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Tham khảo tài liệu 'machinery components maintenance and repair episode 1 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 90 Machinery Component Maintenance and Repair the fact that coarse aggregate has been exposed. Also note that the coarse aggregate is fractured in the process of chipping. Fracturing of coarse aggregate while chipping confirms the good bond of the cement to the aggregate. This observation is a good indication of quality concrete. In summary regardless of the bleeding or the evaporation rate the concrete surface will be weak. The internal tensile strength of concrete can be estimated to be about 8 to 10 percent of its compressive strength. For example the tensile strength of 4 000 lb concrete is usually 320 to 400 psi. The tensile strength of concrete at the surface is only about 50 psi. Because of this weakness the surface of the concrete must be removed prior to grouting if good bonding is expected. Repairing Failures Between Block and Mat Until recent years foundation failures on reciprocating equipment between the concrete block and concrete mat were a rare occurrence. At present this type of failure is becoming more common. This increase in failures can be attributed to poorer construction practices and postponement of equipment maintenance. Before the concrete block is poured the mat must be chipped to expose course aggregate. This is the only good method of removing the laitance from the surface of the mat and providing an anchor pattern between the block and mat. This requires chipping away at least 1 2 to 1 from the surface of the mat. Sandblasting raking the concrete surface prior to curing or roughening the surface with a bushing tool as a means of surface preparation is unacceptable. These methods do not remove all the laitance nor do they expose course aggregate in the concrete. Lateral dynamic forces are generated by most reciprocating equipment and in particular with gas-engine compressors. Consider what happens when maintenance is postponed. One would certainly expect distress from an automobile engine with each cylinder operating at a different .
