Report No. : GEO Report No. 102
Report Title : A Study of the Effects of Blasting Vibration on Green Concrete (2000), 159 p.
Author : A.K.H. Kwan & P.K.K. Lee
In order to study the effects of blasting vibration on concrete including freshly placed concrete (green concrete) so that blasting vibration control limits may be established to avoid causing damage to concrete structures, a literature review and an experimental programme have been conducted.
The findings of the literature review have been reported separately in the Literature Review Report submitted in October 1994. Basically, very large discrepancy exists in the vibration control limits currently adopted by different building authorities. There have been relatively few rigorous studies on the possible effects of blasting vibration on concrete and up to now the vibration resistance of concrete has never been directly measured. Regarding the estimation of vibration resistance of concrete, most engineers assume that the vibration resistance of concrete is proportional to the strength of the concrete but it is not quite clear whether the strength should be compressive strength or tensile strength. The situation is rather confusing and to date no rational framework exists for establishing vibration control limits to avoid causing blasting damage to nearby concrete structures.
The experimental programme involved the testing of 6 typical concrete mixes and 1440 concrete specimens. A method of testing concrete for their shock vibration resistance by subjecting prisms cast of the concrete to hammer blows was developed. Vibration intensities up to a peak particle velocity of 3000 mm/s have been applied. After hammering, the prismatic specimens were continued to be cured until the age of 28 days and then tested for their compressive and tensile strengths. It was found that even at intensities high enough to break the concrete specimen into pieces, the shock vibrations applied have little effect on the compressive strength of concrete. Vibration damages were mainly in the form of cracking and reduction in tensile strength. Hence, the vibration resistance of concrete is dependent more on tensile strength than compressive strength. The vibration resistance results (in terms of peak particle velocity of vibration) were quite scattered but the large number of experimental data allowed us to determine statistically the relationship between the lower bounds or characteristic values of the shock vibration resistance to concrete age, compressive strength, tensile strength and ultrasonic wave velocity with fairly good correlation coefficients. This is the first time that the vibration resistance of concrete is correlated to other mechanical properties. Comparison of the vibration resistance values of different types of concrete with or without PFA and cured at different temperatures revealed that the correlation of vibration resistance to other mechanical properties is not dependent on PFA content or curing temperature. Thus, a single set of vibration control criterion may be applied to concretes within the range studied herein. From these vibration resistance results, vibration control limits that may be applied to Hong Kong and perhaps also to other places may be obtained by dividing the vibration resistance values by a suitable factor of safety. In view of the large variation of vibration resistance results, a factor of safety of 5 is recommended. Despite the use of a fairly large factor of safety, the vibration control limits so derived are higher than most existing limits currently being used in actual construction. Thus if the vibration control limits derived in the present study are adopted, higher intensity blasting and therefore quicker and more economical construction would be allowed.
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