastic tension in prestressed concrete reinforcing?
The steel has not been permanently deformed and is still trying to spring back to its original
length. This allows for the concrete to be placed under compression even before a load has
been applied. Any tensile load has to overcome the elastic nature of the steel before the
concrete needs to accept the load.
Activity 4
What is the meaning of the following terms






Exothermic chemical reaction
Endothermic chemical reaction
Elastic tension
Seepage
Slump
Composite material
Exothermic chemical reaction
The ability of two or more substances to combine chemically and produce a net loss of heat.
In other words if you combine cement powder and water in a bucket you would find after a
short time the mass of cement would get warm.
Endothermic chemical reaction
The opposite of exothermic reaction, the substances would react and absorb heat from the
surroundings.
Elastic tension
Where a material stretches, but does not break any bonds and so can return to its original
shape. In metals the crystal lattices will deform to a limited extent without any slip occurring.
If the load is removed the lattices will spring back to their original positions.
Seepage in dams
The slow movement of water that occurs in dams due to small cracks and structural
imperfections. Whilst it remains within certain parameters it is unavoidable but acceptable.
Once it exceeds the parameters it may indicate future structural weaknesses or impending
failure.
Slump
The physical collapse or change in shape of a volume of concrete "slurry" once the mould has
been removed. This may occur in a "slump test" when the cone is removed. It may also occur
where the formwork allows concrete to leak out of formwork joints.
Composite material
A material made up of two or more different materials with complimentary characteristics. In
this way the composite material can be designed to benefit from the strengths of both
materials by overcoming any weaknesses from either material.
OTHER RESOURCES
Dams Safety
is the website for the Dams Safety Committee.
Go to YouTube for a video on the monitoring of reinforcing and cables
Go to YouTube for a video on Post Tension Concrete Construction
.
.
The NSW Board of Studies provides support and resources about the Sydney
Harbour Bridge .
New procedures
Go to YouTube for a video on Concrete-Jet Robot
.
Go to YouTube for a video on GPS Assisted Excavator - Topcon MC
.