Presentation on theme: "Introduction to Concrete"— Presentation transcript:
1 Introduction to Concrete Unitec New ZealandIntroduction to ConcreteSession Content:Insitu ConcreteClassification and Types,WorkabilityAdmixturesTransportation, Placing & CuringSurface FinishesJoints
2 Our cities are defined by…Concrete! Unitec New ZealandA ubiquitous construction material2006 – 7 billion cubic meters produced annuallyThat is more than 1m3 for every person on Earth!A $35 billion industry in US only40% of current world production used in China
3 The GOOD, the BAD and the UGLY “Welcome to (concrete) jungle”
4 Zen and the Art of Concrete? Tadao Ando: Japanese ArchitectAn outstanding proponent of unadornedfair-face off-shutter concreteWhat qualities of space is being pursued here?Chichu Art Museum - Tokyo
5 More compression please, but don’t get too heavy! American Air Museum at Duxford, Cambridge, by Ove Arup and Partners.Due to its versatile plastic nature of RC it has an extensive history of massive shell structures (see Nervi, famous pioneering engineer). This aviation museum was designed to house 32 planes including B52 bomber with 61m wingspan. Spans 90 x 100 meters, made of two layers of interconnected precast shells. Heavier and costlier than steel hangar, but outperformed steel in terms of life-cycle costing. Used only six different formwork moulds, lower elements inverted Tees with ribs to stitch upper panels to, strong enough to carry suspended aircraft. Note edges tapering form 100 to 45mm to allow in-situ conc stitching.Tough Shells: Duxford Auronautical Museum Roof StructureInverted T-beams: capable of suspending a variety of Aircraft!
6 Concrete so slender it seems to be taking flight! Sunscreens that open and closes…Voted Building of the Year by Time Magazine (?? not AJ?)Santiago Calatrava - Milwaukee Art Museum, 2001
7 Why choose concrete? – many advantages! Concrete is durable, strong in compression, easy to use.Versatile: reflect mould forms and shapesFlexibility of size - no modular restrictionsRelatively low cost - compared to steelLow level of maintenance – if correct spec used externally!Resistance to fire – compared to steel framesResistance to insect attack – unlike timber & toxic treatmentsHigh thermal mass - useful for heat retentionMay incorporate insulation - i.e. rib-raft & sandwich wall panelsA good range of surface textures & colors available.However, a few notes of caution is in order:Concrete is weak in tensionSubject to shrinking during setting processSubject to discoloration if left unprotected
8 Comparative Strength Chart Compressive strength may be verified by producing test cylinders or test cubes at the time of pouring the concrete.As test results may vary, a number of samples need to be taken of each pour on commercial projects.Properties of raw material: mix of Portland cement, fine and course aggregate. Weak in tension but achieves compressive strengths from 10 to 130Mpa, (25-45Mpa typical range). Achieves working strength in 28 days, a year to achieve final strength. Water/cement ratio by volume critical to strength of mix, thus max slump specified. Test cubes or cylinders regularly taken for lab tests, but results only available after the fact! (It is difficult and costly to replace substandard concrete).
10 Workability Tests The Slump Test - used for site mixed concrete Water combines readily withcement to form a paste whichfills the voids between theaggregate (stone) particles andglue them togetherHowever:the “water:cement” ratio is themost critical element in the mixThe Slump Test - used for site mixed concrete
11 Workability Considerations – how fluid? Method used for placing determines workability requirementsWorkability depends on water content & aggregate shape and sizeHigher workability is more expensive - due to extra cement req.May be modified by using specialized admixtures (plasticisers)Workability levels are often determined by the Contractor (i.e. not specified by the structural engineer)However, we need to limit the amount of water!Excessive water leads to bleedingLow ultimate compressive strengthPolluted or impure water may lead to setting problems
12 Concrete Admixtures Definition: Admixtures, such as plasticisers and air-entraining agents, are very effective in making concrete more suitable for the conditions on siteDefinition:Chemical Admixtures for Concrete is a material added to the concrete batch in controlled amounts, to produce a specified result.(Refer New Zealand Standards NZS 3113)
13 Admixture Types - by primary function Air Entraining – for light weight concreteSet accelerating – shortens setting timeSet retarding – delays setting of concreteWater reducing – increases workability (plasticiser)Super plasticiser – increases fluidity (self consolidating concrete)Thickeners – increases viscosity for pumping, reduces segregationPermeability reducing – reduce transmission of moisture after settingA summary:We need to know the main types and their functions/usage!Concrete Admixtures and their usages (Taylor)
14 Admixtures study aid: Refer to table on page 9 of Notes for details regarding admixturetypes and effects
15 Why use “Ready Mixed Concrete”? Convenient, thus may be cost effective:obviates the setting up of a mixer, ordering and storing the bulk ingredients, dry space for cement storage, hiring the skilled staff to operate etc.Most important considerations are:Increased quality control when ordering concrete ready-mixedMaking use of the supplier’s expertise and the resources
16 Concrete Classification: Method 1 – Density Dense concretenormal density or high densityLight weight concretelightweight aggregates and/orcreating internal voids in concrete by usingair entrapment additives, orno-fines concrete (less structural strength)higher thermal insulationeasier to cut or fix into.However, lightweight concrete has:greater shrinkage and moisture movementgreater cover of concrete required to protect reinforcementweaker, due to a lower modulus of elasticity.
17 Classification Method 2: Type Regular concrete10 MPa to about 40 MpaHigh-strength concretegreater than 40 MPA.made by lowering the water-cement (W/C) ratio to 0.35 or lower.silica fume is added to prevent the formation of free calcium hydroxide crystals.High-performance concrete (HPC)is set against higher specialised performance standards.And many other types, see notes
18 Concrete by Type - Self-consolidating concrete (SCC) is a self-compacting concreteextreme fluidity, placement being easier.uses a polycarboxylate plasticizerno need for vibrators to compact the concreteno bleed water, or aggregate segregationSCC can save up to 50% in labor costs, due to 80% faster pouringreduced wear and tear on formwork.As of 2005, self-consolidating concretes account for 10-15% of concrete sales in some European countries.In the US precast concrete industry, SCC represents over 75% of concrete production.
19 Concrete by Type: 3 Shotcrete (or gunite) uses compressed air to shoot concrete onto (or into) a frame or mesh reinforced structureCommonly used in retaining structures, or in conjunction with soil nailing techniques
20 Concrete by Type…..4 Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete.Also called “no-fines concrete”
21 Concrete by Type cont… GRC (Glassfibre Reinforced Concrete) Alkali resistant glassfibres are added as reinforcement to a mortar or concrete mix.Developed some 30 years ago and is used increasingly for a wide range of building elements, due to:homogeneous reinforcementincreased tensile strength and high impact resistancethin section: light weight, durability (no steel to corrode)typical use as building cladding,GRC offers:the possibility of large surface area lightweight panelsfreedom of design and mouldability,maintenance free performance.
22 GRC FabricationGlass fibre reinforced concrete allows considerable reduction of wall thickness, typically 13 – 16 mm, the process is described in Allen.The art centre in Singapore may have been a candidate, the architects settled for curved metal elements made out of aluminium sheeting. Why?
23 GRC cladding panels Right: Skin and Frame composite panels Due to the thinness of the panels it needs to be stiffened for transport and flexing under imposed loads. Stiffening by ribs or as shown above lightweight metal studframe bonded to rear of panel with sections of GRC placed by hand
24 Is GRC a suitable choice for the sunscreen here? St Rita Church by Ottavio di Blasi, Bari Italy. Thin planar elements on subframe – would you consider GRC suitable for this type of application?Is GRC a suitable choice for the sunscreen here?
25 Vierendeel tubular galvanised mildsteel framework projects through apertures in curtainwall and permits spandrel panels to span at angle between front and rear member of steel lattice.
26 Finish is an elastomeric paint coating that bonds with the silicate in the concrete to waterproof panels and permits touching up the noticeable blemishes on site.
27 Some current experimentation: Do these forms seem feasible?GRC website
29 Concrete Transportation Concrete mixers, skips and pumps are used to placethe concrete exactly where it is needed.Placing must be done at the correct speed:Too fast and the compacting gang will not be able to keep upToo slow and the mix will stiffen making compaction very difficultConcrete should be placed in layers and compacted properly:With poker vibrators the layers should match the length of the headWith vibrating beams, layers should not exceed 150 mmHand tamper may be required with thin slabs (100mm).Note:Only small amounts of water may be added to a mix on siteWhen dropping concrete from a height use tubes or baffles
30 Method of Transportation Depends on: Volume and of concrete to be pouredPosition/height it is to be placedRate at which concrete is to be placedSite access and ground surface conditionsClearances required for the moving equipmentMaximum aggregate size specified (pump mix – smaller agg.)Ensuring that the compacting equipment will be able to copewith the rate of concrete supply
31 Transportation Wheelbarrows Dumpers (or buggy) small quantities , over short distances < 70m.only 0.03m (30 litres) per barrow (a small mixer is 200 litres capacity).high labour costs(6 men needed to move 2.5m3 of concrete per hour, along a 70m run)Dumpers (or buggy)Manually or hydraulically operated0.3m m3 with 0.5m3 most commonused for difficult site access and poorsurface conditions
32 Transportation Truck mixers plan the deliveries typical truck load is 4.8m3.may travel a considerable distanceNote:a ready mixed concrete truck is not an “off road” vehiclethus trucks needs temporary roading,to get to the area to be concreted
33 Truck Types & Sizes Large capacity (10m3) Mixer Truck Novel front-loading truckdelivering into bucketskip
34 Transportation - Vertical transport Bucket and skips (or Hoppers)The constant- attitude skipThe lay-back orroll-over skipBucket skip
35 Transportation by Pumping Vertical and horizontal transportationPumping of Concretea popular and convenient placement methodreach is at least 60m vertically or 300m horizontally,In practice, a pump is likely to place approx30 cubic meters per hourSuccessful pumping depends on:the right pump capacity for the joba suitable concrete mix suppliedgood communication between contractorsgood organisation on the siteinform the supplier of the delivery rate
36 Transportation by Pumping Boom reach:Up to five floors in height, or 30m across
37 Placing Concrete Pumping Grade Concrete A different type of concrete is used for pumping:A target slump of mmaggregate grading: maximum of 40% pump diameter.increased sand content - 50 to 70 kg/m3 above that normally used.must be placed carefully so that it does not segregateplaced in a series of approximately equal layers.
38 Compaction Importance of getting rid of entrained air: Air holes reduce the strength of concrete.Air holes in the concrete stop a good bond forming between the concrete and the reinforcing steel making the structure weaker.Large air holes can cause ugly marks on the surface.Use the right compaction equipment for each job.Poker vibrators for beams, columns, walls and deep slabs.Beam vibrators for thin layered slabs.Clamp-on vibrators can be used on special formwork, especially in the precast industry.The contractor should always have spare compacting equipment at hand in case of a breakdown.
39 Compaction: Critical process! Impact of air presence in concrete mix:Air entrapped in the concrete as it leaves the mixer typically may vary from 5 to 20% and has to be removed by compacting (mechanical vibration)For each 1% of voids left within the concrete, the strength is reduced by approximately 5 to 6%Compaction is vital to achieve:maximum strength of the placed concretemaximum durabilityadequate bond to and protection of reinforcementavoidance of visual blemishes, such as honeycombing,on the surface of the concrete
40 Concrete Vibration Use the right equipment for the task Immersion Poker or internal vibratorsExternal External clamp on vibrators(if access is problematic)Flat Surfaces: Vibrating Screeds (Video)Vibrating Tables: For horizontal moulds,(used in precast yards)
41 Poker Vibrators Poker must be placed quickly and withdrawn slowly Poker to be left in one position for at least10 secondsA two stage process: liquefaction and air expulsionPoker vibrator typesOscillating vibrators – normal usagePneumatic vibrators - large diameter,used for dams etc.
42 Poker Vibrator Neither formwork nor reinforcement should be in contact The whole poker head must be placed into the concrete.Poker vibrator should not be placed on top of a heapPoker must extend 100mm into any previous layer.Poker should not be used to make concrete flow.Excessive vibration can cause segregation.100Extending into previous layer Careful levelling process
43 Poker Vibrator Placement Rule of thumb: Poker must be placed at no more than 500mm from last position(or 10 x poker diameter)See:Exact placement formula (right)(ex CCA Guide Ch8)
44 Vibrating Beam Screeds Vibrating Beam Screeds: used to strike off concrete surfactNormally up to 4m wideLarger trusses type Beam Screeds - may span up to 12m
45 Combined with poker vibrators for deeper slabs > 200mmAnd along edges and corners offorms
46 Surface Finish Trowels and Floats Floating: Applied as first process Leaves slightly open textureRemoves imperfectionsEmbeds visible aggregatesBull Floating:A large Aluminium float on along handle, used to reachacross and float slabs ongrade
47 Power Floating Four bladed machine with rapid rotary action Machine floats over surface of setting concreteFollowed by hand trowelling in corners etcLarger dual rotor ride-on models used on large floors
48 Site visited in Parnell, surfacr bed to parking basement being floated Site visited in Parnell, surfacr bed to parking basement being floated. Lower left shows specialised laser controlled unit for achieving high accuracy level surface, say less than 5mm deviation in level along any three metre long straightedge laid on the surface.
49 Concrete Curing : Strength Curing is essential to allow concrete to reach full strengthWith curingStrengthWithout curingTime
50 Concrete Curing : Density Cured concrete is more dense - resistant to chemical attackPoorly cured concretePermeabilityExposure to chemical attack and wear| | | | | | |Days of Curing
51 Water Curing Methods Ponding On flat surfaces such as pavements, footpaths and floorsSprinklingA fine spray of water applied continuously through a system of nozzles provides a constant supply of waterWet coverings:Wet hessian, other moisture-retainingfabrics can be laid onto the concrete as soon as it has hardened enough to prevent surface damage.
52 Sheet Curing MethodsWatertight sheets are placed over and around concrete to prevent water from escaping.Polythene and formwork are often combined for this task.When left in place, formwork gives a barrier which stops water evaporating in the same way that polythene does.
53 Membrane Curing Methods Curing compoundsare either sprayed or rolled onto the concrete and are suitable for vertical and horizontal surfaces.Spraying in set patternhelps to ensure that the whole surface is covered evenly (see picture, left). Some compounds contain a dye - to see if an all-over layer has been applied.Roller applicationIn windy conditions it may be better to apply the compound with a roller than a sprayer.
54 Shrinkage in slabs on grade There are two primary shrinkage solutions:Tied JointsRequires some reinforcement passing through the joint.There are a variety of tied joints to primarily suit the methods of construction.Joint-free slabsJoint free slabs use a plastic grid insert that encourages aclosely spaced network of fine cracks throughout the entire slab.The effects of drying shrinkage is controlled and uniform as possible.Nose Shrinking
55 Construction Joints: slabs on grade Movement or Control Joints:Controls tensile stresses due primarily to moisture change and thermal contraction of the slab, and thus to limit random cracking.Construction JointsAre concrete to concrete joints constructed to prevent future movement across the joint, necessitated by breaks in concreting operations.The area constructed per shift is governed by practical considerations arising from the method of construction and resources available.
56 Joint Type 1: Construction Joint Construction joints: locations must be designed!Size of panels relates to:working hours (daywork joints)weather conditionsshapes to be constructedsupply and placement rate of concrete
57 Joint type 2: Control Joints Shrinkage Control Joint (or Contraction joint)is a surface cut, 25mm deep and 5mm widedesigned to control tensile stresses due to moisture change and thermal contraction of the slabFilled with propriatory sealing compound
58 Joint Type 3: Movement Joints An Expansion Joint is a continuous cut for the full depth of the slab, where reinforcing is stopped at each side.Dowels are coated with bond breaking compound to one endNote:A slab over 24m in any direction must be broken up by or expansion or free joints
59 Joint Type 4: Isolation Joints Isolation Joints are designed to provide a limited degree of freedom of movement in both the vertical and horizontal planei.e. around columns passing through a slab on grade
61 In-situ Concrete Finishes A significant challenge?
62 Windscreen Surveys? Only if you look closely A telling commentary on the foibles of off-shutter concrete…Only if you look closelyare the defects revealed...
63 Site Instructions: Patch the joints with an approved compound! Note variation in tone/textureat joints where additional facework occurredInterior work is not the mostchallenging problem?!Note effect of extra tooling required around joints – why needed?
64 Surface FinishesHorizontal surfaces are finished by floating and trowellingFloating is a process of rapidly smoothing blemishesin the upper surfaceTrowelling is the final operation to achieve a smoothand dense finished surfaceNZS 3114 – Concrete and Surface Finishes:Describes and classifies surfaces finishes for a particular usageHorizontal surface finishes may range from U1 to U11 – see next slide
65 Surface Finishes Horisontal Surfaces; (see page 20 of notes) Alternatively, if specifying by Appearance:F Classes:(F1 to F6) specifies finish by surface quality/appearanceF6 is highest grade, requires careful mould design and productionof samples
66 Surface FinishesAlternatively, may be specified according to Appearance:F1 to F6 specifies finish by surface quality/appearanceF6 is highest grade: requires careful mould design and production of samples on site for approval by the architect
67 Surface Finish: Recoloring Tinted Concrete:Inorganic pigments into topping screeds or precast panels/tilesto provide a durable and colour fast range of attractive tonesOften applied as a dry-shake powder in NZ & troweled into the topping
68 Typical Range of off- shutter & tooled finishes Smooth PlyRotary PlyExposed AggregateBush HammeredRibbed and tooledRibbed precast panelsFinishes as indicated. All involves careful specification and provision for a series of samples. Contingency amount need to allow for variation in mix, I.e. addition of white cement, special sands or aggregate, inorganic pigments etc. For exposed aggregate type mould needs to be coated with retarding agent to permit cleaning of surface layer with high pressure hose. Bush hammer tools come with a range of combs and chisel tips to create various effects, viva Brutalism!
69 Note: Off-shutter finishes subject to streaking, discoloration and in some atmospheres, carbonation (atmospheric pollution changes PH value of concrete and increase permeability to gasses, leading to corrosion of reinforcing steel and consequent spalling of covering concrete.