Australian Society For Concrete Pavements - ASCP … Papers/Paper 29... · Australian Society for...

Australian Society for Concrete Pavements

Concrete Pavements Conference 2017

Challenges for Rigid Airfield Pavements in Australia

Greg White, PhD, MEng, ME, MTech(Pvmts), BE(Civil), CPEng, RPEQ

Director, Airport Pavement Research Program

University of the Sunshine Coast

ABSTRACT

Rigid pavements provide an important element of the Australian airfield pavement inventory.

Airfield pavements are designed differently to road pavements and rigid airfield pavements

utilise specific concrete mixtures and details, reflecting the particular requirements of aircraft

and their safe operation. The divestment in airport ownership and management by the

Commonwealth government in the 1990s resulted in the disbandment of the works

departments that maintained Australian rigid airfield pavement technology. As a result,

some more recently identified technology challenges have not been solved. Important

challenges include the specification of characteristic and flexural strength for concrete

design and acceptance, as well as the absence of local guidance for sub-base selection,

joint and transition pavement detailing and the repair of cracks and spalls. Pavement type

selection based on construction cost, rather than whole of life cost, and the increasing

demand for expedient pavement reconstruction methods also present unsolved challenges.

A program of applied research, aimed at developing solutions to these challenges and the

preparation of independent guidance materials for practitioners, is recommended in the

future.

ASCP Concrete Pavements Conference 2017 2 Challenges for Rigid Airfield Pavements in Australia, Greg White

Introduction

Many rigid airfield pavements in Australia were constructed in the 1950 and 1960s in response to WWII and the subsequent Cold War period. These pavements are now 50-60 years old. At the time of their construction, most Australian airports were owned by the Commonwealth Government and the design, construction and management of airfield pavements was management by the various Commonwealth Works Departments. The Works Department also conducted research, actively interacted with international airfield pavement authorities and updated Australian rigid airfield pavement technology, practice, standard detail drawings and model specifications.

Following the privatisation of most Australian airports in the 1990s [1] the Commonwealth no longer actively maintained airfield pavement technology and practice. The Works Departments were disbanded and consulting engineering firms became the design and maintenance authorities, responsible to the individual private airport corporations. As a result, the previous national and standardised approach to airfield pavement design, construction and maintenance ceased.

However, rigid airfield pavements have continued to be constructed. Major developments occurred at Sydney Airport in preparation for the 2000 Olympics and Melbourne, Sydney and Brisbane Airports all undertook significant development work in preparation for the introduction of the A380 in 2007. Brisbane and Melbourne Airports have also increased their rigid parking aprons and taxiway networks significantly, in response to increased air traffic. Importantly, Australian airports are now entering another cycle of significant development. Brisbane, Melbourne, Perth and Sunshine Coast Airports are preparing to construct new runways, as well as associated taxiways and parking aprons. Sydney is also expecting a second airport to be developed. Although the runways may not be constructed in rigid pavement, all projects are expected to include significant rigid pavement taxiways and/or parking aprons. Also, many of the older rigid airfield pavements are reaching the end of their serviceable life and require upgrading or reconstruction. As a result, there is now increased interest in rigid airfield pavement technology and practice, which has not been the case since the 1990s.

This paper outlines a number of challenges facing the owners, designers, constructors and maintainers of rigid airfield pavements in Australia. Some challenges are economic while others are technical and logistical in nature.

Background

Rigid airfield pavements

The methods of design, construction and maintenance of aircraft pavements are generally similar to those appropriate for roads. However, there are important differences which must be taken into account in order to provide pavements that are satisfactory for aircraft loading and operations. Compared to rigid road pavements, airfield pavements have:

• Increased thickness. Reflecting the higher wheel loads and tyre pressures. Typically, 300-500 mm concrete slab thickness for major airfield pavements.

• Reduced long-grade and cross-fall. Reflecting the instability of aircraft on the ground. Typically, 1% long-grade and 1.5% cross-fall, reduced in aircraft parking areas.

• Increased effective surface texture. Reflecting the inability for pilots to simply slow down during wet weather operations. Typically, broom-finished and then grooved for rigid pavement runways.

• Reduced tolerance of loose material. Reflecting the relatively fragile nature of low slung jet engines. Typically, any cracks and spalls are repaired without delay.


Like many road pavements, rigid airfield pavements are not generally reinforced with steel. Rather, they are comprised of unreinforced concrete slabs, typically 4-6 m in length and width, with sawn transverse contraction joints and dowelled longitudinal construction joints. Aggregate interlock is relied upon to provide load transfer across sawn joints. These differences create a unique set of challenges for rigid airfield pavement owners, designers and maintainers.

Airfield concrete

Airfield concrete differs from other pavement and structural concrete mixtures, reflecting the particular requirements of aircraft pavement. Airfield pavement concrete must be of low slump to prevent plastic flow during paving and curing operations, of low water-cement ratio to reduce the risk of plastic and dry shrinkage cracking and of large aggregate size to allow load transfer across open sawn contraction joints. As a result, airfield concrete mixtures generally comprise:

• 40 mm aggregate. 50-60%.

• Natural sand aggregate. 20-30%.

• Cement. 20-25% (360-400 kg/m3).

• Water. 5-10% (water-cement ratio 0.35-0.45).

• Air content. 3-5% entrained air.

• Additives. Less than 2%.

Airfield pavement concrete mixture design and acceptance testing are based on flexural strength, rather than compressive strength. This reflects the use of flexural strength to characterise concrete stiffness for rigid aircraft pavement thickness design. Correlation between flexural and unconfined compressive strength is mixture-specific and acceptance of concrete based on compressive strength has generally been avoided by airports. Usually, a characteristic flexural strength of 4.5 MPa is specified, requiring an average flexural strength of around 5.0-5.3 MPa, which indicatively correlates to a compressive strength of 45-55 MPa. Flexural strength is not commonly specified for acceptance testing of other concrete mixtures in Australia, with compressive strength more usual.

Aircraft development

Since their first introduction in the early 1900s, aircraft have become progressively larger and heavier. Particularly since WWII, aircraft wheel loads and tyre pressures have increased significantly [2-3]. New aircraft, such as the A350-900, have tyre inflation pressures up to 1.66 MPa and wheel loads up to 31.8 tonnes.

Particularly concerning to rigid airfield pavements is the B777-300ER. Although the tyre pressure is only 1.52 MPa and wheel load only 27.7 tonnes, the six-wheeled dual-tridem landing gear is compact and the wheels are closely spaced. As a result, pavement designers can largely ignore the B747 and even the A380 aircraft movements because the B777-300ER movements will dominate the required concrete slab thickness. Around 75 mm more concrete thickness is required to accommodate the B777-300ER compared to other large commercial aircraft. Further, heavier variants of the B777 are now being considered, creating an additional challenge for rigid airfield pavement designers.

Challenges

Flexural strength testing

As detailed above, airfield pavement concrete is specified by a characteristic flexural strength, usually 4.5 MPa. Two beams are cast from plastic concrete delivered to site.


Typically, 6-12 pairs of beams are cast during each concrete paving work period. Some pairs of beams are tested after 7 days curing and some after 28 days curing.

Because airfield pavement concrete comprises 40 mm maximum aggregate, the beams are 150 mm by 150 mm by 500 mm in dimension (Figure 1). This is much larger than unconfined compressive samples and much larger than flexural beams for the 20 mm sized mixtures more common in road and structural concrete applications. The larger beams are heavy and exceed the recommended weight for a single-person lift. As a result, some suppliers have resisted sampling and testing large beams on some rigid airfield construction projects.

Figure 1. 150 mm by 150 mm by 500 mm flexural beam samples

In one example, the supplier’s concerns regarding flexural beam sample handling were accepted by the airport and smaller (suited to 20 mm aggregate mixtures) beam samples were accepted. However, to achieve the recommended maximum aggregate diameter to beam cross section dimension ratio, all the 40 mm aggregate was raked out of the mixture prior to casting the beams. As a result, smaller beam sizes, devoid of 40 mm aggregate and no longer representative of the approved concrete mixture design, were utilised for compliance testing during the works. This approach is unsound and is not recommended.

On other projects, suppliers have developed project-specific correlations between flexural strength and compressive strength and then relied on unconfined compression strength testing for compliance during the works. Where the aggregate sources, cement source and the mixture design do not change, this is a reasonable approach for large projects, because the cost of developing the correlation in the laboratory is justified by the size of the works. However, seemingly minor changes in concrete composition or cement source can invalidate the correlation, as indicated by the example in Figure 2, which represents a single project during which the cement supplier changed and then the cement content was subsequently increased to compensate for the reduction in flexural strength. Where a mixture-specific correlation is proposed to permit unconfined compressive strength testing for concrete production compliance, it is recommended that the test certificates include a declaration that ‘the concrete sample was produced to the same mixture design and from the same sources of constituent ingredients as utilised for the development of the mixture-specific correlation’.


Figure 2. Example of poor correlation between compressive and flexural strength

Characteristic strength

As described above, airfield concrete strength is usually specified as a characteristic strength. This is consistent with Australian practice for road and structural concrete specification and recognises the variable nature of constituent materials, concrete production, sample preparation and test results reproducibility. However, the statistical basis of characteristic strength is not always well understood, resulting in additional compliance requirements being specified in parallel. Further, the implementation of statistically based acceptance/rejection requires an understanding of the variability of the results for the material being tested. This works well for ‘routine’ mixtures from established production plants but presents a challenge when a large database of historical results is not available for the same mixture type. Because airfield pavement concrete is often a ‘special’ mixture, may utilise unfamiliar material sources and because the measured strength is flexural, rather than compressive, the variability of the results is often not known with confidence until well into the project.

Calculated project-specific coefficients of variation of airfield pavement concrete flexural strength results have ranged from 6-11%. Without a reliable estimate of the true coefficient of variation, suppliers have limited ability to set a target average strength to ensure the specified characteristic strength is reliably achieved.

Further, pavement designers are concerned that statistically based criteria will allow one truly defective batch or load to be accepted, despite its strength falling well below the specified characteristic value. As a result, designers have added multiple criteria to their specifications and this is often statistically unsound. Examples of previously specified airfield pavement concrete 28-day flexural strength acceptance criteria include:

• Example 1.


o Minimum target 5.0 MPa. o Minimum mean for each pair of beams 4.8 MPa. o Minimum for each individual beam 4.6 MPa.

• Example 2. o Minimum mean per Lot 4.8 MPa. o Minimum mean for each pair of beams 4.35 MPa. o Minimum for each individual beam 4.15 MPa.

• Example 3. o Minimum mean for each pair of beams 4.8 MPa. o Minimum for each individual beam 4.8 MPa.

• Example 4. o Minimum mean for four consecutive pairs of beams 4.5 MPa. o Minimum for each individual beam 4.3 MPa.

• Example 5. o Minimum for each individual beam 4.5 MPa.

None of these examples is statistically consistent with a characteristic flexural strength of 4.5 MPa. Example 5 is the simplest and its absoluteness is similar in nature to the specification of other rigid airfield pavement construction acceptance criteria, such as subgrade and sub-base compaction, expressed as relative density ratio. However, specifications usually require removal and replacement of all concrete that does not pass the acceptance criteria. This does not reflect the variable nature of the materials and results. Also, the cost of replacing concrete is a high percentage of the overall cost of construction. At the same time, the designer is often reluctant to accept any result lower than the value assumed when determining the thickness of rigid pavement, exacerbating this challenge. Another complication is the 28-day delay between concrete production/sampling and performing the compliance testing. This is particularly problematic for smaller projects when the works may be complete by the time the first acceptance testing data becomes available. In contrast, asphalt production is assessed during the production shift and insitu density is assessed the following shift. Also, a reliable estimate of the true standard deviation of flexural strength results, required to calculate the characteristic value, is not known until well into the job.

In practice, it is rare for significant distress in rigid airfield pavements to reflect a lack of concrete strength. However, it is a primary acceptance criteria and its specification creates a significant challenge. Also, in reality, despite questionable results, airfield pavement concrete has rarely been removed and replaced due to inadequate strength test results. This likely reflects the difficulty in enforcing statistically unsound acceptance criteria, the high financial replacement cost to the supplier and the high degree of inconvenience to the airport to extend the project to accommodate the removal and replacement. Conditional acceptance, often with extended warranty provisions, is often more palatable to all parties.

Sub-base selection

Rigid airfield sub-base does not provide significant structural contribution to the pavement. That is, increasing the sub-base thickness will not result in any comparable reduction in the concrete slab thickness. As a result, rigid airfield pavement sub-bases are selected primarily to provide uniform support to the concrete slabs and to provide a smooth working platform for concrete placement and finishing operations. The uniform support prevents rocking of the slabs and the smooth surface allows the unreinforced slabs to expand and contract freely without being ‘anchored’ to the sub-base.


Traditionally, rigid airfield pavement designers in Australia almost exclusively selected fine crushed rock sub-bases. The sub-base was proof rolled with heavy pneumatic tyred rollers. In the absence of authoritative Australian-specific guidance, some designers have adopted cement treated crushed rock sub-bases, which are more common in the USA. Guidance in the USA requires stabilised sub-bases for aircraft exceeding 45 tonnes [4]. However, provision is also made for fine crushed rock sub-base if the fine rushed rock is of ‘superior’ quality and has performed adequately in the past. Australian fine crushed rock is fully crushed and of high quality. In combination with proof rolling, fine crushed rock is a viable sub-base option for rigid airfield pavements in Australia. Not only are bound sub-bases more expensive than fine crushed rock, they introduce a number of other challenges.

Cement treated crushed rock has a limited work window, the construction joints present planes of weakness and the surface is often less smooth than is achieved by paving fine crushed rock. These challenges present risks to rigid airfield pavements. It follows that fine crushed rock sub-base is preferred unless justified by project-specific circumstances. Such circumstances may include reactive subgrade and/or poor sub-surface drainage.

Importantly, cement treated crush rock sub-base should not be proof rolled. To be effective, proof rolling must be performed when the material remains near the optimum moisture content, essentially immediately after placement and compaction rolling. Proof rolling of cement treated crushed rock at this time often results in surface ruts that are difficult to correct due to cementitious setting. The ruts anchor the concrete slabs, preventing free movement of the slabs, resulting in uncontrolled cracking.

Proof rolled, fully crushed rock has proven adequate over many years in Australia. However, where project circumstances require a bound sub-base, such as cement treated crushed rock, proof rolling must be omitted. This challenge reflects the difficulty for designers attempting to combine elements of Australian practice with elements of international guidance, in the absence of authoritative Australian guidance on which designers can rely.

Standard details

Designers have also sought international guidance regarding rigid airfield concrete pavement details, including slab dimensions, jointing arrangements, joint details and transitions to flexible airfield pavements. In 1975 the Australian Works Department published three standard detail drawings. These standard details remained in place until the Works Department was disbanded around the time of privatisation of many Australian airports. Since then, rigid airfield pavement detailing has become significantly influenced by the experience and background of the designer. Issues presenting a particular challenge include tied outer slab joints, transition pavement configuration, slab thickening in the absence of load transfer devices and intersection fillet geometry.

Conventional theory requires the outer longitudinal rigid airfield slabs at the unsupported edge of a pavement to be tied to prevent the outer slabs from ‘walking away’. The last two transverse joints at an unsupported end of a runway or taxiway may be similarly tied. Tie bars replace or supplement the dowels across longitudinal joints and are added to transverse sawn contraction joints. The tie bars are deformed, longer but slimmer than dowels and spaced further apart than dowels. Some designers have expressed concern that tie bars in longitudinal construction joints will not transfer the load between slabs and have utilised deformed dowels as ties. However, deformed dowels, at close spacing, essentially lock the joint solid, preventing the slabs from expanding and contracting. This effectively creates a 12 m wide unreinforced slab from what was intended to be two 6 m


wide slabs, increasing the risk of cracking. In practice, most rigid airfield pavements are provided with shoulders, which reduces the risk of outer slabs walking away from the pavement and tied outer joints may not be required.

In locations where flexible pavements adjoin rigid pavements, the rigid pavement must be constructed first. The limited ability to compact the flexible pavement layers immediately adjacent the thick concrete slab creates an unavoidable weakness at the interface. Under aircraft traffic loading, a depression in the unavoidably under-compacted flexible pavement results. A transition pavement is required to provide a smooth change from rigid to flexible pavement and to provide a material that does not require roller compaction immediately adjacent the interface. A wedge of lean mix (5 MPa compressive strength) concrete transitioning to a layer of fine crushed rock and a thick asphalt surface is commonly utilised (Figure 3). However, the absence of standard details has resulted in alternate details intended to provide the transition. Concrete slabs with asphalt surfaces and cement treated base and sub-base in the flexible pavement have been designed. However, these approaches do not address the inability to adequately compact the flexible pavement layers immediately adjacent the concrete slab.

Figure 3. Example rigid to flexible transition pavement.

Rigid aircraft pavement thickness design determines the mid-slab thickness. This also applies to the edge of slabs where load transfer is effective. Load transfer is commonly achieved by dowels across the construction joints and aggregate interlock across the sawn contraction joints. Where no load transfer is provided, such as at an isolation joint, a 25% increase in slab thickness is required. Some designers have detailed the slab thickening to occur over 2-3 m.

However, to reduce the risk of crack initiation at the commencement of the thickening, continual thickening over the full width of the slab (typically 4-6 m) is recommended. This is also more practical for the contractor to construct because a change in sub-base grade is avoided midway between slabs.

Fillets are required in aircraft pavements where runway, taxiway and parking aprons intersect and aircraft turn from one to another. The area of fillet is determined so as to maintain a minimum separation between the outside of the aircraft landing gear and the edge of the high strength pavement. Some designs require odd-shaped or even curved rigid pavement slabs to follow the required alignment of the fillet. However, saw-toothed or stepped slabs with dimensions consistent with the rest of the pavement are recommended.


The linemarking is placed at the required fillet alignment and the usually flexible shoulder is constructed around the rigid pavement slabs that penetrate into the shoulder pavement area. This approach prevents irregular shaped slabs from cracking, avoids small slabs from walking away from the pavement and avoids odd jointing arrangements. It is also reduces construction complexity by maximising uniformity.

The absence of authoritative guidance for Australian rigid pavement details has resulted in some designers adopting or adapting details provided in guidance materials published by the USA or other countries. Many of the details are similar to traditional Australian practice so this is an attractive option. However, the guidance materials published by the USA sometimes reflect what is ‘possible’ and not what is ‘common’. Also, some details may not be compatible with local Australian practice. Therefore, caution must be exercised when adopting overseas guidance, particularly when isolated elements are adopted rather than the whole package.

Spall and crack repairs

Rigid airfield pavements crack and spall with age. Loose fragments of concrete present a hazard to aircraft and are not tolerated. It follows that a significant portion of rigid airfield pavement maintenance comprises the repair of cracks and spalls. These defects are generally related to construction imperfections and environmental effects, rather than deficient concrete strength or pavement thickness.

Cracks may be sealed with a rubberised bitumen bandage or routed and then sealed with a sealant, similar to joint sealing. Minor spalls can also be filled with joint sealant, particularly when located adjacent to an already sealed joint.

Large spalls are more challenging and saw cutting is generally required to form a regular repair shape and to expose sound concrete. Once the excavation is complete, the challenge is selecting the most effective repair material. Some designers have utilised concrete patches or cementitious grouts. However, these often re-fail rapidly, likely the result of shrinkage during curing or imperfect adhesion to the exposed concrete surface. Asphalt patches have been successful in some applications but do not provide the fuel or temperature resistance that the rigid pavement was initially constructed to provide. However, semi-rigid epoxy repairs have proven effective in many cases (Figure 4) and are recommended.


Figure 4. Example concrete repair with semi-rigid epoxy.

Rehabilitation and reconstruction

Rigid airfield pavements are generally designed for a structural life of 40 years. In practice, many Australian rigid airfield pavements have lasted 50-60 years. While long lasting pavements provide many whole of life advantages, rigid airfield pavements present a challenge for reconstruction works. With many of Australia’s current rigid airfield pavements constructed in the 1950s, a significant amount of reconstruction work is now required. As a result, reconstruction technology is an important issue and represents a significant challenge.

The reconstruction of rigid airfield pavement in itself is not particularly challenging. However, when the pavement is in an operationally critical location, it is not feasible to close the pavement for 6-8 weeks while the existing pavement is demolished, the underlying sub-base/subgrade is prepared, new concrete is placed, set and adequately cured for aircraft traffic. Critical locations include apron parking positions at busy capital city airports, critical intersection taxiways and runway ends, which are often constructed with rigid pavement due to slow moving and turning aircraft.

Flexible airfield pavements are less prone to reconstruction issues because available flexible pavement reconstruction materials and techniques are more conducive to expedient construction. Full depth asphalt and foamed bitumen stabilised base course readily allow a pavement to be excavated, reconstructed and returned to service during a short night time closure. Flexible pavements also allow the area of reconstruction to be reduced to accommodate the available closure because the joints are less critical to pavement performance. The more expediently constructible nature of flexible pavements is a disadvantage for rigid airfield pavements where both pavement types are otherwise considered to be suitable.


Some airports have utilised a relatively new rapid setting concrete for expedient rigid pavement reconstruction. The rapid setting is achieved by a specially developed cement with over 95% of the 28 day flexural strength achieved in seven days [5]. The high rate of cement hydration and strength gain requires volumetric-based mixing of concrete on site, rather the conventional plant batching and agitator delivery. The process is labour and equipment intensive, resulting in a low productivity and a high unit-cost of construction. However, when the cost of extended pavement closures can not even be quantified, this provides a viable means of reconstructing rigid airfield pavements in operationally critical areas.

Whole of life cost

One of the greatest challenges for rigid airfield pavements in Australia is the construction cost, which is typically 50-100% higher than for flexible pavements of equivalent strength. The cost premium for rigid pavement construction is greater when the subgrade support is higher, reflecting the greater influence of subgrade support on flexible pavement thickness than on rigid pavement thickness.

Despite the increased construction cost, rigid airfield pavements often provide whole of life benefits. This reflects the 40 year design life and no requirement to resurface every 10-15 years. However, the actual whole of life comparison between rigid and flexible pavements is highly influenced by the residual value assigned at the end of the evaluation period. For a 40 year evaluation period, the rigid pavement would be considered to be at the end of its structural life and therefore the residual value is negative, reflecting the cost of demolition prior to reconstruction. In contrast, a flexible pavement that is resurfaced every 10-15 years, effectively resetting the structural design life, is perpetual in nature. It follows that the residual value of a flexible pavement is much higher than for a comparable rigid pavement.

Even in cases where the whole of life evaluation clearly demonstrates the advantages of a rigid pavement, flexible pavements are often still preferred by airports. This reflects the long period required to realise the cost benefits associated with the rigid pavement, compared to the period for financial performance assessment of privatised airports. In contrast, the reduced construction cost associated with an equivalent flexible pavement solution is almost immediately realised.

Summary and Conclusions

Rigid pavements provide an important element of the Australian airfield pavement inventory. However, outside of runway ends and capital city airport taxiways and parking aprons, flexible pavements are more common. The use of rigid airfield pavements in Australia may increase in the future, which will be assisted if a number of challenges are addressed.

Flexural beam testing presents a logistical and safety risk and flexural strength acceptance criteria are not consistent and are often statistically unsound. Sub-base selection is based primarily on the preference and experience of the designer, rather than on local guidance materials reflecting the design aircraft, subgrade conditions and quality of available materials. Joint layouts and details are similarly subject to the designer’s preference. Maintenance of spalls has also proven challenging in the past with various materials utilised with widely different success. Arguably the greatest challenge for rigid airfield pavements in Australia is cost-effective reconstruction in operationally critical areas.


Many of the challenges faced by rigid airfield pavement designers, constructors and maintainers are exacerbated by local industry circumstances. The absence of a local authority has prevented the upkeeping of technology and practice, as well as a lack of guidance materials. At the same time, aircraft are now more damaging than ever before. A program of applied research, aimed at developing solutions to challenges and independent guidance materials for practitioners, is recommended for the future.

References :

1. Eames, J, Reshaping Australia’s Aviation Landscape: The Federal Airports Corporation

1986-1998, Focus Publishing, Sydney, Australia, 1998.

2. Roginski, MJ, ‘Effects of aircraft tire pressures on flexible pavements’, Proceedings

Advanced Characterisation of Pavement and Soil Engineering Materials, Athens,

Greece, pp. 1473-1481, 20-22 June 2007.

3. Fabre, C, Balay, J, Lerat, P & Mazars, A, ‘Full-scale aircraft tire pressure test’,

Proceedings Eight International Conference on the Bearing Capacity of Roads,

Railways and Airfields, Urbana-Champaign, Illinois, USA, pp. 1405-1413, 29 June -

2 July 2007.

4. FAA, Advisory Circular: Airport Pavement Design and Evaluation, AC No. 150/5320-6F,

Federal Aviation Administration, Washington, District of Columbia, USA, 10 November

2016.

5. Hampton, I, Rapid set concrete technology. USC 2016 Airfield Engineering Seminar,

Sippy Downs, Queensland, Australia, 4-6 May 2016.

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