concrete surfaces – beautiful solutions with .concrete surfaces – beautiful solutions with...
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Concrete Surfaces beautiful solutions with AALBORG WHITE
AALBORG WHITE is white cement a product of natures own raw materials,
refined with unparalleled technology, for use in the creation of beauty and functionalism.
Concrete surfaces the natural solution
Surface texture 3Untreated surfacesTreated surfacesLightly treated surfaces
Coloured concrete 6
Formwork technique 8Formwork oilsFormwork materials
Concrete production 10Concrete element facadesExamples
Concrete vibration 12
Treatment of concrete 14FormworkPlasticSealing agentsTheory
and discoloration 17Discoloration of concrete facades
Pore formation 18
Environmental effects 19DirtyingFacade cleaning
Historical development shows thatsince the end of the 19th century,the use of concrete has progressedbecause it is mainly an inexpensive andflexible material able to solve con-structional problems arising in heavybuilding work. This new material madeit possible to undertake constructionssuch as large foundations, substantialfortifications, towers, etc. in a moreeconomical way.
Concrete was originally seen primarilyas a load-carrying material. Require-ments concerning strength, durabilityand not least appearance were quitemodest. If appearance or durabilitybecame important, the concrete wasclad in natural stone, plastered, ortreated in some other way. With theintroduction of reinforced concrete(not least prestressed concrete), theunderstanding of the significance ofthe w/c ratio, and with the useof air-entrained concrete, progressreached a point where consultingengineers and architects dared useconcrete as a material in its own rightwhere both appearance and strengthwere concerned.
It was the industrialisation of theconcrete production process that
created interest in the material. Newmethods of construction meant that itbecame impossible to conceal concretebehind plaster or other materials.
Only after concrete producers andcontractors had solved variousproblems did cast-in-situ concrete,which stood untreated after formworkremoval, make its breakthrough
a form of architecture that reliesheavily on the utilisation of the mono-lithic structure of cast-in-situ concretebuildings. Such buildings are nowerected using certain numbers ofprefabricated elements, includingfacade elements. It has thus becomepossible to put up buildings consistingof elegant forms with varying surfacestructures and colours.
If grey cement is used, the naturalcolour of the concrete normally be-comes slightly cool grey. However,on closer inspection it can be seenthat there are varying shades, in thatthere are local deviations in thegeneral grey tone.
When looking at the causes of colourvariation, it can be seen that thecolour of concrete depends on thetype of cement used and, to a notinsignificant extent, on the colour ofthe filler used (filler is particles of lessthan 0.25 mm).
Alternating between cement typesor cement dosage carries with it therisk of colour variations. A substantialstep in achieving uniform colour toneis to replace part of the grey cementwith AALBORG WHITE. As regards
filler, natural sand can also vary. Fillermaterial contributes to the colourof the concrete so that even smallfiller variations can result in quitelarge colour differences. For ex-ample, the use of filler from whitemarble, feldspar, etc. gives fargreater colour uniformity and makesit easier to control the colour of theconcrete.
In describing concrete surfaces, this issue
covers much ground from pore-free
surfaces on dividing wall elements to
coloured and exposed facades. The
demands in connection with surfaces
in different colours and profiles thus
depend to a large extent on the
Concrete Surfaces shows a number
of surfacing possibilities offered by white
and coloured concrete. It examines
methods of finishing fresh and hardened
concrete. Examples are given of selecting
concrete recipes and production methods,
including formwork technique, methods of
compacting, and surface finishes. It also deals
with blemishes and discoloration, pore
formation and, finally, the environmental
effects to which the completed concrete
construction is exposed.
Concrete Surfaces is aimed primarily at
architects, the concrete element industry,
Since concrete is a flexible, formable material it can be givenalmost any surface texture desired. The surface texture canbe created in the fresh phase by profiling or similar methods,or in the hardening phase by machining or exposure of thecoarse aggregate. An examination of architecture todayshows clearly that many of the most elegant examples arebuildings where the surface texture of concrete has played alarge part in the design.
Characteristic of untreated concrete surfaces is that theouter cement paste is not removed and helps in creating animpression of the formwork surface.
The simplest way of profiling such a surface is through theuse of unplaned planks. It is wise to be careful when selec-ting shuttering boards to make sure that they have all beensubjected to the same storage conditions, and most impor-tant of all, to make sure that they are all neutralised as faras sugar content is concerned. Boards should only be re-used a limited number of times in order to ensure that theunplaned surface texture is preserved.
A special phenomenon when casting untreated surfaces isthe formation of pores. These arise because air clings to thesurface during vibration. The number and size of pores isclosely connected to the composition of the concrete. Inparticular, water content, formwork surfaces, formwork oiland vibration method have an influence. The best result isobtained by using easily worked concrete, e.g. using airentraining agents and various plasticisers so that the water
content is held at a suitablelow level. Additionally, it isimportant to use goodformwork oil or wax.
Experience shows that anabsorbent and not tootightly sealed formworkgives the least number ofpores resulting from air. Nar-row unplaned boards makeideal formwork materialwhen a minimum numberof surface pores is desirable.
A special effect can also be achieved by using sand-blastedor flame-cleaned wooden shuttering. These processes bringout the grain of the wood in the form of soft curves in thesurface of the concrete. If the wish is for surfaces with thedistinctively coarse texture of wooden formwork on all fa-cades, it is advantageous to manufacture the formworkusing artificial rubber moulded over a wooden matrix.A coarser surface texture can also be obtained by layingmatrixes of different materials in the formwork, such as rub-ber, PVC, polystyrol, etc.
Removal of the outer cement paste on concrete surfacesgives better control of the many aesthetic problems con-nected with untreated surfaces. It also provides the possibil-ity of varying the colour and texture of concrete surfaces.
Exposure by retardation. Washing unset concrete exposesthe coarse aggregate in the surface. This process is per-formed on elements being cast with the visible side upwards.If retarder is applied to the formwork side, the most usualmethod, the facade part is cast towards the formwork. Con-crete surface setting is thus retarded until the formwork isremoved. Washing to expose the concrete is then possible. Inboth cases, the process combines brushing and water flushing.
Exposure is normally only used in connection with concreteelements. In principle, an exposed concrete surface resem-bles a sand-blasted one, but the exposure goes deeper andthe aggregate retains its natural brilliance. The compositionof concrete which is to be exposed more or less follows thesame lines as that of concrete for sand-blasting.
The typical aggregate content of concrete is 70%, oftenwith quite large steps in particle size. Depending on thegrain shape (cubic/flat) there will be a difference in aggregate layering either on vertical or horizontal castsurfaces.
It should be remembered that like most chemical processes,retardation is very temperature-sensitive. Also, retardershould not be applied to formwork in low temperatures. It is
important to test retarder to make sure that it is suitable forthe conditions in which it will be used when final castingtakes place.
The retarder should also be tested in relation to the materialof the formwork being used. The absorption properties ofsuch materials can have a significant effect on retardationdepth. When using wooden formwork for the first time, itcan be necessary to apply the retarder twice.
During final brushing and flushing, it must be remembered
that the penetration depth also depends to a certain extenton how much power is used.
Treatment must be completely finished in one operationwithout stopping. A final mild acid etching can be applied afew days after exposure if the cement slurry has diminishedthe brilliance and colour of the exposed aggregate.
Exposure by mild acid etching. Mild acid etching of planeor profiled surfaces the day after casting will remove cementpaste. It is important to thoroughly wet the surfaces withwater before acid etching to prevent the surface fromabsorbing hydrochloric acid. Acid etching is usually per-formed using a 1:10 acid solution (30% commercial hydro-chloric acid). After acid etching, the surface must be thor-oughly flushed with water. Normally, this form of treatmentonly exposes the sand grains.
Grinding is a surface treatment method normally used forterrazzo floors in bathrooms, on stair ele