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Testing is essential

Tim Gardiner, Business Unit Manager at ELE International, highlights the importance of thorough and accurate concrete testing in tunnelling projects, in order to avoid potentially disastrous failures.

The versatility of concrete ensures that is chosen time and time again for major tunnelling projects. However, this feature makes thorough and accurate testing essential, as without the correct testing processes in place, the almost infinite range of variables in the properties of a mix can result in inconsistent tolerances and compromised safety. For this reason, construction companies and civil engineers are now investing in the latest sample testing technology, to ensure that concrete consistently demonstrates the requisite qualities for a given application.  By making testing more efficient and reliable, this technology helps to increase productivity, reduce costs and improve long term safety.

Tunnels place a challenging set of demands on the material used in their construction, requiring building materials that are sufficiently strong and durable to withstand often high pressures and extreme mechanical stresses, and also flexible enough to allow components to be built and assembled in-situ.  For example, when a tunnel is constructed through soft soil, its lining is required to withstand significant loads created by water pressures and overburden of the soil; similarly, if the tunnel is located close to the surface, then any structures above or close to the tunnel can apply additional forces.

Concrete is ideally suited to meet these requirements as fresh materials can easily be prepared on-site, while component parts can be produced locally to the exact specification and quantities required, to ensure that high quality construction work can be carried out cost effectively.

However, with this flexibility come important issues which need to be addressed. As cement, aggregate and water, the raw materials used for making up concrete, are extremely difficult to regulate, any two batches are likely to be inconsistent, even if accurately mixed. Mixing methods can also vary from batch to batch, or supplier to supplier, potentially resulting in finished pre-cast segments with an unacceptable level of variance. 

The ability to maintain a consistent and high quality standard of both fresh concrete for pouring and spraying (shotcrete), and pre-cast components such as liners and lintels is of the utmost importance. The demanding specification requirements for pre-cast concrete tunnel linings can only be achieved when the concrete materials and mix proportions have been properly evaluated in pre-production mix design trials, including closely controlled mixing, fabrication and curing.

The implications for inadequate quality control can be fatal. As well as the clear danger to the safety of workers and users if tunnel linings fail, there are considerable commercial and legal implications, with the potential costs associated with court action, compensation and repairs, as well as the daunting prospect of countering negative press coverage.

Less immediately damaging, but just as serious, is the risk of premature deterioration in a tunnel lining or other structures, due to incorrect formulation or inconsistencies in mixing. The prevention of reinforcement corrosion, particularly that caused by chloride ingress and carbonation, is of major concern in tunnel design, it is essential to ensure a reliable and long service life. The specification of components must also take this into account to prevent leakage and create a lining that is free from cracks or flaws. 

These risks can be significantly minimised through sample testing, and there are a number of processes that are now an essential part of all construction and tunnelling projects. Indeed, when the relatively low cost of testing is weighed against firstly the total cost of a tunnelling project as a whole, and secondly the potentially unforeseen expenses caused by failure or deterioration, it makes little sense to proceed with construction without effective and accurate testing methods in place.

There are a number of methods and tools available for checking and evaluating the quality of concrete, and although many of these techniques have been in existence for some time, the introduction of new technology is now making a considerable difference to the speed, accuracy and consistency with which the processes can be carried out.

Fresh concrete should be regularly tested throughout the duration of a contract to assess the suitability of a mix to its specific application, and to ensure that the concrete used is of a consistent standard. Three proven methods of testing fresh concrete at different levels of workability are the slump test, the flow table test, and the Vebe consistometer test. 

Fig 1 outlines a standard range of quality control tests for fresh and hardened concrete used in tunnelling.

(Fig 1)

The slump test measures the consistency of freshly mixed concrete of medium consistency, and is the most widely used method for assessing its workability. It provides an indication, rather than an accurate measure, of workability, involving filling a conical mould with a sample of wet concrete mix, compacting the sample and then lifting the cone. The slump is calculated by the difference in height of the mass before and after the cone has been removed.

The second method, the flow table test, is used where high workability mixes with a slump of 180mm or more are specified. This method determines the flow index of the concrete as an arithmetic mean of the diameter of the sample after it has been worked with a flow table that is dropped through a known height.

A third test for fresh concrete is the Vebe consistometer test, and is generally used with particularly stiff mixes. This test begins with the slump test as above, in which the result will be virtually zero. The sample then subjected to vibration in a cylinder, and the time taken for the sample to mould itself to the cylinder measured. A longer time in seconds, measured as Vebe degrees, indicates lower workability.

As well as testing fresh concrete, to assess the suitability and consistency of a mix, hardened concrete should also be tested, both in the laboratory and on-site, to ensure that the component parts to be used, such as tunnel linings and lintels, have the desired properties, and that concrete already in use has retained its desired properties.

It is often advantageous to specify plant-produced concrete, as pre-cast components are manufactured under controlled conditions, offering precisely produced products and high levels of consistency. However, even when this is the case, there remains a requirement for samples to be taken from site, or the production plant, to be tested under controlled conditions. 

The density of both fresh and hardened concrete is of interest to engineers because of its effect on durability, strength and resistance to permeability. Hardened concrete density is determined either by simple dimensional checks, followed by weighing and calculation or by weight in air/water buoyancy methods. The density of hardened concrete specimens such as cubes and cylinders can be quickly and accurately determined using a buoyancy balance, consisting of a rigid support frame and a platform mounted water tank. A mechanical lifting device is used to raise the water tank through the frame height immersing the specimen suspended below the balance. The balance supplied may also be used as a standard weighing device, providing a versatile and comprehensive weighing system in the laboratory.

Drying, shrinkage and moisture movement can also be measured in pre-hardened concrete samples. Firstly, initial drying shrinkage tests can be taken, measuring the length of a moulded and cured specimen under specified conditions and comparing it to its final constant length when dried. Likewise, tests can be carried out for drying shrinkage, which is the difference in length of a matured specimen cut from concrete and saturated, and its final length upon being dried. Thirdly moisture movement, whereby the difference between the constant length of a specimen when dried and its length when subsequently saturated in water is recorded. Each of these tests to determine the change in length of a concrete sample brought about by a change in moisture content enable engineers to make accurate predictions of how a pre-cast section will behave when in contact with moisture.

The most crucial test for hardened concrete used in tunnelling, however, is compressive strength testing, which enables engineers to assess the strength of a concrete sample and its performance under actual loading, as opposed to the design loading. Furthermore, any deterioration, from chemical action, weathering, fatigue or excessive loading, can be precisely measured. 

The latest generation of compression testing equipment combines a high stability load frame, hydraulic ram assembly and high specification loading platens, with incorporated digital and microprocessor technology to provide highly accurate results. Closed loop microprocessor control is invaluable in today’s compression testing equipment, significantly increasing productivity and offering outstanding levels of accuracy and consistency in testing cycles.  

A typical closed loop control system, which provides automatic loading control, is shown in Fig 2.

(Fig 2)

Advanced microprocessor controlled units, such as ELE’s ADR Auto range, can be used to provide fast, accurate and detailed results recording and analysis for high throughput testing, both on site and in the laboratory. These machines are fully automatic and require minimal operator involvement, improving productivity and reducing costs.

These testing machines can accommodate a wide range of different sized samples, and are designed for different frequencies of use, and varying levels of data handling. They are also compliant with the latest EN specifications, and employ specially designed upper plate/ball seating arrangements to ensure there is no movement of the assembly after initial contact with the sample, as research has shown that any movement of the upper platen, coupled with frame deformation during the loading cycle of standard cube samples, can induce tensile cracks and result in variances of measured strength. 

Compressive load is achieved using a hydraulic power unit to provide the required pressure to the ram/cylinder unit of the load frame. As this is closed loop microprocessor controlled, the need for expensive control valves and problematic oil bleed type valves is removed, and the service life of components extended due to reduced operational stress. While much of the operation process is automated, users have access to a simple control panel for calibration, sample selection, test control, and options to save or print or download test data to PC. 

This new technology allows concrete used in tunnelling projects to be tested more efficiently and accurately than previously possible. In critical applications such as tunnelling, poor quality control is not only bad for business, it’s potentially disastrous. Thorough and precise testing of fresh and hardened concrete, before, during and after construction, is essential to ensure that a design brief can be profitably developed into a safe structure with a long life.

For further information contact Tim Gardiner, Business Unit Manager, ELE International, Chartmoor Road, Chartwell Business Park, LEIGHTON BUZZARD, Bedfordshire, LU7 4WG.  Tel: 01525 249 223. Fax: 01525 249 249.  Email: ele@eleint.co.uk.  Web: www.ele.com.