How to mix concrete


Concrete Proportion and Mixture Ratio

Mixing Concrete TruckConcrete is one of the most simple and best known building materials and probably the most vital for the strength of the constructions of your building, parking lot, patio, garden wall, paths etc. That is why it is important to know how to properly mix and pour concrete.

Concrete contains only the following ingredients: cement (Portland), gravel, sand and water. When cement is mixed with water a chemical reaction takes place. The cement does not simply “dry” it reacts with the water binding the materials in a concrete mix firmly together and harden into an incredibly strong rock like mass.

Gravel and sand are called also aggregate and all you need to know how to mix concrete properly is the correct aggregate to cement ratio. The more cement the stronger the concrete.

The strength of a concrete mix is measured in grades. There are different concrete grades for example C15, C20, C25. The grade of the concrete means the concrete compression resistance after 28 days. It is done in Newtons per square millimetre. A concrete of Grade C15 means that the concrete will have a compresion resistance (withstanding a compression) of 15 N/mm2 per square millimetre in 28 days.


Proportions for different grades:

Cement  :  Sand  :  Gravel
   1    :   2    :    5     for grade C15(general purpose concrete)
   1    :   2    :    4     for grade C25(strong)
   1    :   2    :    3     for grade C30(very strong)

Remember the proportion is done in terms of volume. This means the above proportions are correct if using the same measure.

For example: 1 bucket cement + 2 buckets sand + 5 buckets gravel


Simple Calculation for 1 cubic meter of Concrete

0.5m3 sand + 1m3 gravel + cement + water = 1m3 Concrete


Mixture of half cubic meter sand, one cubic meter of gravel and a portion mix of cement (for the required strength of the concrete) plus water, there will be about one cubic meter of volume


For example:
0.5m3 sand + 1m3 gravel + 8 bags x 50kg cement + water = 1m3 Concrete C25 grade



Volume measures for cement, gravel and sand:
Cement (Portland): 1506 kg per cubic meter (
Gravel (loose, dry): 1522 kg per cubic meter
Sand (dry): 1602 kg per cubic meter

Pressure calculators: 1 Newton (N)= 0.1019716 Kg force
1 Pascal = 0.1019716 Kg/m2= 1 N/m2
1 Megapascal = 101971.6 Kg/m2 = 0.1019716 kg/mm2 = 1 N/mm2
1 Megapascal = 145.0377 psi = 10.19716 kg/cm2

The usual required strenght of the concrete are:
For C30: 30 Megapascal(N/mm2)=4351 psi = 305 kg/m2;generally 4500 psi
For C25: 25 MP (N/mm2) = 3625 psi = 255 kg/m2; generally 4000 psi
For C20: 20 MP (N/mm2) = 2900 psi = 203 kg/m2; gen. 3000 psi
For C15: 15 MP (N/mm2) = 2175 psi = 152 kg/m2; gen. 2000 psi

Another factor which determines the strength and hardness of the finished concrete product is proper mixing of the ingredients. If the ingredients are not thoroughly mixed there will be differing degrees of hardness in various parts of the finished product. Needless to say this is something to avoid at all costs.

GravelCementSand

Cement can be mixed by hand by a reasonably fir person, by a cement mixer or it can be ordered to be delivered ready mixed.
Mixing concrete by hand is exhausting and can be done only for small quantities - max. 1 cu. metre. For larger projects it is better to be be delivered ready to your site.


While it is very easy to order concrete you have to know what to oder:
- how much concrete (how many cubic metres)
- what grade
- time of the delivery

Once you have the concrete is delivered you have to be prepared to handle it. You should also consider access for the lorry, and you should be aware that the driver won’t be willing to wait long. That is why you should be fully prepared for the concrete also considering that concrete is getting hard pretty fast and you will not have more than 1-2 hours to handle it


Foundation concrete (for 1 cu metre) General purpose concrete (for 1 cu metre)
6 x 50kg bags cement 7 x 50kg bags cement
1.76 tonnes ballast (sand and gravel) 1.6 tonnes ballast (sand and gravel)

How to mix concrete by hand

  1. Select mixture formula from the table above. A 1:2:3 (cement:sand:gravel) is also a bullet proof formula.
  2. Use a shovel to mix to ingredients in a wheelbarrow or on a large plate of sheet iron on the ground.
  3. Add some water and continue mixing by hand. Use minimum volume of water possible. The less water the stronger the concrete.
  4. While keeping water minimal be sure that there are no dry areas in the mix
  5. Work fast as concrete dries out fast.

Tip

  • You can also buy pre-mixed bags of ready-to-mix concrete at DIY supermarkets and lumber yards. These require water and mixing; the measuring is already done for you.
  • If you have doubts about your ability to “work” and “finish” concrete, hire professionals. Any job over a few square feet in size may be more than the average homeowner wants to tackle.

Concrete Thickness

Recommended minimum thickness of the concrete for the following projects are:

  • Footpaths - 75mm (3 inches)
  • Patio - 100mm (4 inches)
  • Driveways and Parking areas - 125mm (5 inches)
  • Base of light structures - 75mm (3 inches)

Slope of the concrete

Water will gap in depressions without a slab and if there is no natural slab of the land it should be calculated.
Any concrete against a building or a garden wall must slope away from it. Here the reccomended slopes:

  • Footpaths - minimal, just to avoid areas where rain water will collect
  • Patio - 1:60 (16mm in 1 metre, 5/8in in 3 ft)
  • Driveways and Parking areas - 1:40 to 1:60 (25mm to 16mm in 1 metre, 1in to 3/8in in 3ft)
  • Base of light structures - 1:80 (12.5mm in 1 metre, ½in in 3 ft)

See full Metric system conversion tables (opens new window)

What does it mean to “cure” concrete?

Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. The hydration occurs only if water is available and if the concrete’s temperature stays within a suitable range. During the curing period - from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill

Hot weather concreting

There are certain problems which may occur when mixing concrete in hot weather. The high air temperature and the low humidity which are typical for the summer months are factors that may affect the process of concrete curing and may spoil the quality of the concrete structure.

Most of the problems associated with pouring concrete in hot weather conditions relate to the increased rate of cement hydration at higher temperatures and the increased rate of evaporation of moisture from the fresh concrete.



The properties of concrete that may be affected by hot weather conditions include:

Setting time: Due to the high ambient temperatures increases also the concrete temperature which reduces the time for mixing, pouring, compacting and finishing of the concrete.

Workability and slump: Higher temperatures reduce the slump of the concrete more rapidly with time and more mixing water is needed to improve the workability of the material.  Adding more water decreases the strength and increases the permeability of the concrete, thus affecting its durability.

Compressive strength: Higher water demand and higher concrete temperature could lead to reduced 28-day strengths. If more water is added to the concrete mix at higher temperatures to maintain or restore workability, the water-cement ratio in the concrete will be increased, resulting in a loss of both potential strength and durability. This may also increase the drying shrinkage of the hardened concrete. Where water is not added, the reduced setting time and workability increase the potential for inadequate compaction (itself of a major influence on strength), the formation of cold joints and poor finishes. The water/cement ratio however should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.

Poor surface appearance: With the increased rate of evaporation, the surface of the concrete will dry out and stiffen. In the case of flatwork, this may lead to premature finishing of the surface, trapping an amount of bleed water within the mix. The compacted surface layer (from finishing) may cause the rising bleed water to be trapped below the surface, resulting in debonding of the surface layer and subsequent flaking. Also, colour differences on the surface may result from different rates of hydration and cooling effects.

Plastic shrinkage cracking: Hot weather conditions accelerate the loss of moisture from the surface. If the rate of evaporation is greater than the rate of bleeding (rate at which water rises to the surface), surface drying will occur, resulting in shrinkage of the concrete. When the shrinkage stresses exceed the tensile capacity of the concrete, cracking will occur. The likelihood of plastic shrinkage cracking is therefore greater whenever hot weather conditions increase evaporation or the concrete has a reduced bleeding rate. Plastic shrinkage cracks in the concrete can be quite deep, as the plastic concrete has little capacity to resist shrinkage stresses, and cracks continue to widen and propagate until the shrinkage stresses are relieved.

Thermal cracking: Concrete is at risk of thermal cracking when it is first placed, and the heat of hydration raises the temperature of the interior of the concrete. Rapid changes in the temperature of the external concrete surface, such as when concrete slabs, walls or pavements are poured on a hot day followed by a cool night, lead to thermal gradients between the warm/hot interior and the colder external surface. The warmer interior provides a restraint to the colder external surface, which wants to contract. Depending on the temperature differential, cracking of the concrete may result. Massive or thick concrete elements are more at risk because of the insulating effect that the concrete provides to the interior of the element.


Plastic shrinkage cracking
Plastic shrinkage cracking

Poor surface finishing
Poor surface finshing

Concrete flaking
Concrete flaking

Thermal cracking
Thermal cracking

How to pour concrete in hot weather:

  • The good planning is very important factor not only in concreting but also in every other construction process. Because of the reduced placement and finishing time, you should have enough manpower and equipment available on site to quickly place, finish and cure the fresh concrete.
  • Use cold water to dampen the substrate and form work prior to concrete placement.
  • Construct temporary sun-shades and wind-breaks to reduce and slow-down the moisture loss from the fresh concrete.
  • Use cool mixing water to reduce initial concrete temperatures.
  • After placement, start curing the fresh concrete immediately. Ponding with water, covering with wet hessian, cotton mats, or plastic sheeting, and applying special concrete curing substances are the most common methods in this case.
  • Placing the concrete early in the morning or later in the evening when the air temperatures are cooler is also a way to avoid the harmful effects of the hot weather.



Sprinklers are effective to keep concrete moist while curing

Cold weather concreting

Cold weather is when the average daily temperatures are around 0° C for more than three days. These conditions require special precautions when placing, finishing and curing concrete. The construction site should be fully prepared for the concreting if you want to have a successful cold weather pour. This means that you need to have all the necessary insulation and heating equipment required to protect the fresh concrete from freezing on site before the pour begins. The traditional, and still the best way, to protect concrete from the cold is to cover it with blankets after it has been finished. Concrete generates its own heat and blankets will keep it warm even if the temperature goes below -5°C. The protection should continue only for a couple of days, if the concrete is warmer than 10°C. If the concrete freezes in its plastic state (usually this happens if its temperature falls below - 4° C) its potential strength could be reduced by more than 50%. For every 10°C reduction in concrete temperature, the times of setting of the concrete double, thus increasing the amount of time that the concrete is vulnerable to damage due to freezing.


Frozen concrete during curing
Frozen concrete during curing

Freezing in concrete
Freezing in concrete

To protect the concrete against the effects of the cold weather certain measures have to be taken:

  • Do not place concrete on frozen ground or onto snow or ice. Clean all snow and ice from the formwork and the sub base. If there is any standing water present, it should be removed in order not to mix with the concrete.
  • Everything which will come in contact with the plastic concrete must have temperature of at least 0° C. If the temperature is below 0° C, the sub base, the formwork and the embedments have to be warmed before the pouring.
  • The finished concrete should be insulated to retain heat from the exothermic hydration reaction of curing. Insulation blankets or heated enclosures have to be used to maintain the concrete temperatures above 10° C for 3 - 7 days.
  • The insulation should be removed gradually to prevent cracking from rapid temperature change when the concrete reaches 3500 psi compression strength (this is strong enough to carry a compressive stress of 3,500 psi. Conventional concrete has strengths of 7,000 psi or less at 28 days.
  • The dimensions of the finished concrete construction also affect the risk of freezing during curing. Thinner forms are more likely to freeze. Edges and corners are especially susceptible to rapid temperature variations. A high risk of cracking exists when the difference between interior and exterior concrete temperatures is more than 2 - 3° C.



Straw and hay insulate concrete in freezing weather

For further reading:
- Concrete mixing on DIYdata.com
- Mixing concrete on DIYdoctor.org.uk
- Protection and curing of concrete


Builder in Bulgaria - Contact You can contact us if you are looking for a Builder in Bulgaria. We are general builders covering all aspects of construction including Property Renovation, Repairs and Property finishes in Bulgaria. Click here to >> Contact us <<


People reading this were also interested in:

 


Tags: ,