Concrete

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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.

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.

Table 3.11 Typical Strength Development of Concrete

(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

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.

Table 3.12 Suggested Use for Various Concrete Grades and Nominal Mixes

Aggregate

Aggregate or ballast is either gravel or crushed stone. Those aggregates passing through a 5mm sieve are called fine aggregate or sand and those retained are called coarse aggregate or stone. The aggregate should be hard, clean and free of salt and vegetable matter. Too much silt and organic matter makes the aggregate unsuitable for concrete.

Testfor Silt is done by putting 80mm of sand in a 200mm high transparent bottle. Add water up to 160mm height. Shake the bottle vigorously arid allow the contents to settle until the following day. If the silt layer, which will settle on top of the sand, is less than 6mm the sand can be used without further treatment. If the silt content is higher, the sand must be washed.

Test for Organic Matter is done by putting 80mm of sand in a 200mm high transparent bottle. Add a 3% solution of sodium hydroxide up to 120mm. Note that sodium hydroxide, which can be bought from a chemist, is dangerous to the skin. Cork the bottle and shake it vigorously for 30 seconds and leave it standing until the following day. If the liquid on top of the sand turns dark brown or coffee coloured, the sand should not be used. "Straw" color is satisfactory for most jobs, but not for those requiring the greatest strength or water resistance. Note however that some ferrous compounds may react with the sodium hydroxide and cause the brown colour.

Grading of the aggregate refers to proportioning of different sizes of the aggregate material and greatly influences the quality, permeability and workability of the concrete. With a well-graded aggregate the various sizes of particles intermesh leaving a minimum volume of voids to be filled with the more costly cement paste. The particles also flow together readily, i.e., the aggregate is workable, enabling less water to be used. The grading is expressed as a percentage by weight of aggregate passing through various sieves. A well-graded aggregate will have a fairly even distribution of sizes.

Moisture Content in sand is simportant since sand mixing ratio often refers to kg dry sand and the maximum amount of water includes the moisture in the aggregate. The moisture content is determined by taking a representative sample of 1 kg. The sample is accurately weighed and spread thinly on a plate, soaked with spirit (alcohol) and burned while stirring. When the sample has cooled it is weighed again. The weight-loss amounts to the weight of the water which has evaporated, and is expressed as a percentage by dividing the weight lost by the weight of the dried sample. Normal moisture content of naturally moist sand is 2.5 to 5.5%. That much less water is added to the concrete mixture.

Density is the weight per volume of the solid mass excluding voids, and is determined by putting one kilo of dry aggregate in one litre of water. The density is the weight of the dry aggregate ( I kg) divided by the volume of water forced out of place. Normal values for density of aggregate (sand and stone) are 2600 to 2700 kg/ m3 and for cement 3100 kg/m3.

Bulk density is the weight per volume of the aggregate including voids and is determined by weighing I litre of the aggregate. Normal values for coarse aggregate are 1500 to 1650 kg/m3. Completely dry and very wet sand have the same volume but due to the bulking characteristic of damp sand it has a greater volume. The bulk density of a typical naturally moist sand is 15 to 25% lower than coarse aggregate of the same material, i.e., 1300 to 1500 kg/m3.

Size and Texture of Aggregate affects the concrete. The larger particles of coarse aggregate may not exceed one quarter of the minimum thickness of the concrete member being cast. In reinforced concrete the coarse aggregate must be able to pass between the reinforcement bars, 20mm being normally regarded as maximum size.

Aggregate with larger surface area and rough texture, i.e., crushed stone, allows greater adhesive forces to develop but will give less workable concrete.

Stock piles of aggregate should be close to the mixing place. Sand and stone should be kept separate. If a hard surface is not available, the bottom of the pile should not be used to avoid defilement with soil. In hot, sunny climates, a shade should be provided or the aggregate sprinkled with water for cooling. Hot aggregate materials make poor concrete.

Batching

Measuring is done by weight or by volume. Batching by weight is more exact but is only used at large construction sites. Batching by volume is used when constructing farm buildings. Accurate batching is more important for higher grades of concrete. Batching by weight is recommended for concrete of grade C30 and higher. Checking the bulk density of the aggregate will allow greater accuracy when grade C20 or higher is batched by volume. A 50 kg bag of cement can be split into halves by cutting across the middle of the top side of a bag lying flat on the floor. The bag is then grabbed at the middle and lifted so that the bag splits into two halves.

A bucket or box can be used as a measuring unit. The materials should be placed loosely in the measuring unit and not compacted. It is convenient to construct a cubic box with 335mm sides, since it will contain 37 litres, which is the volume of one bag of cement. If the box is made without a bottom and placed on the mixing platform while being filled, it is easily emptied by simply lifting it. The ingredients should never be measured with a shovel or spade.

Figure 3.19 Relation between comprehensive strenght and water cement ratio

The sum of the ingredient volumes will be greater than the volume of concrete, because the sand will fill the voids between the coarse aggregate. The materials normally have 30 to 50% greater volume than the concrete mix; 5 to 10% is allowed for waste and spill. The cement added does not noticeably increase the volume. The above assumptions are used in Example 1 in roughly estimating the amount of ingredients needed. In Example 2, a more accurate method of calculating the amount of concrete obtained from the ingredients is shown.

Example 1

Calculate the amount of materials needed to construct a rectangular concrete floor 7.5m by 4.0m and 7cm thick. Use a nominal mix of 1:3:6. 50 kg of cement is equal to 371.

Total volume of concrete required = 7.5m x 4.0m x 0.07m = 2.1m³

Total volume of ingredients, assuming 30% decrease in volume when mixed and 5% waste = 2.1m³ + 2.1(30% + 5+)m³ = 2.84m³

The volume of the ingredients is proportional to the number of parts in the nominal mix. In this case there are a total of 10 parts ( 1 +3+6) in the mix, but the cement does not affect the volume so only the 9 parts for sand and stone are used.

Cement = (2.89 x 1)/9 = 0.32m³ or 320

Sand = (2.84 x 3 ) / 9 = 0.95m³

Stone = (2.84 x 6 ) / 9 = 1.89m³

Number of bags of cement required = 320/37 = 8.6 bags, i.e., 9 bags have to be bought.

Weight of sand required = 0.95m³ x 1.45 tonnes/ m³ = 1.4 tonnes

Weight of stone required = 1.89m³ x 1.60 tonnes/m³ = 3.1 tonnes

Maximum size of stones = 70mm x 1/4 = 17mm

Example 2

Assume a 1:3:5 cement - sand - stone concrete mix by volume using naturally moist aggregates and adding 62 litres of water. What will the basic strength and the volume of mix be if 2 bags of cement are used. Additional assumptions:

Moisture content of sand: 4%

Moisture content of stones: 1.5%

Bulk density of the sand: 1400 kg/m³

Bulk density of the stones: 1600 kg/m³

Solid density of aggregate materials: 2650 kg/m³

Solid density of cement: 3100 kg/m³

Density of water: 1000 kg/m³

1 Calculate the volume of the aggregate in the mix.

2 bags of cement have a volume of 2 x 37l = 74l

The volume of sand is 3 x 74l = 2221

The volume of stones is 5 x 74l = 3701

2 Calculate the weight of the aggregates.

Sand 222/1000 m³ x 1400 kg/m³ = 311 kg

Stones 370/1000 m³ x 1600 kg/m³ = 592 kg

3. Calculate the amount of water contained in the aggregate

Water in the sand 311 kg x 4/100= 12 kg

Water in the stones 592 kg x 1.5/100= 9 kg

4 Adjust amounts in the batch for water contents in aggregate.

Cement 100 kg (unaltered)

Sand 311 kg - 12 kg = 299 kg

Stones 592 kg- 9 kg= 583 kg

Total amount of dry aggregate = 299 kg + 583 kg = 882 kg

Water = 62 kg + 12 kg + 9 kg = 83 kg

5 Calculate water- cement ratio and cement - aggregate ratio.

Water - cement ratio = (83 kg water) / 100 kg cement = 0 83

Aggregate - cement ratio = (882kg aggregate) / 100 kg cement = 8.8

The water - cement ratio indicates that the mix has a basic strength corresponding to a C10 mix. See Appendix V: 12.

6 Calculate the "solid volume" of the ingredients in the mix, excluding the air voids in the aggregate and cement.

Cement 100 kg/3100 kg/m³ = 0.032m³

Aggregate 882 kg/ 2650 kg/m³ = 0.333m³

Water 83 kg/ 1000 kg/m³ = 0.083m³

Total = 0.448m³

The total volume of 1:3:5 mix obtained from 2 bags of cement is 0.45m³.

Note that the 0.45m³ of concrete is only 2/3 of the sum of the volumes of the components - 0.074 + 0.222 + 0.370.

Table 3.13 Requirements per Cubic Metre for Batching Nominal Concrete Mixes

These quantities are calculated with the assumption of sand having a bulk density of 1450 kg/m³ and stone 1600 kg/m³. The density of the aggregate material being 2650 kg/m³.

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.

Table 3.14 Mixing Water Requirements for Dense Concrete for Different Consistencies and Maximum Sizes of Aggregate

³ Includes moisture in aggregate. The quantities of mixing water are maximums for use with reasonably wellgraded, well-shaped, angular coarse aggregate. 2 For slump see table 3.15.

Figure 3.20 Batch mixer.

A simple hand-powered concrete mixer can be manufactured from an empty oil drum set in a frame of galvanized pipe. Figure 3.21 shows a hand crank, but the drive can easily be converted to machine power.

Figure 3.21 Home-built concrete mixer.

Hand mixing is normally adopted on small jobs. Mixing should be done on a close-boarded platform or a concrete floor near to where the concrete is to be placed and never on bare ground because of earth contamination.

The following method for hand mixing is recommended:

• 1 The measured quantities of sand and cement are mixed by turning over with a shovel at least 3 times.
• 2 About three-quarters of the water is added to the mixture a little at a time.
• 3 Mixing is continued until the mixture becomes homogeneous and workable.
• 4 The measured quantity of stones,. after being wetted with part of the remaining water, is spread over the mixture and the mixing continued, all ingredients being turned over at least three times in the process, using as little water as possible to get a workable mix.

All tools and the platform should be cleaned with water when there is a break in the mixing, and at the end of the day.

Slump Test

The slump test gives an approximate indication of the workability of the wet concrete mix. Fill a conically shaped bucket with the wet concrete mix and compact it thoroughly. Turn the bucket upside down on the mixing platform. Lift the bucket, place it next to the concrete heap and measure the slump as shown in Figure 3.22.

Placing and Compaction

Concrete should be placed with a minimum of delay after the mixing is completed, and certainly within 30 minutes. Special care should be taken when transporting wet mixes, since the vibrations of a moving wheelbarrow may cause the mix to segregate. The mix should not be allowed to flow or be dropped into position from a height greater than 1 metre. The concrete should be placed with a shovel in layers no deeper than 15cm and compacted before the next layer is placed.

When slabs are cast, the surface is levelled out with a screed board which also is used to compact the concrete mix as soon as it has been placed to remove any trapped air. The less workable the mix is, the more porous it is and the more compaction is necessary. For every per cent of entrapped air the concrete loses up to 5% of its strength. However excessive compaction of wet mixes brings fine particles to the top resulting in a weak, dusty surface.

Manual compaction is commonly used for construction of farm buildings. It can be used for mixes with high and medium workability and for plastic mixes. Wet mixes used for walls are compacted by punting with a batten, stick or piece of reinforcement bar. Knocking on the formwork also helps. Less workable mixes like those used for Doors and pavings are best compacted with a tamper.

Figure 3.22 Concrete slump teset.

Table 3.1 5 Concrete Slump for Various Uses

Figure 3.23 Manual compaction of foundation and floor slab.

The stiffer mixes can be thoroughly compacted only with mechanical vibrators. For walls and foundations a poker vibrator (a vibrating pole) is immersed in the placed concrete mix at points up to 50cm apart. Floors and pavings are vibrated with a beam vibrator.

Figure 3.24 Mechanical vibrators.

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.

Formwork

Formwork provides the shape and surface texture of concrete members and supports the concrete during setting and hardening.

The simplest type of form is possible for pavement edges, floor slabs, pathways, etc.

Figure 3.25 Simple type of formwork for concrete slab.

In large concrete slabs, such as a floor, cracks tend to occur during the early setting period. In a normal slab where watertightness is not essential, this can be controlled by laying the concrete in squares with joints between allowing the concrete to move slightly without causing cracks in the slab. The distance between the joints should not exceed 3 metres. The simplest type is a so called dry joint. The concrete is poured directly against the already hardened concrete of another square.

A more sophisticated method is a filled joint. A gap of 3mm minimum is left between the squares and filled with bitumen or any comparable material.

Forms for walls must be strongly supported, because concrete, when wet, exerts great pressure on the side boards. The greater the height, the greater the pressure. A concrete wall will not normally be thinner than 10cm, or 15cm in the case of reinforced concrete. If it is higher than one meter it should not be less than 20cm thick to make it possible to compact the concrete properly with a tamper. The joints of the formwork must be tight enough to prevent loss of water and cement. If the surface of the finished wall is to be visible and no further treatment is anticipated, tongued and grooved boards, planed on the inside can be used to provide a smooth and attractive surface. Alternatively 12mm plywood sheets can be used. The dimensions and spacing of studs and ties are shown in Figure 3.26. The proper spacing and installation of the ties is important to prevent distortion or complete failure of the forms.

Forms must not only be well braced, but they must be anchored securely to prevent them from floating up, allowing the concrete to run out from underneath.

The forms should be brushed with oil and watered thoroughly before filling with concrete. This is done to prevent water in the concrete from being absorbed by the wooden boards and to prevent the concrete from sticking to the forms. Soluble oil is best, but in practice used engine oil mixed with equal parts of diesel fuel is the easiest and cheapest material to use.

Wooden forms can, if handled carefully, be used several times before they are abandoned. If there is a repeated need for the same shape it is advantageous to make the forms of steel sheets.

The form work can be taken away after 3 days, but leaving it for 7 days makes it easier to keep the concrete wet.

In order to save on material for the formwork and its supporting structure, tall silos and columns are cast with a slip form. The form is not built to the full height of the silo, but may in fact be only a few metres high. As the casting of concrete proceeds the form is lifted. The work has to proceed at a speed which allows the concrete to set before it leaves the bottom of the form. This technique requires complicated design calculations, skilled labour and supervision.

Curing Concrete

Concrete will set in three days but the chemical reaction between water and cement continues much longer. If the water disappears through evaporation, the chemical reaction will stop. It is therefore very important to keep the concrete wet (damp) for at least 7 days.

Premature drying out may also result in cracking due to shrinkage. During curing the strength and impermeability increases and the surface hardens against abrasion. Watering of the concrete should start as soon as the surface is hard enough to avoid damage, but not later than 10 to 12 hours after casting. Covering the concrete with sacks, grass, hessian, a layer of sand or polythene helps to retain the moisture and protects the surface from dry winds. This is particularly important in tropical climates.

Temperature is also an important factor in curing. For temperatures above 0° C and below 40° C strength development is a function of temperature and time. At temperatures above 40°C the stiffening and hardening may be faster than desired and result in lower strength.

The approximate curing time needed to achieve characteristic compressive strength at various curing temperatures for concrete mixes of ordinary portland cement. Show in figure 3.27

Figure 3.26 Dimensions and spacing of studs and ties in formwork for walls.

Figure 3.27 Curing times for concrete.

Finishes on Concrete

The surface of newly-placed concrete should not be worked until some setting has taken place. The type of finish should be compatible with the intended use. In the case of a floor, a non-skid surface for humans and animals is desirable.

Tamped finish: The tamper leaves a coarse rippled surface when it has been used to compact the concrete.

Tamper drawn finish: A less pronounced ripple can be produced by moving a slightly tilted tamper on its tail end over the surface.

Broomed finish: A broom of medium stiffness is drawn over the freshly tamped surface to give a fairly rough texture.

Wood floated finish: For a smooth, sandy texture the concrete can be wood-floated after tamping. The float is used with a semi-circular sweeping motion, the leading edge being slightly raised; this levels out the ripples and produces a surface with a fine, gritty texture, a finish often used for floors in animal houses.

Steel trowelled finish: Steel trowelling after wood floating gives a smoother surface with very good wearing qualities. However, in wet conditions, it can be slippery.

Surfaces with the aggregate exposed can be used for decorative purposes but can also give a rough, durable surface on horizontal slabs. This surface can be obtained by removing cement and sand by spraying water on the new concrete, or by positioning aggregate by hand in the unset concrete.

Reinforced Concrete

Concrete is strong in compression but relatively weak in tension. The underside of a loaded beam, such as a lintel over a door, is in tension.

Figure 3.28 Stresses in a concrete lintel

Concrete subject to tension loading must be reinforced with steel bars or mesh. The amount and type of reinforcement should be carefully calculated or alternatively, a standard design obtained from a reliable source should be followed without variation.

Important factors relative to reinforced concrete:

• 1 The steel bars should be cleaned of rust and dirt before they are placed.
• 2 In order to obtain good adhesion between the concrete and the steel bars, the bars should be overlapped where they join by at least forty times the diameter. When plain bars are used the ends of the bars must be hooked.
• 3 The reinforcement bars should be tied together well and supported so they won't move when concrete is placed and compacted.
• 4 The steel bars must be in the tensile zone and covered with concrete to a thickness of three times the diameter or by at least 25mm to protect them from water and air which causes rusting.
• 5 The concrete must be well compacted around the bars. 6 Concrete should be at least C20 or 1:2:4 nominal mix and have a maximum aggregate size of 20mm.

Concrete floors are sometimes reinforced with welded steel mesh or chicken wire, placed 25mm from the upper surface of the concrete, to limit the size of any cracking. However, such load-distributing reinforcement is necessary only when loadings are heavy, the underlying soil is not dependable, or when cracking must be minimized as in water tanks.

Figure 3.29 Placing reinforcement bars.

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