Architectural concrete provides higher aesthetic finish and serves the structural function for a building. Mix proportions, features and uses of architectural cast-in-place concrete is discussed.
History of Architectural Cast-in-Place Concrete
The first use of architectural concrete was recognized at the Corbusier’s Carpenter Center at the Harvard University. This particular project was a big excitement among many designers in United States.
It was in the year 1965 to 1990 the application of architectural concrete bloomed extremely. This concept brought sculpturing of the forms, innovative mix design for concrete, better quality of form liners and gave improved placing and consolidation procedures. These finally would provide improved color and texture throughout the structure uniformly.
Fig.1. Hradec Kralove Library – X Shaped Floor Plan
Features of Architectural Concrete
Whatever be the area of interest say civil engineering, economic concerns or aesthetic reasons, the architectural concrete is a modern solution meeting all the needs.
The architectural concrete is presently an important means of design in architecture. It has been proved that no other material be used and performed in such a versatile manner. All the construction tasks will demand for architectural concrete.
Given appropriate formwork systems and form lining, any shape of desired quality and strength can be obtained by using fresh architectural concrete.
Design Elements Influencing the Architectural Concrete
There are certain factors that will influence the creation of architectural concrete. They include the following:
- The formwork and the form lining system employed.
- The concrete mix. This will include the type of cement and the aggregates used.
- The type and quantity of pigments.
- The selection of release agent for architectural concrete mix.
- Surface treatment employed; This will include sanding, washing, polishing and the sand-blasting procedures.
- Other procedures like color varnishing, coatings, and hydrophobizing impregnation
As the development of concrete incorporating fibers and other materials in concrete will expand the application of architectural concrete.
Considerations in Mix Design for Architectural Consideration
It is necessary to have adequate workability for the architecture concrete in order to conduct proper consolidation and to gain desired strength for the structure. The architect should specify the design requirements that is specific for the structure that is planned and constructed.
Engineer must determine the design strength of the structure, keeping in mind the service requirements, the practical considerations in constructions, precast works, handling and the erection works.
If the concrete mix is designed for pure structural purpose, all the grades of aggregates shall be present, that makes the mix denser to the optimum value.
A good concrete mix for the manufacture of architectural concrete aim in bringing a mix that provide uniformity in color, texture, avoid segregation and surface blemishes.
When texture of concrete is a primary factor, the use of a single sieve size range of coarse aggregate with masonry or the concrete sand in smaller amounts is advisable. This mix provides uniform distribution for aggregates that helps in bringing good texture.
The use of gap graded aggregates, that does not include coarser particles of sand and the fine particles of coarse aggregates helps in providing a mix that can produce a surface that is free from bug holes. This is achieved only if proper consolidation is carried out.
Color variations can be brought by varying the sand content. The aggregate transparency is obtained by the use of aggregates that are darker in color.
It is not necessary that the mix design of architectural concrete is similar to that of the conventional concrete.
When a contractor accepts the order of architectural concrete, it is his responsibility to ensure that the concrete made will gain adequate strength and good workability so that a concrete surface is produced that gains minimum blemishes. This is a big responsibility for the designer in charge and not to be completely assumed by the contractor.
For gap graded mixes a ratio of fine to coarse aggregate by weight must be equal to 1:2:5 to 1:3:5. Masonry sand is the most commonly used fine aggregate.
If the mix gains higher cement content, the amount of coarse graded sand is in satisfactory amount, as the cement can act as better fine for workability. But in the case of a mix with lower cement content, there is need for fine aggregates particles in desired amount to gain adequate workability.
A high heat evolution will be caused by the use of higher cement content in concrete mix. In order to avoid rapid cooling of concrete and result in thermal cracks, thermal curing blankets can be employed so that the concrete exterior is protected.
The demand of self-consolidating concrete (SCC) is increasing in the concrete plant. This have a greater application in precast construction plants also. When ignoring the additional need for quality control and materials in SCC manufacture and focusing on the advantage of SCC like:
- Decreased skilled labor
- Air bubbles reduction
- Improvement in product consistency
Helps in overcoming the problems of higher cost of mix and the process. SCC cannot be implemented on every site. But the application must be based on proper checking on the locally available raw materials and resources, the environmental constrains and the experience of the concrete producer.
Mix Considerations for Smooth Textured Architectural Concrete
The amount of air, water, cement and fine aggregate that is desired to have proper consolidation of concrete that is continuously graded is in an amount of 50 to 66 percentage by absolute volume. The absolute volume will be 45 to 60 percentage by weight.
This value is be lower if rounded aggregates are used and higher if crushed aggregates are used. Almost 35 to 45 percentage by weight or the volume of the total aggregates that is used are fine aggregates for obtaining smooth texture for the architectural concrete.
Those aggregates that have a size lesser than 3mm must not be used in excess of 5%. The air voids can be reduced to a large extent in number as well as in size, if proper form materials, release agents, and proper placing techniques is carried out.
Mix for Very Light to light Textured Architectural Concrete
For a very light sandblast texture, coarse aggregate of size 9.5mm is recommended. The very light texture is often called as the Indiana Limestone finish. The aggregates used can be either continuously graded or gap graded.
Mix for Medium Textured Architectural Concrete
The coarse aggregates which are in the middle range when used in concrete mix seem to adhere to the concrete surface during the removal of cement/sand matrix from the exposed surfaces.
The use of such a range of aggregates have to be removed else there will be large unevenness seen on the surface of the concrete. The maximum size that is recommended in the gap-graded mixes is 25mm. Larger size aggregates will be very difficult to be placed.
The larger aggregates won’t undergo movement even under a higher vibrating effort. This will lead to segregation.
Mix for Heavy Texture Architectural Concrete
In order to achieve a heavy exposed aggregate finish, the aggregates that have size in the range of 4.75 mm to 9.5mm have to be omitted. This will help in avoiding the concrete liable to segregation problems.
The fine aggregate used must be 100 percent passing through the 2.36mm sieve and must be 0 to 10 % passing through the 150mm sieve. It is also recommended to make use of industrial or masonry sand that pass 2.36mm sieve than the use of normal fine aggregates. The sand used must have a fineness modulus equal to lesser than 2.4.
The architectural concrete for heavy texture will contain more coarser aggregates. These mixes must have a slump value lesser than 125mm during their placing.