Custom Search

Sunday, February 22, 2009

Method of underwater concreting - Tremie method


This is a method on how to place concrete underwater, this method place a big role in offshore concreting, since cement looses its strength and fade away under water, Tremie method is to be used. Tremie Concrete is done by using a formwork/pipe which will have one end of the formwork/pipe above water and other bottom end immersed under the water and with the help of gravity. 

A tremie is a watertight pipe, generally 250mm in diameter, having a funnel shape hopper at its upper end and a loose plug at the bottom or discharge end. The valve at the discharge end is used to de-water the tremie and control the distribution of the concrete. The tremie is supported on a working platform above water level, and to facilitate the placing it is built up in 1 to 3.5m section.

During the concreting, air and water must be exclude from the tremie by keeping the pipe full of concrete all the time; and for this reason the capacity of the hopper should be at least equal to that of the tremie pipe. In charging the tremie a plug formed of paper is first inserted into the pipe as the hopper is filled the pressure of fresh concrete forces the plug down the pipe, and the water in the tremie I displaced by concrete.

For concreting, the tremie pipe is lowered into position and the discharge end is kept as deeply submerged beneath the surface of freshly placed concrete as the placed concrete as the head of concrete in tremie permits. As concreting proceeds the pipe is raised slightly and the concrete flows outwards. Care should be taken to maintain continuity of concreting without breaking the seal provided by the concrete cover over the discharge end. Should this seal is broken, the tremie should be lift and plugged before concreting is recommended. The tremie should never be moved laterally though freshly placed concrete. It should be lifted vertically above the surface of concrete and shifted to its new position.

When large quantities of concrete are to be placed continuously, it is preferable to place concrete simultaneously and uniformly through a battery of tremies, rather than shift a single tremies from point to point. It has been recommended that the spacing of tremies be between 3.5 and 5m and that the end tremies should be about 2.5m from the formwork. The risk of segregation and non-uniform stiffening can be minimized by maintaining the surface of concrete in the forms as level as possible and by providing a continuous and rapid flow of concrete.




How is the Underwater Concrete Mixes?


For Structural concrete following must be considered 

  • Coarse Aggregate: Gravel of 3/4” (20mm) max. size.  Use 50-55 % of the total aggregate by weight.
  • Sand, 45-50% of the total aggregate by weight.
  • Cement: Type II ASTM (moderate heat of hydration), 600 lbs/yd3
  • Pozzolans: ASTM 616 Type N or F, 100 lbs/ yd3
  • Water/Cement Ratio: 0.42 (0.45 Maximum).
  • ƒWater-Reducing Admixture (preferably it is also plasticizer):  Do not use superplasticizers.
  • ƒAir-Entrainment Admixtures:  To give 6% total air.
  • ƒRetarding Admixture:  To increase setting time to 4-24 hours, as required.
  • ƒSlump:  6 1/2" ± 1"
  • ƒThis mix will develop compressive strength in the range of 5,600 – 7,000 psi at 28 days.



Method of underwater concreting - Tremie method (inside view)
Basic principle behind Method of underwater concreting - Tremie method


Maldives bridge work using this method

Conditions of underwater concrete

Concrete poured underwater must have good workability and, thus, should meet the following conditions:

(1) The mixture must incorporate the proper proportions of sand and gravel (preferably not crushed material) in a rich paste of Portland cement and freshwater.

(2) The mixing water must not exceed 5.5 gallons per bag of cement. (Mixing water includes the water entering the batch in the form of free, surface moisture on the sand and/or gravel; this free water must, therefore, be deducted from the total water to be
added.) If the aggregate particles are surface-dry and not saturated, they will absorb some of the gross mixing water; allowance must, therefore, be made for extra mixing water, taking care that the W/C ratio of 5.5 gallons per bag is not exceeded.

(3) The mixture should not contain less than 8 bags and not more than 10 bags of cement per cubic yard of ASTM Type V concrete.
- Type V is the recommended product for such structures because of its high resistance to sulfate attack (a form of disintegration occurring in seawater and other high salt environments)

(4) For improved workability, the concrete should incorporate an admixture to provide not less than 3°% and not more than 6% entrained air as determined
by standard ASTM methods.

(5) The sand and gravel should be physically sound, and the maximum gravel size should be 3/4 inch The aggregate should be graded as indicated in Table 1 .


(6) The formwork in which the concrete is poured must be rigid, carefully fitted, and designed so that no underwater currents can pass through it. Provision must be made for the seawater displaced by the concrete to escape from within the form. Timber is generally the most suitable material for construction of the formwork. Joints between the formwork and the intact portion of a structure should be caulked.

(7) Low temperatures during mixing and curing of concrete (i.e., below 50°F) can delay strength development for periods as long as one year and so should be avoided.

(8) An enclosed chute or "trunk" should be specified so that there is no mixing with water during placement

Underwater concrete

Placing of concrete in water is a very difficult operation all aspects from mixing transportation placing and control of the work have to be carefully evaluated and should only be preformed be very experienced engineers and workers the aim in placing concrete underwater is to keep the fresh concrete and water a part as much as possible during the placing of the concrete, and to avoid a rapid flow of either of them and when they come in contact. So that cement will not be washed out.
For this reasons, the correct placing method is the most important factor with respect to final quality.


Underwater concreting is not a new technique: it has been experimented with since about 1850. In 1910 the Norwegian August Gunderson, took a Norwegians patent on a method of underwater casting for concrete columns and the like. In the same year, the method was tried for the first time In Norway for underwater concreting of reinforced structure. This method is, nowadays, the Maine underwater concreting method and is known as termite pipe method.

Since 1980s, admixtures that increase the cohesion of the concrete and make direct contact with water passable without signification changing the properties of the concrete have been developed and are widely used. The ant washed out (AWO) admixtures.

E.g. Rescan T from Norway and similar product, has certain properties that influence the fresh concrete, and the setting and hardening of it knowledge about this properties
is crucial for all parties involved.


Tuesday, February 17, 2009

Contaminants that may be found in drinking water

There is no such thing as naturally pure water. People are increasingly concerned about the safety of their drinking water. As improvements in analytical methods allow us to detect impurities at very low concentrations in water, water supplies once considered pure are found to have contaminants. We cannot expect pure water, but we want safe water. The health effects of some contaminants in drinking water are not well understood, but the presence of contaminants does not mean that your health will be harmed.

Drinking water can become contaminated at the original water source, during treatment, or during distribution to the home.

• If your water comes from surface water (river or lake), it can be exposed to acid rain, storm water runoff, pesticide runoff, and industrial waste. This water is cleansed somewhat by exposure to sunlight, aeration, and micro-organisms in the water.

• If your water comes from groundwater (private wells and some public water supplies), it generally takes longer to become contaminated but the natural cleansing process also may take much longer. Groundwater moves slowly and is not exposed to sunlight, aeration, or aerobic (requiring oxygen) micro-organisms. Groundwater can be contaminated by disease-producing pathogens, leachate from landfills and septic systems, careless disposal of hazardous household products, agricultural chemicals, and leaking underground storage tanks.

• Erosion of natural rock formations

• Substances discharged from factories

• Discharged from farmlands

• used by consumers in their homes and yards

In general all water contains some impurities

Monday, February 9, 2009

Penetration test for bitumen



Introduction

Penetration test is the measures consistency expressed as the distance that a needle vertically penetrates a sample under known conditions of loading, time, and temperature. In the penetrometer (figure 1) the 150 mm diameter dial is graduated in 400 divisions of 0.1 mm. The indicating pointer is fitted with a frictiongeared knob, which can be rotated freely to any position on the dial. Resetting the needle can be carried out quickly and positively ready for the next measurement.
Figure 1; Penetrometer

Methodology

Steps;

1. A sample of bitumen is heated to a temperature of 100 to 150 deg C for not more than one hour and poured in to a penetrometer cup.

2. The sample is left in room temperature till it cools and solidifies (Figure 2).

3. The sample is placed in water bath at temperature of 25 deg C for 45 min since the this experiment requires the sample to be cool inside and out.

4. Transport the sample in a water transport can.

5. The needle is kept close enough to the sample just where it touches the sample( Figure 3).

6. The needle must not be close to the mouth of penetro meter less than 10mm and the test can be done with one sample three days.

If the measurements show its 80 to 100, that means is ok. Take average of three reading and than conclude.
Figure 2; Bitumen sample solidified after heating
Figure 3; Sample kept in penetro meter
Dear reader, pls rate this post.. thanks




"Cant FIND what your looking for!! Try Google search"
Custom Search