On March 15, a 174 foot long span of the recently erected FIU Sweetwater UniversityCity pedestrian bridge in Florida collapsed onto the Tamiami Trail.
The collapse sent 950 tonnes of concrete and steel plummeting onto the road below, resulting in eight vehicles underneath being crushed, eight injuries and six fatalities – one bridge worker and five vehicle occupants.
Whilst investigations into the cause remain ongoing (the National Transport Safety Board released a preliminary report into the accident in May), the incident underscores the importance of quality control in major building and civil infrastructure projects.
One important area in this regard involves having the concrete used in these projects tested against the appropriate standards and specifications. That raises questions about the types of testing involved, what should be done with the results and strategies which engineers, construction contractors and clients should adopt in relation to having their concrete tested.
For answers, Sourceable spoke with three respected industry sources: Greg van Rooyen, technical director – materials technology at civil engineering firm GHD; John Franceschini, director, laboratory manager and principal scientist at chemical and forensic analysis and testing services firm Sharp & Howells; and Hamish McDonald, concrete manager Newcastle/Hunter Valley at engineering, geotechnical, environmental, materials testing and project management services firm Coffey.
Primarily, each of the above sources said, concrete testing is about quality control and gaining assurance that the material being used conforms to the required standards and specifications.
To be used, concrete must meet the requirements of the National Construction Code (NCC), relevant Australian standards which are referenced under the NCC and any specifications which are given by clients, architects, engineers or builders. Important Australian standards in respect of concrete include AS 3600 (concrete structures) and AS 3700 (masonry structures). Methods for testing concrete are outlined in AS 1012.
“Quality control for any structure is critical to the performance of that structure – whether it’s a bridge, a high-rise building or a domestic house,” Franceschini said.
“That’s why we are testing. We are testing to ensure that the materials being used on the site are adequate for the construction process and in accordance with the specification of the standards that are relevant.
“A lot of things can go wrong from the supply of raw material through to the batching of the concrete through to the placement of the concrete on the site. The only way to monitor that is to conduct testing.”
McDonald says testing is not so much about what could go wrong but rather gaining assurance that your road, bridge or high-rise building will perform to the standard.
“You’ve got to make sure that what you are getting is right for the job,” McDonald said.
“If you don’t test it, you can’t be assured of the quality of the product that you are getting. By testing, you are assuring the relevant road authorities (when dealing with road projects in their respective states) and the people using the end product of the quality of that product.”
As well, he says, testing also provides asset owners with assurance about the life cycle of their asset and informing them of the appropriate maintenance schedule.
Concrete is manufactured from a range of raw materials. These include cement, supplementary cementitious materials such as fly ash and ground blast furnace slag, coarse and fine aggregate (stone and sand), water and admixtures. Each of these is either derived from natural resources or is manufactured.
Concrete’s variability, van Rooyen says, arises out of variations within each of the raw materials, as well as with the different batching and delivery processes of concrete manufacture.
According to van Rooyen, the material can be separated into two classes: Normal (N class) or Special (S class). N class concrete allows the manufacturer to determine the mix proportions to achieve standard performance requirements. These include strength and slump (the two primary properties) as well as other properties such as shrinkage, and the chemical and aggregate sources.
Concrete becomes S class where the designer includes additional criteria in the specification. These requirements can include limitations in cement concrete, water/cement ratio, or other properties particular to the performance requirements.
When it comes to concrete, the two main tests are the tests for compressive strength and slump tests. Compressive strength tests indicate the capacity of the concrete to support the load of the building or structure. The slump test, meanwhile, assesses the consistency of the concrete and confirms that the correct volume of water has been added to the mix before the concrete is placed within the structure.
Depending on the type of application, other types of testing may be involved. In the case of pavements, for example, the material is often tested for flexural strength to determine how the material flexes – the ability of the concrete to bend without being broken. Since concrete is known to expand or contract, with, for example, changes in temperature, materials used in some other applications such as large slab floors, testing for shrinkage (the degree to which the concrete contracts) may be performed.
Once testing is done, results need to be compared with the requirements of the Australian Standards and those of the specifications.
When looking at test results, McDonald says, the analysis is either a pass or a fail and there is no room for interpretation. Say, for instance, the strength specification in the design is 40 megapascals. If the result is over 40, then all well and good. If a result of 35 was achieved, this would represent a fail.
In terms of common mistakes in respect to concrete testing, Franceschini points to several areas.
First, there is a failure to undertake sufficient levels of testing or ordering the wrong types of tests. On occasion, he says, clients will be tempted to reduce the amount of testing in order to save on costs. Where this happens, the compliance of the material to standards and specifications cannot be assured.
Alternatively, the incorrect type of tests can be ordered and thus testing can be performed for parameters which may not be pertinent to the performance of the material.
Beyond that, testing should be properly supervised and independently performed. Whilst internal testing performed by concrete suppliers is allowed, Franceschini says only independent testing assures that the testing is rigorous, correct for the application and not guided or directed. Independent testing, he says, provides ‘good or bad’ answers depending on the results and is not performed under any constraints or biases.
On the flip side, he says successful strategies involve ensuring that the right type of tests are performed for the application in question and that testing is performed by an independent third party.
van Rooyen agrees that the tests should be independent and that the right type of tests need to be undertaken for the applications in question. As an example, he says shrinkage tests may not be needed on concrete which will either be submerged or used in humid environments.
van Rooyen says testing should be undertaken by a laboratory which is authorised by the National Association of Testing Authorities. The lab should also be compliant with the requirements of ISO 17035. This is an international standard for the general competencies which should be required for carrying out testing.
McDonald says the primary issue for contractors and engineers is to understand what testing involves and how it is done. Particularly on government infrastructure projects, he says the tests required are generally stipulated by the client. Where you are doing a road project in anywhere in Australia, the requirements for testing will be specified by the relevant authority.
When constructing any building or structure with concrete, being certain that the material will perform to standards and specifications is essential.
Where the appropriate testing is done, contractors, engineers and clients are able to proceed with confidence that this will be the case.