Friday, May 24, 2024
HomeCivil Products & ServicesPrecast / Precast EquipmentDifferent materials used for Precast Concrete Buildings

Different materials used for Precast Concrete Buildings

Precast elements could be made of reinforced or prestressed concrete. The behaviour of these two under service loads is dependent on the properties of the material composition as well as the manufacturing process. It is important to understand the nature of the materials used in the precast concrete buildings.

Given below is an overview of different materials used in precast concrete buildings.

Cement for Precast Concrete Buildings

Cement should comply with the requirements of IS 456;2000, for gaining satisfactory performance in a structure. The standard classifies cement under two divisions;

General-purpose- Ordinary Portland Cements [OPC] 43 grade [IS:8112] and 53 [IS:12269] are normally used in precast concrete construction.

Special purpose-These cements are specified for elements in aggressive environments, eg. tidal and splash zones, in sulfate bearing soils and in chemically polluted environments. Portland Pozzolana Cement [IS 1481] and Portland Slag Cement [IS 455] are preferred

Avoid high silica cement as it suffers reversion and loses a large portion of its strength in warm and humid conditions.

Cement used in precast construction

Supplementary Cementitious Material [SCMs] for Precast Concrete Buildings

Supplementary cementitious materials are natural materials or industrial byproducts that exhibit cementitious behaviors when combined with either water or water and other compounds. SCMs enhance the results of ordinary portland cement (OPC) hydration reactions in concrete and are either incorporated into concrete mixes as a partial replacement for portland cement or blended into the cement during manufacturing. SCMs augment cement’s actions and can improve fresh concrete characteristics, strengthen hardened concrete properties and reduce raw material costs.

Supplementary cementitious materials include fly ash, ground granulated blast- furnace slag, metakaolin and silica fume. They should comply with the requirements of the appropriate parts of IS; 3812 for fly ash, IS;12089 for GGBS and IS;15388 for silica fumes. The benefits of Supplementary cementitious materials include reduced cost, improved workability, lower heat of hydration, improved durability and chemical resistance.

Aggregates for Precast Concrete Buildings

Aggregates for precast concrete should be durable. They should not deteriorate or disintegrate under the action of the weather. Items for consideration under weathering action are freezing, thawing, variations in moisture content, and temperature changes. General aggregates [ coarse and fine aggregates including manufactured sands] should comply with requirements of IS:383. Methods of the test of aggregates are covered by IS; 2386. Special Aggregates may be required to give desired features such as color and texture for exposed aggregates surfaces. Where special aggregates are used it is better to stockpile them at the beginning of a project for avoiding variations in supply. Gap- graded aggregates will give the most uniform exposed aggregate surface.

Aggregates used in Precast Concrete Buildings

Water for Precast Concrete Buildings

Water should be free from matter which will reduce the strength and durability of the concrete. The use of recycled water may lead to a rise in the proportion of soluble salts and alkalis in the concrete. Testing should be carried out to ensure the limits are not exceeded and where potentially alkali-segregates are being used. Mixing water/curing water shall conform to the requirements as per IS 456-2000.

Waterproofing admixture for Precast Concrete Buildings

Waterproofing admixture for precast construction should comply with the requirements of IS; 9103-1999, when compared with the manufacturers’ declared values. Where two or more admixtures are to be used in combination they should be checked for compatibility with the cement. It is also desirable to conduct trials with admixtures using the specific materials to be used in the project to ascertain the dodge for the desired performance.

Crystalline waterproofing admixture is a great option for precast buildings. Integral crystalline admixture powder specifically formulated to interact with concrete capillary pore structures to provide a waterproofing system that is a permanent part of the concrete. These can be used in above- and below-grade applications. Active chemicals combine with the free lime and moisture present in the capillary tracts and pores, to form insoluble crystalline complexes. These crystals block the capillaries and minor shrinkage cracks in the concrete to prevent any further water ingress.

Waterproofing admixture for Precast Concrete

Pigments [Oxides] for Precast Concrete Buildings

Oxide pigments are commonly used to colour precast concrete. Pigments are available in a wide range of colours from deep to pale pastel hues. The major non-blended standard pigments are green, black, red, brown, yellow, blue and white. These can be obtained in the commercially blended form to produce many intermediate colours.

Pigments for use in precast concrete should be-

  • chemically inert and particularly alkaline resistant
  • insoluble
  • chemically inorganic to prevent fading by photochemical degradation.

Pigments may be natural or synthetic. Mineral oxide pigments are the most widely used materials, fulfilling all the above requirements.

Pigments [Oxides] for Precast Concrete Buildings

Reinforcement bars for Precast Concrete Buildings

Reinforcement bars should comply with IS; 1786-2008 for high strength deformed bars for concrete reinforcement, IS; 432 for mild steel and medium tensile bars. Bars are classified by shape, ductility, class, strength grade and size. The physical properties, chemical compositions and geometry of ribs and lungs in case of steel reinforcement shall be in conformity with IS; 1786-2008.

Welded wire fabric reinforcement is widely used in developed countries and its use in India is also seeing a rise.

Reinforcement bars for Precast Concrete

Prestressing tendons for Precast Concrete Buildings

Steel tendons for prestressed concrete may be wires, strands or bars. Wires and strands are commonly used for pre-tensioned members. 7- wire strands systems are the most common system used for post-tensioned members. Prestressing tendons should be clean and free of harmful matter such as loose mill scale, loose rust, mud, oil grease, or any other coating which could reduce the bond between the concrete and the steel. Prestressing steel should confirm to IS;1343.

Prestressing tendons for Precast Concrete

Protective coatings on reinforcement for Precast Concrete Buildings

Appropriate quality of concrete and amount of cover in combination with proper detailing is considered sufficient to protect reinforcing steel for the normal life of a structure. The most commonly used coating is zinc-based. It provides appropriate thickness in the form of hot-dip galvanization. It should not be coupled with dissimilar metals.

Protective coatings on reinforcement for Precast Concrete

Joints for Precast Concrete Buildings

The purpose of Joint in precast structure is: to transmit forces between structural components and provide overall stability. Joint is designed to resist unpredictable loads. Load transfer mechanism and ductility offered by the joint connections play a key role in determining the overall seismic performance of precast structures, thus connections require proper design for adequate transfer of seismic forces between the precast panels. For precast architectural and structural joints are used. 

Architectural joints are designed to hold the cladding, connection points by evaluating  the effects of structural movement on the cladding. The precast concrete manufacturer designs the cladding for the specified loads, erection loads and connection details. The precast panel is independently supported to the building structure using an assemblage of metal components and anchors. Joints around each of the precast panels are usually filled with sealant.

Structural joints generally placed at various intervals to allow for the expansion/contraction of the materials. There are two kinds of joints that can be distinguished Dry and Wet joints Dry Joint is accomplished by simple placing of two members by means of welding or fastening. Wet Joint requires not only casting with cement but also concreting and grouting material.

Joints for Precast Concrete Buildings

Lightweight Aggregate concrete for Precast Concrete Buildings

Lightweight aggregate concrete can be made by injecting air into the composition of concrete. It will make the concrete contained with an air bubble which can reduce the weight and the density of concrete. The other way to produce this concrete is by replacing the aggregate by hollow, cellular and porous aggregate. The aggregates used in structural lightweight concrete may be a combination of fractions of both lightweight coarse and fine materials and lightweight coarse material with an appropriate, natural fine aggregate. The advantage of Lightweight aggregate concrete is reduction in dead loads due to low density as compared to conventional concrete.

Lightweight Aggregate concrete for Precast Concrete

Fibers for Precast Concrete Buildings

Fibers can be used as reinforcement in precast concrete, either as a fiber-only solution or as a combined solution with conventional rebar reinforcement. Fiber-reinforced concrete (FRC) in precast construction increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Blends of both steel and polymeric fibers are often used in construction projects in order to combine the benefits of both products; structural improvements provided by steel fibers and the resistance to explosive spalling and plastic shrinkage improvements provided by polymeric fibers. In certain specific circumstances, steel fiber or macro synthetic fibers can entirely replace traditional steel reinforcement bars in reinforced concrete. This is most common in industrial flooring but also in some other precasting applications.

Fibers for Precast Concrete Buildings

Non-shrink grouts for Precast Concrete Buildings

When complete filling of spaces or bond to reinforcement in core holes is essential, non-shrink grouts are used. Non-shrink grout when hardened under stipulated test conditions, does not shrink, so its final volume is greater than or equal to the originally installed volume. It is often used as a transfer medium between load-bearing members.

Non-shrink grouts for Precast Concrete Buildings

Epoxy grouts for Precast Concrete Buildings

Epoxy grouts are used when very high strength is desired to the concrete. Design, detailing, and proper material specification of field-cast grouts are critical for a number of reasons. They are mixtures of epoxy resins and filler material, mostly sand. The physical properties of epoxy vary widely so the user should be familiar with the compounds to be used.

Epoxy grouts for Precast Concrete Buildings

Conclusion

Precast concrete materials help you maximize your project’s potential while making sure it is completed on time. They are also among the most versatile products in construction, combining a strong structure with the ability to meet compatibility requirements.

Image Source: esolution-pt.com, monocrete.com, techdry.com.au, bft-international.com, precast.org, designingbuildings.co.uk, slideserve.com, aggregate.com, trpreadymix.com, parex.co.uk

RELATED ARTICLES

Most Popular

Hot News