Soft Concrete Safety Net Stops Aircraft Overruns

An airport runway end, utilizing a unique slowing down agent, and being an enhanced version of the Engineered Materials Arresting System (EMAS), can act as a more effective speed bump when aircraft overrun the runway due to emergencies during take off and landing. Researchers from China Building Materials Academy Co., Ltd. (CBMA), Aviation Academy (Beijing) Technology Development Co., Ltd. and China Academy of Civil Aviation Science and Technology have developed a new ultra-light foam concrete EMAS, which can stop a plane gently and stably in a few seconds.

Finding the right material

The International Civil Aviation Organization requires modern large airports to set up Runway End Safety Areas (RESAs) at their runway end to reduce the risk of damage to an airplane undershooting or overrunning the runway.

An EMAS can act as a substitute of an RESA, when the full recommended length of a RESA cannot be realized in certain airports due to geological or other reasons, or it may be adopted as an extra buffer where dangerous geographic conditions like cliffs immediately follow the end of a full length RESA.

According to Fang Jun, R&D engineer at CBMA, an ultra-light foam concrete has been developed to be used in the EMAS. With an inner porosity of over 80 percent, it weighs only about 200 kilograms per cubic meter, which is one tenth the weight of ordinary concrete. This ultra-light foam concrete can crack and absorb the kinetic energy generated when a large plane enters the EMAS, enabling the slow landing of the plane.

The slowdown material should neither be too hard nor too soft. If it is too hard, it cannot be broken and absorb the energy. If it is too soft, the buffering effect will be weakened. According to Fang, the strength of the foam concrete for stopping a Boeing-747 plane should be controlled between 0.3-0.35 MPa.

Sulphoaluminate cement was used as a solution for its speedy hardening. However, EMAS made of material produced by foreign producers is relatively expensive, and the material is easily pulverized during prolonged service, which will greatly lower its strength and weaken its function.

Researchers decided to develop a new generation ultra-light foam concrete. Thanks to CBMA's rich experience in developing inorganic nonmetal materials, the research team quickly mastered the basic theories for producing ultra-light foam concrete, Fang said.

But the stringent requirements in mechanical properties, durability and weather resistance when it is used in EMAS, pushed the team to find the right production solutions.

Foaming is a critical link in the production of the ultra-light foam concrete, and the research team conducted countless experiments with different choices of foaming agents, air entraining agents, temperatures and humidity.

A maleated rosin based Gemini type air entraining agent was finally introduced by the research team to enhance the thickness and strength of the bubble liquid film, thus avoiding the collapse of the ultra-light foam concrete.

Stable long term performance

EMAS needs to work outdoors for prolonged periods, and a key issue is to maintain the stable mechanical properties of the material in long-term exposure to wind and rain.

The research team figured out a two-level strength control mechanism. Through the precise control of the process, the strength is formed, and can be released layer by layer. In this way, strength loss caused by the environment can be made up by the slow release of the strength of ultra-light foam concrete itself.

In traditional concrete, there are active admixtures like slag and coal ash besides cement, and they can provide certain strength. The multiple source of strength makes precise control difficult.

The best way to precisely control the formation process of concrete's strength is to make cement the only source of strength, as it can be realized via the precise control of the relevant parameters of cement, Fang said. The research team suggested replacing active admixture with inert admixture to dilute the cement clinker, thus eradicating the influence of secondary hydration on the increase of strength.

To allow strength release layer by layer as designed, the particle size of cement and the hydration speed need to be balanced. The research team designed a multi-level hydration cementitious system with "concentrated distribution in narrow intervals, gradient distribution in multiple intervals" through the precise proportioning of cement with different particle sizes.

The system can control the collapse strength of the ultra-light foam concrete based on the relationship between particle size and hydration time, thus letting the strength release layer by layer. The lost strength of the concrete in long-term service can be supplemented dynamically under this system, realizing a dynamic balance of strength.

Statistics showed that the overall fluctuation of mechanical properties of the EMAS by ultra-light foam concrete applied in 2018 at Nyingchi Mainling Airport, southwest China's Xizang Autonomous Region, was only three percent, much lower than the designed range of ±10 percent. Fourteen domestic airport runways have adopted this research achievement, safeguarding each takeoff and landing.

Source: Science and Technology Daily