Preparation And Performance Of Ultra-lightweight Concrete For Engineered Material Arresting System

In recent years,with the number of flights increasing significantly,the amount of air crash caused by aircraft veering off the runway is on the rise,which threatens the passengers’ safety of life and property.The Engineered Material Arresting System(EMAS)is an energy-absorbing system assembled using ultra-lightweight concrete(ULWC)as its unit body.EMAS can stop the aircraft veering off the runway,ensuring the safety of passengers and crew.ULWC used for EMAS is lightweight and low-strength,with a design service life of up to 20 years.During EMAS’s service life,it is a challenge for the ULWC component to avoid aging and chalking due to the direct exposure to outdoor atmosphere,especially in the complex plateau environments characterized by huge temperature differential,strong evaporation,etc.As a result,current ULWC often does not meet the design requirements and has to be withdrawn from service earlier than expected.To improve the service life and service performance of ULWC for EMAS in complex environments of plateaus,a cementitious material system(P·I cement + inert additive)was designed to improve the stability of the mechanical properties of ULWC.Meanwhile,a stepwise hydration control method was put forward on the cementing system based on the cement particle size distribution to precisely control the crushing strength of ULWC following the design requirements.The results are as follows:1.The cementing material system of "P·I cement + inert additive" of ULWC was designed using P·I cement as active hydraulic cementing material,and a large dosage of inert additive was used to replace the active additive and part of the cement.The effect of raw materials on the performance and pore size distribution of ULWC is studied.Within the test dosage,it is found that the dosage of foaming agent determines the dry density of ULWC;increasing the dosage of foam stabilizer made the pore size distribution of ULWC more concentrated;increasing the water-binder ratio made the pore size smaller,and increasing the dosage of the inert additive reduced the crushing strength of ULWC.Based on the change law of dry density and the crushing strength of ULWC,the inert additive dosage of ground calcium carbonate,foaming agent,foam stabilizer,and water-to-binder was determined to be 63.0% ～ 64.3%,4.4%,5.0%,and 0.54 ～ 0.56 respectively.2.The preparation process,pore structure characteristics,mechanical properties,and durability of ULWC prepared by P·I cement system,P·II cement system,and sulphoaluminate cement system were comparatively studied.It showed that the properties of ULWC prepared by the P·I cement system is the best,but still need further improvement.(1)From the preparation process,the P·I cement system and P·II cement system had higher slurry alkalinity(p H 10.41 and 10.47 respectively)and a faster foaming rate.On the contrary,the sulfoaluminate cement system’s slurry alkalinity was the lowest(p H 9.01),and the foaming rate was also the slowest.(2)Pore size distribution of the ULWC prepared by the P·I cement system and P·II cement system were similar,and the pore sizes were concentrated in 1.0 mm ～ 2.0 mm.Meanwhile,the ULWC prepared by the sulfoaluminate system had a larger pore size concentrated in the range of 1.5 mm ～ 2.5 mm.(3)The stability of the mechanical properties of ULWC prepared by the P·I cement system is much higher than the others.The crushing strength change rate from 60 d to 150 d was 4.81%(about 1 /4 of the design requirement).(4)The semi-crush energy softening coefficient and frost resistance coefficient of ULWC prepared by the P·I cement system were 0.67 and 0.70,respectively,which were slightly lower than the other two cementing systems and design requirements(Design requirements are 0.8 ～ 1.2.The closer to 1,the better the water resistance and frost resistance).The mass water absorption rate and mass loss rate were 22.66% and 1.91%,respectively,which were also slightly lower than the other cement systems(The lower the mass water absorption rate and mass-loss rate,the better the water resistance and frost resistance).3.Based on the hydration contribution of cement with different particle size intervals made to the crushing strength of ULWC,the P·I cement was divided into three particle size ranges: coarse(>35 μm),medium(8 ～ 35 μm),and fine(<8 μm).The hydration rate of cement with different particle sizes was studied.It was found that the hydration rate of fine-grained cement was fast.The hydration degree of 1 d and 3 d were 58.6% and 80.9%,respectively,which can quickly develop strength and avoid collapse during the preparation process.The hydration rate of medium-grain cement was moderate,and the hydration degree of 7 d and 28 d were 69.4% and 76.2%,respectively,which can obtain high crushing strength after 7 d and 28 d.The coarse-grained interval cement has a low degree of hydration at each age,which can compensate for the late strength loss of ULWC.4.By optimizing the particle size distribution of cement particles,the hydration process of ULWC was controlled in a stepwise manner.The crushing strength of ULWC developed according to the design requirements.The 1 d,7 d,28 d crushing strength of ULWC reached 56%,68%,86% of that of 300 d,the crushing strength change rate between 90 d and 300 d is 1.8%,semi-crush energy softening coefficient is 0.874,frost resistance coefficient is 0.885.This indicates that ULWC with the cementing material system of "graded P·I cement + inert additive" has stable mechanical properties,better water resistance,and freeze-thaw resistance.