Construction Joints

The casting should be planned so that the work on a member can be completed before the end of the day. If cast concrete is left for more than 2 hours it will set so much that there is no direct continuation between the old and new concrete. Joints are potentially weak and should be planned where they will effect the strength of the member as little as possible. Joints should be straight, either vertical or horizontal. When resuming work, the old surface should be roughened and cleaned and then treated with a thick mixture of water and cement.

Mixing

Mechanical mixing is the best way of mixing concrete. Batch mixers with a tilting drum for use on building sites are available in sizes from 85 to 400 litres. Power for the drum rotation is supplied by a petrol engine or an electric motor whereas the tilting of the drum is done manually. The pear-shaped drum has blades inside for efficient mixing. Mixing should be allowed to proceed for at least 2.5 minutes after all ingredients have been added. For small scale work in rural areas it may be difficult and rather expensive to get a mechanical mixer.

Suggested Use for Various Concrete Grades and Nominal Mixes

Grade	Nominal mix	Use
C7 C10	1:3:8 1:4:6 1:3:6 1:4:5 1 :3:5	Strip footings; trench fill foundations; stanchion bases; non reinforced foundations; oversite concrete and bindings under slabs; floors with very light traffic; mass concrete, etc.
Cl5 C20	1:3:5 1:3:4 1:2:4 1:3:3	Foundation walls; basement walls; structural concrete; walls; reinforced floor slabs; floors for dairy and beef cattle, pigs and poultry; floors in grain and potato stores, hay barns, and machinery stores; septic tanks, water storage tanks; slabs for farm yard manure; roads, driveways, pavings and walks;stairways.
C25 C30 C35	1:2:4 1:2:3 1:1.5:3 1:1:2	All concrete in milking parlours, dairies, silage silos and feed and drinking troughs; floors subject to severe wear and weather or weak acid and alkali solutions; roads and pavings frequently used by heavy machinery and lorries; small bridges; retaining walls and dams; suspended floors, beams and lintels; floors used by heavy, small-wheeled equipment, for example lift trucks; fencing posts, precast concrete components.
C40 C50 C60		Concrete in very severe exposure; prefabricated structural elements; pre-stressed concrete.

Cement

Ordinary Portland cement is used for most farm structures. It is sold in paper bags containing 50kg or approximately 37 litres. Cement must be stored in a dry place, protected from ground moisture, and for periods not exceeding a month or two. Even damp air can spoil cement. It should be the consistency of powder when used. If lumps have developed the quality has decreased, but it can still be used if the lumps can be crushed between the fingers.

Typical Strength Development of Concrete

Age at test	Average crushing strength
	Ordinary Portland cement
	Storage in air 18°C 65%, R H N/mm2	Storage in water N/mm2
1 day	5.5	-
3 days	15.0	15.2
7 days	22.0	22.7
28 days	31.0	34.5
3 months	37.2	44.1

(1 cement - 6 aggregate, by weight, 0.60 water - cement ratio).

In some literature the required grade of concrete is noted by the proportions of cement - sand - stone, so called nominal mixes rather than the compressive strength. Therefore some common nominal mixes have been included in Table 3.12. Note, however, that the amount of water added to such a mix will have a great influence on the compressive strength of the cured concrete.

The leaner of the nominal mixes listed opposite the C7 and C10 grades are only workable with very well-graded aggregates ranging up to quite large sizes.

Ingredients

Properties of Concrete

Concrete is associated with high strength, hardness, durability, imperviousness and mouldability. It is a poor thermal insulator, but has high thermal capacity. Concrete is not flammable and has good fire resistance, but there is a serious loss of strength at high temperatures. Concrete made with ordinary portland cement has low resistance to acids and sulphates but good resistance to alkalies.

Concrete is a relatively expensive building material for farm structures. The cost can be lowered if some of the portland cement is replaced with pozzolana. However, when pozzolanas are used the chemical reaction is slower and strength development is delayed.

The compressive strength depends on the proportions of the ingredients, i.e., the cement-water ratio and the cement aggregate ratio. Since the aggregate forms the bulk of hardened concrete, its strength will also have some influence. Direct tensile strength is generally low, only l/8 to 1/14 of the compressive strength and is normally neglected in design calculations, especially in design of reinforced concrete.

Compressive strength is measured by crushing cubes having l5cm per side. The cubes are cured for 28 days under standardized temperature and humidity and then crushed in a hydraulic press. Characteristic strength values at 28 days are those below which not more than 5% of the test results fall. The grades used are C7, C10, Cl5, C20, C25, C30, C40, C50 and C60, each corresponding to a characteristic crushing strength of 7.0, 10.0, 15.0 N/mm2, etc.

Concrete

Concrete is a building material made by mixing cement paste (portland cement and water) and aggregate (sand and stone). The cement-paste is the "glue" which binds the particles in the aggregate together. The strength of the cement-paste depends on the relative proportions of water and cement; a more diluted paste being weaker. Also the relative proportions of cement-paste and aggregate affects the strength; a higher proportion of the paste making stronger concrete. The concrete hardens through the chemical reaction between water and cement without the need for air. Once the initial set has taken place concrete cures well under water. Strength is gained gradually, depending on the speed of the chemical reaction.

Admixtures are sometimes included in the concrete mix to achieve certain properties. Reinforcement steel is used for added strength, particularly for tensile stresses.

Concrete is normally mixed at the building site and placed in forms of the desired shape in the place the unit will occupy in the finished structure. Units can also be precast either at the building site or at a factory.