Foundations Example

The function of any foundation is to safely sustain and transmit to the ground on which it rests the combined dead, imposed and wind loads in such a manner as not to cause any settlement or other movement which would impair the stability or cause damage to any part of the building.

Subsoil beneath foundation is compressed and reacts by exerting an upward pressure to resist foundation loading. If foundation load exceeds maximum passive pressure of ground (i.e. bearing capacity) a downward movement of the foundation could occur. Remedy is to increase plan size of foundation to reduce the load per unit area or alternatively reduce the loadings being carried by the foundations.

Power Float

A hand-operated electric motor or petrol engine, surmounted over a mechanical surface skimmer. Machines are provided with an interchangeable revolving disc and a set of blades. These are used in combination to produce a smooth, dense and level surface finish to in-situ concrete beds.

The advantages offset against the cost of plant hire are:

* Eliminates the time and materials needed to apply a finishing screed.
* A quicker process and less labour-intensive than hand troweling.

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.