Farm structures ... - Ch3 Building materials: Concrete (2024)

Concrete

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Concrete is a building material made by mixing cement paste(portland cement and water) and aggregate (sand and stone). Thecement-paste is the "glue" which binds the particles inthe aggregate together. The strength of the cement-paste dependson the relative proportions of water and cement; a more dilutedpaste being weaker. Also the relative proportions of cement-pasteand aggregate affects the strength; a higher proportion of thepaste making stronger concrete. The concrete hardens through thechemical reaction between water and cement without the need forair. Once the initial set has taken place concrete cures wellunder water. Strength is gained gradually, depending on the speedof the chemical reaction.

Admixtures are sometimes included in the concrete mix toachieve certain properties. Reinforcement steel is used for addedstrength, particularly for tensile stresses.

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

Properties of Concrete

Concrete is associated with high strength, hardness,durability, imperviousness and mouldability. It is a poor thermalinsulator, but has high thermal capacity. Concrete is notflammable and has good fire resistance, but there is a seriousloss of strength at high temperatures. Concrete made withordinary portland cement has low resistance to acids andsulphates but good resistance to alkalies.

Concrete is a relatively expensive building material for farmstructures. The cost can be lowered if some of the portlandcement is replaced with pozzolana. However, when pozzolanas areused the chemical reaction is slower and strength development isdelayed.

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

Compressive strength is measured by crushing cubes having l5cmper side. The cubes are cured for 28 days under standardizedtemperature and humidity and then crushed in a hydraulic press.Characteristic strength values at 28 days are those below whichnot more than 5% of the test results fall. The grades used areC7, C10, Cl5, C20, C25, C30, C40, C50 and C60, each correspondingto 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 - cementratio).

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

The leaner of the nominal mixes listed opposite the C7 and C10grades are only workable with very well-graded aggregates rangingup to quite large sizes.

Ingredients

Cement

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

Table 3.12 Suggested Use forVarious Concrete Grades and Nominal Mixes

Aggregate

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

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

Test for Organic Matter is done by putting 80mm of sand in a200mm high transparent bottle. Add a 3% solution of sodiumhydroxide up to 120mm. Note that sodium hydroxide, which can bebought from a chemist, is dangerous to the skin. Cork the bottleand shake it vigorously for 30 seconds and leave it standinguntil the following day. If the liquid on top of the sand turnsdark brown or coffee coloured, the sand should not be used."Straw" color is satisfactory for most jobs, but notfor those requiring the greatest strength or water resistance.Note however that some ferrous compounds may react with thesodium hydroxide and cause the brown colour.

Grading of the aggregate refers to proportioning of differentsizes of the aggregate material and greatly influences thequality, permeability and workability of the concrete. With awell-graded aggregate the various sizes of particles intermeshleaving a minimum volume of voids to be filled with the morecostly 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 aggregatepassing through various sieves. A well-graded aggregate will havea fairly even distribution of sizes.

Moisture Content in sand is simportant since sand mixing ratiooften refers to kg dry sand and the maximum amount of waterincludes the moisture in the aggregate. The moisture content isdetermined by taking a representative sample of 1 kg. The sampleis accurately weighed and spread thinly on a plate, soaked withspirit (alcohol) and burned while stirring. When the sample hascooled it is weighed again. The weight-loss amounts to the weightof the water which has evaporated, and is expressed as apercentage by dividing the weight lost by the weight of the driedsample. Normal moisture content of naturally moist sand is 2.5 to5.5%. That much less water is added to the concrete mixture.

Density is the weight per volume of the solid mass excludingvoids, and is determined by putting one kilo of dry aggregate inone litre of water. The density is the weight of the dryaggregate ( I kg) divided by the volume of water forced out ofplace. 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 aggregateincluding voids and is determined by weighing I litre of theaggregate. Normal values for coarse aggregate are 1500 to 1650kg/m3. Completely dry and very wet sand have the same volume butdue to the bulking characteristic of damp sand it has a greatervolume. The bulk density of a typical naturally moist sand is 15to 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 largerparticles of coarse aggregate may not exceed one quarter of theminimum thickness of the concrete member being cast. Inreinforced concrete the coarse aggregate must be able to passbetween the reinforcement bars, 20mm being normally regarded asmaximum size.

Aggregate with larger surface area and rough texture, i.e.,crushed stone, allows greater adhesive forces to develop but willgive 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 notavailable, the bottom of the pile should not be used to avoiddefilement with soil. In hot, sunny climates, a shade should beprovided or the aggregate sprinkled with water for cooling. Hotaggregate materials make poor concrete.

Batching

Measuring is done by weight or by volume. Batching by weightis 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 ofconcrete. Batching by weight is recommended for concrete of gradeC30 and higher. Checking the bulk density of the aggregate willallow greater accuracy when grade C20 or higher is batched byvolume. A 50 kg bag of cement can be split into halves by cuttingacross the middle of the top side of a bag lying flat on thefloor. The bag is then grabbed at the middle and lifted so thatthe bag splits into two halves.

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

Figure 3.19 Relation betweencomprehensive strenght and water cement ratio

The sum of the ingredient volumes will be greater than thevolume of concrete, because the sand will fill the voids betweenthe coarse aggregate. The materials normally have 30 to 50%greater volume than the concrete mix; 5 to 10% is allowed forwaste and spill. The cement added does not noticeably increasethe volume. The above assumptions are used in Example 1 inroughly estimating the amount of ingredients needed. In Example2, a more accurate method of calculating the amount of concreteobtained 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.

See Also

Concrete: The Basic MixConcrete Calculator - How Much Concrete Do I Need? - Concrete NetworkWhat's a Mix Ratio?Ratio of 3 to 5 (3:5)

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

See Also

What happens when sugar is added in to concrete? - The Constructor

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 CubicMetre for Batching Nominal Concrete Mixes

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

Mixing

Mechanical mixing is the best way of mixing concrete. Batchmixers with a tilting drum for use on building sites areavailable in sizes from 85 to 400 litres. Power for the drumrotation is supplied by a petrol engine or an electric motorwhereas the tilting of the drum is done manually. The pear-shapeddrum has blades inside for efficient mixing. Mixing should beallowed to proceed for at least 2.5 minutes after all ingredientshave been added. For small scale work in rural areas it may bedifficult and rather expensive to get a mechanical mixer.

Table 3.14 Mixing WaterRequirements for Dense Concrete for Different Consistencies andMaximum Sizes of Aggregate

³ Includes moisture in aggregate. The quantities ofmixing water are maximums for use with reasonably wellgraded,well-shaped, angular coarse aggregate. 2 For slump see table3.15.

Figure 3.20 Batch mixer.

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

Figure 3.21 Home-builtconcrete mixer.

Hand mixing is normally adopted on small jobs. Mixing shouldbe done on a close-boarded platform or a concrete floor near towhere the concrete is to be placed and never on bare groundbecause 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 hom*ogeneous 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 whenthere is a break in the mixing, and at the end of the day.

Slump Test

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

Placing and Compaction

Concrete should be placed with a minimum of delay after themixing is completed, and certainly within 30 minutes. Specialcare should be taken when transporting wet mixes, since thevibrations of a moving wheelbarrow may cause the mix tosegregate. The mix should not be allowed to flow or be droppedinto position from a height greater than 1 metre. The concreteshould be placed with a shovel in layers no deeper than 15cm andcompacted before the next layer is placed.

When slabs are cast, the surface is levelled out with a screedboard which also is used to compact the concrete mix as soon asit has been placed to remove any trapped air. The less workablethe mix is, the more porous it is and the more compaction isnecessary. For every per cent of entrapped air the concrete losesup to 5% of its strength. However excessive compaction of wetmixes brings fine particles to the top resulting in a weak, dustysurface.

Manual compaction is commonly used for construction of farmbuildings. It can be used for mixes with high and mediumworkability and for plastic mixes. Wet mixes used for walls arecompacted by punting with a batten, stick or piece ofreinforcement bar. Knocking on the formwork also helps. Lessworkable mixes like those used for Doors and pavings are bestcompacted with a tamper.

Figure 3.22 Concrete slumpteset.

Table 3.1 5 Concrete Slump forVarious Uses

Figure 3.23 Manualcompaction of foundation and floor slab.

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

Figure 3.24 Mechanicalvibrators.

Construction Joints

The casting should be planned so that the work on a member canbe completed before the end of the day. If cast concrete is leftfor more than 2 hours it will set so much that there is no directcontinuation between the old and new concrete. Joints arepotentially weak and should be planned where they will effect thestrength of the member as little as possible. Joints should bestraight, either vertical or horizontal. When resuming work, theold surface should be roughened and cleaned and then treated witha thick mixture of water and cement.

Formwork

Formwork provides the shape and surface texture of concretemembers 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 offormwork for concrete slab.

In large concrete slabs, such as a floor, cracks tend to occurduring the early setting period. In a normal slab wherewatertightness is not essential, this can be controlled by layingthe concrete in squares with joints between allowing the concreteto move slightly without causing cracks in the slab. The distancebetween the joints should not exceed 3 metres. The simplest typeis a so called dry joint. The concrete is poured directly againstthe already hardened concrete of another square.

A more sophisticated method is a filled joint. A gap of 3mmminimum is left between the squares and filled with bitumen orany comparable material.

Forms for walls must be strongly supported, because concrete,when wet, exerts great pressure on the side boards. The greaterthe height, the greater the pressure. A concrete wall will notnormally be thinner than 10cm, or 15cm in the case of reinforcedconcrete. If it is higher than one meter it should not be lessthan 20cm thick to make it possible to compact the concreteproperly with a tamper. The joints of the formwork must be tightenough to prevent loss of water and cement. If the surface of thefinished wall is to be visible and no further treatment isanticipated, tongued and grooved boards, planed on the inside canbe used to provide a smooth and attractive surface. Alternatively12mm plywood sheets can be used. The dimensions and spacing ofstuds and ties are shown in Figure 3.26. The proper spacing andinstallation of the ties is important to prevent distortion orcomplete failure of the forms.

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

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

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

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

In order to save on material for the formwork and itssupporting structure, tall silos and columns are cast with a slipform. The form is not built to the full height of the silo, butmay in fact be only a few metres high. As the casting of concreteproceeds the form is lifted. The work has to proceed at a speedwhich allows the concrete to set before it leaves the bottom ofthe form. This technique requires complicated designcalculations, skilled labour and supervision.

Curing Concrete

Concrete will set in three days but the chemical reactionbetween water and cement continues much longer. If the waterdisappears 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 toshrinkage. During curing the strength and impermeabilityincreases and the surface hardens against abrasion. Watering ofthe concrete should start as soon as the surface is hard enoughto avoid damage, but not later than 10 to 12 hours after casting.Covering the concrete with sacks, grass, hessian, a layer of sandor polythene helps to retain the moisture and protects thesurface from dry winds. This is particularly important intropical climates.

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

The approximate curing time needed to achieve characteristiccompressive strength at various curing temperatures for concretemixes of ordinary portland cement. Show in figure 3.27

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

Figure 3.27 Curing timesfor concrete.

Finishes on Concrete

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

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

Tamper drawn finish: A less pronounced ripple can be producedby moving a slightly tilted tamper on its tail end over thesurface.

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

Wood floated finish: For a smooth, sandy texture the concretecan be wood-floated after tamping. The float is used with asemi-circular sweeping motion, the leading edge being slightlyraised; this levels out the ripples and produces a surface with afine, gritty texture, a finish often used for floors in animalhouses.

Steel trowelled finish: Steel trowelling after wood floatinggives 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 decorativepurposes but can also give a rough, durable surface on horizontalslabs. This surface can be obtained by removing cement and sandby spraying water on the new concrete, or by positioningaggregate by hand in the unset concrete.

Reinforced Concrete

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

Figure 3.28 Stresses in aconcrete lintel

Concrete subject to tension loading must be reinforced withsteel bars or mesh. The amount and type of reinforcement shouldbe carefully calculated or alternatively, a standard designobtained from a reliable source should be followed withoutvariation.

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 steelmesh or chicken wire, placed 25mm from the upper surface of theconcrete, to limit the size of any cracking. However, suchload-distributing reinforcement is necessary only when loadingsare heavy, the underlying soil is not dependable, or whencracking must be minimized as in water tanks.

Figure 3.29 Placingreinforcement bars.

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