Concrete, Formwork & Reinforcing

Lesson 4

Two methods of fabricating reinforced concrete◦ Pouring liquid concrete into forms on a building

site. This is called ‘in-situ’◦ Precast concrete – components are manufactured

in a central plant and brought to site for assembly

Concrete

In- situ concrete is a mixture of aggregate, Portland cement and clean water. The ratio of which determines the strength of the concrete. Concrete strength is defined in mpa 25 Mpa, 30 Mpa etc. The cost of varies with the strength of the concrete

Additives are sometimes added to the concrete to make them either harden quick or slower. These are an additional cost to the price of standard concrete

Coloured concrete is more expensive. Care must be taken when pricing coloured concrete as the oxide percentage specified has a bearing on the cost. You must always check with the supplier

Insitu Concrete

There is a range of concrete formats and finishes available For a relatively smooth finish it is first necessary to screed

(make level) the concrete once poured and compacted, then ‘float’ the surface using any one of a range of floating devices. These machines have up turned blades that sit parallel to the surface and flatten any exposed aggregate at high speed

For a perfectly smooth closed finish the concrete will need to be trowelled after the floating process. A power trowelling machine has sharper blades that ‘close’ the concrete surface.

Other finishes are possible including the rough exposed aggregate look, which creates a non-slip finish.

The type of finish is classified from U1 class to U5E class. The cost of the finish varies depending on the complexity

Insitu concrete

Formwork◦ This is a type of boxing into which the wet concrete is

poured. Formwork is commonly built from wood planks and boards but in commercial buildings plastic and steel are commonly used. Formwork is a temporary structure in most cases but it can be left permanently

◦ When pouring a concrete ground slab formwork may not be needed as the brick or block foundation walls act as the sides and the hardcore/metal acts as the base

◦ For a suspended slab the formwork is shaped like a tray and is often supported by temporary props until the concrete is set

◦ Different types of finish F1 – F4 will also affect the price of the concrete

Insitu Concrete components

Formwork examples

• Reinforcing If the slab is to be reinforced then rebars are

positioned within the formwork before the concrete is poured. Plastic bar chairs/spacers are used to hold the rebar away from the bottom and sides to the formwork so when the concrete sets it covers the all the rebar.

Mesh reinforcement is also used in insitu concrete slabs

Insitu Concrete components

Reinforcing examples

Considerations when pricing ◦ Sloping sites need to add 5% to labour for

concrete laid up to 15 degrees◦ Waste includes losses due to small quantities and

irregular levels for beds and blindings. Part load charges should be considered for small quantities

◦ Outputs include for labour to protect and cure fresh concrete

Insitu concrete

Materials Cost of the material + wastage Depends upon the type of concrete used.

The concrete may be ready mixed (delivered in a truck) or site mixed

labour constants used depends whether the cost of the materials included for pumped concrete or concrete delivered

Calculations for insitu concrete

Depending on the labour constants used impacts the way the labour rate is calculated

Labour rates based on hourly rates of an operative

Labour rates based on labour gang hours Labour rates based on output per day

Calculations for insitu concrete

Hourly rate for a concrete gang Labour

◦ Concreter 1 hr @ $35.00 hr = $35.00

◦ Labourers (3 no) 3 hrs @ $20.00 = $60.00 ◦ Carpenter in attendance 1 hr @ $45.00 =

$45.00

◦ Labour total = $150.00◦ Cost of one operative (/5no)

$30.00/hr

Calculations cont’d

Plant◦ Tamper 2 hrs @ $20.00 = $40.00◦ Concrete pump 1 hr @ $90.00 = $90.00◦ Vibrator 1 hr @ $25.00 = $25.00

◦ Total plant $155.00/hr◦ Cost of one operative (/5 no) $

31.00/hr

Calculations cont’d

Concrete placed◦ C Concrete 1 m3 @ $220.00/m3 = $220.00/m3◦ C Delivery (incl.) = $ 0.00/m3◦ W Wastage say 2.5% = $ 5.50/m3◦ F Fixings (nil) = $ 0.00/m3◦ e Plant hire-1.20 hr/m3@ $31.00 = $ 37.20/m3◦ L Concrete operative

1.20hr/m3 @ $30.00 = $ 36.00/m3

◦ TOTAL COST OF INSITU CONCRETE = $298.70/m3

Calculations [example]

Concrete poured per day◦ Reinforced insitu concrete bed; ave. thickness

200mm, per m3◦ For a concrete slab 40 x 20 m◦ Assume 1 bay is 4m wide can be poured in one

day◦ Volume poured 40 x 4 x 0.2m = 32 m3

Calculations cont’d

Concrete poured per day

C Concrete 32m3 @ $ 220.00 = $7,040.00/dayC DeliveryW Wastage 2.5% = $ 176.00/dayF Fixings (incl.)e Plant

Tamper (2no) 18hrs @ $20.00 = $ 360.00/dayPump 9 hrs @ $90.00 = $ 810.00/dayVibrator 9 hrs @ $25.00 = $ 225.00/day

L Labour Concreter 9 hrs @ $ 35.00 = $ 315.00/dayLabourer (3 no) 27 hrs @ $20.00 = $ 540.00/dayCarpenter in attendance 2 hrs @ $45.00 = $ 90.00/day

Total Cost $9,556.00/dayCost per m3 = (/32m3) = $298.63/m3

Calculations cont’d

Considerations when pricing◦ Examine drawings to consider the degree of

repetition◦ Consider the standard of finish required◦ Consult programme for time constraints◦ Investigate better quality shutters to increase

uses◦ Will there be salvage value after the work has

finished

Formwork

Formwork (basic finish) to sides of foundations(250 – 500 high, per m2)

First calculate cost of 1m2 of shutter to manufactureC Plywood - 1m2 @ $25.00 = $25.00

Timber - 0.04m3/m2 @$255.00 = $10.20 $35.20

C Cartage say $0.50/m2 $ 0.50W Waste 10% $ 3.52F Bolts & nails 1 kg @ $5.00/kg $ 5.00e nil $ 0.00L Carpenter 0.80 hr @ $35.00/hr = $28.00

Labourer (1 per 4 carpenters) 0.20 @ $20.00 = $ 4.00 $32.00

Total cost of making shutter $76.22 / m2If shutter is used 6 times $76.22/6 = $12.71/m2

Formwork calculation

Formwork (basic finish) to sides of foundations (250 – 500 high per m2)

Additional materials to Fix and Strike shutterConsumables

mould oil 0.3 ltrs @ $3.00 = $0.90/m2

nails 1 kg @ $3.00 = $3.00/m2

TOTAL additional materials $3.90/m2

Formwork calculation

Formwork (basic finish) to sides of foundations (250 – 500 high per m2)

Labour to Fix and strike shutter

Carpenter 1.5 hr @ $35.00 = $52.50/m2Labour (1 per 4 carpenters)

0.375 hr @ $20.00 = $ 7.50/m2

TOTAL Labour $60.00/m2

Formwork calculation

Formwork (basic finish) to sides of foundations (250 – 500 high per m2)

Total cost for formworkMake formwork $ 12.71/m2Mats to fix and strike $ 3.90/m2Labour to fix and strike $ 60.00/m2

TOTAL $ 76.61/m2

Formwork calculation

Formwork (basic finish) to sides of foundations (250 – 500 high per m2)

If 500mm high Cost of formwork $76.61/m2 x .50 = $38.31/m

Formwork calculation

Reinforcing bar considerations◦ Suppliers will charge lower prices for straight bars◦ The fixing times for straight bars can be reduced

by approximately 10 – 15%◦ Basic prices normally include delivery costs◦ Small loads will attract a delivery charge◦ Additional charges for lengths over 12m◦ Bars are rarely cut and bent on site unless a late

design information

Reinforcing calculations

16mm reinforcing bars , bent per tonne

C Steel per tonne 1t @ $600.00 = $600.00C Cartage (incl)W Waste 0.025t = $ 15.00F Tie wire 8 kg @ $2.00 = $16.00 Spacers 40no. @ 0.50 = $20.00 = $ 36.00e Plant (nil)L Labour

Unload & distribute -2.5hrs @ $15.00 = $ 37.50 Skilled fixers - 22 hrs @ $25.00 = $550.00 Labour to assist -5.5 hrs @ $ 15.00 = $ 82.50 =$670.00

Total cost per tonne $1,321.00

Therefore cost per kg = $ 1.32/kg

Reinforcing calculations