Abstract: After a mortar layer is formed on the surface layer of placed concrete, an ultrasonic vibrator is caused to vibrate while being pressed against the mortar layer surface, and a thin surface layer is scraped off, obtaining a dense water-shielding layer. Mortar with a low water-cement ratio is spread onto the surface of the water-shielding layer, and adhesion between the mortar and the water-shielding layer is strengthened by using the ultrasonic vibrator. The pressing force of the ultrasonic vibrator is then increased to make the mortar and the water-shielding layer densely integrated together, thereby forming a hard concrete surface excelling in abrasion resistance.

Abstract: A cement composition is provided which has the property of being inseparable in water, having improved strength and no effect in delaying setting time or reducing strength. It is produced at a cheaper cost by reducing the amount of a polyacrylamide compound to a considerable great extent. The polyacrylamide compound is present in an amount of 0.1 to 3% by weight based on the weight of cement, when a dialdehyde is added thereto.

Abstract: A concrete composition is provided which has the property of being inseparable in water and no effect in delaying setting time or reducing strength. A polyacrylamide partial hydrolysate having a molecular weight of one million or higher and a hydrolysis degree of from 0.5 to 20% moles is present in an amount of 0.5 to 4% by weight based on the cement constituent proportion.

Abstract: A concrete breaker, in which in breaking a concrete structure, the inclination of the main chisel is maintained within a desired angle by a limit switch to prevent the chisel receiving bed and the ram from being damaged at least two mini-chisels disposed opposite the main chisel are slidably arranged and driving means for these chisels are encased in a machine frame provided with machine beds so that the machine body may be formed smaller and lighter in weight.