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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.
A detailed video on conducting a slump test can be found at:
http://www.youtube.com/watch?v=iOBK6NGfMtE 
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.
A detailed video on using a dumpy level can be found at:
http://www.youtube.com/watch?v=L54T4uvpMTg
A detailed video on using a laser level can be found at:
http://www.videojug.com/webvideo/how-to-use-laser-levels-for-concrete-forming
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.
Thermal imaging
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
A website dealing with modern bridge monitoring methods:
http://www.bluerr.com/applications/civil.php
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 composite 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.
A table showing the various components of the hydrated gel can be found at http://www.epcserver.com/Structural/Analysis/concrete/info_hydration.asp
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.
Draw and label a micrograph showing the microstructure of a plain concrete sample.
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?
What is the significance of elastic tension in prestressed concrete reinforcing?
What is the meaning of the following terms
- Exothermic chemical reaction
- Endothermic chemical reaction
- Elastic tension
- Seepage
- Slump
- Composite material
Other resources
Dam Safety web site
http://www.damsafety.nsw.gov.au
Monitoring of reinforcing and cables
http://www.youtube.com/watch?v=g1ex9pg8_Uc
Post tensioned building
http://www.youtube.com/watch?v=7JsuNg5r4Is&NR=1
New procedures
Concrete Jet Robot.
http://www.youtube.com/watch?v=TyOgDlUWfFE
GPS guided excavating machinery.
http://www.youtube.com/watch?v=W7-6v_RpZZQ&NR=1
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