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Introduction to Concrete

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1 Introduction to Concrete
Unitec New Zealand Introduction to Concrete Session Content: Insitu Concrete Classification and Types, Workability Admixtures Transportation, Placing & Curing Surface Finishes Joints

2 Our cities are defined by…Concrete!
Unitec New Zealand A ubiquitous construction material 2006 – 7 billion cubic meters produced annually That is more than 1m3 for every person on Earth! A $35 billion industry in US only 40% 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 Architect An outstanding proponent of unadorned fair-face off-shutter concrete What 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 Structure Inverted 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 shapes Flexibility of size - no modular restrictions Relatively low cost - compared to steel Low level of maintenance – if correct spec used externally! Resistance to fire – compared to steel frames Resistance to insect attack – unlike timber & toxic treatments High thermal mass - useful for heat retention May incorporate insulation - i.e. rib-raft & sandwich wall panels A good range of surface textures & colors available. However, a few notes of caution is in order: Concrete is weak in tension Subject to shrinking during setting process Subject 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).

9 Construction Accuracy?

10 Workability Tests The Slump Test - used for site mixed concrete
Water combines readily with cement to form a paste which fills the voids between the aggregate (stone) particles and glue them together However: the “water:cement” ratio is the most critical element in the mix The Slump Test - used for site mixed concrete

11 Workability Considerations – how fluid?
Method used for placing determines workability requirements Workability depends on water content & aggregate shape and size Higher 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 bleeding Low ultimate compressive strength Polluted 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 site Definition: 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 concrete Set accelerating – shortens setting time Set retarding – delays setting of concrete Water reducing – increases workability (plasticiser) Super plasticiser – increases fluidity (self consolidating concrete) Thickeners – increases viscosity for pumping, reduces segregation Permeability reducing – reduce transmission of moisture after setting A 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 admixture types 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-mixed Making use of the supplier’s expertise and the resources

16 Concrete Classification: Method 1 – Density
Dense concrete normal density or high density Light weight concrete lightweight aggregates and/or creating internal voids in concrete by using air entrapment additives, or no-fines concrete (less structural strength) higher thermal insulation easier to cut or fix into. However, lightweight concrete has: greater shrinkage and moisture movement greater cover of concrete required to protect reinforcement weaker, due to a lower modulus of elasticity.

17 Classification Method 2: Type
Regular concrete 10 MPa to about 40 Mpa High-strength concrete greater 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 concrete extreme fluidity, placement being easier. uses a polycarboxylate plasticizer no need for vibrators to compact the concrete no bleed water, or aggregate segregation SCC can save up to 50% in labor costs, due to 80% faster pouring reduced 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 structure Commonly 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 reinforcement increased tensile strength and high impact resistance thin section: light weight, durability (no steel to corrode) typical use as building cladding, GRC offers: the possibility of large surface area lightweight panels freedom of design and mouldability, maintenance free performance.

22 GRC Fabrication Glass 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

28 Concrete Transportation

29 Concrete Transportation
Concrete mixers, skips and pumps are used to place the 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 up Too slow and the mix will stiffen making compaction very difficult Concrete should be placed in layers and compacted properly: With poker vibrators the layers should match the length of the head With vibrating beams, layers should not exceed 150 mm Hand tamper may be required with thin slabs (100mm). Note: Only small amounts of water may be added to a mix on site When dropping concrete from a height use tubes or baffles

30 Method of Transportation Depends on:
Volume and of concrete to be poured Position/height it is to be placed Rate at which concrete is to be placed Site access and ground surface conditions Clearances required for the moving equipment Maximum aggregate size specified (pump mix – smaller agg.) Ensuring that the compacting equipment will be able to cope with 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 operated 0.3m m3 with 0.5m3 most common used for difficult site access and poor surface conditions

32 Transportation Truck mixers plan the deliveries
typical truck load is 4.8m3. may travel a considerable distance Note: a ready mixed concrete truck is not an “off road” vehicle thus trucks needs temporary roading, to get to the area to be concreted

33 Truck Types & Sizes Large capacity (10m3) Mixer Truck
Novel front-loading truck delivering into bucket skip

34 Transportation - Vertical transport
Bucket and skips (or Hoppers) The constant- attitude skip The lay-back or roll-over skip Bucket skip

35 Transportation by Pumping
Vertical and horizontal transportation Pumping of Concrete a popular and convenient placement method reach is at least 60m vertically or 300m horizontally, In practice, a pump is likely to place approx 30 cubic meters per hour Successful pumping depends on: the right pump capacity for the job a suitable concrete mix supplied good communication between contractors good organisation on the site inform 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 mm aggregate 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 segregate placed 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 concrete maximum durability adequate bond to and protection of reinforcement avoidance 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 vibrators External 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 seconds A two stage process: liquefaction and air expulsion Poker vibrator types Oscillating vibrators – normal usage Pneumatic 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 heap Poker must extend 100mm into any previous layer. Poker should not be used to make concrete flow. Excessive vibration can cause segregation. 100 Extending 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 surfact Normally up to 4m wide Larger trusses type Beam Screeds - may span up to 12m

45 Combined with poker vibrators
for deeper slabs > 200mm And along edges and corners of forms

46 Surface Finish Trowels and Floats Floating: Applied as first process
Leaves slightly open texture Removes imperfections Embeds visible aggregates Bull Floating: A large Aluminium float on a long handle, used to reach across and float slabs on grade

47 Power Floating Four bladed machine with rapid rotary action
Machine floats over surface of setting concrete Followed by hand trowelling in corners etc Larger 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 strength With curing Strength Without curing Time

50 Concrete Curing : Density
Cured concrete is more dense - resistant to chemical attack Poorly cured concrete Permeability Exposure to chemical attack and wear | | | | | | | Days of Curing

51 Water Curing Methods Ponding On flat surfaces such as pavements,
footpaths and floors Sprinkling A fine spray of water applied continuously through a system of nozzles provides a constant supply of water Wet coverings: Wet hessian, other moisture-retaining fabrics can be laid onto the concrete as soon as it has hardened enough to prevent surface damage.

52 Sheet Curing Methods Watertight 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 compounds are either sprayed or rolled onto the concrete and are suitable for vertical and horizontal surfaces. Spraying in set pattern helps 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 application In 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 Joints Requires some reinforcement passing through the joint. There are a variety of tied joints to primarily suit the methods of construction. Joint-free slabs Joint free slabs use a plastic grid insert that encourages a closely 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 Joints Are 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 conditions shapes to be constructed supply 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 wide designed to control tensile stresses due to moisture change and thermal contraction of the slab Filled 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 end Note: 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 plane i.e. around columns passing through a slab on grade

60 Part 2: Concrete Finishes

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 closely are the defects revealed...

63 Site Instructions: Patch the joints with an approved compound!
Note variation in tone/texture at joints where additional face work occurred Interior work is not the most challenging problem?! Note effect of extra tooling required around joints – why needed?

64 Surface Finishes Horizontal surfaces are finished by floating and trowelling Floating is a process of rapidly smoothing blemishes in the upper surface Trowelling is the final operation to achieve a smooth and dense finished surface NZS 3114 – Concrete and Surface Finishes: Describes and classifies surfaces finishes for a particular usage Horizontal 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/appearance F6 is highest grade, requires careful mould design and production of samples

66 Surface Finishes Alternatively, may be specified according to Appearance: F1 to F6 specifies finish by surface quality/appearance F6 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/tiles to provide a durable and colour fast range of attractive tones Often applied as a dry-shake powder in NZ & troweled into the topping

68 Typical Range of off- shutter & tooled finishes
Smooth Ply Rotary Ply Exposed Aggregate Bush Hammered Ribbed and tooled Ribbed precast panels Finishes 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.

70 End

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