Effect of Blast Induced Ground Vibrations on Green Concrete
In civil construction projects,the strive for a more time-efficient construction process naturally focuses on the possibilities of reducing the time periods of waiting between stages of construction.As an example,the rock excavation in open cut excavations in metro rail projects requires coordination between the bed concreting and adjacent rock blasting so that blast induced vibrations would not affect the green concrete which is hardly at a distance of 5 ~ 10 m.There also arise similar problems in mining,where concrete supporting is required in drivages of weak rock.The grid of drifts in recent mines is very dense for excavating as much ore volume as possible.This means that supporting systems in one drift are likely to be affected by vibrations in a neighbouring drift.A criterion for how close,in time and distance,to the young concrete,blasting can take place would be an important tool in planning for safe and economical mining projects.Another example is the foundation excavation of thermal power projects,where the concreting and rock excavation go simultaneously,adjacent to each other,to crash the time.In actual practice,numerous occasions arise when it is necessary to build concrete structures at the same time when excavations by blasting are being carried out.For example,lining during tunnel driving,foundation for the primary crushing buildings near open pits,etc.Tab.1 gives prevention criteria given by Oriard (1980) depending upon general curing or hardening time of the concretes.This criterion is based on some assumptions of curing and cannot be made extensive to all types of concrete.The basis for the existing criterion,during the hardening period of 0 to 4 hours,the concrete is still not hard and the admissible levels are relatively high.From 4 to 24 hours,it begins to harden slowly,and after 7 days it reaches the strength that is approximately 2/3 of the final product (28 days),allowing a progressive intensification of the vibrations.The guidelines given by the previous research findings are broad and general in nature.Therefore,the authors that it requires further experimentation to correlate the strength acquired by the concrete with the threshold maximum vibration level.With the advent of insitu concrete strength measuring instruments,now a days,it is convenient to monitor strength and corresponding effect of vibrations.In this way the exact effect of vibrations on green concrete can be clearly known to avoid possible damage and deterioration to the structure in long run.Some experiments were carried out during base slab concrete works of a Metro construction works in India to find out the vibration criteria for freshly poured concrete of age 0 to 4 hours.From the temperature history graphs it was found that the temperature of concrete starts increasing after 2 hours,which is the indication of curing process.At the experimental site the structural concrete was exposed to a blast induced vibration with peak particle velocity (PPV) level of 46 mm/s,which was still less that the threshold limit suggested by Oriard (1980).The vibration level was exposed at 3 hour 15 minutes from concreting.The vibration exposure took place in twice with a gap of 5 minute.There was another site with same structural concrete kept without any exposure of vibrations.After 15 days of curing samples of both the sites were tested for strength.Very interestingly the strength of vibration free concrete sample measured was 40.2 MPa and that of vibration exposed samples was 31.5 MPa.This indicates the vibration exposure resulted in strength reduction of 22%,which is very substantial one.Similar experiments were conducted on samples of 3 hours age and with exposure of PPV 48.5 mm/s and observed 23.5% reduction in strength.This indicates that no chance of exposure of vibrations should be taken for the freshly poured concrete of age 2 to 4 hour,although the existing global vibration criteria is liberal for concrete up to 4 hour age.Therefore,the paper recommends that the green concrete up to 4 hour age is also vulnerable for vibrations above 40 mm/s and stresses for need of more research and experiments in this direction.
green concrete Young shotcrete blasting vibration peak particle velocity (PPV)
M.Gurharikar M.Ramulu
Rocktech Systems & Projects,Nagpur,India Central Institute of Mining & Fuel Research,Regional Centre,Nagpur,India
国际会议
The 4th Asian-Pacific Symposium on Blasting Techniques(第四届亚洲太平洋地区爆破技术研讨会)
深圳
英文
433-437
2014-11-01（万方平台首次上网日期，不代表论文的发表时间）