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
Welcome to (concrete) jungle The GOOD, the BAD and the UGLY
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 Zen and the Art of Concrete?
Tough Shells: Duxford Auronautical Museum Roof Structure Inverted T-beams: capable of suspending a variety of Aircraft! More compression please, but dont get too heavy!
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 http://www.arcspace.com/architects/calatrava/milwaukee_art_museum/
1) Concrete is durable, strong in compression, easy to use. 2) Versatile: reflect mould forms and shapes 3) Flexibility of size - no modular restrictions 4) Relatively low cost - compared to steel 5) Low level of maintenance – if correct spec used externally! 6) Resistance to fire – compared to steel frames 7) Resistance to insect attack – unlike timber & toxic treatments 8) High thermal mass - useful for heat retention 9) May incorporate insulation - i.e. rib-raft & sandwich wall panels 10) 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 7
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.
10 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
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 11
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) 12
Admixture Types - by primary function 1.Air Entraining – for light weight concrete 2.Set accelerating – shortens setting time 3.Set retarding – delays setting of concrete 4.Water reducing – increases workability (plasticiser) 5.Super plasticiser – increases fluidity (self consolidating concrete) 6.Thickeners – increases viscosity for pumping, reduces segregation 7.Permeability reducing – reduce transmission of moisture after setting A summary: We need to know the main types and their functions/usage!
Admixtures study aid: Refer to table on page 9 of Notes for details regarding admixture types and effects
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 suppliers expertise and the resources 15
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. 16
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 17
18 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. Concrete by Type -
19 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 Concrete by Type: 3
Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete. Also called no-fines concrete 20 Concrete by Type…..4
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. 21 Concrete by Type cont…
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 30
Wheelbarrows 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 0.3m3 -0.75m3 with 0.5m3 most common used for difficult site access and poor surface conditions 31
33 Large capacity (10m3) Mixer Truck Novel front-loading truck delivering into bucket skip Truck Types & Sizes
Bucket and skips (or Hoppers) 34 The constant- attitude skip The lay-back or roll-over skip Bucket skip
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 35
36 Transportation by Pumping Boom reach: Up to five floors in height, or 30m across
Pumping Grade Concrete A different type of concrete is used for pumping: A target slump of 100-125mm 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. 37
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. 38
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) 40
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 41 Poker vibrator types Oscillating vibrators – normal usage Pneumatic vibrators - large diameter, used for dams etc.
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. 42 100 Extending into previous layer Careful levelling process
43 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) Poker Vibrator Placement
44 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 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 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 Power Floating
49 Strength Time Without curing With curing Curing is essential to allow concrete to reach full strength
50 Permeability |||||||1234567|||||||1234567 Days of Curing Poorly cured concrete Exposure to chemical attack and wear Cured concrete is more dense - resistant to chemical attack
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. 51
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. 52
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. 53
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. 54 Nose Shrinking http://www.youtube.com/watch?v=4KBOalf5T2o
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. 55
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 56
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 57
58 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 Joint Type 3: Movement Joints
59 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 Joint Type 4: Isolation Joints
In-situ Concrete Finishes A significant challenge?
Only if you look closely are the defects revealed... Windscreen Surveys?
Note variation in tone/texture at joints where additional face work occurred Interior work is not the most challenging problem?! Site Instructions: Patch the joints with an approved compound!
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 Horisontal Surfaces; (see page 20 of notes) Surface Finishes 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 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
a)Smooth Ply b)Rotary Ply c)Exposed Aggregate d)Bush Hammered e)Ribbed and tooled f)Ribbed precast panels Typical Range of off- shutter & tooled finishes