What is the purpose of keeping tremie pipe’s tip immersed in freshly-placed concrete for about 1m in underwater concreting?

  What is the purpose of keeping tremie pipe’s tip immersed in freshly-placed concrete for about 1m in underwater concreting? The size of tremie pipe is about 300mm with sections having flange couplings fitted with gasket to prevent water leakage. The tremie pipe should be closed initially to prevent water from entering the pipe. It should be designed with sufficient thickness and weight so that it would not be buoyant when empty inside water. The placement of tremie concrete is commenced by putting the closed pipe underwater to the location for concreting, followed by partial filling of tremie pipe with concrete. In order to have tremie concrete flowed out of the pipe, it is necessary to fill the pipe with concrete of sufficient height to overcome the water pressure and frictional head. After that the tremie pipe is raised about 150mm to allow concrete to flow out. To enhance sufficient bonding, each succeeding layer of concrete should be placed before the preceding layer has reached
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what are the differences between epoxy grout, cement grout and cement mortar??What is the difference between foam concrete and cement grout?

differences between epoxy grout, cement grout and cement mortar

Epoxy groutIn fact, there are various types of resin used in construction industry like epoxy, polyester, polyurethane etc. Though epoxy grout appears to imply the presence of cement material by its name, it does not contain any cement at all. On the other hand, epoxy hardener serves to initiate the hardening process of epoxy grout. It is commonly used for repairing hairline cracks and cavities in concrete structures and can be adopted as primer or bonding agent.

Cement groutratio of cement of water is more or less similar to that of concrete. Owing to the relatively high water content, the mixing of cement with water produces a fluid suspension which can be poured under base plates or into holes. Setting and hardening are the important processes which affect the performance of cement grout. Moreover, the presence of excessive voids
would also affect the strength, stiffness and permeability of grout. It is versatile in application of filling voids and gaps in structures.

Cement mortarproportion by weight is 1:0.4:3). It is intended that cement mortar is constructed by placing and packing rather than by pouring. They are used as bedding for concrete kerbs in roadwork. They are sometimes placed under base plates where a substantial proportion of load is designed to be transferred by the bedding to other members.

What is the difference between foam concrete and cement grout?

Foam concrete is mainly composed of cement, water and air pores with filler (such as PFA, sand etc.) without any coarse aggregates. The air pores are formed by agitating air with a foaming agent. The typical size of air bubbles is around 0.3-0.4mm in diameter. For cement grout, it mainly consists of cement and water.

Foam concrete is characterized by have low density and low cost when compared with normal concrete. The density of foam concrete is around 400 – 1600 kg/m3. Therefore, the low density enhances low dead load and has extensive applications when low loadings are required. Foam concrete does not require compacting and hence imposes no lateral forces on
adjacent structures. Moreover, it also displays good resistance to water and produces high level of sound and thermal insulation. However, it suffers from the demerit that it have low compressive strength only (e.g.less than 15 MPa) which is drastically different from cement grout which possesses high compressive strength. There is recent development of foam concrete as road sub-base.

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