Concrete Cutting Cutter Dracut MA Mass Massachusetts

Concrete Cutting Dracut Massachusetts

Concrete Cutting Cutter Dracut MA Mass Massachusetts

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Dracut Massachusetts Concrete Cutting and Core Drilling

Whatever the method of waterproofing used the protection must form a continuous surface around pits under boilers and so on, and the concrete must be structurally sufficient to resist all stresses due to water pressure. The strength of concrete is very variable, depending on many factors, and it requires skilled workmanship and control to secure even reasonable uniformity in quality. Average values for the 28-day compressive strength for various mixes are given in Article 18. This table gives results to be expected with specimens made and stored in accordance with standard laboratory practice. A test piece obtained by drilling a core from the concrete in place on the job should not be expected to give as great strength. In comparing test results it is important to note whether the test specimen was a cube or a cylinder since cubes show materially higher strength than cylinders on account of the lack of opportunity for free fracture along the inclined planes of maximum shear. The tensile strength of concrete is small, averaging about 10 per cent of the compressive strength. It is common to neglect it in design. The shearing strength is approximately 60 per cent to 80 per cent of the compressive strength. Working stresses for design are generally expressed as percentages of the .28-day compressive strength of cylinders. At this time the concrete has attained approximately two-thirds of the strength it will have at six months and about one-half of that at three years. Working stresses 25 per cent higher than those used in design for concrete 28 days old are usual for computing the safe loads for old structures and in designing for future additions. The average strength to be expected from various mixes and the classes of construction where they are employed may be roughly summarized as follows: Foundation walls, plain concrete, retaining walls, piers, abutments, machine foundations. Concrete is not an elastic material, strain increasing faster than stress from the very first and permanent deformation occurring under low stress. On the average the stress-strain curve in compression may be taken as approximately a parabola with vertex at the point of ultimate strength and axis vertical, stress being plotted vertically and strain horizontally. Within the range of working stresses this curve does not deviate greatly from a straight line and it is universally the custom to assume that the modulus of elasticity is a constant for working stress conditions. The values of the modulus given by different authorities differ greatly, 3,000,000 pounds per square inch being perhaps the most accepted average for ordinary 1-2-4 concrete at 28 days. As the concrete ages it gets harder and stiffer and the modulus increases in design computations a lower value than the actual is used for reasons that will appear in the discussion of the mechanics of reinforced concrete. The Joint Committee recommendations for the design modulus are given in Appendix B. Alumina cement concrete has a considerably higher modulus than Portland cement concrete of the same proportions. Strictly speaking there is no elastic limit for concrete. The term is often used inexactly however to indicate the limit of stress that may be applied repeatedly without causing increase in permanent deformation. This limit is about 50 per cent of the ultimate; loads beyond this limit and below the static ultimate when applied repeatedly, because continually increasing deformation, and, finally, rupture. The modulus of elasticity of steel is taken as 30,000,000 pounds per square inch for all grades of steel in all computations of reinforced concrete. This value is slightly higher than the average given by tests. A certain concrete has an ultimate compressive strength of 2200 lbs./sq. in. with a corresponding strain of 0.0018.