Pre-cast concrete
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Manufacturing / extraction processes
Pre-cast concrete is concrete items that are factory poured in molds such as pier blocks, drain pipes, stepping stones, and decorative landscape accessories.
Extraction process of materials
Portland cement: Basic ingredients are concrete, mortar, and grout, which consists of a mixture of oxides of calcium, silicon, aluminium, ferric and magnesium. These raw materials are usually quarried from local rock. Sometimes needing the addition of clay, limestone iron ore, bauxite or recycled materials.
Aggregate: Aggregate is made up of gravel and sand. Gravel contains crushed stone - generally limestone, dolomite or granite that has been crushed, or rounded stone known as creek rock. Both the gravel and sand are extracted from either a quarry, which is the primary source of stone where it is obtained from blasting the side of a rock face, or can be dredged or scooped from river and creek beds, which are formed by natural erosion of existing rock and then naturally transported by either ice or water movement.
Manufacturing process
Pre-cast units are produced at ground or bench level, and after hardening are lifted into place to form part of the structure. Precasting may be done on the same site as the finished structure, but is usually done away at a pre-cast factory. Some firms specialize entirely in the manufacture of precast units, while others continue on by doing the erection work on site as well.
Modern casting processes include many different methods of making the mold, filling it and
The cast can be removed from the mold after 24hours and left to stand until the curing process has taken effect (usually 28 days). The pre-cast concrete can then be transported by special transportation equipment to the site, and erected by cranes to either form a building or be placed where it is needed.
Material properties
Pre-cast concrete most commonly consists of aggregates held together by hardened cement paste (the binder), which is formed by reacting Portland cement with water.
Portland cement consists of a mixture of oxides of calcium, silicon, aluminum. Made by heating limestone with clay or sand, and grinding the product with a source of sulfate (most commonly gypsum). Aggregate is usually a mixture of sand and gravel or crushed rock. Admixtures are organic or non-organic materials in form of solids or fluids that can be added to the concrete to give it certain characteristics such as workability and freeze proofing.
The ratio and mixing of the various ingredients contributes to the material properties of pre-cast concrete. The compressive and tensile strength is determined by this as well as other factors such as, the curing environment. For this reason the practical implication is that the large pre-cast elements such as free-standing walls, tunnels etc are subjected to tensile stresses which must be reinforced. Concrete also has a very low coefficient of thermal expansion which is determined by the ratio of the mixture.
The mixture of concrete hardens, by chemically reacting with water, also known as the hydration process. This process combines the aggregates and reinforcing materials together to construct the needed feature. The curing process then takes about 28 days to reach it full strength, but is strong enough to be gently removed from the cast after 24hours.
Format & finish of materials
Pre-cast manufacturers have developed a variety of techniques for producing a range of formats and finishes.
Rough-Floated Finishes:
In order to leave a smooth but open finish, the concrete needs to be leveled (screed) once poured into the cast. The concrete is then ‘floated’ with a wooden hand float; if the surface is very large, a darby or bull float is often used. These devices work by flattening, at high velocity, any exposed aggregate. A steel trowel is then used to make sure the entire surface is smooth.
Smooth polished Finishes:
In order to leave a smooth polished face, the surface of the finished concrete is to be grinded to remove the rough outer cement layer. This finish is achieved by either trowelling the surface once or several times with a stainless steel trowel or by using a powered trowelling machine which has blades that ‘close’ the surface of the pre-cast concrete. The pieces of aggregate which have been ground away appear in cross-section in the polished surface.
Exposed-Stone and Coarse-Aggregate Finishes:
In pre-cast concrete one method of giving a rough, textured, non-slip surface is to paint the relevant parts of the cast with a retarding agent just before pouring the concrete. This has the effect of slowing the rate of hardening of the cement at the surface in contact with the cast, so that when the unit is removed from its mould, and firmed the outer cement layer can be removed by gentle brushing under water, allowing the aggregate particles to elevate from the cement.
Grooved, Stamped and Broomed Finishes:
Grooved and stamped finishes are produced by pressing masonry stamping tools and other objects into slightly stiff concrete. Stamping tools can be used to imprint simulated paving brick, stone, tile and other patterns. Brooming or etching the concrete while curing can achieve a wide range of effects.
Rubbed and Hammered Aggregate Finishes:
A sandy effect can be produced by applying a thin layer of sand onto a concrete surface that has partially set. Shake the sand over the entire surface, then rub it in using a piece of clean burlap.
Coloured Finishes:
To achieve a consistent colour throughout the concrete, white cement, types of aggregate (such as light coloured limestone), or pre-mix liquid cement colour can all be added to the dry concrete mix to change the colour of the final appearance. A second colouring method is to apply paint, coloured coating, or pigmented stain to the concrete after it has cured. All these can be applied with a brush, roller, or airless sprayer.
Common fixing methods
The designer or engineer when providing working drawings for a casting will usually state a preferred fixing method they would like a precast to utilize. This has become increasing important as these connections can make their own statements in regard to the over all design and look of a finshed product.
Fixing concrete to another surface is now not just limited to the use of mortor. With the development of Polymers and Expoxy resins that adhere to almost any surfaces, concrete can be used in many more imaginative ways.
Tradtional methods such as the use of embedded reinforcing steel is still used. Other connecting devices have also been developed. Various locking nuts, metal plates, bolts and a variety of fasteners such as weld plates can now enhance a concrete surface and be used as part of the design element of the finished product.
A drossbach can be used to fix panels. Grout is poured in one end until it has filtered out one and eventually filled the tubing completely, and is then left to set.
Durability and maintenance requirements
Before the manufacturer can forward precast elements to clients, strength testing is carried out to approve the strength of the concrete. A concrete testing cylinder (200mm long, 100mm diameter) is filled with the same batch of concrete used to make the precast, and is then sent away to be tested to make sure it is the right strength.
Concrete is an incredibly durable product but is also prone to numerous problems. Products such as fruit juice (hydrofluoric acids and sugars wil cause disintergration) and seawater (slow disintergration) can effect the life span of concrete.
To ensure the life span of concrete, maintenance is required. Initial sealing of the product helps protect the concrete. Ongoing maintenance and removal of harmful substances with approved products also adds to the life span of the product.
When chips, cracks and holes appear maintenace and repairs may be carried out in the form of: Grinding, Mortor, Dry pack, Epoxy-Bonded Dry pack, Expoxy-Bonded epoxy mortor, Injected Portland Cement and cutting and replacing of the effected area
Life-cycle / recycling possibilities
Concrete starts life is the form of 3 materials: water, aggregate and cement.
In addition to these materials the addition of Chemical Admixtures can be used to produce modification to the properties of concrete. The use of steel, fibre and polymers embedded in concrete increases the strength, durability and lifespan of the product.
Concrete is a durable and robust product often having an infinate lifespan. In areas where temperatures create a freeze and thaw effect on the concrete, the lifespan can be greatly diminished. Also the use of de-icing salts on concrete can weaken the product. In factories and plants were concrete is exposed to chemicals, on-going maintneance, repair and replacement may be required.
Concrete recyling basically involves the crushing of the material to be later used as back fill or base course material.
Currently recycling of concrete products is prevalent in the northern half of the North Island with other areas slowly adopting the practice rather than using the material as landfill.
Smaller plant operations has allowed recyling to take place on sites and helps minimise recyling costs.
List of New Zealand manufacturers / contractors / suppliers
Concrete Repairs Ltd, Auckland: (09) 634-7686
Concrete repairs.
Concrete Solutions NZ, Kaiwharawhara, Wellington:(04) 381-4952.
Concrete protection and repair, flooring, injection technology, leak investigation, roofing membranes, sealants, waterproofing.
Emmett Civil Construction, Gonville, Wanganui:(06) 349-1788 or 0508 414-515.
Manufacturer of prestressed and/or precast concrete, and concrete repairs.
Firth Industries Ltd, Richmond, Nelson:(03) 543-9068.
New Zealand's largest manufacturer of concrete products, systems and solutions for both commercial and residential projects. Outdoor supplies include concrete, bricks and paving for courtyards, pool surrounds, fences, paths and retaining walls. http://www.firth.co.nz
Fulton Hogan Ltd - Concrete Division, Balclutha:(03) 418-2880.
Design, manufacture and supply of concrete products to the building and civil markets. The prestressed concrete manufacturing facility operaines in Balclutha, producing prestressed flooring systems and beams to suit most specificiations. http://www.fh.co.nz
Pre-Cast Components (Wgtn) Ltd, Otaki:(06) 364-8355.
Manufacturers of precast and prestressed concrete components for the building and construction industry. Manufacture and supply precast concrete beams, columns, panels and stairs, and prestressed concrete flooring including Spiroll Hollowcore, Double Tees, Multispan ribs with timber infills and flat slabs. Also manufacture bridge beams, culverts and other components for civil construction work. Other products manufactured include precast concrete power poles and GRC (Glass Reinforced Cement) products.
R & C Precast Ltd Porirua (04) 238 2530.
Precast and prestressed concrete manufacturer.
Reid Engineering Systems, Seaview, Hutt City, Wellington:(04) 568-4505.
Concrete reinforcement, anchoring, fastening and fixing technologies and specialists in the design of precast construction. www.reids.co.nz
Rockbond Special Concrete Products, Petone, Hutt City, Wellington:(04) 568-5401 or 0800 762-526.
3 part system epoxy adhesives, grouts & motars, acid etching concrete, concentrate acrylic concrete, concrete colourings, concrete curing compound, concrete floor sealer.
Stevenson Precast Systems Ltd, East Tamaki, Auckland:(09) 921-8688 or 0800 610-710.
Building panels, concrete bench tops, concrete building components, concrete building panels, pavers, paving slabs, pre-cast panels, structural precast concrete elements. http://www.stevensons.co.nz
Stresscrete, Hornby, Christchurch: (03) 344-3014 or 0800 373-977. Mt Maunganui, Tauranga:(07) 575-3103 or 0800 373-977. Invercargill:(03) 218-6709. Cromwell:(03) 445-3246.
New Zealand's largest national supplier of precast & prestressed concrete components to the building industry. http://www.stresscrete.co.nz
Thelin Construction Ltd, Richmond, Nelson:(03) 544-8906.
Specialists in the manufacturing and placing of pre-cast and pre-stressed concrete components which include commercial and residential foundations, bridges, suspended concrete floors, culverts, pile driving, wharf construction, reservoirs, and under passes.
Thomson Concrete Ltd, Otorohanga:(07) 873-8775 or 0800 876-844.
Produce concrete products distributed throughout the North Island. Located in Otorohanga with a branch in Taumarunui. http://www.thomsonconcrete.co.nz
Product information / specifications
Positives of using concrete?
There are not many things concrete isn't good for because of its strength, durability, mouldability. The advantage of using precast concrete, is the repetition factor, as the same mould can be used over and over again producing the same end product, which proves to be cost effective and practical.
What does it mean to have higher/lower MPa?
The higher the MPa, the quicker the concrete gains its strength - and the sooner you can take the precast out of its mould. Concrete only needs to set to 15 MPa overnight to lift from a mould.
What is the common MPa to use in structural precast panels etc?
Generally for a large panel 30/40/55 MPa is used, depending on the strength needed.
What are the size limitations with precasts?
Are different concretes used for different situations..ie are some concretes better in high water areas?
There is a whole smorgasboard of possibilities. For example, a pre cast concrete sea wall could incorporate silica fume to cope with sea water, as it makes the concrete less porous.
What product specifications do clients need to provide?
It is general practise that a client provides a structural drawing, an architectural drawing and a booklet of contractual specifications for the manufacturer to follow. From these three things, there is communication back and forward to make sure both parties have the same vision, and then the precast is made. It is important that the manufacturer understands exactly what is being created and what it is going to be used for so that the end product is exactly what is wanted.
The client often designs the lifting system to get the precasts off the truck/crane. The client is usually responsible for the design performance of the precast unless the manufacturer provided the design. The manufacturer is responsible for making sure the precast matches the engineered design exactly - the right length, width, concrete etc.
Precedent 1
PRE-CAST CONCRETE AND WELLINGTON WATERFRONT
Freyberg Beach
Pre-cast slabs have been carefully arranged at the end of a walkway providing opportunities for people to interact with the ocean. Stacked and connected together using contrasting stainless steel connections, this installation allows for people of all spectrums the chance to enjoy this landscape. As the tide rises and falls the wave’s impact chances as it hits these slabs, splashing people, challenging people, capturing people. The growth of Algae and Blue Lipped muscles of its lower surfaces also creates a point of interest. The use of small aggregate creates a textured surface allowing for a safer walking surface.
Len Lye Water Whirler
Another Wellington Sculptural Trust addition to the waterfront. This typical Lye piece offers a different day and night time experience. The Water Whirler is cleverly done and when viewed at night offers a few illusionary surprises. The use of pre-cast concrete as a base for the sculpture is an obvious choice but does not reflect the beauty of the Water Whirler and I considered it to be an unchallenged use of the material. With the ability to create texture and pattern in pre-cast concrete, this technique could of be used to enhance the sculpture or portray elements of the work and it's creator.
Waitangi Park Climbing Wall
This shows great use of vertical pre-cast concrete wall. With the colours of concrete, rust and vegetation dominating the surrounding area, these small flecks of bright colour are well used in this child’s play area.
Herd St Entrance Lighting and Bollards Cubes
These pre-cast cubes act as sentinels to the Herd St apartments. Every 3rd cube contains a light that illuminates the pedestrian walkway and also acts as a solid bollard providing a safe zone beyond. The size of these cubes also offers pedestrians the opportunity to stop and sit upon them, children to climb and jump from them and the local canine a place to say “hi” to his comrades. These elements for me stand out as an attractive and useful feature.
Denis Glover Poem
Part of the Wellington Writers Walk this pre-cast creation stands out from the others as it contrasts against Lambton Harbour and the rocks it sits upon. After much discussion, trials and errors this Stresscrete pre-cast concrete product was chosen to create the Writers Walks Plagues. After initial installation, vandals removed many of the letters from various plagues. Again trial and error has provided an adhesive securing the letters of the works to their base. An innovative use of pre-cast concrete, these plagues offer pedestrians the opportunity to reflect on their surroundings as they walk along Wellington Waterfront.
Albatross
Sculpture by Tanya Askin 1985/86 Commissioned in 1982 this sculpture established the Wellington Sculpture Trust. Using pre cast concrete it is a lament to the albatross. As you move around the sculpture you can see the wings of the birds against the sky. When water is flowing it invites images of feathers extending from the art work. Round, smooth and fluid form - this sculpture shows how versatile this material can be.
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Precedent 2
Isthmus group, a team of landscape architects, was resposible for the Oriental Bay development, finished in 2003. As part of the redesign of Oriental Parade, this artistic installation of concrete slabs was put in place for public interaction with the sea. Different views of the installation are visible at different times of day due to the tide - it is a continually changing installation. The tide, sea erosion, and plant growth mean that the installation is never viewed the same twice. The higher slabs are always dry, and can be used as a place to eat fish n chips, and the lower ones which are submerged at various levels, can be used to paddle in the water, so the utilisation is entirely up to the public.
The precast concrete panels on the exterior of the St Joseph's Church in Mt Victoria, Wellington bear a design which resembles an abstract fish motif. The fish is a significant symbol in the Christian faith, and combined with the concrete material, they come together in a modernised, respectful way. The panels are so large and bold, it could be argued that from a distance, the detailed panels are now infact part of the landscape, not just an architectural statement. The designs would have been made by crafting the pattern out of another material and attaching to the bottom of the mould, so it would leave an imprint in the precast element.
Modern public areas have begun to use simple concrete precasts as street furnishings. From bollards, to fencing, to pavings, to seating and rock climbing walls, street furnishings have begun to take on a new look and elements such as standardised bench seats are becoming less utilised in new landscapes. Concrete is durable, strong economical, so it isn't suprising that it has become so popular in public spaces. Also, the clean lines that can be achieved with precasting match the simplistic design look which is so sought after.There is a quote which states 'form follows function,' and all of these concrete components are purely functional in their simplicity of form.
Precedent 3
Pre-cast precedent's in the Waitangi Park and Waterfront Landscape, Wellington, New Zealand.
References
Richard Henderson, R & C Precast Ltd, 3-5 Sunlight Grove, Porirua, New Zealand.
Stresscrete. Information and Photo Credits, National Services Office. Address Level 1, Wright Stephenson House, 585 Great South Road, Penrose, Auckland, New Zealand, http://www.stresscrete.co.nz
Alsop, D & Faber, J. (1976). Reinforced Concrete Simply Explained. Oxford University Press: Great Britian
Edwards, L. (1990). Materials in action series: Manufacturing with Materials. The open University: Great Britian.
Moavenzadeh, F. (1990). Concise Encylopedia of Building & Conctruction Materials. Pergamon Press plc.
Schwautz, M. (2002). Materials, Parts & Finishes. CRC Press LLC.
Spander, F. (1987). The David & Charles Manual of Concrete, Brickwork, Plastering and Tiling. David & Charles Inc: Great Britian.
Cement and Concrete Association of New Zealand
http://www/cca.org.nz/arh_des/
Concrete http://en.wikipedia.org/wiki/Concrete
Concrete Lifespan and Modelling Research Group http://www.genie.uottawa.ca/research/rsrch_cvg.php?section=&lang=e&dpt=1&id=34
Concrete technology http://www.cement.org/tech/index.asp
History of concrete building construction http://www.ce.memphis.edu/1101/notes/concrete/section_3_properties.html
Isthmus group, landscape architects http://www.isthmus.co.nz/index.html
Methods of Concrete Repair. http://www.arch.psu.edu/thinshells/module%20III/concrete_repair.htm
St Joseph's Church http://www.city-gallery.org.nz/mainsite/st-joseph-s-hato-hohepa1.html
