Results obtained by numerous researchers concerning concrete strength anisotropy are analyzed. In some cases, the researchers have found that the compressive strength of specimens loaded at a right angle to a direction of concrete placing, f′c, is greater than for those loaded parallel to the above direction, f″c; in other cases, it is lower. This fact has been elucidated. In the cases where anisotropy is caused by the “internal” mix segregation, we come across f′c/f″c > 1; when the cause is in the “external” mix segregation, the result is f′c/f″c < 1. Given are recommendations for taking into account such anisotropy in core testing.
Experimental investigations of within-test variability of non-destructive methods to determine concrete strength are reported. Rebound, indentation, ultrasonic (with direct and surface transmissions) and shearing rib methods were among those studied.
AustraliaThe authors on behalf of the R1LEM Technical Committee "Use of Recycled Materials" have presented an interesting and useful paper. The definition of sustainability in construction, which carries two meanings -both durable and sustainable, is correct.The authors of the discussed paper correctly point out that construction and demolition waste contain iron, wood, mastic, bitumen, chipboard, cardboard, plastics, aluminum, zinc, cooper, etc. The list is not full of course, but enough to appreciate roughly the variety of different materials presented in this waste. Further, the authors write: "During the past decades, mechanical 'reprocessing' systems have largely replaced manual processes, primarily in the metal sector both also for construction and demolition waste (CDW), so that CDW can now be used on a large scale in the construction sector".Since the mechanical "reprocessing" systems in the metal sector have been mentioned in the discussed paper, the following information could be of interest. Even the separation of steel from the slag in the basic oxygen steelmaking and electric arc furnace steelmaking is not currently perfect [1]. And this is a relatively "easy" case for the separation of the materials, since both slag and steel are in liquid forms, the specific gravity difference is threefold plus the steel is a magnetic material. The separation rubbles from the slag (as a more valuable by-product than rubbles) in steel production practically is an unresolved problem. In the construction and demolition waste, the situation is far more complicated. We are dealing with solidified (mainly non-magnetic) materials in the very wide range of specific gravity. There are materials, which after heavy handling turn into the dust, for instance, gypsum plaster. It is needless to explain potential problems associated with its presence in concrete. It is unclear how to remove
This paper considers problems of improving the non-destructive control of concrete strength from the standpoint of: selecting the most economic methods; determining the required number of individual tests at a location to obtain an average in-place test result with the same degree of confidence by various methods; and modifying the acceptance criteria of ACI 318 to reduce the consumer's risk without changing the producer's risk. The ways of reducing labor consumption of nondestructive tests are recommended.
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