Concrete

To concrete something means to form it into a mass, or to solidify it.

As far as building is concerned, the term concrete means an artificial stone made by mixing sand, stone, Portland cement and water. This mixture, cast into a form of the desired shape and size, hardens into a stone-like mass: the concrete.

There are basically three materials we start with to make concrete:

- The aggregate, which is made up of the fine and coarse aggregates together, ie. the sand and broken stones. The aggregate makes up the main mass of the concrete; its function is mostly just to add bulk.

• The water.
• The binding material, which is usually Portland cement.

When the three materials are mixed together, the cement and water combine chemically to make a cement paste, which surrounds the particles of the aggregate and holds them together.

CEMENT PASTE

The cement paste component of concrete is what causes it to harden, the aggregate simply remains passive (inactive).

Thus the cement paste must completely cover the surface of every single particle of the aggregate. This means that each stone, no matter whether tiny or big, must be covered all over by a thin layer of cement paste.

This is achieved by mixing all three components very thoroughly and in the correct proportions (see Batching, and Mixing The Mortar).

The cement paste fills up all the spaces between the particles of the aggregate and bonds them firmly together as it hardens.

The hardening process requires a certain amount of water; how much depends on how much cement is added to the mix. The correct proportions can be found in the Table below.

After it is set, the hardened cement paste cannot be dissolved again (except by the use of certain acids).

An undesirable further reaction of the cement paste is the drying shrinkage as it hardens. Because of the evaporation of the extra water, the volume of the concrete is gradually reduced. The concrete shrinks and develops cracks.

This reaction can be effectively reduced, if not prevented, by correct curing; as will be discussed later in this book.

Also to prevent cracking, large areas that are covered with concrete; such as floors, should be divided up into bays.

PROPERTIES OF CONCRETE

Concrete has many properties, but most of them are of little interest to the Rural Builder. Therefore this chapter deals only with the three most important properties:

• a - Compression strength
• b - Tensile strength
• c - Protection against corrosion.

COMPRESSION STRENGTH:

It is conuncniy known that concrete becomes very hard and can withstand enormous pressures; a property which is called compression strength.

This compression strength depends mainly on the properties and quality of che cement paste and the aggregate.

• If the aggregate consists of a soft or weak material, the concrete will be weak also.
• If the aggregate is so dirty that there is no direct contact between the surface of the particles and the cement paste, the concrete will again be weak.

Provided that all the rules for producing a goou concrete are observed, the strength of the concrete can be controlled by choosing the mix proportions. For example, a mix proportion of 1: 10 is weaker than a 1: 3 mix. This is because in a 1: 10 mix the particles of aggregate are not completely coated with cement paste, but in the 1: 3 mix they are fully embedded in it.

• If not enough water was added to the mix, the cement paste remains too dry and stiff and the concrete will be weak.
• If too much water was added, making the cement paste too thin, the concrete will again be weak.
  

Therefore the Rural Builder must always carefully follow the correct concrete recipe.

TENSILE STRENGTH:

The tensile strength of a material means its capability of being stretched to a certain extent without breaking.

Although concrete becomes very hard, its tensile strength is very limited. It is so low that in practice, the tensile strength of concrete is regarded as being nonexistent. This is why sometimes concrete members of a structure must be reinforced by steel bars embedded in them.

Some types of wood, while they are softer and have a much lower compression strength than concrete, have a far higher tensile strength because of their fiber structure. The wood fibres act in a way like the reinforcement iron embedded in concrete.

Wood is a good building material because of its tensile strength. However, its flexibility makes it subject to bending under loads. Because of this problem, short-span constructions are chosen; or, among other possibilities, reinforced concrete can be used instead of wood.

PROTECTION AGAINST CORROSION:

Corrosion means a wearing away, a slow destruction caused by a reaction with air, water or chemicals.

Reinforcement iron which is left unprotected and exposed to air and humidity will eventually start to corrode on the surface and become rusty.

If this process is not halted in time, the rust goes into the bar and it becomes too weak to be used.

In order to maintain the strength of steel-reinforced concrete, the steel has to be protected from rust. This is partly done by the hardened cement paste and partly by structural means.

Ideally, the hardened cement paste hermetically seals the iron so that direct contact with air and humidity is cut off. Even slight rust stains on the iron cannot do any harm because the cement paste protects it against further corrosion.

The protection will not be enough however, unless the builder observes the following rules:

• The reinforcement bars must be completely covered by concrete which is well compacted and without voids.
• The concrete cover must be sufficiently thick, and without cracks.In most cases ordinary Portland cement is used and the mix proportion should be no less than 1: 5 for reinforced concrete, (see Table below).

Apart from these, all the other rules for producing a good concrete must be observed.

NOTE: Quality concrete is not a brand. It does not have a trademark on it to say "This is quality concrete". Sometimes the concrete does not even look different from poor concrete, but It is different. This depends not only on the mix proportion, but on the awareness and skill of the builder.

BUILDING MATERIALS REQUIREMENT FOR ONE CUBIC METRE CONCRETE

(approximation values)

USE	MIX	CEMENT		WATER		AGGREGATES (in head pans)
codes (keybelow)	prop.	bags (50kgs)	head-pans	buckets size no. 28	head-pans	fine sand	coarse sand	small broken stones	medium broken stones
1	1:15	2	4,5	4,5	3	21	19	23	21
1;4	1:12	2,5	5,5	5,5	3,75	21	18,75	23	21
1;4;5	1:10	3	6,75	6,75	4,5	20,5	18,5	22,5	20,5
2;4;5	1:9	3,25	7,5	7,5	5	20,25	18,25	22,25	20,25
2;3;4;5	1:8	3,5	7,75	10	6,5	20	18	22	20
3;4;5	1:7	4	9	11	7,5	19,5	17,5	21,5	19,5
3;4;5	1:6	4,5	9,5	12,5	8,5	19	17,25	21	19
3;5	1:5,5	5	11,25	13,75	9	19	17	21	19
6;7;8	1:5	5,5	12,5	14,5	10	18,75	22,50	33,75	-----
6;7;8	1:4,5	6	13,5	16,5	11	18,25	22	33	-----
6;7;8;9	1:4	6,5	14,5	18	12	18	21,5	32,5	-----
8;9	1:3,5	7	15,75	20	13,5	17,25	20,75	31,25	-----
8;9	1:3	8	18	22	14,5	16,5	20	30	-----

KEY FOR USE CODES

• 1  = foundations
• 2  = sandcrete blocks*
• 3  = mortar*
• 4  = plaster/render*
• 5  = floors
• 6  = columns
• 7  = beams/lintels
• 8  = slabs
• 9  = screed mortar*

* The items marked with a star are those which contain only sand as an aggregate. The amount of sand required is obtained by adding together the amounts of all the aggregates: fine and coarse sand, small and medium broken stones.

NOTE: Any reinforced concrete member of the structure must contain at least 270 kg of cement per cubic metre; and must be free of medium and large sized stones. Therefore, the figures in the upper part of the table are never used in mixing reinforced concrete.

HOW TO USE THE TABLE

When the plans for the building are completed, the builder can make calculations to find how much cement needs to be ordered. From the dimensions in the plan, the volume in cubic metres can be found for the various parts of the structure such as the foundation, floor, footings, etc. This is done by multiplying the width, length, and height of a part to get its cubic volume.

The examples below show how to calculate the cement required once you have found the volumes.

FOUNDATIONS:

Mix proportion = 1:10, volume = 5,75 cubic metres.

FOOTINGS:

Mix proportion of sandcrete blocks and mortar = 1:8, volume = 3,3 cubic metres.

FLOOR:

Mix proportion = 1:7, volume = 2,4 cbm.

CALCULATION:

FOUNDATIONS:

According to the table, the mix proportion of 1:10 requires 3 bags of cement per 1 cbm. Therefore, we multiply 5,75 cbm x 3 = 17,25 bags of cement.

FOOTINGS:

The mix proportion of 1:8 requires 3,5 bags of cement per cbm. We multiply the volume of 3,3 cbm x 3,5 = 11, 55 bags of cement.

FLOOR:

The mix proportion of 1:7 requires 4 bags of cement per cbm. We multiply the volume of 2,4 cbm x 4 = 9,6 bags of cement.

We now add up the three results above and obtain a final result: 17,25

+11,55

+ 9,6

38,40 bags The total of 38,40 means that 39 bags of cement have to be ordered.

Do not forget to include all the members which contain cement in your calculations: landcrete blocks, mortar, lintels, concrete ring beam, etc.

The cement requirements for landcrete blocks varies according to the soil used. For approximate values see the table in that sections.

When the cement requirements have been determined, we can use the table to find the quantities of aggregates that are needed. This is done by multiplying the same volume measurement by the appropriate number in the table.

FOUNDATIONS:

5,75 cbm x   20,5   equals approximately 118 headpans of fine sand.

5,75 cbm x   18,5   equals approximately 106 headpans of coarse sand.

5,75 cbm x   22,5   equals approximately 129 headpans of small broken stones.

5,75 cbm x   20, 5   equals approximately 118 headpans of medium broken stones.

FOOTINGS:

Since the sandcrete blocks and the mortar require only sand, all four quantities under aggregates (fine sand, coarse sand, small and medium broken stones) are added together and the result is multiplied by the volume of the footings: 20 + 18 + 22 + 20 = 80; 80 x 3,3 cbm = 264 headpans of sand.

FLOOR:

2,4 cbm x   19, 5   equals approximately 47 headpans of fine sand.

2,4 cbm x   17,5   equals 42 headpans of coarse sand.

2,4 cbm x   21. 5   equals approximately 52 headpans of small broken stones.

2,4 cbm x   19. 5   equals approximately 47 headpans of medium broken stones.