Reinforced Concrete Structure

• What is RC Concrete?

– Concrete that needs to be strengthened in tension

– Concrete good in compression poor in tension

• How?

– Steel bars embedded into the concrete

– Cut to a variety of shapes

– Cut & bent on site or delivered cut & bent

– Shape Codes

Formwork

• Before a concrete structure can be cast a mould has to be made

– This called formwork

• Formwork can be made from timber, metal, GRP and Polystyrene

• Formwork can be made with patterns in it and/or motifs

– Often called ribbed formwork

– Labour intensive, therefore expensive

• Formwork can be proprietary

• Fixing and supporting formwork is crucial.

• Accurate calculations are required if the formwork is not to burst open.

• Formwork can be massive and supported/reinforced by steel strongbacks and walings.

• Formwork design is serious business.

• Accurate setting out is vital

• Re-use?

Falsework

• Falsework is the part of formwork that supports the forms.

– Bridge decks, bridge abutments, multi-storey (table forms)

• It is important to keep the reinforcement in the formwork in position

– Reinft. must not move when concrete is poured.

Concrete

• Designed mixture of cement, sand, aggregate and water

– Water/cement ratio crucial

– Cement is most expensive

• Manufactured to a BS 810

– Aggregates must be clean and chemically inert

– Course aggregates can be pit gravels, beach gravels, crushed angular stone and/or crushed blastfurnace slag

– Air entraining

• Testing

– Specification will describe tests

Reinforcement

• Hot rolled mild steel bars, BS 4449

– Most common, 6mm to 40mm, max length generally 12m

• Hot rolled & processed high tensile alloy bars, BS 4486

– Less common

• Steel wire, BS 4482

• Steel fabric, BS 4483

– 200mm x 200mm, 1.54kg to 6.16kg per square metre

• Reinforcement cutting, bending and placing is a skilled job

• Accuracy is crucial

• Reinforcement must not move during pouring

• Can be constructed in or out of mould

• Needs careful handling/lifting to install to ensure no deformation

Placing Concrete

• By hand

• By ready-mix lorry

• By skip

• By cableway

• By pumping

• By conveyor

– Which ever way discharge needs careful control to prevent segregation

– Not less than 5C

– Within 30min of discharge

– Needs compacting – vibrating

– Needs curing

Joints

• Contraction joints

• Expansion joints

• Joggle joints

• Sliding joints

• Temporary joints

Waterbars

• Used if shrinkage of concrete would cause leaks in water retaining structures

Piling, Diaphragm and Retaining Wall Systems

Sheet Piling

Sheet piles are normally formed of reinforced concrete or steel

Reinforced concrete sheet piles are of value in the construction of permanent embankments to rivers, canals and other forms of water-related structures.

An interlocking mechanism joins piles together

Steel sheet piling is the most common form of sheet piling and is used in both temporary and permanent works.

It is used in such structures as cofferdams, retaining walls, river frontages, quays, wharves, dock and harbour works, land reclamation and sea defence works.

Advantages are: high structural strength combined with watertightness and can be easily driven into most types of ground.

Steel sheet piles are available in four basic forms in the UK:

· Normal sections

· Straight web sections

· Box sections

· Composite sections or high modulus piles.

Normal section sheet piles include Larssen and Frodingham sheet piles.

Straight web piling is used to construct cellular cofferdams.

Such piles are interlocked and driven to form cells which are then filled with gravel or broken rock.

Box piles are formed from two or more sheet pile sections welded together.

Used where local heavy loads are anticipated and can be positioned in a normal section pile wall

Composite sheet piling or high modulus section piling has been developed to support bending moments which are in excess of the capacity of normal sheet pile sections.

Methods of driving

When piles are being driven they have a tendency to lean in the direction of driving;

There are various ways in which piles may be guided during driving, the two principal methods are:

· Driving in panels

· Use of trestles and walings.

Pile driving equipment

· Percussion drivers - which includes diesel and air hammers

· Hydraulic drivers

· Vibratory drivers.

Examples of hydraulic drivers is the ‘Still Worker’ Designed by Tosa, in Japan, the ‘Still Worker’ is a hydraulic piling machine which installs and extracts sheet piles and H-piles.

Hydraulic drivers work on the principle of pushing sheet piles into the ground by means of a hydraulic ram acting against a firm reaction. The reaction is provided by the skin friction of piles already partially driven.

Vibratory drivers are very suitable for the initial driving of sheet piles, H-piles and tubular piles but must be avoided in heavy clays since the clay tends to dampen the vibrations.

The unit vibrates the piles, which reduces the resistance of the ground around them, allowing the piles to pass into the ground. The vibrations can be either low or high frequency depending on the nature of the soil - high frequency can reach 2300 vibrations per minute.

Continuous-flight augered piles with grout or concrete intrusion

Continuous-flight auger (CF A) piles are now widely available in sizes generally up 750 mm diameter, and exceptionally 900 mm diameter. The commonly used diameters are 300, 450, 600, and 750 mm. These piles offer considerable environmental advantages during construction. Vibration is minimal, and noise outputs are low. In permeable soils with a high water table, their use removes the need for concreting by the tremie method, and temporary support for the borehole walls using casing or bentonite slurry is not necessary. The method of pile construction is suitable in sands, gravels and clays.

HIGHWAY DRAINAGE

The main drainage system consists of:

· Foul water drainage to carry away safely, and without causing unacceptable pollution.

· Storm water drainage to limit or prevent flooding

Foul Water Drainage

Three systems are in existence:

· Totally Separate

· Partially Separate

· Combined System

Totally Separate

· Two sewer networks are required

· One carrying foul sewage and the other storm water.

· Reduces pollution of watercourses to a minimum.

Partially Separate

· Compromise system

· One carrying storm water

· The other foul water plus the storm water off house roofs and backyards.

· Provides a flushing of the foul sewer by the roof water

· The foul sewer must be provided with storm overflows

· Possibility of pollution of watercourses remains

Combined System

· Carries both foul and storm water

· Found in many old built-up areas

· Was the cheapest and quickest way of providing the drainage required under the Public Health Acts.

· The storm water flow is many times greater than the quantity of foul sewage carried

· Sewer is designed as a storm water drain.

· Size of pipe required tends to be very large

· When the storm flow exceeds a predetermined limit, the overflows discharge he excess flow

· Overflows are designed to operate when the flow in the sewer exceeds six times the dry weather flow

· Mixture of foul and storm water causes pollution

· Today unacceptable

Foul Drains

· Emphasis in roadwork is on storm drainage

· Basic knowledge of foul drainage is needed

· A foul sewer is normally designed to carry the foul sewage of the area which it serves

· Allowance for future developments

· Liquid wastes from Industry must also be allowed for in the design

· Even with a totally separate system there will be some storm water getting into the foul drain, yard gullies in factories and garages should be connected to it, not to the storm sewer.

Storm Water Drains

· Estimation of the quantity of storm water to be carried is difficult to determine. Affected by:

· Variation of rate of intensity of rainfall during a storm.

· The unpredictable direction of movement of the storm.

· The degree of impermeability of the area on which the rain falls

· The storage capacity of the sewer system itself.

· The time taken for rain water to get into the sewer.

Road Drainage Systems

Main drainage requirements fall into two categories:

· Sub-soil Drainage

· Drainage of the Carriageway

Sub-soil Drainage

· Must be a provision of sub-soil drainage to cope with water in the ground and to ensure that the water table can be kept low enough

· Sub-soil drainage also helps to prevent frost damage to the road structure by keeping it drained.

· Usually associated with the construction of new roads

· Care must be taken to keep the water table below the road formation level.

· May require a system of land drains, collector drains and a long outfall pipe before the ground can be drained sufficiently for the Works to commence.

· Before excavation for cut and fill begins, it is good practice to lay French drains

Drainage of the Carriageway

· Surface of the road must be kept clear of standing water

· Roads should be cambered when straight

· Laid to crossfalls on bends

· Adequately provided with gullies or grips to dispose of water

· In many cases, the drainage of surface water and sub-soil water are connected to the same system

Drainage Theory

Aim of Drainage:

· Provide drains/sewers with sufficient capacity to deal with he most severe storm conditions

· Laid to falls so that he water entering the sewers will be conducted away from the road, discharging into watercourse

· Outfall to the watercourse must high enough to be clear of the highest water level of the river or stream to prevent backing up

Self-cleansing Velocity

· Laid uphill from the outfall point with the gradient of the pipeline being sufficient for any water to move with at least ‘self-cleansing velocity’

· Gradient to be such that the minimum velocity of flow will allow solid particles to remain in suspension in the water

Gradient

Gradients at which the various sizes of pipes are self- cleansing are approximately as follows:

· 100 dia 1 in50

· 150 dia 1 in75

· 225 dia 1 in 112

· 300 dia 1 in 150

NB Self-cleansing can be determined by dividing the pipe diameter (measured in millimetres) by 2.

Manholes

· All sewer pipelines must he laid in straight lengths to a constant grade

· Manholes are required at every change of:

· a) direction ;

· b) gradient;

· c) pipe size;

· d) pipe type;

· For long straight lengths, manholes or inspection chambers are often provided at not more than 100 m intervals.

· Manholes are provided to enable the sewer to be rodded in the event of it becoming blocked

Gullies

· Connections to storm sewers made by laying junction pipes along the sewer pipeline.

· Not less than 100 mm dia.

Drainage Excavation

Reasons for Collapse

· Failure of the soil to support its own weight.

· Steeply angled bedding planes which encourage slips

· Water sheds or seepages

· Breakdown of the cohesion of the soil by frost or heavy rain.

· Changes in the type of soil such as weak material underlying sound rock or layers of sand and clay

· Failure due to trenching on or near the position of an earlier excavation.

· Vibration due to the close passage of vehicles and plant.

· Failure due to loads placed too near the edge of the face.

· Impact of heavy loads such as pipes, striking the sides of a trench when being lowered.

· Inadequate timbering in supporting a face.

· Lack of cohesion (or shearing) of soil