Question

Discuss the properties and uses of concrete that can be used under water construction.​

Answer 1

Underwater concrete was specially designed to enhance constructability and performance in water environments. ... It can be characterized by the concrete resistance to washout, segregation and bleeding and is affected by the mix proportioning, aggregate shape and gradation, admixtures, vibration and placement conditions.

Answer 2

Answer:

Explanation:

Underwater concrete “UWC” is one special type of high performance concrete used in the past, present, and in the foreseeable future as long as there is need to construct bridges, with foundations in soil with high water levels, and almost all off- and on-shore structures. The term high performance concrete refers to concrete that performs particularly well in at least three key performance indicators: strength, workability, and service life. Therefore, underwater concrete should meet these performance criteria and it remains a viable and economic choice for consultants and contractors. UWC requires special and careful monitoring during all stages of construction; i.e. special considerations for selecting the right materials, specialized apparatus for the quality control, design, and methods of construction. Underwater concrete was specially designed to enhance constructability and performance in water environments. Using the underwater concrete technique may avoid engineers using the “old-style” of construction by isolating the water, and therefore, minimize interruption to plant operation and result in high savings.

“UWC” is a highly flowable concrete that can spread into place under its own weight and achieve good compaction in the absence of vibration, without exhibiting defects due to segregation and bleeding. Underwater concrete technology has developed dramatically in recent years, so that the mix can be proportioned to ensure high fluidity as well as high resistance of washout and segregation.

The construction of a wide range of structures including bridge piers, harbors, sea and river defences over many decades, and the development of offshore oilfields, has required placement of concrete underwater. This process can be successfully carried out and sound, high-quality concrete can be produced if sufficient attention is paid to the concrete mix design and the production method applied.

The stability of fresh concrete depends on the rheological properties and placement conditions. It can be characterized by the concrete resistance to washout, segregation and bleeding and is affected by the mix proportioning, aggregate shape and gradation, admixtures, vibration and placement conditions. The differential velocity at the interface between the freshly cast concrete and surrounding water can erode some cement and other fines. Such erosion can increase the turbidity and contamination of the surrounding water, and impair strength and durability, as well as bond to reinforcement steel and existing surfaces.

The improvement of the in-situ properties of underwater concrete is related to the enhancement in washout resistance [1–3]. A superplasticizer (SP) is used to ensure high fluidity and reduce the water/powder ratio (W/P). An anti-washout admixture (AWA) is incorporated to enhance the yield value and viscosity of the mix and hence the washout resistance and segregation resistance [1,4]. The majority of AWAs are water-soluble polymers that increase the yield value and viscosity of cement paste and concrete [4,5]. A statistical design approach was used to establish statistical models and to provide an efficient means of evaluating the influence of key mix variables on the fresh and hardened concrete characteristics that affect the performance of underwater concrete [6,7]. The derived models include mixes with 380 to 600 kg/m3 of powder, W/P ratios of 0.34 to 0.46, sand/aggregate ratios of 0.42 to 0.50, as well as AWA and SP dosages varying between 0.005% and 0.265% and 0.05% and 2.65%, respectively. The slump flow and the washout resistance are influenced, in order of importance, by the concentrations of AWA and cement, then by the water/cement ratio and dosage of SP, and various combinations of these parameters. The sand/aggregate ratio had a secondary effect on these properties [6,7].