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How much water should be used in concrete?
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CURRENT TOPICS.
lJ. F. L
T e s t i n g S t r e n g t h of J o i n t Glues. ANON. (Forest Products Laboratory, Madison, 147isconsin, Technical Notes No. F - i 6 . ) - - I r t using glues for high-grade joint work, a knowledge of the s t r e n g t h of the joint is important. A m e t h o d which is inexpensive, accurate, and suitable for use in a w o o d w o r k i n g factory has not y e t been developed. T h e following method, w h i c h is used at the F o r e s t P r o d u c t s L a b o r a t o r y , can be e m p l o y e d if a universal timber testing machine is available. Two blocks of selected hard maple, about I inch by 2 ~ inches by I2 inches in size are glued together. After the glue has aged sufficiently they are cut into shear specimens and these are placed in a testing machine so that the base of the long half of the block rests on a metal seat. Pressure is then exerted on the short half, causing it to slide past the long half at the glued point. The pressure required to separate the blocks in this way is measured and the percentage of the area of wood surface torn out by the glue estimated. If the failure occurs entirely in the glue, a measure of the strength of the glue joint is obtained, but if the failure is entirely or p a r t l y in the wood, as f r e q u e n t l y happens, the full strength of the glue is not developed and the test m a y have to be repeated, using s t r o n g e r blocks. As the same m e t h o d has been used in securing data on the s t r e n g t h of wood in shear, when the strength of glue has been determined, it can be compared with that of any wood whose average shearing strength is known. T h e F o r e s t P r o d u c t s L a b o r a t o r y has made thousands of tests on specimens glued with casein and animal glues, and when properly used, these glues have shown shear values of 24oo pounds or more per square inch. Few commercial American woods average more than 24o0 pounds per square inch in shearing strength, and the majority of them average less than 2ooo pounds. Many glue tests have averaged as high as 3000 pounds. H o w Much Water Should be Used in Concrete?
ANON.
(Scientific American, vol. cx, No. I5, p. 365, April I2, I 9 ~ 9 . ) - - T h e E m e r g e n c y Fleet Corporation, in connection with its work on concrete vessels, has developed an apparatus for testing the a m o u n t of w a t e r which should be used in concrete work. An open metal cylinder is employed resting upon a glass plate. T h i s serves as a mold which is filled with concrete and smoothed off level on top. T h e n the cylinder is raised, leaving the concrete on the glass plate. If the mixture is v e r y dry, the concrete will maintain its cylindrical form, but the w e t t e r the concrete the more it flows out at the bottom, so that a measure of the consistency of the mixture can be obtained by m e a s u r i n g the height of the cylinder or cone of concrete after the metal cylinder has been withdrawn.
CURRENT TOPICS.
lJ. F. L
T e s t i n g S t r e n g t h of J o i n t Glues. ANON. (Forest Products Laboratory, Madison, 147isconsin, Technical Notes No. F - i 6 . ) - - I r t using glues for high-grade joint work, a knowledge of the s t r e n g t h of the joint is important. A m e t h o d which is inexpensive, accurate, and suitable for use in a w o o d w o r k i n g factory has not y e t been developed. T h e following method, w h i c h is used at the F o r e s t P r o d u c t s L a b o r a t o r y , can be e m p l o y e d if a universal timber testing machine is available. Two blocks of selected hard maple, about I inch by 2 ~ inches by I2 inches in size are glued together. After the glue has aged sufficiently they are cut into shear specimens and these are placed in a testing machine so that the base of the long half of the block rests on a metal seat. Pressure is then exerted on the short half, causing it to slide past the long half at the glued point. The pressure required to separate the blocks in this way is measured and the percentage of the area of wood surface torn out by the glue estimated. If the failure occurs entirely in the glue, a measure of the strength of the glue joint is obtained, but if the failure is entirely or p a r t l y in the wood, as f r e q u e n t l y happens, the full strength of the glue is not developed and the test m a y have to be repeated, using s t r o n g e r blocks. As the same m e t h o d has been used in securing data on the s t r e n g t h of wood in shear, when the strength of glue has been determined, it can be compared with that of any wood whose average shearing strength is known. T h e F o r e s t P r o d u c t s L a b o r a t o r y has made thousands of tests on specimens glued with casein and animal glues, and when properly used, these glues have shown shear values of 24oo pounds or more per square inch. Few commercial American woods average more than 24o0 pounds per square inch in shearing strength, and the majority of them average less than 2ooo pounds. Many glue tests have averaged as high as 3000 pounds. H o w Much Water Should be Used in Concrete?
ANON.
(Scientific American, vol. cx, No. I5, p. 365, April I2, I 9 ~ 9 . ) - - T h e E m e r g e n c y Fleet Corporation, in connection with its work on concrete vessels, has developed an apparatus for testing the a m o u n t of w a t e r which should be used in concrete work. An open metal cylinder is employed resting upon a glass plate. T h i s serves as a mold which is filled with concrete and smoothed off level on top. T h e n the cylinder is raised, leaving the concrete on the glass plate. If the mixture is v e r y dry, the concrete will maintain its cylindrical form, but the w e t t e r the concrete the more it flows out at the bottom, so that a measure of the consistency of the mixture can be obtained by m e a s u r i n g the height of the cylinder or cone of concrete after the metal cylinder has been withdrawn.