Devendra Kumar Pandey
Head – Technical & Value Added Concrete, UltraTech Cement Ltd – RMC, Mumbai, India
Zonal Head – Technical Services – South, UltraTech Cement Ltd, India
Concrete as a construction material has gone beyond being merely a cement-aggregate mixture due to advancement in material sciences. Availability of additives like microfine pozzolans, superplasticisers, processed lightweight aggregates and composite fibres etc. enable concrete technologists to modify multiple properties of concrete. World of construction poses large variety of design and application requirements in order to build fast, efficient, economical, durable and sustainable structures. Enhancement in concrete properties like modulus of elasticity, rapid chloride penetration, water permeability, temperature rise, early age strength development, tensile strengths, and equilibrium density have led to development of marketable specialty concrete products. Engineers have developed specifications and knowledge base to utilise benefits available from these concrete varieties. To add more value to the project, structural designs need to increasingly incorporate additional benefits derived from these. For example, high rise buildings in India are designed using concrete grades beyond M80. However, apart from strength criteria, such high end projects require further attention towards issues like durability and constructability in order to minimise potential repair and maintenance costs. Futuristic, smart, sustainable buildings and infrastructure in this century are being built while the stakeholders involved are taking cognisance of these new facets of concrete as a material and making the most appropriate selection of the concrete variety. The new genre of concrete products are helping us to build durable, rapid, economical, green, maintenance-free, easy structures which enhance quality of life during as well as post construction.
Concrete as a Versatile Building Material
Conventionally, concrete matrix comprises of cement and water chemically reacting to form a paste that binds the aggregates together. Compressive strength and workability are by far the most specified characteristic parameters of concrete. However, unlike other building materials such as natural stone or steel, concrete can be engineered to meet a wide variety of performance specifications, which have not been sufficiently leveraged. Design and utilisation of specialty concrete opens the utilisation of various other modified properties of concrete.
For example, Durability of concrete is its ability to resist any process of deterioration when exposed to environment. Well-proportioned concrete mixes, using supplementary cementitious materials, produce durable concrete. Such concrete exhibits dense microstructure and high packing density. To reduce the dead load of the structure, density of conventional concrete (ranges from 2200 kg/m3 to 2600 kg/m3) can be modified using lightweight aggregates. Proper proportioning and characterising of materials helps in producing concrete with density ranging from 300 kg/m3 to 1850 kg/m3. Plain concrete has limited ductility, little resistance to cracking and very low tensile strength leading to its brittle failure. Addition of small, closely spaced fibers into the concrete have found to act as crack arrester and improve its static and dynamic properties. Workability of concrete can be super-enhanced, by improving multiple characteristics like cohesiveness, flowability, passing ability and self consolidation etc. Properly engineered usage of a combination of fourth generation super-plasticizers, mineral additives and viscosity modifying agents in a well proportioned concrete mixture achieves such performance. Likewise, characteristics like thermal conductivity, fire resistance, early strength, aesthetics, and permeability of concrete can be improved by using special aggregates, pigments and additives. Some of the advanced specialty concrete examples below illustrate this approach.
Pervious Concrete for Water Conservation
Pervious Concrete is a special concrete which allows the water to percolate and reduce the runoff. This ensures recharge of ground water, making it an excellent alternative to expensive storm water management methods. It is a mixture of cement, water, and coarse aggregate, and little to no sand. The strength of Pervious is in the range of 10-25 MPa and can be used for hard landscape such as walkways in garden and drainage pits.
With growing urbanization, it is important to restrict uncontrolled replacement of permeable areas with impervious hardscapes and avoid the imbalance in the ecosystem. Use of pervious concrete must be aggressively taken up in Indian context to resolve water shortage and a host of problems including erosion, flash floods, water table depletion etc.
Lightweight Concrete as Screed and Filler
Lightweight concrete is smart filler produced by distributing foam or polystyrene beads in a uniform mixture throughout a precisely designed concrete with a density ranging from 600 to 1500 kilograms per cubic metre. Litecon is generally used for filling the sunken slabs, roof screed and other non-structural areas. Other typical filler alternatives are twice or thrice in density. Apart from relative reduction in dead load imposed on the structure, it also has excellent thermal and sound insulation properties. This concrete also categorises in green category product and is energy efficient.
Structural version of lightweight concrete is also being produced using artificial sintered flyash or calcined clay aggregates. The concrete density normally ranges from 1500 to 1850 kilograms per cubic metre with compressive strengths upto M30 grade. The typical application is for composite composite construction.
Ultra-High Performance Concrete (UHPC) for Thin Members
UHPC is a high performance and efficient alternative material for making lighter and intricately designed building facades, architectural monuments and thin shell structures. Compressive strengths of 140 to 200 MPa, along with low permeability can be achieved depending on the curing process. To improve ductility, suitable fibres are added, which enables partial or complete replacement of reinforcing steel. UHPC is being used for designing building facades by offering versatility and functionality that cannot be matched with conventional concrete. In infrastructure construction this special concrete is used as a highly efficient field grout for connection between the elements. It is highly mouldable material, allowing the production of customized, precast shapes. This material offers architects a new range of alternatives for lightweight, durable facades with slim, streamlined shapes. The mechanical performances of UHPCs enable façade elements to be much lighter and very easy to implement. The thin-ness of the cement matrix also ensures that all the patterns in the mould are reproduced, enabling significant textural freedom.
Water Resistant Concrete for Damp Proofing
Concrete contains network of pores, capillaries and micro cracks. These are generally 0.1mm to 0.4mm in thickness and are potential avenues for water transport inside the structure. Aquaseal water resistant concrete is crystalline waterproofing technology based solution that alters concrete’s performance by transforming it from a hydrophilic sponge into a hydrophobic, waterproof, construction material. The crystalline compound impregnated within concrete reacts with the water and calcium hydroxide to form insoluble crystals which fill these voids and cracks. This further reduces the porosity, brings self-healing effect and resists the passage of water through concrete structure. It is a first level of defense against seepages for basements, UG tanks, STP, elevated pits and other underground structures as well roof slabs.
Decorative Stamped Concrete for Durable Landscapes
Stamped concrete, sometimes referred to as patterned or imprinted concrete, is concrete that is dyed, altered and shaped to resemble a number of other construction materials — like brick, flagstone, tile and wood. Normally this concrete is laid in 3 to 6 inch thickness and a dry on wet colour hardener layer is applied in order to achieve specified colour shades. Customized patterns, textures and designs can be produced by stamping and additional surface treatments like stencils, staining and exposed aggregate finishes. Such concrete is not only aesthetically pleasing but also s at an affordable price. Color hardener and sealer used in stamped concrete makes the concrete stronger, more resistant to abrasion and help block the penetration of water, stains, dirt and chemicals. Since the surface aesthetics is monolithically integrated within the concrete, such material is much more durable than traditional alternatives.
Vermicuilite Firesafe Concrete for Lift Shaft Firestops
Opening in walls or floors which are necessary to allow passages of all building services like cables, wiring and pipes are required to be sealed at floor levels of high rise buildings. Passive fire protection of this nature protects the buildings from spread of fire across various floors. The concrete suitable for such application should demonstrate a fire resistance of not less than 120 minutes. Vermiculite is a lightweight natural aggregate which enables high fire ratings in this variety of concrete. The space between the electrical cables/ conduits and the wall/slabs shall be filled in by a fire stop material having a fire resistance rating of not less than 120 minutes. This concrete can easily withstand high elevated temperatures; thus helping in controlling the fire spread and its effect by providing sufficient fire resistance to prevent loss of structural stability within the prescribed time period. The material is a green product.
High Durability Concrete
Specialty concrete designed by careful proportioning of combination of binders in order to control permeability and enhance the durability of the structure contributes towards substantially increasing the life of construction. Often, in this case the strength keeps increasing beyond 28 days to upto later ages. Highly recommended for coastal belts and underground construction. Often designed with excellent workability that makes compaction, much easier and more effective. The performance requirement is normally dependent on the environmental conditions for which the structure is exposed during its service life. The common durability requirements of structures exposed to extreme environment conditions are as follows:
– Rapid Chloride Permeability Test (RCPT) as per ASTM C 1202 – less than 1000 coulombs
– Water Permeability as per BS EN 12390/ DIN 1048 – less than 10mm
Matching Construction Challenges with Concrete Varieties
Not every concrete solution is appropriate for every construction situation. There is need to educate the users about approach for selection of various concrete varieties with respect to most common construction needs. The benefits can be mapped with respect to both structural performance and constructability. Apart from these, there are many more specialized needs that special concrete performances that can be addressed viz. anti-microbial concrete, sound insulation, controlled low strength backfills, glow-in-the-dark concrete etc. Following matrix proposes a simplistic guide for product type selection.
The enhanced properties of the specialty concrete must be studied with respect to changes in structural and construction design approach. By proper selection and leveraging the characteristics, the design process can be reoriented to improve the performance of the structures and thus reduce the holistic service life cost. The concrete solutions referred here bring about improvement in surface finish, better insulation, sleek elements, reduced expansion joints, early formwork removal, timely post-tensioning, reduction in steel etc.
These above factors lead to significant cost reductions. Moreover, these also provide an opportunity for further savings by reducing repair and maintenance costs during the building’s lifespan. The implications on costs due to reduction in project execution time and increase in sellable area need to be explored further to depict the degree of cost advantage possible through such concrete types. By judicious selection and use of specialty concrete, savings in the tune of 5-15 percent for the entire project can be achieved. Many of these products support green building ratings and enable superior quality of construction environment. Concrete is adapting to the change for constructing a smart future.