Concrete to Simple Forms - npstag.comnpstag.com/.../2016/10/CPCPCO2013A-Concrete-to-Simple-Forms.pdf · CPCPCO2013A Concrete to Simple Forms 2 Learning Guide Formwork can collapse

Concrete to Simple Forms

L E A R N I N G G U I D E

CPCPCO2013A

Concrete to Simple Forms CPCPCO2013A

Learning Guide 1

Contents

1. WorkProcedures 1

Observe Safety Signs 1Concrete Strength 2Water-Cement Ratio 3Compaction 3Curing 4Protection 5Mixing the Concrete 5Site Batched Concrete 6Ready-Mixed Concrete 7Testing of Concrete 8Quantities 10

2. Typesandusesofreinforcementinconcrete 11

TypesofReinforcement 13

CuttingFabricandRodReinforcing 14

3. Erection,dismantlingandmaintenanceofformwork 17

Slab-on-Ground 17Formwork Requirements 18Stripping 20

4. Preparationofformsforconcreting 23

Cleaning 23Cleaning of Formwork 23

5. Installationofformworkforslab-on-ground 24

for Installing Formwork for a small Slab-on-Ground 24Storage 26Proportioning of Materials 26

7. Useandmaintenanceofconcretingtools 30

HandtoolsandEquipment 30

ScreedsandStraightEdges 30

Bull Floats 31Wood Floats 31Plastic Floats 32Steel Trowels 32Edging Tools 32Jointing Tools 33

8. Assistwithfinishingconcrete 34

Finishing Concrete Slabs 34Screeding 34Edging and Jointing 35Floating 37Brooming 38Exposed Aggregate 39Curing 40


Learning Guide 1

WorkProcedures

The safety of yourself and other workers on-the-job can be ensured if safe working procedures are practised at all times. The following safety factors must be observed:

• Keep the work area clear of off-cuts and loose materials.

• Make sure that all structural components are firmly fixed before leavingthe job; even for a few minutes.

• When you have finished using a particular tool, put it in your tool box. Thispractice will prevent it form being lost or from falling on a work mate.

• Ensure electric leads and air hoses do not create tripping hazards.

• Always report unsafe equipment or structural components to yoursupervisor immediately they are noticed and warn your fellow workers.

• Ensure that handrails and kickboards are in place and securely fixed.

• Observe safety signs and wear your safety clothing and equipment at alltimes while on the job.

ObserveSafetySigns

CPCPCO2013A Concrete to Simple Forms

2 Learning Guide

Formwork can collapse as a result of faulty design or carelessworkmanship. A collapse may result in costly structural damage or, more seriously, injury or even loss of life.

Safety is of particular importance to workers involved in the construction and removal of formwork. Always ensure that you pay attention to constructional details and observe correct working practices.

ConcreteStrength

Grade of Concrete

Concrete is designated by its grade, eg. 25 grade concrete means 25 MpA. This is the strength the concrete is expected to reach after 28 days. The grade is what is specified when ordering concrete.

Normal concrete generally reaches 70 percent of its grade strength after 7 days.

For general information only at this stage:

MPa is an abbreviation of the term “megapascal” which is used to describe compressive strength of any material. The term can be broken down into two words:

• Mega, which means 1 000 000 (one Million), and

• Pascal, which is one newton of force over an area of one square metre, anewton being 0.1 or 1/10th of a kilogram.

The strength and quality of concrete depend on the following:

• Water/cement ratio.

• Compaction.

• Curing.

• Batching of materials.


Learning Guide 3

Water-CementRatio

Compaction

Hand Compaction Ordinary hand tools of compaction consist of rodding, tamping and spading with suitable tools.

• Vibration

Although hand compaction produce satisfactory results for some purposes, the use of vibration allow the use of drier mixes, resulting in higher strength and reduced shrinkage for given mix proportions.


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Curing

After placing and compaction of the concrete, it is important that the settling and hardening process be controlled as far as possible. This is curing. It is done to reduce cracking and shrinking and to allow the concrete to achieve its designed strength under controlled conditions. In general, the measures taken to cure concrete are designed to:

• prevent it drying out too quickly

• control the temperature of the hardening concrete. Common methods ofcuring include:

1 Keeping concrete damp or wet for two or three days, by:

– spraying with water

– flooding with water.

2 Covering with plastic sheet or similar material.

3 Spray on chemicals.

4 Admixtures included in the concrete at the mixing stage.


Learning Guide 5

Protection

After concrete is placed and finished it is necessary to ensure that it is protected from damage until it has hardened sufficiently to be walked on or loaded. This will require temporary barriers or signs and strict control on access.

MixingtheConcrete

All concrete should be mixed thoroughly until it is uniform in appearance and all ingredients are uniformly distributed.

Very small quantities of concrete for patching or small post bases, etc, can be mixed by hand using a shovel. Small quantities of concrete can be mixed on site using a small mixing machine. This would be used where the small quantities make it uneconomical to use truck mixed or ready mixed.

The vast majority of concrete is now provided to the sites as ready mixed or truck mixed concrete discharging directly into the formwork by the truck, or pumped, or wheeled to its place.


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SiteBatchedConcrete

Very large sites, eg. Large civil engineering projects have their own batching and mixing plants on the site.

Mixing time will depend on several factors:

• the speed of the machine

• size of the batch

• the condition of the mixer.

Generally, the mixing time should be at least one minute for mixes up to one cubic metre, with an increase of twenty seconds for each half a cubic metre. Mixing time should be measured from the time all materials are in the mixer.

Mixers should not be loaded above their capacity. They should be run at the speed for which they were designed. Mixers coated with hardened concrete or having badly worn blades will not perform as efficiently as they should. These conditions should be corrected.

Generally, about 10 percent of the mixing water is placed in the mixer before the aggregate and cement is added. Water should then be added uniformly along with the dry materials. The last 10 percent of the water is added after all the dry materials are in the mixer.


Learning Guide 7

Ready-MixedConcrete

Ready-mixed concrete is purchased directly from a central plant. It is convenient and usually of high quality. In some ready-mixed operations, the materials are dry batched at the central plant and then mixed en route to the site in truck mixers. Another method is to mix the concrete in a stationary mixer at the central plant just enough to intermingle the ingredients. The mixing is then completed in a truck mixer en route to the job site.

Most truck mixers have a capacity of 1.2 cubic metres to 5.0 cubic metres, and carry their own water supply.

Initial Set Initial set of concrete usually takes place two or three hours after the cement is mixed with water. It then continues to harden for many years to come.

Admixtures for Concrete Admixtures are substances added to concrete during the mixing process. The substances differ according to their purpose.

Some of the purposes for using admixtures are:

• To slow down the setting process -Retarders.

• to speed up the process – Accelerator eg. in hot or cold weather.

• to reduce water -Add Strength.

• to expand the product during drying -Ensure a firm fit eg. underpinning.

• to thicken or increase the viscosity of the cement paste.

• to improve resistance to varying temperatures eg. freezing and thawing- these are called Plasticisers.

The types of admixtures used will depend on the purpose the concrete is to be used for and the principal desired effect.

There are other advantages to the use of intentionally entrained air in concrete and it is these which have led to its widespread use even in the more temperate regions of Australia. For example, the workability of the fresh concrete is increased, enabling a reduction in the water content of the mix. In addition, the cohesiveness of the concrete is increased.


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TestingofConcrete

Slump Test Except for drier mixes this test is a measure of workability. It is the simplest method of ensuring that the consistency of the concrete does not alter throughout the job. The equipment required consists of a slump cone, a bullet - pointed 15mm diameter steel rod 600mm in length, a trowel, straight edge and rule.


Learning Guide 9

The test is carried out in the following way:

• Make sure the cone is clean and stand it on a smooth, hard surface, preferably a sheet of metal.

• Stand on the footrests and fill the cone in three layers, rodding each layer asrequired by the Australian Standard. (Usually 25 times)

• Overfill the cone and strike off the surplus.

• Clean round the base and lift the cone vertically, placing it upside downbeside the resulting mound of concrete.

• Place the straight edge across the cone and measure down to the topmostpoint with the rule.

This dimension is termed “the slump” and it should be reasonably constant throughout the job.

To measuretheslump, the rod is rested on the cone and the distance from the underside of the rod and the top of the concrete is measured.

Collapse as shown in the adjacent figure, is caused by excess water in the mixture.


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Quantities

Volume Volume is important in relation to concrete, as before any quantities of materials can be obtained, the volume (amount) of the required concrete must be known. Volume is the cubic content and is calculated as cubic metres.

Volume = Length x Breadth x Thickness or; the area of the base multiplied by the perpendicular height.

Cubic Measurement

1.0m x 1.0m x 1.0m = 1.0m3

NOTE: All work is to be kept in metres and the decimal point used. This simplifies and minimises mistakes.

Example: Calculate the amount of concrete required for a floor, 5.6m long, 4.5m wide

and 150mm thick.

Volume: = Length x Breadth x Thickness

= 5.6m x 4.5m x 0.150m

= 3.780m3 rounded to the next 0.2 of m3

= 3.8m3

NOTE: Ready mixed concrete is purchased in cubic metres (m3) in increments of

0.2m3 . Therefore, concrete calculations are rounded off to the next 0.2m3. .


Learning Guide 11

2. Typesandusesofreinforcementinconcrete

Reinforcement is a material used to strengthen the hardened concrete, which is inserted prior to or during concrete placement and is almost always made from steel.

This unit of learning relates to terms used in reinforcement, sizes, position bar chairs and spacers. The main stresses which occur in concrete are explained to a basic level of understanding.

Concrete when first made is a fluid material and needs to be confined in a mould until it sets. In large works, such as bridges and dams, the mould is a large construction in its own right.

When hardened and cured, concrete has great compressive strength. (This means it has the ability to support great loads placed directly upon it.) However, it has very little strength to resist stresses or forces that tend to bend or pull it apart.

The compressive strength of concrete is about 10 times its tensile strength.

Steel reinforcing is used to give concrete tensile strength.

When hardened and cured, concrete is very strong, particularly in compression but has very little strength when under tensile loads.

A beam when loaded compresses on its top side and the bottom tends to stretch as it is in tension. The concrete beam has an inherent weakness.


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To overcome this, reinforcing steel is used. The hooked ends of the reinforcing steel rod transfer load from the concrete to the reinforcement.


Learning Guide 13

TypesofReinforcement

Steel is regarded as the best reinforcing material for concrete because it has almost the same contraction and expansion rate due to temperature changes as concrete. It may also be purchased in many sizes and forms.

The two main types of steel reinforcing used in concrete are reinforcing bars and welded wire fabric (or mesh). Bars may be smooth or deformed. Smooth bars are generally smaller in diameter. Deformed bars have ridges along the sides. These improve the bond between the concrete and steel.

Steel reinforcing is used in footings, slabs, columns, beams, walls and other concrete work to add strength and control stresses. Reinforced concrete is a composite material which utilises the concrete in resisting compression forces, and some other material, usually steel bars or wires, to resist the tension forces.

Reinforcing for Footings and Ground Piers

• Y Bar Deformed bar called Tempcore - High Strength

• S Bar Deformed bar called Structural GRA08 - Mild Steel

• R Round Bar Mild Steel Rod

• F Fabric Mesh Is made from Hard Drawn Wire - High Strength

The two most common types of steel reinforcement used in strip footings are:

• welded fabric, called trench mesh; and

• individual bars.


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CuttingFabricandRodReinforcing

• Sheets are cut to size using either abrasive cut-off wheels or bolt cutters.

• Large bolt cutters can cut up to 19mm diameter mild steel reinforcing rods.

Positioning Steel Reinforcement in Beam Footings

The following points must be observed when placing reinforcement in position:

• Place the cage in the trench clear of the trench sides. Support the bottombars on chairs or hung from a timber support across the top of the trench. See Figure 31

• Ensure the laps at change in direction are properly made and the cages arelocated firmly in position.

• Steel must be lapped at the corners for the full width of the cage and atjoins in the length a minimum of 500 mm or as specified.


Learning Guide 15

Supporting the Reinforcement

There is a wide range of bar chairs for on-ground or in formwork application which can support the reinforcing steel at the height specified in the reinforcing schedule.

Reinforcing must be held firmly enough to support the weight of workers during the placement of concrete. Bar chairs are spaced in accordance with details given on drawings, but should be greater than 1 metre for large diameter wire but closer for small diameter wires.

The most common type used for fabric consists of a wire chair which clips into a plastic base. The base provides support, and prevents damage to the plastic moisture barrier, if used in a ground slab.


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Learning Guide 17

3. Erection,dismantlingandmaintenanceofformwork

This unit refers to the basic work practice of selecting, erecting and dismantling formwork.

Erection, dismantling and maintenance refers to the “putting together” (assembly), taking down and taking care of the formwork used on a construction site, which is used to form a mould for the concrete to be placed into.

Assessment of this unit is carried out in practical tasks involving the erection of formwork for slab-on-ground and pavement concrete work.

These tasks must involve a demonstration of the skills required in stripping, cleaning and storage of formwork.

Formwork can best be described as a mould that shapes concrete, and is used extensively in the building industry.

Some typical applications of formwork are:

• Footings

• Columns

• Slab-on-Ground

• Suspended Slabs

• Suspended Beams

• Walls

Slab-on-Ground

Formwork for slab-on-ground construction usually consists of one of the following:

1. Timber or metal forms to the edges of the slab

2. Brickwork

3. Concrete blockwork.

Suspended Concrete Slabs Formwork for suspended slabs usually consists of form ply or special metal sheeting supported and braced above ground level.


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FormworkRequirements

Formwork is the temporary construction that gives form to and supports the freshly placed concrete. The loads carried by the formwork are considerable and it must be strong enough to support workers, materials and equipment as well as the concrete without movement or fear of collapse. It is often specially designed and supervised by an engineer.

There are three main principles that must be observed in the design and construction of formwork. They are:

• Quality

• Safety

• Economy.

Quality The forms which shape the freshly placed concrete will imprint in the surface of the concrete any defect present on the face of the form. Forms must be true to size, plumb, square and correctly aligned in accordance with acceptable tolerances as stated in the specification. Four factors are important to good craftsmanship.

• Accuracy

• Rigidity

• Tightness of joints

• Finish

Accuracy The plasticity of fresh concrete allows it to be moulded into any desired structural or architectural shape.

The moulds or forms are equivalent to a photographic negative of the concrete construction, any inaccuracy or blemish in the formwork being reproduced identically in the structure. It is therefore essential that forms be designed and constructed accurately, so that the desired size, shape, position and finish of the cast concrete structure are obtained.

Wherever possible, forms should be set level with the finished top of the proposed concrete so that they can be used as a screed.

Rigidity Formwork should be of substantial construction to prevent distortion when supporting wet concrete.

It should be sufficiently rigid to prevent:

• bulging

• sagging

• movement.


Learning Guide 19

Bulging can be prevented by ensuring that the supporting members are strong enough to support all of the loads and are spaced in accordance with the specified centre-to-centre dimension.

Formwork must be braced to prevent movement in any direction. For suspended beams and slabs, lateral ties prevent the vertical supports from being displaced by knocking or from bending while under load.


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Tightness of Joints Joints between intersecting members in direct contact with the concrete during placement, must be tight enough to prevent leakage of the cement paste.

Leakage of cement paste will form unsightly fins and result in a honeycomb surface to the concrete.

Any flaws in the concrete will require a considerable amount of work to restore the surface finish.

Planed timber or formply will provide a neater finish and is much easier to clean for re-use than sawn timber.

Finish There are several surface finishes which can be obtained by treating the surface of the form. Industrial buildings generally require a smooth finish; however, simulated wood grain finishes can be produced by exposing the grain of timber forms by sand blasting. Special plywood sheets can also be made for this purpose. Joints, nails, screws, and tie wire leave unsightly imprints on the concrete surface. Always ensure that you provide a clear and even surface to the face of the forms.

Stripping

When formwork is stripped, ensure that safe and damage limiting procedures are used, for example, supporting components during the process.


Learning Guide 21

Safety There are two aspects of safety which must be observed:

• Strength, design and construction of formwork and supporting components.

• Work procedures.

Strength - Design & Construction

The “dead load” of the fresh concrete and reinforcing steel is approximately 2500kg (per cubic metre). It is essential that the formwork and formwork components are strong enough to support the concrete and all other imposed loads.

While the design and the supervision is the work of a specialist, you can contribute to the safety of the job by ensuring that directions and specifications are strictly observed.

Economy Formwork components and forms are usually designed for re-use, either on the same project or on others. You can contribute to the economic operation of the job by observing the following procedures:

• Timber and plywood form panels are expensive. Never cut these materialswithout checking your measurements.

• Never cut from a large piece of material when a smaller piece will produceless waste.

• When stripping the forms take care not to damage the ends or edges withyour hammer or pinch bar.

• Remove nails and other fixings with care.

• Always store and stack materials and equipment in the designated storagearea when not in use.


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Learning Guide 23

4. Preparationofformsforconcreting

Cleaning

All dirt, mortar, wood chips, sawdust, etc must be removed from inside the forms before concreting can commence. If the bottom of the form cannot be reached, then clean out holes should be provided at suitable points to permit the removal of foreign matter. A jet of air or water can often be used effectively to remove debris. All clean out holes must be carefully closed after cleaning out the forms

CleaningofFormwork

Any concrete adhering to the forms must be removed before re-use of the forms. The sooner it is removed the easier it will be and less sustained by the forms.

Different means are used on the various materials:

• Steel Forms: -metal scraper

• Plywood Forms: -stiff brush

• Sawn Timber: -metal scraper

• Glass Reinforced Plastic: -brass brush


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5. Installationofformworkforslab-on-ground

Formwork for slab-on-ground construction usually consists of one of the following:

a. Timber or metal forms to the edges of the slab;

b. Brickwork in place and used to contain the concrete, or

c. Blockwork in a similar situation.

forInstallingFormworkforasmallSlab-on-Ground

• Excavate if necessary, a level area free of vegetation.

• Drive pegs securely into the ground in the required positions. (Rememberto allow for the thickness of the form boards).

• Mark the form height onto the pegs.

Nail form boards onto the pegs at the required height, (support should beplaced directly behind where nail is being driven so as not to loosen peg).

Where necessary bracing should be installed.

All pegs protruding above formwork should be trimmed to provide auniform flat surface for screeding.

• In some situations it may be necessary to construct and assemble formworkfirst, then position and secure.

• Fill and level inside forms with sand or coarse gravel as necessary torequired level. Compact filling to ensure a solid base for concrete.


Learning Guide 25


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Storage

If the forms are to be re-used at a later time it is important that they are:

• stacked to prevent distortion

• stacked to prevent rising dampness

• protected from heat/wind/rain

• protected from contact surfaces.

ProportioningofMaterials

Course aggregate contains approximately 50% of voids (spaces). Fine aggregate is used to fill these voids, therefore, fine aggregate required equals half of the course aggregate. Fine aggregate contains approximately 30% voids and cement is used to fill these voids. Therefore, cement content equals 30% of the fine aggregate quantity.

Experiments show, however, that in order to coat all surfaces with cement a further 20% is required, thus making 50% of cement to fine aggregate.

From the figures above we obtain a mix of 4:2:1 or 4 parts course, 2 parts fine and 1 part cement or 4:2:1 is considered a good general mix. See Figure 55.

Water required is approximately 20 litres for every bag of cement, depending on the moisture content of the aggregate.

Accurate proportioning is a major factor in concrete strength. Figure 55 shows the sand occupying the voids (spaces) formed by the gravel. It also shows the voids between the sand grains filled with cement.


Learning Guide 27

Figure55: Proportioning of Aggregates and Cement

Pre-mixed Concrete Concrete must be placed quickly and simply.

Direct from a transit mixer truck is easiest and best.

The most convenient and least labour intensive of concreting is to use ready mixed concrete. A phone call to a ready mix plant will confirm your order. Details given when ordering must include:

• The volume in m3.

• The slump required.

• The strength of concrete stated in MPa.

• The nominal mix of aggregates.

Concrete Mix The concrete mix should be such that there is just sufficient fines (cement and sand) to allow a mortar to be worked to the surface with vibration and a little tooling effort.

Too many fines will make finishing easier but will probably lead to surface crazing, as well as being more expensive than a well proportioned mix. Too much water in the mix (high slump) will create delays in finishing, as well as producing a weak surface layer of mortar, resulting in a dusty and crazed surface with low resistance to wear and abrasion.

A satisfactory uniformed concrete surface requires:

• a properly proportioned concrete

• adequate mixing, handling and placing methods which will minimisesegregation

• adequate compaction

• controlled finishing techniques

• adequate curing.


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Placing and Compacting the Concrete Order concrete by strength-grade and slump, as required by the project specification.

As soon as the concrete is properly mixed, it is delivered to site. Placing should begin without delay and should be completed within half an hour. If a long time is allowed to elapse, the concrete will begin to stiffen and become difficult to place and compact.

If water is added the mix will become sloppy and easier to work into place - but the concrete will be weaker, crack more and have a poor surface finish. For this reason nowaterwouldbeaddedtoconcreteduringtheplacementandfinishingoperations.

Place each load of concrete next to the previous load. Start at one end and work along the slab or beam making sure that each new load is well mixed into the load before.

As far as possible concrete should be placed in one operation, and succeeding batches placed one against the other before initial set has commenced. Each batch of concrete should be well rodded to compact it and to weld it with the preceding batch.


Learning Guide 29

Finishing Irrespective of the type of surface finish required the essential requirements are: Initial finishing should be completed as soon as possible after placing and vibration.

Final finishing, floating and trowelling should be delayed until the surface is ready the final work should also be kept to the minimum necessary to produce the required surface.

NO finishing operations should be performed in any areas where there is free surface water.

Precautions Precautions must be taken when placing concrete to ensure that:

• Formwork and reinforcement are not damaged or dislodged.

• The concrete does not segregate.

The placing of concrete should start from the corners of formwork and from the lowest level if the surface is sloping.

Each load of concrete should be placed into the face of the previously deposited concrete, not away from it.

Where a layer of concrete cannot be placed before the previous layer hardens, as on the morning after an overnight stop, a construction joint should be formed.

Concrete should not be placed in heavy rain without overhead shelter, otherwise the rain may wash cement from the surface.


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7. Useandmaintenanceofconcretingtools

HandtoolsandEquipment

The following is a list of hand tools commonly used in concreting:

• wheelbarrows

• planks

• shovels

• straight edges and screeds

• floats

• steel trowels

• edging and jointing tools

• brooms.

Again, it is very important that these tools are kept clean of hardened concrete and concrete residue.

• Wheelbarrows

• Used to transport concrete from the delivery point (usually the mixertruck) to the final place in the forms.

• Planks

• Used to make a smooth path across ground, over formwork and acrossreinforcement for the wheelbarrow to be wheeled along and reduce theeffort required by the concrete worker.

• Shovels

Used for mixing, moving concrete around in the forms, initial placing and tamping, and cleaning up.

ScreedsandStraightEdges

These are used to compact concrete by tamping and to screed off to the required levels.

Special care must be taken when storing, handling or using straight edges to ensure that they do not bend and are protected from damage.


Learning Guide 31

Floats There are several types of floats, each designed to produce a different finish.

BullFloats

A back and labour saving tool used for large surface areas. Made from magnesium alloy and designed to have a long handle, it aids surface finishing to concrete surfaces and is used immediately after screeding off.

WoodFloats

The wood float is used to level out any slight imperfection left after screeding off with the straight edge or bull float. It can also be used to provide a textured finish to the surface of concrete.


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PlasticFloats

Has a similar use to the wood float but produces a finer finish.

SteelTrowels

The steel trowel is used after wood floating or bull floating to provide a dense smooth surface to concrete.

EdgingTools

When the concrete has been poured and has stiffened, it is often required that the edges be worked to a neat rounded finish. A bricklayer’s large laying trowel is used to develop a clear line between the existing formwork and concrete edge. The edging tool is then used to form a neat rounded finished edge to the work.


Learning Guide 33

JointingTools

Used for putting smooth edges on construction joints or induced cracking joints, after the joint has been formed or cut, or just in the surface of thin slabs.

Brooms Used for producing a non-slip finish on concrete (Brooming or Broom Finish). Also used for general clean up.


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8. Assistwithfinishingconcrete

FinishingConcreteSlabs

Finishing concrete refers to the final top finish process which is completed in various ways according to the specified use of the concrete or the appearance required.

This unit will introduce you to the techniques of surface finishing and you will be given the opportunity to apply the knowledge and skills gained in this and previous units of this section, by completing Practical Activities incorporating all aspects of preparing for, placing and finishing concrete.

During the course of this unit you will learn to:

• Use a screed to level concrete.

• Use a wood float and steel trowel to surface finish a small concrete slabsmooth and in accordance with engineer’s specifications.

• Use a range of nosing, edging and grooving tools in line with specifiedtolerances.

• Produce non-slip surfaces to concrete pathways and slabs.

Concrete slabs may be finished several ways. It depends on the effect desired and the use of the product. Some surfaces may be left rough, others broomed, floated or trowelled, and other surfaces may be textured, coloured or have exposed aggregate.

Screeding

Screeding is usually the first finishing operation after the concrete is placed in the forms. It is performed with a screed.

Screeding is the process of striking off the excess concrete to bring the top surface to the proper grade or elevation.

The edge of the screed may be straight or curved depending on the surface requirements. The screed rests on the top of the forms and is moved across the concrete with a sawing motion. It is drawn backwards slightly.

An excess of concrete should be carried along in front of the screed to fill low places as the tool is moved forward. But if too much concrete is allowed to build up in front of the screed, it may tend to leave hollows behind it.

In normal concrete the dry materials used are heavier than water. They will begin to sink or settle to the bottom of a plastic concrete mixture shortly after placement. This settling action causes excess water to rise to the surface.

This condition is called bleeding. Bleeding does not usually occur with air-entrained concrete. It is very important that the first operations of placing, screeding and wood floating be performed before any bleeding takes place.


Learning Guide 35

If any finishing operation is performed on the surface while the bleed water is present serious scaling, dusting or crazing can result.

The high and low spots may be eliminated and large aggregate embedded using a bullfloat. The operation should follow immediately after screeding to prevent bleeding.

Some surface finishes may not need any further finishing, but most will require one or more of the following operations.

EdgingandJointing

If edging is necessary, this could be the next operation.

Edgingprovides a rounded edge or radius to prevent chipping or damage to the edge. The edger is run back and forth until the desired finish is obtained. Care should be taken to cover all coarse aggregate and not to leave too deep a depression on the top of the slab. This indentation could be difficult to remove during subsequent finishing operations.


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Edging produces a radius on the edge of the slab which prevents chipping. In step one, above, trowel is inserted between form and concrete, to provide track of an edging operation shown in step two below.

As soon as edging has been completed, the slab is jointed or grooved. The bit (cutting edge) of the jointing tool cuts a groove in the slab which is called a control or contraction joint.

Any cracking due to shrinkage caused by drying out or temperature change will occur at the joint. These cracks are not noticeable when controlled. The joint weakens the slab and induces cracking at that location rather than some other place.

In sidewalk and driveway construction, the tooled joints are generally spaced at intervals equal to the width of the slab, but not more than 5m. They should be at right angles to the edge of the slab.

Use a straight edge as a guide when making a groove. A 25 x 200 or 25 x 250 board will be ideal. Be sure the board is straight.

Figure72: The Jointing Operation


Learning Guide 37

Large concrete surfaces may be jointed by cutting with a power saw using an abrasive or diamond blade. When grooves are cut rather than jointed the operation should be performed 4 to 12 hours after the slab has been finished. The cutting must be done before random shrinkage cracks develop, but after the concrete is hard enough not to be torn or damaged by the blade.

Floating

After concrete has been edged or jointed, it should be allowed to harden enough to support a person and leave only a slight foot imprint. Floating should not begin until the water sheen has disappeared.

When all bleed water and water sheen has left the surface, the concrete has started to stiffen.

The surface is floated with wood or metal floats or with a finishing machine using float blades. Aluminium or magnesium floats work better especially on air-entrained concrete. Metal floats reduce the amount of work required by the finisher. Drag is reduced and the float slides more readily over the surface.

It provides a dense smooth surface that could be slippery when wet. A wood float tends to stick to the surface and produces a tearing action. It provides a rougher texture that is not as slippery in wet conditions. The light metal float also forms a smoother surface texture than the wood float.


38 Learning Guide

Generally, finishing begins when the sheen has left the surface (in the case of air-entrained concrete there may be little bleed water and no visible sheen and it may be possible to finish this type of concrete after a short delay).

Normally when the sheen has left the surface, the concrete will support your weight. This may cause indentation of 5mm or more so finishers must use foot and knee pads to distribute their weight. As shown above (Figure 75).

Where power equipment is used the delay period can be increased so that the concrete can support the weight of a person with little marking.

NOTE1: Finishing should NOT be attempted in any area where there is free surface water.

NOTE2:Cement should NOT be used to dry up surface moisture as this will cause cracking later on.

Brooming

A broomed finish is very popular. It provides a non-slip surface which is particularly useful for paths and driveways.

This finish is achieved by pulling a damp broom across freshly floated surfaces. Coarse textures, suitable for steep slopes or heavily trafficked areas are produced by stiff-bristled brooms. Medium to fine textures are obtained with a soft-bristled broom. The broom should be kept damp during the work.

Broomed textures can be applied in straight lines, curved line or wavy lines. It is advisable when applying the curved or wavy lines to take care to keep the lines regular in curve and pitch - and not to be too flamboyant with this finish.

On paths and driveways the broomed lines should be at right angles to the direction of the traffic. For long paths or areas with many panels it may be useful to broom diagonally changing direction for each adjoining panel.


Learning Guide 39

As with the floated finishes, after brooming it will be necessary to clean up the grooves and edged with another pass of the grooving and edging tools.

ExposedAggregate

Exposed aggregate offers a simple way of obtaining decorative finishes which are textured, colourful and highly immune to wear and weather. Various sizes and colours of gravel are available - but it is advisable to use aggregate 5 to 20mm diameter which is smooth - not crushed rock with abrasive sharp edges.

While the correct delay before removing the surface cement to expose the aggregate is most important to the production of a quality finish, it is difficult to nominate the specific time as it depends on many variables. Some of these are concrete temperature and age, type of cement, admixture type and the quantities of water, cement and admixtures used. The delay also depends on weather conditions, depth of pour, type of aggregate, type of sub-grade, etc.


40 Learning Guide

Curing

Curing is the drying out of concrete under conditions of humidity and temperature which help in the proper setting of the concrete.

The properties of concrete such as strength, watertightness, wear resistance and stability, improve with age as long as conditions are favourable.

The improvement in the properties is rapid at first and continues at a diminishing rate for an indefinite period as long as moisture is present and the temperature is favourable.

Evaporation of water from newly placed concrete can cause the cement hydration process to stop at an early age. Loss of water also causes the concrete to shrink thus creating tensile stresses at the drying surface. If these stresses develop before the concrete has attained adequate strength, surface cracking will result.

It follows that concrete should be protected so that the moisture is not lost during the early hardening period. Concrete should also be kept at a favourable temperature.

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