DIY

Concrete has a wide range of end uses, from the small DIY job to the largest civil engineering project.

This section describes general purpose concrete use around the home; namely small slabs and paths, mixed on-site using Golden Bay Cement in bags. Advice on the use of ready-mixed concrete for larger jobs may be obtained on request.

Considerations before you start

Ground and Formwork Preparation

  • Clear the site beyond the edges of the slab to allow enough room for setting out pegs and formwork.
  • Dig out to the required depth of the slab (nominal 100mm) allowing additional depth for a layer of sub base (100mm).
  • Ensure sufficient compaction of the sub base is achieved.
  • Formwork acts as a mould for the concrete and is commonly made from 25mm thick timber. Timber should be clean, smooth and not flex under the weight of the concrete.
  • Run string lines first to the desired level, then put the formwork up to the line and peg as you go.
  • Allow for a fall for drainage if required.

 

Concrete Mixing and Placing

  • Accurate proportioning (or batching) of materials and thorough mixing are essential for making good quality concrete. Either method of batching, by weight or by volume, may be used.
  • During the mixing and placing of concrete, air is entrapped in the form of voids. The voids lower the performance of concrete and may lead to the following problems: 
    • reduced strength
    • increased permeability
    • reduced durability - the concrete will be less watertight and more vulnerable to aggressive materials
  • To produce a concrete that is strong, dense and impermeable, the mix should be thoroughly compacted to remove the entrapped air. Compaction may be achieved by hand tamping, but the best method is by mechanical vibration.

 

Concrete Finishing

  • To ensure shrinkage cracking occurs where you want it to, install a control joint at least every 3 metres for a 100 mm slab or pour in small bay sizes.

 

There are several different types of surface finishes to consider:

  • Tamped - a rippled finish left by tamping. The final finish can range from fine to bold, depending on how the tamping beam is used.
  • Brushed - a variety of finishes can be achieved by brushing the concrete surface. For example: a smooth finish is obtained with a soft broom used on fresh concrete immediately after bleed has disappeared or a lightly rippled finish is obtained with a stiff brush or nylon broom as the concrete starts to stiffen.
  • Exposed aggregate - a textured finish is obtained by brushing and washing the surface when the concrete has stiffened sufficiently to hold the coarse aggregate firmly in place, i.e., when the concrete is just hard enough to walk on.
  • Float - a variety of finishes can be achieved by using a wooden or steel float.

 

Curing

  • Curing is a process that is completed after the concrete has been placed and finished.  It is designed to allow satisfactory hydration of the cement and to allow the concrete to achieve its potential strength and durability. Methods include flooding with water, covering with wet hessian or polythene or the use of a curing membrane. 

Raw Materials

Cement

Fresh Golden Bay EverSure™ Type GP or EverFast™ Type HE Cement – bags must be stored in a dry place off the ground and locked away.

Aggregates

Aggregate is the term used to describe gravels, crushed rocks and sands, and they are classified as coarse or fine. Fine aggregate (PAP or sand) is material passing through a 5-7mm sieve. Combined coarse and fine aggregate is described as all-in or builders mix.

Use a clean, well-graded builders mix,  in accordance with NZS3104 grading requirements (if possible).

Water

Clean water should be used.

How to mix concrete

Machine mixing

A 100 litre tilting drum mixer is a convenient size for general concreting.

  • Load half the coarse aggregate or builders mix and half the anticipated amount of water into the mixer.
  • Add most of the cement and sand (if added separately)
  • Add the rest of the materials alternately, avoiding build-up of dry or hardened material on the blades and drum.
  • Mix for at least two minutes to obtain a concrete of uniform consistency and colour.

 

Hand mixing

Mixing by hand is hard work, but is satisfactory if the correct procedures are followed. Always work on a hard and reasonably smooth surface such as a sheet of plywood or a concrete drive or yard.

  • Measure out the aggregate or builders mix into a compact pile.
  • Form a crater in the top and add the measured quantity of cement.
  • Turn over at least three times in the dry state until uniform in colour and texture.
  • Form a crater in the pile and add some of the water.
  • Bring the dry material to the water and keep mixing, adding water as necessary for workability until uniform in consistency and colour.

 

For both machine and hand mixing, use only sufficient water in the mix to ensure workability and allow thorough compaction.  Unnecessary added water will affect the concrete quality.

How to place and compact concrete

  • Place the concrete quickly, evenly and steadily without causing segregation.
  • Rake or shovel the concrete into place to a level of about 15mm above the formwork.  Make sure concrete gets right into the corners.
  • Use a length of thick timber on its edge to tamp or compact and screed the concrete to the correct level.  A vibrating beam could be used which will achieve greater and faster compaction.

The tamped finish achieved during compaction and screeding can be used if a washboard type finish is desired.

How to finish concrete

  • Float or trowel the concrete as soon as possible after tamping/screeding but don’t overwork the surface.
  • Protect concrete surface with evaporative retardant in drying conditions to avoid the surface drying out and plastic shrinkage cracking occurring.
  • Let the concrete bleed and do not touch until all bleed water has disappeared.
  • Use an edging tool to finish the edges while the forms are still in place so that they end up slightly rounded.
  • Apply the desired finish to the concrete.

How to cure and protect concrete

  • Cover with polythene sheeting as soon as the concrete is hard enough not to be marked by the sheeting.
  • Weigh down the edges of the sheeting so that the wind cannot blow underneath.
  • Leave the covering in place for at least seven days.
  • Alternatively a curing membrane can be applied to the surface of the concrete.

 

Inclement weather precautions may need to be followed during the carrying out of the work:

  • Do not mix concrete when the air temperature is at or below 5°C and falling, or below 2°C and rising.
  • Do not place concrete during heavy rain.
  • If there is a risk of frost or the air temperature will drop significantly overnight once the concrete has been placed, protect with an insulation quilt sandwiched between two sheets of polythene sheeting and weighted down; or protect with a layer of sand.
  • Equally, during hot and/or windy weather, the concrete mix should be protected from drying out too quickly to avoid surface crazing and plastic shrinkage cracking.

Recommended mixes and working out quantities

Table 1 shows the recommended concrete mixes and raw material quantities required for general DIY use.

 

Table 1 - Recommended concrete mixes and raw material quantities

PURPOSE SUGGESTED   MIX RATIO FOR SMALL QUANTITIES SUGGESTED   ORDER QUANTITIES PER M3 OF COMPACTED CONCRETE
40kg bag Cement Builders' Mix Water Cement Builders Mix
Footpaths,   mowing strips etc 1 part 6 parts 3/4 part 6 bags  1.25m3
Driveways 1 part 5 parts 2/3 part 71/2 bags 1.20m3
Fence posts,   bedding, clotheslines etc 1 part 8 parts 1 part 5 bags 1.25m3
Foundations,   walls etc 1 part 5 parts 3/4 part 8 bags 1.20m3
Masonry   mortar, wall plaster 1 part 3 parts sand To suit 10 bags 1.20m3 sand

 

Table 2 - Amount of concrete required for Slabs, Driveways and Paths

M3   Concrete Required

Area m²

Thickness (mm)

 

75

100

125

150

5

0.4

0.6

0.7

0.8

10

0.8

1.1

1.4

1.7

15

1.2

1.7

2.1

2.5

20

1.7

2.2

2.8

3.3

25

2.1

2.8

3.4

4.1

30

2.5

3.3

4.1

5.0

35

2.9

3.9

4.8

5.8

40

3.3

4.4

5.5

6.6

45

3.7

5.0

6.2

7.4

 

Calculate the area required (Sq. /m2 = length (m) x width (m) and then use table 2 to determine quantity of concrete required (Table 2 allows for 10% for wastage)

Jobs that require >3m3 consider the option of Readymixed concrete from a Readymix Concrete Supplier.

 

Table 3 - Amount of concrete required for Fenceposts and other round excavations

M3   Concrete Required

Hole Depth (mm)

Hole diameter (mm)

 

300

400

500

600

700

200

0.01

0.03

0.04

0.06

0.08

400

0.03

0.05

0.08

0.11

0.15

600

0.04

0.08

0.12

0.17

0.23

800

0.06

0.1

0.16

0.23

0.31

1000

0.07

0.13

0.2

0.28

0.38

 

Measure the hole diameter and  depth and then use Table 3 to determine the quantity of concrete required.

 

Post hole calcualtions based on hole diameter of 300mm and depth of 500mm

 

Step 1

Calculate   entire hole volume.

Formula:

p  r² h
= 3.14 x (0.15m x 0.15m) x 0.5m
= 3.14 x 0.0225
x 0.5
Volume = 0.0353m³

 

 

Step 2

Calculate Post Area

Formula:

p r² h
= 3.14 x (0.075 x 0.075) x 0.5m
= 3.14 x 0.00563 x 0.5
Volume = 0.00884m³

 

 

 

 

Step 3

= Volume  Step 1 – Volume Step 2
= 0.353m³ – 0.0088m³
= 0.02646m³

 

Heatlh and Safety

 

Danger       Warning       Danger corrosive 2

     DANGER                 WARNING               DANGER

 

  • Causes serious eye and severe skin burns
  • May cause respiratory irritation
  • May cause damage to lungs by inhalation through prolonged or repeated exposure
  • Harmful to aquatic life with long lasting effects.

  

Prevention

  • Keep out of reach of children.
  • Read label before use.
  • Do not breathe dust.
  • Wash hands and exposed skin thoroughly after handling.
  • Use only outdoors or in well-ventilated area.
  • Contaminated work clothing should not be allowed out of the workplace.
  • Avoid release to the environment.
  • Wear protective gloves and eye/face protection.

 

First Aid

  • IF INHALED: remove to fresh air and keep at rest in a position comfortable for breathing. 
  • If experiencing respiratory symptoms: call the POISON CENTRE at 0800 POISON (0800 764 766) or a doctor/physician.
    IF IN EYES: rinse cautiously with water for several minutes.  Remove contact lenses, if present and easy to do.  Continue rinsing.
    IF SWALLOWED: rinse mouth.  Do NOT induce vomiting.  Immediately call the POISON CENTRE or a doctor/physician.
    IF ON SKIN: remove immediately all contaminated clothing.  Rinse skin with water.  If skin irritation occurs: get medical advice/attention.
    If medical advice is needed, have product container or label at hand.

 

Storage and Disposal

  • Store locked up in a dry, ventilated area off the ground. 
  • Reuse or recycle where possible. 
  • Alternatively, ensure product is covered with moist soil to prevent dust generation and dispose of to an approved landfill site.
  • Do not discharge to storm water drains and waterways. 

 

Environment

  • DO NOT ALLOW CEMENT OR SLURRY TO ENTER DRAINS OR NATURAL WATERWAYS.
  • Do not allow cement or water that comes into contact with uncured cement, concrete, concrete slurry or wastewater, to go down any drains, waterways or into the sea. The high pH water can kill fish, insects and other aquatic life.
  • Dispose bags  away from waterways and in accordance with local council guidelines.
  • Reuse or recycle where possible. Alternatively, ensure the product is covered with moist soil to prevent dust generation and dispose of it to an approved landfill site.

 

The information above is for general guidance only and is not a substitute to the services of a professional consultant on particular projects, no responsibility can be accepted by Golden Bay Cement by its use.