Abstract: A concrete composition for underwater use contains about 0.1 to 4% by weight based on cement content of a water soluble vinyl polymer containing vinyl units having sulfonic groups. The concrete composition is easy to mix and prevents separation of concrete composition constituents as it is laid under water.

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: Agitation shafts provided with agitation vanes at lower portions thereof are fastened rotatably to the outer circumferential surface of a support cylinder to form a soft ground hardening machine. Connecting means are provided between upper and lower portions of the support cylinder. The connect-means are adapted to be tensed and slackened for eliminating the inclination of the machine in operation. In a soft ground hardening method using this machine, the inclination thereof can be eliminated as the direction in which the machine is inserted into or withdrawn from the soft ground is measured momentarily.

Abstract: Agitation shafts provided with agitation vanes at lower portions thereof are fixed rotatably around a support cylinder to form a soft ground hardening machine. A direction control unit is provided at a lower portion of the support cylinder so as to control the direction in which the machine is inserted into and withdrawn from the soft ground. In a method of hardening the soft ground by using this machine, the direction in which the machine is inserted into and withdrawn from the soft ground is controlled as the direction in which the machine is being inserted into or withdrawn from the soft ground is measured.