Manufacturing / extraction processes
In-situ is most commonly a mixture of aggregate (known as builder's mix) blended with portland cement and clean water, that is free of oils, acid etc.
Portland cement is made from lime, silica, alumina and gypsum; With the raw materials being readily available throughout the country.
Aggregate meanwhile is most commonly sand, gravel and crushed stone and constitutes 60 to 75pc of the volume of most concretes. In Wellington it is sourced from quarries along the Western Hutt hills and in Ngauranga, where Greywhacke (sandstone) and argillite (mudstone) are sorted as aggregates and coarse sand before being blended as builders' mix. Dune sand from Kapiti can also be added.
Where materials are found:
LIME Lime is found throughout New Zealand, high quality lime from Te Kuiti and Nelson is processed for export. A crystallised form of lime, is mined in Nelson. It is used as a filler and in building construction.
SILICA Also a form of sand, found in Northland, North Auckland and Canterbury have deposits of silica sand.
ALUMINA is extracted from coal ash and oil deposits.
GYPSUM Gypsum has been buried by the accumulation of silt and dust millions of years ago. There is no gypsum in New Zealand, we import ours from Australia.
In-situ concrete is strong, durable, stable, readily available and relatively economic in terms of construction and life time maintenance. It is the ideal structural material, in building sites that have difficult access.
Other qualities that make it an ideal construction medium include :
• the ability to control of form and shape
• the enclosure of space and structure in one material
• the ability to form integral surface finishes and colour
• its compatibilty with most other materials
• and excellent acoustic and fire resistant properties.
Also admixtures and other materials can be added to serve various functions ranging from freeze proofing to creating transluscency. It should be noted that the quality of such constituents has a considerable effect on material properties.
Format & finish of materials
There is a range of concrete formats and finishes available to the designer/architect.
For a relatively smooth but open finish, it is first necessary to screed (make level) the concrete once poured and compacted, then ‘float’ the surface using any one of a range of floating devices.Hence the term 'floated finish.' They typically have up-turned blades sitting parallel to the surface that flatten, at high velocity, any exposed aggregate.
For a perfectly smooth closed finish, trowelling is necessary well after the floating process. A power trowelling machine has sharper blades that ‘close’ the concrete surface. A honed surface, like the closed surface, is perfectly smooth, but is sectioned at a depth whereby the aggregate is visible and forms part of the surface itself.
Other finishes are possible including the rough exposed aggregate look, which creates a non-slip finish. Here the top layer of cement paste has been removed to reveal the aggregate. This is commonly achieved using a bristle broom followed by water to wash off this top layer once the concrete has firmed sufficiently.
Formwork is also a popular medium in which to shape, or mould a desired surface in order to create a patterned or stamped finish. Rough timber boxing for instance will leave an imprint on your finished surface. Etching or brushing the concrete while curing are other options, while glass embedded in the concrete itself will give the material a translucent quality.
Different concrete finishes can be achieved by:
• abrasive processes
• chemical processes
• mechanical processes
• combination processes
Colour is another way in which to alter finish. Coloured concrete is achieved by mixing a powdered or liquid pigment into the concrete at the time of batching. This has the advantage of providing a consistent colour finish. For example Liquid Colour is a high solid liquid oxide dispersant, produced from synthetic iron oxides available, which is normally added to ready mixed Concrete at a rate 5% of the cement content in the mix.
Spray Concrete: differs from conventional cast-in-place concrete with regard to the reinforcement and application method and surface, resulting in a considerable range of finishes. Spray concrete’s flexibility in application over complex surfaces means that is used extensively in creating simulated natural objects and environments. As well as in civil engineering applications, such as roads, roadway repairs and tunnels, and in commercial projects. Spray-applied concrete may be applied to wood, steel, polystyrene or any other surface to which it will adhere long enough for the concrete to set.
Spray-applied concrete, "shotcrete" is separated into either a wet process or a dry process. For a dry-mix process, dry cement and aggregate are driven through an applicator hose, with water only added at the nozzle. In a wet-mix process the concrete is pre-mixed, pumped through the delivery hose, and spray-applied with compressed air added at the nozzle via a separate hose.
Common fixing methods
In-situ concrete is only as good as the formwork, which must cope with dynamic loading during the placement and consolidation phases of work. Shuttering can bulge, settle and lean, all of which have the potential to induce small variations in the structure. In addition, the concrete will shrink as it dries. Detailing must therefore consider these and allow adequate tolerances.
The relative cost can be reduced significantly when the formwork is able to be re-used. However repetitive uses on the same project suggests the need for construction joints. Construction joints occur between different placements of concrete.
Durability and maintenance requirements
In-situ concrete is a robust material which does not require much maintenance. Its strength is determined by three main factors; being the quality of cement and aggregates used, the ratio of dry 'ingredients' to water during mixing, and subsequent exposure to water during the curing process. Concrete gains 70pc strength after seven days' curing due to the hydration of the tricalcium aluminates and silicates but needs a further three weeks for complete strength to be realised.
Hydration occurs when water is mixed with cement causing a hardening of the cement paste. During this time it will evelope and lock in position around such reinforcing as deformed bars wired to a grid of mesh. However the concrete won't reach full strength until after 28 days' curing.
Strength is measured as MPa where 20MPa, as recommended for driveways for example, equals one part cement to 5.5 parts building mix (ready-mixed aggregates and coarse sand). Concrete's required strength depends on its purpose: mowing strips can be 15MPa but any concrete exposed to sea-spray must have 25MPa.
It should also be noted that Admixtures (any substance that is added before or during the mixing of the concrete) can act to reduce or augment the potential strength of the concrete mix.Long term durability is compromised if the mixing, pouring, compacting and curing is not handled carefully.
Dry but mild conditions are ideal. As the presence of water is is necassary to continue the chemical reaction and increase strength. Hot days will cause the concrete's superficial water content to evaporate too quickly, leaving a cracked surface, while rain damage may cause the concrete to dust after curing. Too higher water content produces a concrete that is more porous and considerably weaker.
Concrete is stronger in compression than in tension. When combined with steel mesh or rebar, the tensile and flexural strength increases and plain concrete becomes reinforced concrete; a versatile and extremely strong structural material commonly used in landscape construction.
Life-cycle / recycling possibilities
Although in-situ concrete cannot be reused as a structural building material, it can be recycled for other uses such as hardfill and as a buffer to coastal erosion. For use as hardfill, the concrete is stripped free of steel and any other material then crushed using special machinery. As a buffer, the concrete is simply broken down and often piled with other building rubble. "Ward Demolition along with Nikau Demolition accounts for about 50% of the demolition being undertaken in the Auckland Region."
List of manufacturers / contractors / suppliers
Ready mixed concrete suppliers' (Wellington):'
Allied Concrete, 28 Landfill Road.
Firth Certified Concrete, regional office Hebden Crescent, Belmont.
Higgins Concrete Limited, Johnsonville.
Mix Concrete Worx Limited, Park Road, Miramar.
Upper Hutt Ready Mix, The Quarry, Maymourn Road, Upper Hutt .
Winstone Aggregates A Division of Fletcher Concrete & Infrastructure Ltd, Reikorangi Rd,Waikanae .
Product information / specifications
In-situ concrete is the traditional form of concrete construction. Today in-situ concrete above ground level is used less in New Zealand with the advent of precast systems, however it is still widely used in many other countries. This is due to the fact that it is often not cost effective to design traditional load bearing cast-in-situ concrete structures in the face of competition from pre-cast and modular systems.
The written specification for in-situ can follow the form of specifications for commercial work with a greater emphasis placed on workmanship issues. Relevant standards include:
NZS 3104 Concrete production – high grade and special grade. With a maximum aggregate size of 19 mm.
NZS 3109 Concrete construction With concrete strength as specified by the engineer
NZS 3114 Concrete surface finishes
NZS 3402 Steel bars for the reinforcement of concrete
NZS 3422 Welded fabric of drawn steel wire for concrete reinforcement
NZS 3101 Concrete structures standard
NZS 3610 Formwork for concrete. All formwork and reinforcing is to be inspected and approved prior to the concrete being placed.
Pricing varies depending on the qualitly of concrete used and application method. For example:
Ready mixed concrete
Firth Concrete 20MPa $186.00 per cubic meter.
Local Landscaping $200.00 per cubic meter.
Name? $230-280 per cubic meter.
Placemakers 40kg bag aprox $14.00
Concrete is well known to many New Zealanders as a building material, a quick look around Wellington reveals its prominence. From a design perspective it would be difficult to think of the work by Le Corbusier, Adolf Loos and more recently Herzog and deMuron without conjuring up images of concrete. But is this phenomenon positive? Is society sacrificing green space for grey space because it is practical and chic?
The skate park in Waitangi Park is a pertinent example of insitu-concrete for several reasons. Newly constructed and situated in the heart of Wellington, it speaks volumes for contemporary society and popular culture. The design encompasses concrete bowls, rails, dips, ramps and steps, all laid out so skaters, bikers, and roller-bladers can test their skills, regardless of their proficiency.
In terms of materials and construction, it makes sense to use insitu-concrete, proven to be a quiet and easy material to maintain, it provides the most dynamic skating experience. Nevertheless the construction of a skate park and in particular the bowl is one of the most complex and physically demanding applications of insitu-concrete which is part of what makes it so interesting. Few obstacles present as big a challenge as concrete bowls. Briefly construction involves forming and filling, screeding, finishing (a smooth even surface finish is desirable, quality skate parks have a hard troweled finish.) and curing. The structural support comes from using number three (3/8 inch) rebar, placed and tied on a minimum of 12-inch centers.
Waitangi Park’s skate park is a beautiful example of form following function. A subtle change in bowl curvature or the angle of a ramp opens up a world of possibilities for users; it pushes boundaries in recreational sport and material application and construction. The controversial vandalisation of the skate bowl morphs the concrete’s surface and intended effect, adding colour and spunk to an otherwise dull palate. It serves as a reminder that although the concrete as a material has revolutionised many industries, with innovative processes, finishes and applications, one must remain conscientious of overuse particularly in the landscape. As a precedent Waitangi park demonstrates the success of utilising insitu-concrete with purpose to construct a landscape that articulates and defines space, resulting in an area which is simultaneously arresting and functional.
My precedent is concrete finishes on Tory Street and surrounds. In this inner-city zone alone there are several examples of proven or potential finishes for landscaping purposes. The area is also an interesting case study of changing trends towards concrete's materiality and function across four decades in Wellington.
In-situ concrete is a powerful material used widly in this modern day world. After finishing this project I cannot help but to notice how much concrete there is in the world. People today often under estamate how In-situ concrete shapes our world. The skate board ramps, public benches, walkways you name it, in-situ concrete is the ultamite material for a growing nation. It is easy to get and cheap to buy, yet it serves so much purposes.
Landscape Construction : A comprehensive guide to building construction within the landscape, Trevor Pringle; BRANZ, Wellington 2004
Timesaver Standards for Landscape Architecture, Charles Harris, Nicolas Dines; McGraw-Hill Publishing, USA 1998
'Concrete' trade magazine, Cement and Concrete Association of New Zealand; Wellington 2006
Wellington Yellow Pages, Telecom 2005/2006
B Storey, Morten Gjerde, Andrew Charleson, Maibritt Pedersen. Centre of Building Performance Research, VUW, NZ 2006.
Winstone Aggregates, Hebden Road, Lower Hutt
Concrete rocks http://www.digiacomoinc.com/
Cement and Concrete Association of New Zealand http://www.cca.org.nz/concrete_def/in_situ.htm
New Zealand Decorative Concrete Ltd http://www.nzdeco.co.nz/
Skate park construction http://www.skateparkguide.com/Construction/concrete_skatepark_construction.htm
Manufacturing / extraction processes
In-situ concrete is created by mixing cement, aggregate, and water. The most common cement used toady is Portland cement. Portland cement is a powder consisting mostly of Calcium Silicate which comes mostly from limestone. There is also Gypsum, Magnesium, and Alumina in a lot smaller amounts.
With water being used a precise amount is needed. The more water that is used the weaker your concrete mixture is and the less water the stronger your concrete mixture will be. Also the ease of use is effected by the amount of water.
Aggregate is the last main ingredient to in-situ concrete. Aggregate consists mostly of loose materials like sand, natural gravel, and and created gravel from demolished buildings or excavation excess.
These materials come from:
Gypsum: Found nearby in Iowa and Wyoming
Magnesium: Manufactured in Utah on the Great Salt Lake
Alumina: There are major refineries in Louisiana and Texas.
Aggregate: All around the state of North Dakota
Silica and Lime: From limestone found in large quarries in New York and Utah
In-situ Concrete is know for many unique characteristics like durability, strength and its structural ability. In-situ concrete also has important characteristics like being waterproof and being very stable.
It is also used when designers and architects want walls, beams, suspended floors and suspended ceilings. In-situ concrete is also easy to use and is very flexible. It is flexible due to its ability to do pretty much anything and it allows further ideas for design solutions. Designers also use it because in-situ concrete is good for uses of design.
In-situ concrete is has a low thermal conductivity or a low ability for it to conduct heat and is used to essentially "fireproof" steel buildings. In-situ concrete is also is used because it is easy to be molded without losing any of the mixtures strength properties.
Format & finish of materials
Concrete can have many different appearances depending on which way is chosen to finish the concrete. If done improperly the concrete can become discolored, have cracks, or even worse have durability problems.
Bull-Floating or Darbying Bull-Floating or Darbying should be done after the screeding process. the purpose of Bull-Floating is to level an ridges or voids created by screeding. The bull-floater must be careful however because if the concrete is sealed off the air bubbles and water rises to the top and get trapped under the sealed concrete creating blisters that get bigger with age. Blisters reduce the durability and strength of concrete.
Troweling Troweling is a type of finishing that requires a metal machine with blades used to smooth out the concrete. This type of finishing creates a hard and very smooth surface. These surfaces tend to be extremely slippery when wet but can be roughed up to create or more non-slip surface.
Brushing or Brooming This type of technique for finishing is used to create a textured finish and create a non-slip surface. To create a non-slip surface a brush with stiff or soft-bristles damped with water is used. The broom is then dragged across the still wet concrete and in turn creates the desired non-slip surface.
Jointing Jointing is a finishing method to prevent the concrete from cracking in places you don't want it to. When jointing is done properly the cracks will happen in the joints. Using the jointing tool the joint should be at least 1/4 the thickness of the concrete to make sure cracking happens on the joints and not where you don't want it.
Common fixing methods
Cracks in concrete can be very troublesome and unpleasing to the eye. These cracks can be caused by shifting in the concrete, water, or shrinking. There are a few ways with fixing these problems.
One way of fixing cracks in concrete is by Low-pressure injection. This method of fixing injects an expandable polyurethane. The material expands and remains flexible so that the concrete can still flex. Also, the polyurethane bonds to wet or dry concrete to make sure it seals out the water. This material doesn't help the structural integrity of the building, floor, or wall but works as an impenetrable barrier against water.
Another way of fixing cracks is by injecting epoxy into the crack. By injecting the epoxy into the crack it literally glues the two pieces of concrete together preventing water from seeping through. This process also strengthens the structural integrity of the epoxied surface.
A final way of fixing concrete is by applying a surface sealer. This method involves putting a material over the crack to prevent further wearing and tearing. This method keeps out surface water but eroding from underneath can still occur. Also this method does not improve structural integrity.
Durability and maintenance requirements
In-situ concrete is a very durable material. Most buildings plan for the concrete use to last to at least 30 years but most buildings last 50 to 100 years if not longer. Most of all concrete's durability and maintenance are mostly influenced by weathering, use, and other abrasive actions. Concrete can be made to resist freezing and thawing, chemicals like sulfates and chlorides, seawater, and other forms of abrasion.
In-situ concrete can come under certain stresses that effects looks and structural integrity. Some types of problems that can arise with concrete are splotches, cracks, and stains. Splotches and stains can be diminished and non-existent with sealing concrete. By sealing the concrete, water and other corrosive materials cannot eat away at the concrete and damage it. Sealing will need to be done every few years but it will extend the life of the concrete. Another way is to seal and polish the concrete. This method of maintenance seals the concrete and makes it resistant to UV and abrasion, it requires little maintenance after polishing, and it improves the structural integrity of the concrete by making it harder.
Cold Weather In-situ Concrete Pouring
During cold weather special precautions must be made to ensure that the concrete sets properly. One precaution needed to be taken is to make sure that it is warm enough to actually pour concrete. If the concrete does not have enough time to dry and harden the water in the concrete can freeze and create cracking. This will cause significant problems with for the concrete. Also make sure that the cement is not poured on frozen soil because that can also cause problems with settling. After the concrete is poured in rather cold weather water from the surface needs to be moved immediately to prevent freezing. Also, thermal blankets and plastic can make sure that the concrete hardens properly.
Life-cycle / recycling possibilities
Concrete used to be disposed of in landfills. Concrete now is mostly recycled because of environmental laws and construction costs. Concrete collected from demolished building sites is sent through a rock crusher. From there the concrete pieces are sorted based on size with the largest ones being sent back to the crusher. These pieces crushed are usually a mix of asphalt, concrete, rocks, and bricks and free of any materials except metal. After the metal is removed these pieces are then reused for new construction projects. The new crushed mixture is also used as aggregate for new cement. The newly recycled concrete no longer takes up space in landfills.
In-situ concrete can also be used towards many other projects. One project is the breakwater for a harbor or port in cites near the water. Concrete can also be used as a dam or as another barrier towards water.
List of manufacturers / contractors / suppliers
Ready Mixed Concrete Suppliers of Fargo-Moorhead Area:
Strata Corporation, 12th Ave North West, West Fargo, North Dakota
Ames Sand and Gravel, 7th Ave North, Fargo, North Dakota
Prime Concrete Incorporated, Hwy 13, Wahpeton, Minnesota
Aggregate Industries, Holiday Drive. Unit 240, Fargo, North Dakota and Moorhead, Minnesota
Camrud Foss Concrete Construction Incorporated, 8th Street South, Moorhead, Minnesota
Product information / specifications
ASTM C 150 - Portland cement color
ASTM A 615 - Steel reinforcing bars for concrete
ASTM C 39-86 - Compressive strength of concrete mix
ASTM C 642 - Water absorption level of concrete mix
ASTM C 979 - Pigment characteristics
ASTM C 33 - Aggregate type either being fine or coarse
ASTM C 138 - Unit weight, yield, and air content of concrete
Regular Concrete This type of concrete is made and produced by following the directions on the packaging. This concrete is typically made by adding water to a mix and made in a container of opportunity like a bucket or wheelbarrow. This type is concrete is also available at most major hardware stores and is typically used for small projects.
High-Strength Concrete This type of concrete is very dense and when made uses a lot less water then normally used. Also this type of concrete requires an elaborate framework or re-bar for added strength.
Self Compacting Concrete This type of concrete was developed in the 1980's by a few European nations. This type of concrete has numerous intriguing characteristics like enhanced elasticity, easier placement, no need for compaction, and there is no bleed water that can create air bubbles in concrete.
Concrete has been used mainly in the construction of sidewalks, roads, parking lots, and patios. Over the past few years the construction of skateparks has been increasing all over the world. The concrete used in skateparks is a perfect example on how it can be molded into many different forms. The photos taken above are from the Dike West Skatepark in Fargo, ND. The skatepark has a large variety of ramps, stairs, and bowls to meet the expectations of the people who go there to enjoy themselves.
In order to construct a skatepark, there has to be a large amount of considerations to take. The concrete has to be 4000 psi, it needs a 1 inch maximum aggregate, it needs a minimum of 520 pounds of cement per cubic yard, and no fire water or other curing accelerators. The reinforcement for concrete in skateparks has to have a minimum number of three (3/8 inch) tied rebar. The drainage of the skatepark would have to have a 1% slope. Finally, the curing for the cement requires saturating the area with water daily for 28 days, with a minimum of 7 days.
The concept process behind a skatepark involves having flow throughout the spaces. The same concept can be said when designing a landscape with vegetation. The important thing to understand about designing landscapes is to make the spaces enjoyable and intriguing for people in order for the landscape to be successful.
The Red River bridge connects Downtown Fargo and Moorhead. The first two images are pictures of the steps leading up to the bridge. The majority of in situ concrete in the area is very plain and uneventful, but the way these steps and lights works together make an everyday feature seem like artwork. The second two pictures are of a parking garage in moorhead. A parking garage is a perfect example of the plain and uneventful use of in situ concrete. These two sites are less than a block away and instead of being related, they are very different. The bridge is a great example of how the function of concrete can fall into the background and the form becomes the most important aspect of the site. The parking garage is an example of how the form, although massive in size, becomes almost invisible and function takes the stage. These two examples show how concrete has a unique property in the sense that it can be used for form or function and is effective either way.
Aside from normal finishing methods there are many ways that concrete can look after it is finished. In the first picture larger aggregate is used to give a rough looking finish. Also with the larger aggregate it makes the wall or column look more detailed. The second picture was concrete with very very fine aggregate to give a smooth looking finish. Also grooves can be placed to give the concrete a more decorative look to it. The third and forth pictures each show a type of finishing that makes a wall look very detailed. The smooth grooves in the concrete can be created using plastic or metal to form a mold for the concrete to dry around to create a desired finish. Another way of creating this type of look but with a little more texture is by hanging polypropylene rope at the base of the bed to give a textured finish. The stairs in picture five can be created to any size with proper column supports and re-bar framework. The mold creates a nice clean and crisp set of stairs without much maintenance after that. In pictures six through eight columns are created by placing a heavy cardboard mold with re-bar strung through out to old it all together. The concrete is poured and allowed to set. From there the cardboard is stripped away and the rings are shown. These can be easily removed by sanding them down or painting over them. The lines could also be for decorative purposes. These columns can be used for supports for overhead walkways or bridges from one building to another.
Major In-situ Concrete Projects Throughout the United States
Hoover Dam, Arizona
Intake Tower of [|Hoover Dam] in Arizona
San Francisco, California
Fargo/Moorhead/West Fargo Telephone Book Yellow Book 2006-2007
Chemical Admixtures in Concrete Educational Bulletin E2-03 American Concrete Institute 2003
History of Concrete: http://inventors.about.com/library/inventors/blconcrete.htm
Pouring cement: http://www.ozinga.com/resources/concrete/Placing_Concrete.pdf
Concrete Basics: http://www.concrete.net.au/pdf/concretebasics.pdf
Concrete Crack Repair: http://www.radonseal.com/crack-repair.htm
Skatepark construction: http://www.skateparkguide.com/Construction/specs.htm