Poker Vibrators

These consist of a hollow steel tube casing in which is a rotating impeller which generates vibrations as its head comes into contact with the casing

Poker vibrators should be inserted vertically and allowed to penetrate 75 mm into any previously vibrated concrete.

Clamp or Tamping Board Vibrators

Clamp vibrators are powered either by compressed air or electricity whereas tamping board vibrators are usually petrol driven

Placing Concrete

This activity is usually carried out by hand with the objectives of filling the mould, formwork or excavated area to the correct depth, working the concrete around any inserts or reinforcement and finally compacting the concrete to the required consolidation. The compaction of concrete can be carried out using simple tamping rods or boards or alternatively it can be carried out with the aid of plant such as vibrators.

Concrete Pumps

These are used to transport large volumes of concrete in a short time period (up to 100 m3 per hour) in both the vertical and horizontal directions from the pump position to the point of placing. Concrete pumps can be trailer or lorry mounted and are usually of a twin cylinder hydraulically driven format with a small bore pipeline (100 mm diameter) with pumping ranges of up to 85„000 vertically and 200„000 horizontally depending on the pump model and the combination of vertical and horizontal distances. It generally requires about 45 minutes to set up a concrete pump on site including coating the bore of the pipeline with a cement grout prior to pumping the special concrete mix.

The pump is supplied with pumpable concrete by means of a constant flow of ready mixed concrete lorries throughout the pumping period after which the pipeline is cleared and cleaned. Usually a concrete pump and its operator(s) are hired for the period required.

CONCRETING

this site activity consists of four basic procedures †

1 . Material Supply and Storage - this is the receiving on site of the basic materials namely cement, fine aggregate and coarse aggregate and storing them under satisfactory conditions. (see Concrete Production † Materials on pages 284 & 285)

2. Mixing - carried out in small batches this requires only simple hand held tools whereas when demand for increased output is required mixers or ready mixed supplies could be used.

3. Transporting - this can range from a simple bucket to barrows and dumpers for small amounts. For larger loads, especially those required at high level, crane skips could be used:

For the transportation of large volumes of concrete over a limited distance concrete pumps could be used.

4. Placing Concrete - this activity involves placing the wet concrete in the excavation, formwork or mould; working the concrete between and around any reinforcement; vibrating and/ or tamping and curing in accordance with the recommendations of BS 8110: Structural use of concrete. This standard also covers the striking or removal of the formwork. Further ref. BS 8000-2.1: Workmanship on building sites. Code of practice for concrete work. Mixing and transporting concrete. Also, BS EN 1992-1-1 and -2: Design of concrete structures.

Tower Cranes

Most tower cranes have to be assembled and erected on site prior to use and can be equipped with a horizontal or luffing jib. The wide range of models available often make it difficult to choose a crane suitable for any particular site but most tower cranes can be classified into one of four basic groups thus:-

1. Self Supporting Static Tower Cranes † high lifting capacity with the mast or tower fixed to a foundation base - they are suitable for confined and open sites. (see page 194)

2. Supported Static Tower Cranes † similar in concept to self supporting cranes and are used where high lifts are required, the mast or tower being tied at suitable intervals to the structure to give extra stability. (see page 195)

3. Travelling Tower Cranes † these are tower cranes mounted on power bogies running on a wide gauge railway track to give greater site coverage - only slight gradients can be accommodated therefore a reasonably level site or specially constructed railway support trestle is required. (see page 196)

4. Climbing Cranes † these are used in conjunction with tall buildings and structures. The climbing mast or tower is housed within the structure and raised as the height of the structure is increased. Upon completion the crane is dismantled into small sections and lowered down the face of the building. (see page 197)

All tower cranes should be left in an `out of service' condition when unattended and in high wind conditions, the latter varying with different models but generally wind speeds in excess of 60 km p.h. would require the crane to be placed in an out of service condition thus:

Mast Cranes

These are similar in appearance to the familiar tower cranes but they have one major difference in that the mast or tower is mounted on the slewing ring and thus rotates whereas a tower crane has the slewing ring at the top of the tower and therefore only the jib portion rotates. Mast cranes are often mobile, self erecting, of relatively low lifting capacity and are usually fitted with a luffing jib. A wide variety of models are available and have the advantage over most mobile low pivot cranes of a closer approach to the face of the building.