Study of underwater concrete using two-stage (preplaced aggregate) concrete in Libya

Placement of concrete underwater is necessary in the implementation of most in-shore, and off-shore structures. The pouring of underwater concrete is considered to be a challenge for engineers, even during the design stage or during implementation and supervision. This is because many precautions must be taken to ensure the success of the casting process. The most important of these is to protect the fresh concrete from the water during the casting process to avoid the risk of wash-out of the cement past and segregation of aggregates. Concrete can be placed underwater successfully through good design of the concrete mix and choosing the most suitable method for placing of the concrete. There are new techniques for underwater concreting such as grouted aggregate; this is known as the two-stage concrete method. The main objective of this paper is to present the capability of pouring concrete underwater using this method. A laboratory model was prepared, visually investigated and tested by extracting core samples, then performing compressive, tensile and ultrasonic pulse velocity tests. From the results obtained it has been observed that concrete can be poured successfully underwater using the two-stage method, and it is recommended that this research may be developed by using different water-cement ratios and cement-sand ratios to obtain the optimum mix design; also, different types of aggregates, which are available in local quarries, may be used.