Concrete Cutting Cutter Dunstable MA Mass Massachusetts

Concrete Cutting Dunstable Massachusetts

Concrete Cutting Cutter Dunstable MA Mass Massachusetts

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Compute the value of the modulus of elasticity for a stress of 700 lbs./sq. in. This modulus is the slope of the chord connecting the given stress with the origin and is called the secant modulus. Concrete which may be better expressed in words, using the terminology of the surveyor offsets vary as the squares of the distances. On hardening in air concrete shrinks about 0.0005 of its length due apparently to the drying out. Hardened in water it expands slightly but if later removed from water it shrinks nearly the same as though originally hardened in the air. In consequence of this shrinking compressive stresses are set up in any embedded steel which are generally ignored so far as the principal reinforcement is concerned. When any mass of concrete is not free to contract on shrinking or with temperature fall it becomes necessary to provide reinforcement to compel the formation of many small cracks in place of a few large ones.

For this purpose small deformed rods of high carbon steel with a cross-sectional area of 0.002 to 0.005 of that of the concrete are conservatively used. "Expansion from a rise of temperature rarely causes trouble except at angles where the lengthening of the surface may produce a buckling." The coefficient of expansion for concrete has an average value of about 0.000006 per degree Fahrenheit. The 1924 Joint Committee recommends the use of 0.5 per cent of steel to prevent the opening of construction joints. Steel and concrete change in length with temperature variations very nearly the same amount so that there is practically no stress set up on this account tending to break the bond between the two materials. The coefficient of expansion for steel is 0.0000065 per degree Fahrenheit. It would be impossible to reinforce concrete with steel rods but for the adhesion of the concrete to the steel so that there is no slipping between the two materials as the combined member deforms under load. In design care must be taken that there is no excessive tendency for the steel to slip from the grip of the surrounding concrete since in general a small movement will result directly or indirectly in the destruction or serious damage of the piece. This bond, or resistance to sliding, is of two kinds: an adhesion between the two materials and a sliding resistance that develops after the adhesion is broken and movement begins.

Tests made at the Structural Materials Research Laboratory' by pulling out, ordinary plain round rods from 8 inch by 8 inch concrete cylinders of different ages, where the only resistance to the pull was the force developed by bond on the surface of the rod, showed that there was no slip until the bond stress reached an average value of about 10 per cent to 15 per cent of the compressive strength of the concrete, and that the maximum bond resistance, reached when the slip was about 0.01 inch, equaled approximately 24 per cent of the concrete strength. Earlier tests made at the University of Illinois' showed that square bars give results about 75 percent of those obtained with plain round bars. The same series of tests proved that deformed bars begin to slip at about the same bond stress as plain rounds and that the resistance to sliding offered by the bearing of the projecting lugs on the concrete, while considerably larger than that for the plain bars, does not become effective until a considerable slip has occurred. "The large slip and the high bearing stresses developed in the later stages of the tests show the absurdity of seriously considering the extremely high values that are usually reported to be the true bond resistance of many types of deformed bars."

The reputed value of square twisted bars as offering high bond resistance was also denied by these tests, these bars showing lower bond stresses at small slips than the rounds and developing high resistance only at extremely large slips. The use of deformed bars of proper design may be expected to guard against local deficiencies in bond resistance due to poor workmanship and their presence may properly be considered as an additional safeguard against ultimate failure by bond. However it does not seem wise to place the working bond stress for deformed bars higher than that used for plain bars.