CONCRETE BASICS A Guide to Concrete Practice
Contents
1. 2 Collect a sample See Sampling 3 Stand firmly on the footpieces and fill 1 3 the volume of the cone with the sample Compact the concrete by rodding 25 times Rodding Rodding means to push a steel rod in and out of the concrete to compact it into the slump cone Always rod in a definite pattern working from outside into the middle 4 Now fill to 2 3 and again rod 25 times just into the top of the first layer 5 Fill to overflowing rodding again this time just into the top of the second layer Top up the cone until it overflows lL 6 Level off the surface with the steel rod using a rolling T action Clean any concrete from around the base and top of the cone push down on the handles and oy step off the footpieces 7 Carefully lift the cone straight up making sure not to move the sample 8 Turn the cone upside down and place the rod across the upturned cone Fi Cement Concrete amp Aggregates Australia 11 Concrete Basics Concrete Testing CONTENTS 12 9 Take several measurements and report the average distance to the top of the sample 10 If the sample fails by being outside the tolerance ie the slump is too high or too low another must be taken If this also fails the remainder of the batch should be rejected THE COMPRESSION TEST The compression test shows the compressive strength of hardened concrete The testing is done in a laboratory off site The only work done on site is to mak2. CONTENTS 21 DELAYS Delays can cause the concrete to dry out and stiffen Delays are more of a problem on hot and or windy days when the concrete will dry out and stiffen more quickly To avoid delays plan ahead Check that all labour tools and containers are ready and that all preparations for placing have been done before the concrete is delivered NEVER just add water to the concrete to make it more workable Cement Concrete amp Aggregates Australia 21 Concrete Basics Transporting and Placing CONTENTS 22 SEGREGATION Segregation is when the coarse and fine aggregate and cement paste become separated Segregation may occur when the concrete is mixed transported placed or compacted Segregation makes the concrete WEAKER LESS DURABLE and will leave A POOR SURFACE FINISH To avoid segregation Check the concrete is not too wet or too dry Make sure the concrete is properly mixed It is important that the concrete is mixed at the correct speed in a transit mixer for at least two minutes immediately prior to discharge The concrete should be placed as soon as possible When transporting the mix load carefully If placing concrete straight from a truck place vertically and never let the concrete fall more than one and a half metres Always place new concrete into the face of concrete already in place When compacting with a poker vibrator be sure to use it carefully See CHAPTER 8
3. LI Prevention Dampen the subgrade and forms and protect concrete from the wind Keep all materials cool on hot days Place compact and cure as quickly as possible on hot days so concrete won t dry out Once the concrete has been compacted screeded and floated apply a uniform spray film of EVAPORATIVE RETARDER eg Aliphatic Alcohol to prevent rapid loss of surface moisture then continue with finishing Try to place at the cooler times of the day Repair Cracks may be closed by reworking the plastic concrete Ive ALIPHATIC ALCOHOL Cement Concrete amp Aggregates Australia 48 Concrete Basics Cracking CONTENTS 49 Formwork Movement If formwork is not strong enough it may bend or bulge Formwork movement may happen at any time during placement and compaction Prevention Make sure formwork is strong If the concrete collapses strengthen the formwork and re vibrate the concrete Thermal Shock Applying cold water for curing over concrete on a hot day can result in cracks from the sudden contraction Prevention Use warm water or other curing method CRACKS IN HARDENED CONCRETE Cracks after hardening may be caused by drying shrinkage movement or settling of the ground or placing higher loads on the concrete than it was designed to carry Little can be done with cracks after hardening Careful and correct placement helps prevent serious cracking after hardening Only uncontrolled cra
4. can also be used to control drying shrinkage cracking over limited spacings and for odd shaped slabs They also increase the flexural or bending strength of concrete Cement Concrete amp Aggregates Australia 52 Concrete Basics Formwork CONTENTS 53 CHAPTER 18 Formwork Formwork gives concrete its SHAPE Formwork provides a mould into which concrete is placed When concrete has hardened the formwork is removed Formwork must be ACCURATE STRONG and WELL MADE This is necessary so that the concrete will not leak from the joints and so that the formwork will not sag bulge or move and especially in large construction will be safe The surface of the forms in contact with concrete affects how concrete will look If the final appearance of the concrete is important choose a material that will give the required surface texture PLACEMENT Be sure that formwork is placed so it can be removed If formwork is placed in awkward positions or tight corners it may be difficult to remove when the concrete has hardened It is helpful if formwork is SIMPLE to build EASY to handle and RE USEABLE Formwork sections should be of simple design not too big and of standard sizes if they are to be re used MATERIALS Formwork is normally made from steel or timber Timber formwork is easier to make while steel formwork will allow a greater number of re uses Formwork can be made on site or bought from formwork su
5. colouring material onto the concrete surface spreading it evenly with a float Leave for a minute or so to soak up some moisture Cement Concrete amp Aggregates Australia 36 Concrete Basics Colouring and Surface Finishes CONTENTS 37 Shake the other 1 3 of the dry colouring material onto the concrete at right angles to the first application if possible and again after it has dampened up spread evenly with a float The two applications help to give a more uniform colour and thickness Re tool any edges and joints See CHAPTER 11 Joints After a while the surface must be re floated Full Depth Colour The colour is added to the concrete during mixing so all the concrete is coloured then the concrete is compacted and finished as for normal concrete To achieve the required colour the pigment additives should generally be in the range of 3 7 by weight of the cement Higher quantities may affect the strength and durability of the concrete Check manufacturer details for colour selection See CHAPTER 2 Concrete Properties Each batch must be accurately proportioned thoroughly mixed and well floated to give an even colour See CHAPTER 9 Finishing The colour of the cement powder may effect the shade of the final colour ie a dark grey cement may affect light colours Coatings Coatings including paints tinted sealers and trowelled on coatings provide a wide range of colours and are easily applied
6. grinding to expose the aggregates colouring staining and using special aggregates to achieve other desired effects For further details refer to Polished Concrete Floors Briefing 05 Cement Concrete amp Aggregates Australia 39 Concrete Basics Defects CONTENTS 40 CHAPTER 14 Defects A EASY To Some defects are obvious only to a trained eye Sere others such as cracking are obvious to anyone Some common defects their causes and how to prevent and repair them are explained below If in doubt consult an expert COLOUR VARIATION Variations in colour across the surface of concrete May appear as patches of light and dark Causes Uneven or variable compaction and curing conditions See CHAPTER 8 Compacting and CHAPTER 10 Curing Addition of excess water Segregation of materials in coloured concrete Variable colour dosage Prevention Use uniform concrete mix and use consistent placing compacting finishing and curing procedures Do not use driers Repair Many colour variations from workmanship will be permanent To hide the variation a SURFACE COATING can be applied Rectification of colour variation from stains is a very difficult operation and may need repeated gentle treatments with a weak acid See CHAPTER 15 Removing Stains CRAZING A network of fine cracks across the surface of concrete Causes Crazing is caused by minor surface shrinkage in rapid drying conditions ie low humidi
7. of the forms by two VA workers Always keep a small amount or surcharge of concrete in front of both beams of the screed to avoid hollows forming in the surface If a hollow develops the screed will not compact the concrete The mechanical screed compacts the concrete as it vibrates e LLL Cement Concrete amp Aggregates Australia 23 Concrete Basics Compacting CONTENTS 24 INTERNAL VIBRATION Internal vibration is done with a mechanical vibrator or poker vibrator The POKER is put into concrete and vibrates it from the inside INTERNAL VIBRATION ae Method Put the poker into the concrete QUICKLY Take the poker out very SLOWLY otherwise a hole or weak spot may be left in the concrete Make sure there are enough workers so some can compact while others continue to place 7 The SIZE of the poker determines how much concrete is vibrated at one time The area vibrated at one time is called the RADIUS OF ACTION This can be seen by the radius over which air bubbles rise to the surface The radius of action will be greater with a LARGER poker and more workable concrete Always compact in a definite pattern so the radius of action overlaps and covers the whole area of the concrete Where concrete is placed in layers the poker should be long enough to reach and enter into the layers of concrete under the one being compacted Cement Concrete amp Aggregates Au
8. small handstamps for the edges and any difficult to reach areas Stencilled Brick tile or stone appearances can be obtained by floating a paper template into the concrete surface before applying a dry shake The template forms the joint lines in the pattern land tamper THE PATTERN IS VAD PAD 2 TEXTURED FINISHES Broomed To give a skid resistant surface a TEXTURES broom can simply be drawn across the surface of yp Ne concrete The broom can be drawn in straight or s shape lines The depth of the texturing will depend on the stiffness of the broom s bristles and the pressure applied Exposed Aggregate Exposing the aggregate gives an attractive decorative finish Different sizes and colours of aggregates allow many different appearances Cement Concrete amp Aggregates Australia 38 Concrete Basics Colouring and Surface Finishes CONTENTS 39 The concrete can be made as normal and the course aggregate exposed This is done by waiting until the surface is firm but not dry then brushing washing or brooming away any cement paste until the aggregates are exposed The concrete is then cured A surface retardant can be used to aid the process Alternatively there are two ways to get an exposed aggregate finish by adding special aggregates to the surface immediately after the concrete has been placed METHOD 1 Place compact and level the concrete to about 10 mm below the top of the forms Sprea
9. APTER 11 Joints 4 La WASTAGE Good planning and site preparation reduces wastage Reducing wastage can cut costs since up to 15 of concrete can be lost this way Cement Concrete amp Aggregates Australia 19 Concrete Basics Transporting and Placing CONTENTS 20 CHAPTER 7 Transporting and Placing When transporting and placing concrete avoid DELAYS SEGREGATION and WASTAGE TRANSPORTATION The method used to transport concrete depends on which one is the most cost effective and easiest for the job size and site Transport methods include a concrete truck a concrete pump a crane and bucket a chute a conveyor or a hoist For small jobs on which concrete cannot be placed directly from the truck chute a wheelbarrow is the easiest way to transport it Always transport concrete as little as possible on site to reduce problems of segregation and wastage PLACING When placing concrete be careful not to damage or move the formwork and reinforcement Place concrete as near to its final position as possible FoRMWORK should stand upto concrete PLACED against it Start placing from the corners of the formwork or in the case of a sloping site from the lowest level IMPORTANT SAFETY INFORMATION When handling and using cement or fresh concrete avoid skin contact Wear suitable protective clothing Cement Concrete amp Aggregates Australia 20 Concrete Basics Transporting and Placing
10. CONCHETE BASICS A Guide to Concrete Practice E l PETE IS Gabe BY Faxina y CEMENT N We a IL 2 se Nor KS oN oe 24 2 o E e 2 w 2 FGA ND fa S CRUSHED E f ROC Sa eee AGGREGATES CA CEMENT CONCRETE amp AGGREGATES AUSTRALIA CEMENT CONCRETE amp AGGREGATES AUSTRALIA Cement Concrete amp Aggregates Australia is a not for profit organisation established in 1928 and committed to serving the Australian construction community CCAA is acknowledged nationally and internationally as Australia s foremost cement and concrete information body taking a leading role in education and training research and development technical information and advisory services and being a significant contributor to the preparation of Codes and Standards affecting building and building materials CCAA s principal aims are to protect and extend the uses of cement concrete and aggregates by advancing knowledge skill and professionalism in Australian concrete construction and by promoting continual awareness of products their energy efficient properties and their uses and of the contribution the industry makes towards a better environment Cement Concrete amp Aggregates Australia ABN 34 000 020 486 MORE Information CCAA OFFICES SYDNEY OFFICE Level 6 504 Pacific Highway St Leonards NSW Australia 2065 POSTAL ADDRESS Locked Bag 2010 St Leonards NSW 1590 TELEPHONE 61 2 9437 9711 FACSI
11. Compacting Never spread concrete sideways with a poker vibrator as this may cause segregation of the mix Always be sure to vibrate concrete evenly NEVER Wie vitro WASTAGE Wastage can be costly especially on small jobs To minimise wastage mix load transport and place carefully Cement Concrete amp Aggregates Australia 22 Concrete Basics Compacting CONTENTS 23 CHAPTER 8 Compacting WHAT IS COMPACTION Compaction is the shaking or vibrating of the concrete to liquify it and expel any trapped air The concrete settles filling all the space in the forms e fk you shake vibrate the Corcrete the AIR is pushed out WHEN TO COMPACT Compaction must be done as concrete is placed while it is still plastic Never let concrete dry out and stiffen because it will be too difficult to compact WHY COMPACT Properly compacted concrete is more dense strong and durable Off form finishes will also be better EXTERNAL VIBRATION Screeding Screeding levels and compacts thin concrete slabs and the top layers of thicker slabs A screed board will not compact the concrete very well Mechanical vibration or hand rodding is required to provide adequate compaction The Mechanical Screed A double beam mechanical screed compacts the concrete TWICE The first beam levels the concrete roughly and compacts it The second beam levels and compacts the concrete further The screed is pulled along the top
12. E HARDENING CRACKS Cracks that happen BEFORE concrete hardens while it is still workable HARDENED CONCRETE CRACKS Cracks which occcur AFTER concrete hardens PRE HARDENING CRACKS Pre hardening cracks are cracks which form during placing compaction and finishing caused by movement of concrete before it is dry There are three types of pre hardening cracks PLASTIC SETTLEMENT cracks PLASTIC SHRINKAGE cracks and Cracks caused by MOVEMENT OF THE FORMWORK Pre hardening cracks if they are detected early can be easily rectified by re compacting re trowelling or re floating the concrete surface Plastic Settlement Cracks When do they form They form soon after concrete is placed while it is still plastic They get bigger as concrete dries and shrinks and tend to follow the lines of reinforcement Correction Revibrate the concrete Re trowel the surface Look for cracks as the concrete is setting At this stage they can easily be fixed Cement Concrete amp Aggregates Australia 47 Concrete Basics Cracking CONTENTS 48 Plastic Shrinkage Cracks When do they form On very hot days or in low humidity and even moderate winds They are more common in summer but may occur in winter See CHAPTER 12 Hot and Cold Weather Concreting Plastic shrinkage cracks appear in lines roughly parallel or in a crazed haphazard way They are usually 300 600 mm long but may be between 25 mm and 2 m in length
13. Good watertight formwork Repair If honeycombing occurs only in a thin surface layer it can be rendered ie applying a layer of mortar If honeycombing occurs to a greater depth the concrete may need to be removed and replaced BLISTERING Pee ae Blisters are hollow low profile bumps on the concrete surface filled with either air or bleed water FoRMWoRK Causes They are caused when the fresh concrete surface is sealed by premature trowelling trapping air or bleed water under the surface layer This is more likely to occur in thick slabs or on hot windy days when the surface is prone to drying out Prevention After placing screeding and floating leave the concrete as long as possible before trowelling If blisters are forming delay trowelling as long as possible and take steps to reduce evaporation by using an evaporative retarder Repair Grind off the weakened layer to an even finish or remove blisters and apply a repair mortar or epoxy coating Cement Concrete amp Aggregates Australia 43 Concrete Basics Removing Stains CONTENTS 44 CHAPTER 15 Removing Stains Simple stains and everyday grime can be removed by washing and scrubbing Water jet washing may also be successful Stains from oil rust or paint leave ugly marks on concrete ruining its appearance These stains penetrate the ay concrete surface and can often be very difficult to remove A stain may be removed using a special chemic
14. MILE 61 2 9437 9470 BRISBANE OFFICE Suite 2 Level 2 485 lpswich Road Annerley QLD 4103 TELEPHONE 61 7 3227 5200 FACSIMILE 61 7 3892 5655 MELBOURNE OFFICE 2nd Floor 1 Hobson Street South Yarra VIC 3141 TELEPHONE 61 3 9825 0200 FACSIMILE 61 3 9825 0222 PERTH OFFICE 45 Ventnor Avenue West Perth WA 6005 TELEPHONE 61 8 9389 4452 FACSIMILE 61 8 9389 4451 ADELAIDE OFFICE PO Box 229 Fullarton SA 5063 TELEPHONE 61 8 8274 3758 PREMIXED CONCRETE AND EXTRACTIVE INDUSTRIES OFFICE PO Box 243 Henley Beach SA 5022 TELEPHONE 61 8 8243 2505 FACSIMILE 61 8 8125 5822 TASMANIAN OFFICE PO Box 246 Sheffield TAS 7306 TELEPHONE 61 3 6491 1509 FACSIMILE 61 3 6491 2529 WEBSITE www ccaa com au EMAIL info ccaa com au Cement Concrete amp Aggregates Australia is committed to being the major source of information on cement concrete and aggregates in Australia For a complete listing of all retail and free publications please visit and bookmark CCAA website www ccaa com au d g e S h aring In forma Concrete Basics Concrete Materials 1 CONCRETE BASICS A Guide to Concrete Practice First Published July 1991 CONTENTS Preface page 2 CHAPTER 1 Concrete Materials pages 3 5 CHAPTER 2 Concrete Properties pages 6 9 CHAPTER 3 Concrete Testing pages 10 13 CHAPTER 4 Ordering page 14 CHAPTER 5 Proportioning and Mixing pages 15 16 CHAPTER 6 Planning and Site Preparation pa
15. NITIAL FINISHING Concrete is first screeded to the level of the formwork then bullfloated and left to set In some cases screeding leaves a good enough finish especially if floor coverings are to be used over the concrete Water then appears on the surface of the concrete This water is called bleed water Excess bleed water can be removed by dragging an ordinary garden hose across the surface of the concrete Never try to dry up the bleed water using stone dust or cement as this will weaken the concrete surface in the long run Once the bleed water dries up and concrete can support a person s weight with only a slight marking to the surface the final finishing can begin Cement Concrete amp Aggregates Australia 26 Concrete Basics Finishing CONTENTS 27 FLOATING There may be two stages in floating The BULLFLOAT which is part of the initial finishing The POWER or HAND FLOAT which is part of the final finishing Floating helps compact and level the surface and close minor cracks See CHAPTER 16 Cracking Floating can be done by hand or with a power float Power floating leaves a better finish than hand floating Een N J BULL FLOATING ee aIl FINAL FINISHING This involves floating trowelling edging jointing or patterning the concrete Special finishes such as brooming colouring or patterned finishes can be applied to the surface See CHAPTER 13 Colouring and Surface Finishes TROWELLING T
16. NTENTS 46 ALGAL AND FUNGAL GROWTH Algal and fungal stains are removed with domestic chlorine bleach Wash and scrub the area with bleach Leave for a few days Scrub or scrape growth off Wash with water RUBBER OR TYRE MARKS Try pressure washing If this fails use a rubber removal compound available from hardware stores Brake cleaning products may also work Test products on a small area first and thoroughly rinse surface afterwards S SAFETY When using any chemicals epecially acids mentioned in this chapter always wear protective clothing gloves and shoes Protective goggles and face masks may also be necessary Don t breathe in fumes from any of these chemicals If chemicals come in contact with skin or eyes wash the area with plenty of cold water and seek medical advice Always read the safety directions on the label of any chemical container prior to starting work Cement Concrete amp Aggregates Australia 46 Concrete Basics Cracking CONTENTS 47 CHAPTER 16 Cracking Random cracking in concrete is not desirable it can spoil the appearance and lead to structural weakness of the concrete Reinforcement and joints are used to control cracking Bad cracking leaves the reinforcement exposed to air and moisture which may cause it to rust and weaken concrete See CHAPTER 11 Joints and CHAPTER 17 Reinforced Concrete TYPES OF CRACKS Two types of cracks occur in concrete PR
17. a minimum depth of 1 4 to 1 3 the depth of the concrete eg for 100 mm thick concrete joint depth should be a minimum of 25 mm to 35 mm Za Tth Control joints may be made while concrete is hardening by slicing it with a thin piece of metal The edges of the joints should be finished with a grooving or edging tool See CHAPTER 9 Finishing Alternatively a crack inducer may be cast or pressed into the concrete G rooving tool Control joints may also be sawn but timing is very important Too early and the cut will not be clean too late and the concrete will have already cracked randomly Fiexible Filler A tooled or sawn joint may be sealed with a flexible material to minimise water entry and to prevent stones etc entering which may later cause spalling of the concrete edges The position and number of control joints must be carefully planned Control joints in an unreinforced concrete slab should divide it into roughly square areas If rectangular the ratio of the sides should not exceed 1 1 5 Control joints in unreinforced concrete should be spaced at about 20 to 25 times the thickness of the slab eg in a 100 mm slab the joint spacing should be 3 m maximum In steel reinforced slabs the joint spacing is controlled by the area of steel The more steel there is the further apart the joints can be Cement Concrete amp Aggregates Australia 32 Concrete Basics Joints CONTENTS 33 Isolation Joints An
18. action and during finishing Concrete that is sloppy or wet may be easy to place but will be more difficult to finish A worker leaves footprints in setting concrete SETTING HARDENED STATE After concrete has set it begins to gain strength and harden The properties of hardened concrete are strength and durability Hardened concrete will have no footprints on it if walked on Cement Concrete amp Aggregates Australia 6 Concrete Basics Concrete Properties CONTENTS 7 WORKABILITY Workability means how easy it is to PLACE HANDLE COMPACT and FINISH the concrete Concrete that is stiff or dry may be difficult to handle place compact and finish and if not constructed properly will not be as strong or durable when finally hardened A slump test can be used to measure the workability of concrete See CHAPTER 3 Concrete Testing Workability is affected by THE AMOUNT OF CEMENT PASTE The cement paste is the soft or liquid part of the concrete mix The more paste mixed with the coarse and fine aggregates the more workable a mix THE AGGREGATE PROPERTIES AND GRADING Well graded smooth rounded aggregates improve the workability of a mix See Aggregate Grading under Cohesiveness To make a more workable mix Add more CEMENT PASTE Use WELL GRADED aggregates Use an ADMIXTURE admixtures that improve workability are known as plasticisers Never try to make a mixture more workable by just adding mor
19. aint film and rinse with water Cement Concrete amp Aggregates Australia 44 Concrete Basics Removing Stains CONTENTS 45 OIL AND GREASE These can be difficult to remove completely because they tend to soak into the concrete surface If the oil or grease has hardened it can simply be scraped off If an oil or grease spill has just occurred stop it spreading by encircling it with sand dirt sawdust or cat litter These can also be used along with paper towels and cloths to soak up or remove as much of the oil and grease as possible with Cover residue stain with a poultice of 1 part lime to 2 parts mineral turpentine Spread a 5 mm layer of the paste over the stained area and 50 100 mm beyond ee A it Cover with plastic sheeting and leave for 24 hours Remove the cover and scrape off the powder It may be necessary to repeat this process within a day or so to remove any deeply ingrained oil or grease Scrub with warm water and laundry detergent then rinse with clean water Cleaning products such as sugar soap may be adequate to remove light oil or grease stains TIMBER To remove timber stains scrub vigorously with a domestic chlorine bleach then rinse the surface Stubborn stains may need to be repeatedly covered in a cloth soaked in bleach Scrub between bleach treatments and rinse the surface Cement Concrete amp Aggregates Australia 45 Concrete Basics Removing Stains CO
20. al stain remover or a specially prepared chemical mix In extreme cases if a stain cannot be removed chemically it may be sand blasted This removes the surface of the concrete and may expose the aggregates Some types of stains can be covered by simply painting over with a concrete paint However oil based stains may prevent adhesion of the paint and must be removed first Stain removal procedures are outlined below comprehensive guidance is provided in CCAA s online Data Sheet Removing Stains from Concrete RUST Staining resulting from metal objects placed on the concrete can usually be removed by using a special cleaning chemical available from most hardware stores For stubborn stains a weak solution 1 25 of hydrochloric acid may be successful Prior to applying the acid solution wet the concrete and always wash down the surface with clean water afterwards Be careful where the run off goes as it may create problems on other concrete surfaces or gardens A poultice method may also be used For stains resulting from rusting of the embedded steel reinforcement seek professional advice PAINT New spillages should be soaked up immediately to prevent spreading If the paint is water based scrub the area with an abrasive cleaning powder and rinse with water If the paint is oil based allow it to dry and then scrape off as much paint as possible and apply a paint stripper After 20 30 minutes scrub gently to loosen the p
21. and which will set and harden to give a strong and durable product The relative amount of each material ie cement water and aggregates affects the properties of Jb concrete See CHAPTER 2 Concrete Properties CEMENT The cement powder when mixed with water forms a paste CEMENT This paste acts like glue and holds or bonds the aggregates together Six major types of cement are sold in Australia Type GP General Purpose Portland cement j Type GB General Purpose Blended cement Type HE High Early Strength cement Type LH Low Heat cement Type SR Sulfate Resisting cement Type SL Shrinkage Limited cement JL Each type of cement will produce concrete with different properties CEMENT The most commonly used are Type GP and Type GB PASTE Blended cements contain portland cement and more than 5 of either fly ash ground slag amorphous silica eg silica fume or a combination of these Cement Concrete amp Aggregates Australia 3 Concrete Basics Concrete Materials CONTENTS 4 STORAGE Cement should be stored off the ground in a well aired clean dry place Wrapping the cement bags in plastic sheets gives extra protection Bulk cement will normally be stored in silos AGGREGATES Aggregates are of two basic types COARSE crushed rock gravel or screenings FINE fine and coarse sands and crusher fines Sand should be concreting sand and not brickies sand or plasterers sand Aggreg
22. applying a suitable topping if required RAIN DAMAGE The surface has bits washed away or many small dents Causes Heavy rain while concrete is setting or rainwater being allowed to run across the concrete surface Prevention Don t place concrete if rain looks likely If concrete has already been placed and rain looks likely cover it and prevent water from running across it Cement Concrete amp Aggregates Australia 41 Concrete Basics Defects CONTENTS 42 Repair If the concrete has not hardened and damage is minimal the surface can be refloated and re trowelled taking care not to work excess water into the surface See CHAPTER 9 Finishing If the concrete has hardened it may be possible to grind or scrape off the damaged surface layer and if required apply a topping layer of new concrete or a repair compound This may not always be possible and should be done only on expert advice SPALLING When the slab edges and joints chip or break leaving an elongated cavity Causes Edges of joints break because of heavy loads or impact with hard objects As concrete expands and contracts the weak edges may crack and break Entry of hard objects such as stones into joints may cause spalling when the concrete expands Poor compaction of concrete at joints Prevention Design the joints carefully Keep heavy loads away from the joints and edges until the concrete has hardened Ensure proper compaction Keep joints f
23. ate but is satisfactory for minor projects STRENGTH CEMENT CONTENT As the cement content increases so does strength and durability Therefore to increase the strength increase the cement content of a mix 1 2 3 CEMENT CONTENT STEET WATER CONTENT Adding MORE WATER to a mix gives a WEAKER hardened concrete Always use as little water as possible only enough to make the mix workable Zz 3 WATER CONTENT eas WATER TO CEMENT RATIO As the Water to Cement ratio INCREASES the strength and durability of hardened concrete DECREASES To increase the strength and durability of concrete decrease the Water Cement ratio WATER CEMENT RATIO See CHAPTER 2 Concrete Properties Cement Concrete amp Aggregates Australia 15 Concrete Basics Proportioning and Mixing CONTENTS 16 Mec compaction AGGREGATES No stone in top few millimetres Too much fine aggregate gives a sticky mix Too much coarse aggregate gives a harsh or boney mix Se D 26 Qo 00 O MIXING Concrete must be mixed so the Cement Water Aggregates and Admixtures blend into an even mix Concrete is normally mixed by MACHINE Machine mixing can be done on site or by a premixed concrete supplier Premixed concrete is batched proportioned at the plant to the job requirements Truck Mixing The materials are normally added to the trucks at batching plants and mixed for the required time and speed at the plant The truck s drum continue
24. ates should be STRONG and HARD so as to give a strong final concrete crumbly or flakey rock such as sandstone should not be used DURABLE to stand up to wear and tear and weathering CHEMICALLY INACTIVE so that the aggregates don t react with the cement V L CLEAN since dirt or clay sticking to the aggregates will weaken the bond between paste and aggregates Ww Oe A C GRADED be of a range of sizes so that they EY D FO eS fit together well to give a strong and dense concrete 2 e 5 Rounded aggregates give a more workable mix Angular aggregates make concrete harder to place work and compact but can make concrete stronger j STORAGE Aggregates should be stored where they will remain clean separated from other materials and dry If the aggregates are wet less water should be used in the mix Cement Concrete amp Aggregates Australia 4 Concrete Basics Concrete Materials CONTENTS 5 WATER Water is mixed with the cement powder to form a paste which holds the aggregates together like glue YO Water must be clean fresh and free from any dirt unwanted chemicals or rubbish that may affect concrete Many concrete plants now use recycled water IF You CAN DRINK THE WATER Always check bore water before use D Tap Don t use sea water in reinforced concrete IT LL BE Water as it may rust the reinforcing steel OK TO D USE k ADMIXTURES Admixtures are mixed i
25. ay segregation and wastage See CHAPTER 7 Transporting and Placing SAFETY Workers on the site should always wear protective clothing strong boots helmets and eye protection Always avoid direct contact with cement and never kneel in or touch the plastic concrete directly Wear gloves and use barrier creams Ensure that anyone using heavy equipment such as screeds or vibrators has been properly trained The following steps should be taken before any concrete is placed CONSIDER MEASUREMENT Measure and stake out the area to k be concreted and consider how thick the slab must be dhe slab must be The thickness will depend on the weight the concrete must carry ie a driveway carries the weight of a car and needs to be thicker than a garden path THE FINISHED LEVEL Once the thickness of concrete has been established work out where the concrete will finish Concrete cannot finish too high against steps or the external house wall and should not cover any part of weepholes in the wall The finished level determines how much digging or excavation must be done Pavements must slope away from buildings and boundaries DRIVE PATH must STEPS Steps must have equal risers finish at A er with an even step EXCAVATION The ground should be excavated as required by the finished levels Any roots or grass must ene Bo be dug out until there is firm soil to place on Always dig SS a lt the hole wider than needed to allo
26. cks are a possible problem Cracking at control joints or controlled by steel reinforcing is expected and acceptable Cement Concrete amp Aggregates Australia 49 Concrete Basics Reinforced Concrete CONTENTS 50 CHAPTER 17 Reinforced Concrete The steel found in many concrete structures is called REINFORCEMENT Reinforcement helps concrete resist TENSILE and SHEAR forces and helps control CRACKING in concrete CONCRETE PROPERTIES Normal Concrete Reinforced Concrete HIGH compressive strength VERY HIGH compressive strength VERY LOW tensile strength VERY HIGH tensile strength VERY LOW shear strength VERY HIGH shear strength WHY USE REINFORCEMENT As a load is applied compressive tensile and shear CONCRETE IS forces will act on the concrete Concrete naturally m resists compression squashing very well but is COMPRESSION relatively weak in tension stretching Horizontal and or vertical reinforcement is used in all types of concrete structures where tensile VERY WEAK or shear forces may crack or break the concrete L HORIZONTAL reinforcement helps resist tension TENGION forces VERTICAL reinforcement helps resist shear forces Below are some examples of reinforcement use LOAD In a SUSPENDED off the ground concrete slab horizontal reinforcement resists tension while vertical reinforcement in say supporting beams resists shear forces In a SLAB ON GROUND reinforcement increases the tensile stren
27. d selected aggregates over the concrete in a layer and press them into the concrete until completely covered METHOD 2 Place compact and level the concrete to about 5 mm below the top of the forms Mix a Topping Coat a mix of aggregates and cement paste in the ratio 2 1 Use only enough water to make the mix workable See CHAPTER 2 Concrete Properties Spread the topping over the concrete level tamp down and finish with a trowel FOR BOTH METHODS leave the concrete until the cement paste on the surface is firm but not dry then brush or with a fine mist spray wash away some of the cement paste covering the aggregates In both cases extra cleaning can be done with a dilute solution of hydrochloric acid The solution should be 1 part acid to 20 parts water adding the acid to the water Wet the concrete first and rinse off thoroughly afterwards Observe safety procedures See CHAPTER 15 Removing Stains To see what an aggregate finish will look like do a test area first Different colours of cement can be used to get a better effect For instance a white or off white cement may be used with a light stone where a grey cement may create a clash of colours Polished Polished concrete is a finish used on the interior and exterior of dwellings A variety of finishes can be achieved by using different techniques or products The different finishes can be achieved by using liquid polishes latex coatings chemical sealers
28. dges etc Despite its widespsread use many people are unaware of the considerations involved in providing high quality strong durable concrete Concrete Basics provides a clear concise explanation of all aspects of making quality concrete from the Materials and Properties involved through Planning Preparation Finishing and Curing Concrete Basics addresses the needs of unskilled and semi skilled persons undertaking general concreting projects including home and handyman projects Concrete Basics also assists owner builders in the supervision of construction It aims to develop an understanding of technical terms through clear definition accompanied by simple illustrations A general understanding of these terms will help to facilitate communication within the building industry Concrete Basics will help to generate a higher standard of workmanship on site and facilitate better communication among construction workers builders engineers building surveyors architects and anyone interested in understanding the processes involved in making quality concrete Cement Concrete amp Aggregates Australia 2 Concrete Basics Concrete Materials CONTENTS 3 CHAPTER 1 Concrete Materials CONCRETE is made by mixing IS CEMENT Gib Bre WATER COARSE AND FINE AGGREGATES ADMIXTURES if required CEMENT The aim is to mix these materials in measured amounts to make concrete that is easy to TRANSPORT PLACE COMPACT FINISH
29. e achieved by a variety of methods and can be combined with texturing or patterning of the exposed surface A wide range of colours textures and patterns is available The constituents of the concrete itself will also influence its appearance eg white or off white cement can be used instead of the standard grey while aggregates where they are to be exposed at the concrete surface can be selected for their colour To find out how a colour or pattern will look always do a small test area before beginning the bulk of the work CONCRETE GRADE In order to minimise problems arising when using any form of coloured concrete for domestic paving a minimum grade of 25 MPa concrete should be used Other forms of decorative concrete or in commercial work will normally require a higher grade of concrete CURING Adequate curing is a very important step in colouring concrete The concrete surface must stay evenly moist or the colour will be uneven Poorly cured concrete can even affect a painted concrete surface See CHAPTER 10 Curing COLOURING METHODS There are several ways to colour concrete The Dry Shake Method The dry shake method uses a mixture of a mineral oxide yh pigment or colour cement and specially graded fine aggregates The colour is added when compaction screeding and bleeding has finished as part of finishing Uniform reliable results are best achieved if premixed concrete is used Shake 2 3 of the dry
30. e a concrete cylinder for the test The strength is measured in Megapascals MPa and is commonly specified as a characteristic strength of concrete measured at 28 days after mixing The compressive strength is a measure of the concrete s ability to resist loads which tend to crush it Tools CYIUNDER MOULD Cylinders LA Buer ap 100 mm diameter x 200 mm high or 150 mm diameter x 300 mm high the small cylinders are normally used for most testing due to their lighter weight Small scoop Bullet nosed rod 600 mm long x 16 mm diameter Steel float Steel plate Method 1 Clean the cylinder mould and coat the inside lightly with form oil then place on a clean level and firm surface eg the steel plate 2 Collect a sample See Sampling 3 Fill 1 2 the volume of the mould with concrete then compact by rodding 25 times Cylinders may also be compacted by vibrating using a vibrating table Cement Concrete amp Aggregates Australia 12 Concrete Basics Concrete Testing CONTENTS 13 4 Fill the cone to overflowing and rod 25 times into the top of the first layer then top up the mould until overflowing 5 Level off the top with the steel float and clean any concrete from around the mould i a k a i 1 t S 6 Cap clearly tag the cylinder and put it ina cool dry place to set for at least 24 hours 7 After the mould is uD removed the cylinder is sent to the laborat
31. e water because this lowers the strength and durability of concrete Cement Concrete amp Aggregates Australia 7 Concrete Basics Concrete Properties CONTENTS 8 COHESIVENESS Cohesiveness is how well concrete HOLDS TOGETHER when plastic Cohesiveness is affected by THE AGGREGATE GRADING Graded aggregate means that there is a range of size of aggregates from large rocks to small sands Well graded aggregates give a more cohesive mix too much coarse aggregate gives a boney mix WATER CONTENT A mix that has too much water will not be cohesive and may allow materials to separate segregate STRENGTH AND DURABILITY Well made concrete is a naturally strong and durable material It is DENSE reasonably WATERTIGHT impermeable able to resist changes in TEMPERATURE as well as wear and tear from WEATHERING and TRAFFIC Strength and Durability are affected by the density of the concrete Denser concrete is stronger and more watertight or less permeable Concrete durability INCREASES with strength Well made concrete is very important to protect the steel in reinforced concrete See CHAPTER 17 Reinforced Concrete Strength of concrete in the hardened state is usually measured by the COMPRESSIVE STRENGTH using the Compression Test See CHAPTER 3 Concrete Testing Strength and Durability are affected by COMPACTION Compaction is removing the air from concrete Proper compaction results in concrete with an increas
32. ect is temporary therefore once the concrete is finished normal curing techniques should still be used immediately HOW LONG TO CURE Concrete keeps getting HARDER AND STRONGER over TIME Household concrete jobs MUST be cured for at least 3 DAYS For better strength and durability cure concrete for 7 DAYS The LONGER concrete is cured the closer it will be to its best possible strength and durability See CHAPTER 2 Concrete Properties and See CHAPTER 3 Concrete Testing E CURED CONCRETE O NOT CURED CONCRETE STRENGTH DAY 1 DAY 2 DAY 3 TIME Cement Concrete amp Aggregates Australia 30 Concrete Basics Joints CONTENTS 31 CHAPTER 11 Joints WHAT ARE JOINTS Joints are PLANNED BREAKS in concrete which allow it to move and thus prevent random cracking i C WHEN TO MAKE JOINTS Joints can be made at two different times BEFORE any concrete is placed for Construction joints or Isolation joints AFTER concrete has been placed and compacted for Control joints Joints are used to control CRACKING in concrete Random cracking can weaken the concrete and spoil its appearance Cement Concrete amp Aggregates Australia 31 Concrete Basics Joints CONTENTS 32 TYPES OF JOINTS Control Joints Wet formed tooled joints are provided by the use of a grooving tool to create a plane of weakness to control where shrinkage cracks will occur and to conceal them To be effective the joint must be tooled to
33. ed density which is stronger and more durable See CHAPTER 8 Compacting Concrete CURING Curing is keeping concrete continuously damp for a period to allow it to reach maximum strength Longer curing will give more durable concrete See CHAPTER 10 Curing Concrete Cement Concrete amp Aggregates Australia 8 Concrete Basics Concrete Properties CONTENTS 9 WEATHER Warmer weather will cause concrete to have a higher early strength See CHAPTER 12 Hot and Cold Weather Concreting TYPE OF CEMENT Different types of cement will affect concrete properties eg how quickly or slowly concrete gains strength THE WATER TO CEMENT RATIO Too much water and not enough cement means concrete will be weaker and less durable The water to cement ratio W C is the weight of the water divided by the weight of cement Water 20 litres W C eg 0 5 Note 1 litre of water weighs 1 kilogram Cement 40 kg The lower the ratio the stronger the concrete WATER CEMENT RATIO 0 5 VN N WATER EMENT PATA gt 0 78 WATER CEMENT MNO 1 0 mAy RIFTS haa 5 uIRS UME WER Cement Concrete amp Aggregates Australia 9 Concrete Basics Concrete Testing CONTENTS CHAPTER 3 Concrete Testing There are two main tests to be done on concrete The SLUMP test The SLUMP test indicates the WORKABILITY of concrete Workability shows how easy the concrete is to place handle and compact See CHAPTER 2 Concrete Properti
34. equired around service penetrations at joints and around the perimeter of the slab Cement Concrete amp Aggregates Australia 18 Concrete Basics Concrete Materials CONTENTS 19 REINFORCEMENT Reinforcement can be used to increase the strength of concrete and or to help control cracking For house floors resting on the ground it is placed in the top 1 3 of slabs and in the bottom of thickening and beams In strip footings it is placed in the top and bottom The reinforcement must be covered by a specified amount of concrete which protects the steel from rusting This is called cover The amount of cover depends on whether the slab is inside or outside is in contact with the ground or protected by a membrane It is measured to the top side or bottom of the outer surface Reinforcement should be securely held for slab on ground construction Bars and mesh should overlap by a specified amount and at the corners of strip footings See CHAPTER 17 Reinforced Concrete ACCESS Clear access must be provided to transport the concrete If concrete is to be delivered by trucks make sure they have unrestricted access to the site in all weather conditions PLACEMENT Ensure all planning and site preparation takes into account how concrete will be placed allowing room for trucks ramps for wheelbarrows space for a pump etc JOINTS The position type and number of joints should be planned well before the concrete is placed See CH
35. er lasts longer and better protects the teel rein t a ae ae steel reinforcement awe ey i THUMP SCCM 5 lt I Properly cured concrete is stronger and can 1 carry more weight without breaking HEAVY LOADS DON T WORRY ME Cement Concrete amp Aggregates Australia 28 Concrete Basics Curing CONTENTS 29 HOW TO CURE The most common ways of curing are APPLYING EXTRA WATER to the surface of the concrete or REDUCING the rate of moisture loss from the concrete 7 42 e a4 Applying extra water The simplest method of APPLYING WATER is to put a continuous fine misty spray of water over the concrete BEWARE The spray must be a very fine mist or else it will damage the surface of the concrete _ A FINE MISTY SPRAY FOR EASY CURING Concrete will dry out more quickly in hot weather Keep the concrete continuously moist It is most important to keep the concrete moist at all times Hosing in the morning and again at night and letting the concrete dry out in between is no good THIS PLASTIC SHEETING S So EASY Reducing moisture loss Another way to cure concrete is to cover with PLASTIC SHEETS to slow down moisture loss This method is easy and inexpensive The only problem is that the sheets may cause concrete to become darker in places To avoid this keep concrete EVENLY moist The sheets must be held down to stop them blowing away They can be overlapped and stuck toget
36. es The COMPRESSION test The COMPRESSION test shows the best possible strength concrete can reach in ideal conditions The compression test measures concrete strength in the hardened state Testing should always be done carefully Wrong test results can be costly SAMPLING The first step is to take a test sample from the large batch of concrete This should be done as soon as discharge of the concrete commences The sample should be representative of the concrete supplied The SAMPLE is taken in one of two ways For purposes of accepting or rejecting the load Sampling after 0 2 m3 of the load has been discharged For routine quality checks Sampling from three places in the load as it is discharged aA Ae He e SF A Cement Concrete amp Aggregates Australia 10 Concrete Basics Concrete Testing CONTENTS 11 THE SLUMP TEST The slump test is done to make sure a concrete mix is consistent and workable The measured slump must be within a set range or tolerance from the specified slump SONDAPD Tools lt ory BULLET NOCe Standard slump cone T M _ 100 mm top diameter x 200 mm bottom a diameter x 300 mm high RULER Small scoop id 2 a Bullet nosed rod cui xs S sa Lo 600 mm long x 16 mm diameter Ruler Se Slump plate 500 mm x 500 mm Method 1 Clean the cone Dampen with water and place on the slump plate The slump plate should be clean firm level and non absorbent
37. ges 17 19 CHAPTER 7 Transporting and Placing pages 20 22 CHAPTER 8 Compacting pages 23 25 CHAPTER 9 Finishing pages 26 27 CHAPTER 10 Curing pages 28 30 CHAPTER 11 Joints pages 31 33 CHAPTER 12 Hot and Cold Weather Concreting pages 34 35 CHAPTER 13 Colouring and Surface Finishes pages 36 39 CHAPTER 14 Defects pages 40 43 CHAPTER 15 Removing Stains pages 44 46 CHAPTER 16 Cracking pages 47 49 CHAPTER 17 Reinforced Concrete pages 50 52 CHAPTER 18 Formwork pages 53 54 Cement Concrete amp Aggregates Australia Second Edition 1992 Third Edition 1994 Fourth Edition 1996 Fifth Edition October 2002 Sixth Edition August 2004 Seventh Edition February 2010 Cement Concrete amp Aggregates Australia is a not for profit organisation sponsored by the cement concrete and aggregate industries in Australia to provide information on the many uses of cement and concrete This publication is produced by CCAA for that purpose Since the information provided is intended for general guidance only and in no way replaces the services of professional consultants on particular projects no legal liability can be accepted by CCAA for its use Cement Concrete amp Aggregates Australia 1 Concrete Basics Preface CONTENTS 2 Preface Communities around the world rely on concrete as a safe strong and simple building material It is used in all types of buildings from residential to multi storey office blocks and in infrastructure projects roads bri
38. gth and helps control the width of ESICOMPRESS ION shrinkage cracks C TENSION See CHAPTER 16 Cracking Reinforcement does not prevent cracks but controls their width Another benefit is that it reduces the number of control joints required It is particularly beneficial in odd shaped slabs Cement Concrete amp Aggregates Australia 50 Concrete Basics Reinforced Concrete CONTENTS 51 REINFORCEMENT POSITION The position of reinforcement will be shown in the plans Reinforcement must be fixed in the right position to best resist compressive tensile and shear forces and help control cracking The reinforcement in trenches and slabs rests on BAR CHAIRS and must be securely fixed to the bar chairs so it won t move when concrete is placed around it Concrete Cover The reinforcement must be placed BAR CHAIR so there is enough concrete covering it to protect it from rusting Typical covers are shown in the diagram below Both the concrete cover and strength should be shown in the plans es el MANIN TULA th 70MM To INSIDE aye TTI Oo D Concrete Reinforcement Bond To help control the width of cracks or their location at joints there must be a strong bond between concrete and reinforcement This allows the tensile forces which concrete has a very low ability to resist to be transferred Er the reinforcement To help achieve a strong bond The reinforcement should be CLEAN free from Re
39. her and or held down with sand timber or bricks Always check under the plastic from time to time to make sure the concrete is EVENLY moist If it feels dry sprinkle with water and put back the plastic sheets carefully Condensation on the underside of the plastic is a good sign Cement Concrete amp Aggregates Australia 29 Concrete Basics Curing CONTENTS 30 Concrete may also be cured by applying a CURING COMPOUND which slows moisture loss This should be applied soon after finishing Always follow the manufacturer s instructions carefully Curing compounds may be sprayed rolled or brushed on BEWARE Some types of curing compounds may later make it harder or impossible to apply a surface finish such as paint to concrete or to stick down floor coverings When using a curing compound check with the supplier to ensure compatibility with surface coatings or adhesives for future overlay finishes such as vinyl or tiles In rapid drying conditions ie high wind dry air and or hot air the use of an EVAPORATION RETARDER minimises the rapid loss of surface moisture and as such reduces the incidence of early age plastic cracking See CHAPTER 16 Cracking These products contain a fugitive dye and are applied after initial screeding and floating and reapplied after each successive surface working until finished In severe conditions retarders will require reapplication Evaporation retarders are not curing compounds their eff
40. isolation joint totally separates a concrete element from another concrete element or a fixed object such as a wall or column so that each can move and not affect the other The joint filling should be full depth and soft It can be made of cork foam rubber or some other flexible material Construction Joints A construction joint is a place where work finishes temporarily Formwork is used to support the edge of the concrete already in place so that it doesn t just collapse Concrete is finished square and the reinforcement normally runs through the joint When placing begins again Remove the formwork and brush any loose material from the old surface Roughen the old surface to expose the coarse aggregate to help the new concrete bond properly Place the new concrete against the old surface The concrete should always be finished SQUARE If the concrete is left m formed x it SAE Le this 7 gt mse Rae ee IEJ a4 a PLANS The position of ALL JOINTS should be shown on the YAD a plans for any concrete slab Cement Concrete amp Aggregates Australia 33 Concrete Basics Hot and Cold Weather Concreting CONTENTS 34 CHAPTER 12 Hot and Cold Weather Concreting In extremes of heat and cold concrete must be HANDLED PLACED COMPACTED FINISHED and CURED carefully The main problem caused by extremes of heat and cold is cracking When conditions of heat and cold are e
41. nto the concrete to change or alter its properties ie the time concrete takes to set and harden or its workability HOW THE PROCESS WORKS Measured amounts of the coarse and fine aggregates are mixed together A measured amount of cement is added and mixed in Enough water is added to make the mix workable All the materials are then mixed together well The cement powder and water form a paste which acts like a glue to bond the aggregates together weet etal Vet pee Pt hed Cement Concrete amp Aggregates Australia 5 Concrete Basics Concrete Properties CONTENTS 6 CHAPTER 2 Concrete Properties The Properties of Concrete are its characteristics or basic qualities Concrete has three different states PLASTIC SETTING HARDENED ge ae Concrete has different properties in each state CHANGE CHANGE The four main properties of concrete are WORKABILITY COHESIVENESS STRENGTH and DURABILITY PLASTIC STATE When the concrete is first mixed it is like bread dough It is soft and can be worked or moulded into different shapes In this state concrete is called PLASTIC Concrete is plastic during placing and compaction The most important properties of plastic concrete are workability and cohesiveness A worker will sink into plastic concrete SETTING STATE Concrete then begins to stiffen The stiffening of concrete when it is no longer soft is called SETTING Setting takes place after comp
42. ory where it is cured and crushed to test compressive strength Cement Concrete amp Aggregates Australia 13 Concrete Basics Ordering CONTENTS 14 CHAPTER 4 Ordering Name and address for delivery The use of the concrete ie driveway housing slab commercial The amount you need in cubic metres The Class of the concrete There are two classes that concrete can be supplied as Normal Class and Special Class NORMAL CLASS CONCRETE has a strength grade of N20 N25 N32 N40 and N50 with the corresponding characteristic strength of 20 25 32 40 and 50 MPa at 28 days The slump at the point of delivery should be 20 120 mm and the maximum size of coarse aggregate should be 10 14 or 20 mm Normal class concrete is suitable for most purposes For most domestic applications such as driveways and paths grade N20 and N25 are the common grades ordered SPECIAL CLASS CONCRETE is specified when you have additional or alternative requirements to those for normal class concrete eg lightweight aggregate colour pigments a non standard strength grade Special class concrete will not always be available from every concrete supplier The slump in millimetres mm See slump test in CHAPTER 3 Concrete Testing The maximum aggregate size eg 20 mm How you want to place the concrete eg pump shute wheelbarrow etc Time of the first truckload and the time between truckloads There must be enough time to place and compac
43. ppliers Special forms made from various materials can be purchased for forming waffle slabs circular columns and other special profiles Cement Concrete amp Aggregates Australia 53 Concrete Basics Formwork CONTENTS 54 FORM RELEASE AGENT Form Oil should be applied to the inside of the formwork to stop it sticking to the concrete and thus make removal easier Coat BEFORE the reinforcement is put in place REMOVAL TIMES Formwork may be left in place to help curing See CHAPTER 10 Curing Removal time will vary according to the weather In cold weather concrete will take longer to gain strength than in warm weather removal times will therefore be longer In mild conditions around 20 C 7 days is long enough to leave the forms in place unless the concrete is suspended when other considerations apply Leave FORMS in PACE until concrete CA CEMENT CONCRETE amp AGGREGATES AUSTRALIA Cement Concrete amp Aggregates Australia 54
44. pply cold water to a hot concrete surface as thermal cracking may result due to the sudden cooling IN COLD WEATHER Frozen or very cold water will also slow down the setting time which can cause costly delays Concrete should not be allowed to freeze in the first 24 hours To stop concrete freezing use one or more of the following Keep all MATERIALS warm Use WARM WATER in the mix COVER the formwork and subgrade to keep them free of frost and ice Use a Set Accelerating ADMIXTURE Try to keep concrete as much above 10 C as possible for the first few days NEVER lay concrete on frozen ground CURE concrete carefully to keep it warm The best method will be one that KEEPS HEAT in the concrete The curing methods should not cool the concrete An insulating layer may be needed In cold climates with frequent freeze thaw conditions the concrete may need an Air Entraining Admixture for long term durability In extremely cold weather water turns to ice EXPANDS and can CRACK hardened concrete Cement Concrete amp Aggregates Australia 35 Concrete Basics Colouring and Surface Finishes CONTENTS 36 CHAPTER 13 Colouring and Surface Finishes Concrete can be given many different surface finishes including STAMPING BROOMING EXPOSED AGGREGATE POLISHING COLOURING CONCRETE A coloured and or textured patterned surface finish can make concrete look more attractive and interesting Colouring of concrete can b
45. ree from rubbish Repair For small spalled areas scrape chip or grind away the weak areas until you reach sound concrete making sure you brush any loose material off the slab Then refill the area with new concrete or repair mortar after applying a bonding agent to the old concrete if necessary Compact finish and cure the new patch carefully Care should be taken that all joints are maintained and not filled For large spalled areas seek expert advice EFFLORESCENCE A white crystalline deposit sometimes found on the surface of concrete Causes Water with dissolved mineral salts collect on the concrete surface as water evaporates salt deposits are left on the surface Cement Concrete amp Aggregates Australia 42 Concrete Basics Defects CONTENTS 43 Excess bleeding may also result in efflorescence Prevention Use clean salt free water and washed sands Avoid excessive bleeding Repair Remove efflorescence by dry brushing and washing with clean water Do not use a wire brush If this fails to remove the deposit wash with a dilute solution of hydrochloric acid HONEYCOMBING Coarse stony surface with air voids Causes Poor compaction segregation during placing or paste leakage from forms A poor concrete mix with not enough fine aggregate causing a stony mix Workability too low Prevention Use a better mix design Take care during placing concrete to avoid segregation Compact concrete properly
46. rowelling Trowelling leaves a dense hard smooth and durable surface The surface should be trowelled TWICE A well trowelled surface will be very smooth and can be slippery when wet Trowelling can be done by hand or power trowel Edging and Grooving All the edges of a slab should be finished with a special edging tool This gives a neater and stronger edge less prone to chipping Joints should be planned before placing and are usually formed into the concrete during finishing See CHAPTER 11 Joints Once any surface has been finished the concrete MUST be cured See CHAPTER 10 Curing Cement Concrete amp Aggregates Australia 27 Concrete Basics Curing CONTENTS 28 CHAPTER 10 Curing WHAT IS CURING Curing means to keep concrete MOIST for some time typically 3 7 days By keeping concrete moist the bond between the paste and the aggregates gets stronger Concrete doesn t harden properly if it is left to dry out quickly WHEN TO CURE Curing is commenced just after finishing the concrete surface Precautions When curing leave the formwork in place to help reduce moisture loss In hot weather above 30 C or during high winds and low humidity concrete can dry out quickly In these conditions take extra care with curing See CHAPTER 12 Hot and Cold Weather Concreting WHY CURE Concrete that is cured is LESS LIKELY TO CRACK STRONGER MORE DURABLE Cured concrete has a surface that wears bett
47. s to rotate to agitate the concrete during transportation to the site Site Mixing When site mixing begin by loading a MEASURED AMOUNT of coarse aggregate into the mixer drum Add the sand before the cement both in measured amounts NEVER USE A SHOVEL AS A MEASURE AS VOLUMES CAN VARY WIDELY Mix materials together until there is no visible sand in the mix Add enough water to get a workable mix Be careful not to overload the mixer Too much concrete in the mixer means each batch takes longer to be properly mixed which causes costly delays in the long run or it will not mix at all Always check how much the mixer holds so you know how much concrete can be produced at one time Avoid delays between batches to get maximum output Small quantities of concrete may be mixed ras by hand with a shovel Mixing should be Coarse Aggregate done on a clean board or plate or ina wheelbarrow Mix the materials together until they are even Then dish the material and add water Use only enough water to get an even workable mix Finish mixing Cement Concrete amp Aggregates Australia 16 Concrete Basics Planning and Site Preparation CONTENTS 17 CHAPTER 6 Planning and Site Preparation The most important step in placing concrete is planning Always plan every step before any concrete is delivered Proper planning avoids delays wastage segregation and problems which develop from these To eliminate problems of del
48. stralia 24 Concrete Basics Compacting CONTENTS 25 PRECAUTIONS Taking the poker out TOO QUICKLY will leave a hole in the concrete To close the hole vibrate near the hole and take the poker out VERY SLOWLY NEVER touch the form face with the poker as it can damage the formwork and the concrete and can affect the off form finish NEVER touch the reinforcement with the poker as it may reduce the bond to the concrete NEVER spread or move concrete sideways with the poker as it may cause segregation always use a shovel NEVER leave the poker running when not in use HOW LONG TO COMPACT For concrete of average workability ie slump of 80 mm with a poker size between 25 75 mm concrete should usually be vibrated for between 5 and 15 seconds It is worse to UNDER VIBRATE than to OVER VIBRATE concrete Cement Concrete amp Aggregates Australia 25 Concrete Basics Finishing CONTENTS 26 CHAPTER 9 Finishing WHAT IS FINISHING Finishing is screeding floating and or trowelling the concrete surface to densify and further compact the surface layer of concrete as well as giving it the look you want Fin Finishing takes place in two stages INITIAL and FINAL finishing BLEED WATER J APPEARS ON THE Sg SURFACE No final finishing can begin while bleed water is present Mixing bleed water with the surface paste will weaken it possibly resulting in a dusty surface I
49. t one load before the next arrives Any admixtures you may want in the concrete though this is normally left to the pre mixed concrete supplier Always order more concrete ie 10 than you need to allow for construction variations and or some wastage Concrete is ordered in 0 2 m3 increments Ensure you round up when ordering IF MIXING YOUR OWN concrete The cement powder is ordered in bags by weight eg 20 kg and type eg Type GP The coarse and fine aggregates are ordered in cubic metres by maximum size eg 20 m3 of size 20 mm Water is used by the litre or kilogram 1 litre of water 1 kilogram Cement Concrete amp Aggregates Australia 14 Concrete Basics Proportioning and Mixing CONTENTS 15 CHAPTER 5 Proportioning and Mixing A CONCRETE MIX is designed to produce concrete that can be easily placed at the lowest cost The concrete must be workable and cohesive when plastic then set and harden to give strong and durable concrete The mix design must consider the environment that the concrete will be in ie exposure to sea water trucks cars forklifts foot traffic or extremes of hot and cold PROPORTIONING Concrete is a mixture of cement water coarse and fine aggregates and possibly admixtures The proportions of each material in the mixture affects ae the properties of the plastic and hardened concrete Sand These proportions are best measured by weight Measurement by volume is not as accur
50. to hardened dry concrete Paint finishes are either water based or solvent based They will give an even colour but will wear easily and will need to be reapplied periodically Chemical Stains A chemical stain soaks into the concrete surface and colours it by reacting with the cement paste in the surface layer wearing away only as much as the surface does There is only a limited colour range in concrete stains Cement Concrete amp Aggregates Australia 37 Concrete Basics Colouring and Surface Finishes CONTENTS 38 PATTERNED FINISHES Stamped A number of DIFFERENT PATTERNS can be stamped into the surface of setting concrete This includes cobblestone slate tile brick tile and timber finishes The concrete is placed and compacted normally and floated once A dry shake colour may also be used The pattern moulds or pads mats are then placed carefully on the concrete surface When mats are used a release agent will be needed to allow the mat to be removed At least two moulds are needed to step from one to another giving continuous and matching pattern Step onto the moulds pressing them into the concrete surface to the desired depth up to 6 10 mm for cobblestones and less for other patterns Deep patterns may be a hazard for pedestrians When using pads the grooves may be rounded by laying a sheet of plastic across the concrete surface before stamping The surface may then be broomed to give a non slip finish Use
51. touch zy flakey rust dirt or grease The concrete should be PROPERLY COMPACTED around the reinforcement Reinforcing bars and mesh should be located so that there is enough room between the bars to place and compact the concrete Cement Concrete amp Aggregates Australia 51 Concrete Basics Reinforced Concrete CONTENTS 52 To improve the transfer of tensile forces to the steel the reinforcement is often anchored by BENDING HOOKING or LAPPING the bars Types of Reinforcement Two types of steel reinforcement used are mesh sheets or individual bars Bars are normally deformed while mesh may be made from either smooth or deformed bars Typical bar diameters are 12 16 20 and 24 mm Typical mesh sizes are SL42 52 62 72 and 82 The SL stands for Square mesh Low Ductility and the numbers also have meanings For example for SL42 the 4 is the nominal wire size mm and the 2 refers to the wire spacing 200 mm REA Deformed bar A RECTANGULAR MESN DPQVUARE MESH ZZ BE TRENCH MESH GIRDER WRAP Fibre Reinforcement Synthetic fibres can be added to concrete to help minimise early age plastic shrinkage and can reduce the presence of excessive bleedwater However synthetic fibres are not a replacement for mesh or bar reinforcement In slab on ground construction the control joint spacing is the same as for plain concrete Steel fibres are used for the above and to improve the toughness of concrete They
52. ty and high temperatures or alternate wetting and drying Prevention Use an evaporative retarder and initiate curing immediately after finishing the concrete See CHAPTER 9 Finishing and CHAPTER 10 Curing Cement Concrete amp Aggregates Australia 40 Concrete Basics Defects CONTENTS 41 Repair Repair may not be necessary because crazing will not weaken concrete If the appearance is unacceptable a surface coating of a paint or other overlay sealer can be applied to hide the cracks See CHAPTER 13 Colouring and Surface Finishes DUSTING A fine powder on the concrete surface which comes off on your fingers Le Causes Finishing before the bleed water has dried Finishing during the rain See CHAPTER 9 Finishing Not curing properly or the surface is drying too quickly See CHAPTER 10 Curing Concrete of too low a grade for the end use eg subject to severe abrasion Prevention Let any bleed water dry up before trowelling or in cold conditions remove the water Cure correctly See CHAPTER 10 Curing Protect concrete from drying out too quickly in hot or windy conditions For harsh conditions use a stronger concrete Do not add excess water before placing Repair Where surface dusting is minimal the application of a surface hardener can be beneficial If the surface is showing significant wear it is essential to remove all loose material by grinding or scraping the surface to a sound base and then
53. w for the formwork D Try to keep the edges and corners square it shouldn t finish at Dig aut any roots or grass Suis oo ua m Ae Allow room for UI Cement Concrete amp Aggregates Australia 17 Concrete Basics Concrete Materials CONTENTS 18 SUBGRADE The soil a concrete pavement or floor rests on is called the subgrade If the soil is soft or varies in softness a layer of crushed rock known as a subbase should be provided If there are only a few poor areas these can be dug out refilled and compacted It is important that the soil evenly supports the concrete Many later problems can be avoided by properly preparing the subgrade FORMWORK Formwork gives concrete its shape Formwork should be properly braced so it is strong It should not flex or move See CHAPTER 18 Formwork Pipe for sink Pine fo ler Ss SERVICES Plumbing heating or electrical services often run through a slab These Sub grade or subbase must be in place before ANY SERVICES RUNNING any concrete is placed THROUGH THE CONCRETE SHOULD BE IN PLACE UNDERLAY AND SERVICES The underlay or vapour barrier is a heavy plastic covering the ground to minimise water vapour rising through the hardened concrete Always overlap the sheets a minimum of 200 mm and do not tape them Tape the edges of sheets only around drainage pipes or services which pass vertically through the concrete slab Termite protection may be r
54. xpected some of the following precautions will improve the quality of your final concrete IN HOT WEATHER Workability In hot and or windy weather a concrete mix may stiffen rapidly and not be workable A set retarding admixture may be added to the concrete during mixing to extend the working time See CHAPTER 2 Concrete Properties In hot and or windy weather if concrete stiffens quickly a cold unbonded joint may form between concrete already in place and the new concrete If there is a chance of this happening you may need to make a construction joint V See CHAPTER 11 Joints S A lt To reduce the risk of concrete drying out 7 p and cracking use one or more of the following Use SHADE to keep all materials out of direct sun and keep the aggregates MOIST DAMPEN subgrade and formwork but don t leave excess water lying around Cement Concrete amp Aggregates Australia 34 Concrete Basics Hot and Cold Weather Concreting CONTENTS 35 Erect SHADES and WINDBREAKS or try to place in the cooler parts of the day Avoid DELAYS once placing begins by planning ahead Use a set retarding ADMIXTURE SPRAY concrete with aliphatic alcohol after the initial finishing which reduces Saat a EVAPORATION and CRACKING Yo CURE concrete carefully See CHAPTER 10 Curing Keep it COOL In extreme conditions iced water or other methods may be used in the mix to keep it cool Do not a
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