Types of Foundation
The foundation is the base for the constructed building and as such is required to carry out several functions as follows:

0 The foundation should safely sustain and transmit to the ground the combined dead and imposed loads of the building without resulting in any settlement or other movement of the building or any adjoining works.

1 Foundations should be deep enough or be constructed in such a way as to avoid damage by the swelling, shrinkage or freezing of the subsoil.

2 The foundation should be capable of resisting attack by chemicals such as soluble salts or other deleterious materials present in the subsoil.
Choice of Foundation
The designer is required to choose the most appropriate type of foundation taking account of two main factors:

* The total dead and imposed loads of the building * Possibility of subsoil movement

The foundation design should take account of any subsoil movement caused by changes in the moisture content of the subsoil. These movements are normally changes in volume when the subsoil becomes either wet or dry and will normally occur near the surface of the soil . Similar volume changes can occur due to water freezing and expanding in a soil (this is called frost heave).

Note the average foundation loading for a two-storey domestic dwelling is normally between 30 and 50 kN per metre.

Foundation Types
The most common foundation types used in low-rise domestic and light industrial buildings are:

3 Traditional Strip 4 Deep Strip or Trench Fill 5 Raft 6 Pad 7 Piled
Strip Foundation
A traditional strip foundation is the most commonly used foundation for domestic low rise structures and consists of a ribbon or strip of concrete placed centrally under the walls of the building . Traditional strip foundations are normally 600 millimetres wide and a minimum of 150 millimetres in depth, with the walls positioned centrally to allow a uniform distribution of the load.

Traditional strip foundation without trench timbering.

Strip foundation in firm ground, typical of most housing sites.

Strip foundation in firm ground, typical of most housing sites.

Strip foundation ready to receive brickwork. This foundation is for a two bedroom starter home.

Strip foundation ready to receive brickwork. This foundation is for a two bedroom starter home.

There are various methods of modifying the concept of strip foundations to allow its use in other situations, for instance where some soil has a poor bearing capacity. This may be the case in wet ground, soft clay or filled ground, where the width of the strip foundation can be extended to spread the load over a larger area of soil. This will require the provision of reinforcement to the foundation in order to withstand tension and spread the load over a wider area.

When a strip foundation is to be used on a sloping site the most economic solution is to use a stepped foundation which will reduce the amount of excavation, construction under ground, backfill and trench support. The provision of stepped foundations following the line of the ground requires each step to be between 150 and 225millimetres in order to accommodate brick or block courses. The lap of concrete at the step should be not less than the depth of the foundation concrete and never less than 300millimetres whichever is the greater.

Deep Strip or Trench Fill Foundation

A further modification to the strip foundation is the use of deep strip or trench filled foundations which were developed with a view to reducing the amount of work carried out underground by tradesmen. The idea of the trench fill foundation is to fill a narrow foundation trench , to a depth of 900 millimetres or more, with concrete and this is particularly useful in shrinkable clay soils in order to avoid problems caused by shrinkage and swelling as the seasons change. The foundation width is reduced to around 400-450millimetres with a depth to suit the particular site conditions. Those who support the use of trench fill foundations point to time savings and quick completion of groundwork, although this type of foundation is much more expensive in the cost of concrete.

The use of trench fill does require careful consideration of service entry, as in most cases services will have to be taken through the concrete foundation by providing holes or ducts. The positioning of service holes or ducts will require accurate setting out to in order to avoid expensive mistakes.

Trench fill foundation with substructure brickwork. Finished ground level will be the height of the drain cap.

Trench fill foundation with substructure brickwork. Finished ground level will be the height of the drain cap.

Raft Foundation
A raft foundation is simply a foundation which covers the whole area of the building and will normally be used in soils deemed to have a poor bearing capacity. They are normally used in ground which is soft such as filled sites liable to subsidence, for example in mining areas or ground which is naturally soft. A raft foundation is a reinforced concrete slab up to 300millimetres thick, laid near the surface of the ground. The raft is thickened below walls to accommodate the extra load.

TIE TO SUPPORT INSULATION

Short Bored Piles
For foundations constructed in ground where there is the possibility of movement, such as in shrinkable clays, where trees are planted or infilled ground, it may be uneconomic to use trench fill or deep strip foundations. A reasonable alternative is the use of short bored piles consisting of a series of columns of concrete cast into the ground to a depth of 4.5-5m. The piles will transmit the load of the building to a depth where ground of a suitable bearing capacity can be found. Piles are normally 300mm in diameter and in domestic properties 1.5 – 2.5m apart, depending on the loading.

After the piles have been cast a concrete a groundbeam is placed over them to support the wall construction.

Pad and Beam Foundations
Pad foundations are normally used to support columns but it is possible to use them in domestic work where there are soft spots or ground with an irregular bearing capacity. Isolated concrete pads can be placed in the ground (sometimes on top of needle piles) and light reinforced concrete beams placed between them to support the structural walls.

Foundation and Substructure Concrete
The use of foundation concrete is one of the most basic requirements of all building, and this section of the work deals with the provision of concrete for use on site.

The materials used for concrete and the testing procedure to ensure quality control are dealt with in other construction units.

The unit will deal with site practice related to the following areas of concrete foundation and substructure provision:

* Mixing * Transportation * Placing and Compaction * Curing and Protection
Site Concrete
Concrete used for foundations must be strong enough to resist the loads imposed on it and transmit the loads to the ground beneath, without failing or being effected by weathering or chemicals contained in the ground.

The specification of concrete mixes has since 1990, changed with the introduction of designated mixes. Pre-1990, most concrete mixes, specified for domestic housing used a volume or weight specification and were called prescribed mixes, for example plain concrete for strip foundations was proportioned as follows:

1:3:6
1 unit of cement
3 units of fine aggregate
6 units of coarse aggregate

The disadvantage of prescribed mixes on site has been the lack of consistency and quality control in their application. If measured by volume the bulking of damp fine aggregate could mean an error of up to 30% in the mix. BRE Digest 326 deals with the disadvantages of prescribed mixes in some detail.

Since 1990-91 specifying concrete by its grade and compressive strength has become normal practice. BS 5328 contains the provision of designated concrete mixes. The advantage of designated mixes is that the specifier, concrete producer and contractor can be expected to provide consistently high quality concrete across a wide range of uses and at the same time provide a quality assured product.

BRE Digest 326 contains a table of designated concrete mixes and, for example, the mix used earlier in this note for plain concrete would become C7.5 that is a plain concrete mix which reaches a 28 day compressive strength of 7.5 N/mm2.

The use of a designated mix removes the possibility of a dispute if the concrete does not reach the required strength.

Concreting on Site
Mixing
Concrete mixing on site either by hand or machine only applies to small projects in the modern construction industry, mainly due to the number of errors that can occur in the provision of correct amounts of materials, water and mixing time.

The majority of concrete is produced off site using designated mixes by ready mix companies who are able to guarantee a quality assured product which is convenient and consistent whatever site and weather conditions prevail.

Transportation
Most concrete will be brought to site in a ready mix truck and, on many sites, roads will be in place to allow discharge directly to the point it is needed, using the standard concrete chutes carried by the ready mix truck.

In some situations where concrete cannot be placed directly, other means of site transport may be required and these can be split into horizontal and vertical movement.

CONCRETE TRANSPORT TABLE

HORIZONTAL MOVEMENT | Barrows | Used for small quantities transported over short distances. Wheelbarrows hold 30 litres or 0.03m3. Power barrows hold 150 litres or 0.15 m3 of wet concrete. Runs need to be smooth and well constructed. | Dumpers | Used to transport 0.5m3 per journey. Care has to be taken when discharging – smooth runs needed to prevent segregation. | Lorry Mounted Hoppers | Capacity 2-3 m3 used on large contracts. Hopper uses hydraulic tipping runs and can be fitted with mixing paddles. | Tipper Trucks | Open trucks with tarpaulins to protect concrete from rain or sun. | Ready Mix Lorries | Ideal method of transporting concrete from batching plant to point of discharge. Access roads must be provided. | VERTICAL MOVEMENT | Buckets and Skips | Skips and buckets moved by crane with skips discharged from the bottom and buckets tipped. Capacity from 0.5 m3 to 6 m3. | Conveyor Belts | Not used regularly over the past few years. Most likely to be seen mounted on some truck mixers to allow discharge where chutes are not long enough (normally 10 m of conveyor). | Hoists | Used on tall buildings to transport concrete in containers such as barrows. | Pumping | Pumps are convenient for moving concrete on sites. They can be static or mobile and have the capacity to move large amounts of concrete. Normal coverage is 50 m vertically and 100 m horizontally. Pumps can deliver 30 m3 per hour. |
Transportation Affecting
Quality of Concrete
Segregation
Segregation occurs when concrete is transported over long distances or over rough ground. Segregation is the separation of the aggregates according to weight, the heavier aggregate sinks to the bottom of the container and the water (with cement suspended in it) floats to the top. Segregation itself will not affect quality of concrete as long as material which may have segregated is re-mixed at the point of discharge. Concrete should never be allowed to fall for more than 1 m to avoid segregation.
Workability
The workability of concrete has to be matched with the method of transportation. It should be noted that concrete needs very little water to initiate the setting process, most of the water being present to allow placing and compaction of concrete.

Concrete to be pumped usually has a high workability as this prevents blockage of the pipework.
Rain, Wind and Hot Weather
Winds and sun dry concrete quickly during transport, whilst rain will make the mix too wet. Protection is provided by covering containers with tarpaulins. Concrete which is either too wet or too dry when laid, will affect the strength and quality of the finished concrete.
Leaking Containers
Hoppers, skips, buckets or lorries with ill fitting trap doors or tail gates will allow the loss of water, fine aggregate and cement, making concrete difficult to discharge, place and compact, effecting the quality of the finished product.
Contamination
Oil, dirt, dust and mud must not be allowed to contaminate concrete.
Placing and Compaction
Concrete can be placed into a confined space such as a foundation trench or floor slab or, in the case of lintels and concrete buildings, into formwork. In general the need for a clean dry area free from debris is required for placement of concrete. Where concrete is placed, segregation is prevented by avoiding free drops of more than 1 m.

Adequate compaction in most cases for domestic work can be obtained by tamping by hand using a wooden or lightweight metal beam. Where dry mixes are used or a high density concrete is required mechanical vibrators can be used.

There are three main types of vibrator:

* Poker A cylindrical vibrator connected to a motor by a flexible drive shaft, diameters ranging from 25-75 mm. Concrete should be placed in layers of 300-500 mm depending on the length of poker used. Care must be taken not to touch reinforcement as this will result in de-bonding between the reinforcement and the surrounding concrete. * External Motors which are designed to vibrate are clamped to the formwork
(Clamp On) allowing the formwork to vibrate the concrete. Often used in
Vibrators pre-cast work. Formwork must be designed and constructed to resist the constant vibration. * Beam Vibrators Designed for use on concrete slabs which are too thick for hand tamping (over 100 mm), the beam vibrator is made up of a metal beam with a clamp on vibrator fixed to the top. The need for level side forms which will not move when the vibrating beam is used is essential. The beam is pulled along the side forms and will leave a flat level surface on the floor slab.

Beam vibrator for use on a floor slab
Beam vibrator for use on a floor slab

Curing and Protection
Curing ensures that concrete performs as designed in terms of strength, water resistance, and chemical resistance, and has better wear qualities.

Curing is the name used for the process of ensuring the concrete is kept moist and at the correct temperature to allow the hydration process to take place at the right time and speed.

The curing method will in most cases also provide adequate protection from extremes in temperature and weather conditions. The most common protection for domestic floor slabs is the use of polythene sheeting.

The following table shows the relationship between curing techniques and weather protection.

METHOD OF CURING | WEATHER PROTECTION | COMMENTS | Covering concrete with damp sand or hessian | Protects from mild frosts and light rainfall | Care must be taken to ensure concrete is dry enough to prevent marking or contamination when surface is covered | Ponding – flooding of the surface with water | Protects from sun and wind | Freezing could cause damage to surfaces | Spraying – water sprayed on surface | None | Labour intensive, continual spraying could cause damage if not carefully controlled | Sheet polythene retains moisture in the concrete | Protects from all forms of weather damage | Placed as quickly as possible on a frame if surface is likely to be damaged. Sheet weighted at edges preventing wind under sheet. Tape joints between sheets when curing slabs | Sprayed compounds – resin sprayed on to concrete surfaces to form a protective skin sealing in moisture | Provides protection from rain and wind when dry after application | Resins have to be applied carefully in order to ensure they work properly. Designed to degrade without effecting concrete after the curing period |