Abstract: The changing needs of concrete, coupled with construction schedules and varied availability of raw materials, puts tremendous stress on ensuring production, durability and the sustainability of the concrete produced. The Construction of Tall Buildings is becoming more and more common as the available land bank shrinks and the expansion needs to be solely vertical. Concrete is the material of choice today owing to the moldability, economy, speed and durability aspects it provides to the structures. Handling concrete is as much of a skill as it is a science. In terms of concrete, engineers bring together over 2500 kg of varied raw materials (binders, aggregates, sand, water and additives) per cubic meter, to cast a homogenous material that is expected to last in excess of 50 to 100 years.
Bringing homogeneity and imparting durability to a combination of these materials is the task of construction additives, which fall under the category of Construction Chemicals. As we see it, Concrete Construction without of the use of Construction Chemicals is not possible today. When addressing the concept of tall structures, construction chemicals can help address / enhance the following areas:
1. Durability Enhancement for the Structure
2. Load Reduction in the superstructure
3. Minimizing Operational Maintenance, 4. Fast Finishing Systems, 5. Aesthetics, 6. Speed of Construction
Some of the materials reviewed include:
1. Additives for Floor Screeds, 2. Curing Compounds
3. Waterproofing Systems, 4. Protection Systems
This review paper takes a look at the various state-of-the-art materials and application methods for various systems to be used for tall building construction. The paper reviews the underlying mechanisms of these additives and the latest applications / joint studies undertaken at MC-Bauchemie and other industry partners incorporating these materials in Construction of Tall Structures. Pertaining to the concept of tall structures, these technologies can help improve durability and mitigate maintenance, thus redefining the term affordable over the long run.
According to the Council on Tall Buildings and Urban Habitat (CTBUH), based out of Chicago, USA; “There is no absolute definition of what constitutes a tall building.” A tall building is one that exhibits some element of “tallness” by:
1. Height Relative to other structures in its vicinity or municipal limits: For e.g. a 10 to 20-storey building among the relatively shorter buildings in a state like Goa.
2. Proportion: For e.g. a building giving the appearance of slenderness, i.e. small footprint and a great height.
3. Containing technologies, which may be attributed to being a product of “tall” (e.g., specific vertical transport technologies, structural wind bracing, etc.).
In India and internationally, a building more than 50 meters (165 feet) in height – could perhaps be used as a threshold for considering it a “tall building.” The CTBUH further defines “Supertall” as a building over 300 meters (984 feet) in height, and a “Megatall” as a building over 600 meters (1,968 feet) in height. As of June 2015, there were 91 supertall and 2 megatall buildings fully completed and occupied globally.
The construction of these buildings entails special technologies in terms of specialized formwork, pumpable concrete, automated finishing and vertical transportation facilities. Due to the height and nature of these buildings, it is quite difficult to attend to their repair and maintenance needs on a frequent basis. We, therefore, need to ensure, that the production, transportation, placing, finishing and curing requirements of the concrete and other materials are paid special attention to. The need for these structures will include:
1. Materials pumpable to great heights, easily transportable
2. Curing Systems for the elements (slender elements with high-performance concrete)
3. Waterproofing Systems that behave as one with the structure and need minimum intervention and re-work
4. Specialist Protective Systems for floors and walls of the building to provide durability to the structure as a whole
To aid in achieving these needs, construction chemicals like concrete additives (plasticizers, superplasticizers, integral waterproofers, polymer dispersions, etc.) were designed and introduced. All in all, durability can be achieved by improving the quality of concrete as well as protecting concrete from deterioration mechanism due to chemical, physical, thermal and environmental attacks. We look at durability as providing both sustainability and affordability (reduced maintenance and repair costs) over the life of the structure.
This paper reviews the underlying mechanisms of these additives and the latest applications / joint studies undertaken at MC-Bauchemie and other industry partners incorporating these materials in use for tall buildings.
Additive and Coating Solutions for Flooring
Basic Admixtures are a requirement for tall structures. The admixtures used are generally based on PCE Technology and Stabilizers for Pumping large distances and heights. Another area where specialty admixtures can be applied is in terms of improving the floor finishes for tall buildings. Tall structures have large floor areas. These are mostly cast with a high-performance concrete. Due to finishing and curing problems, these floors are more susceptible to cracking, water ingress and deterioration even during the tiling phase. In addition, floors are often not in level, making them difficult to beautify with an architectural finishing system (marble, tiles, granite or resin based coatings). In addition, the use of self levelling screed systems has not yet picked up substantially due to costs and performance criteria. In General, the Basic requirements of concrete floors are:
– High green strength (quick return to using)
– Quick drying (moisture content less than 4% for resin based toppings)
– Structural stability, no swelling or shrinkage
– Flat / smooth surfaces
– Sharp edge formations
– Compact / Dense Structure
– No bonding on tools, moulds or machinery in green state
– Free of Cracks and Efflorescence
– Resistance to Water / frost
One of the methods that can be effectively used to level and protect the floor areas is a semi-dry earth moist concrete mix. These mixes are characterized by their production methods, which includes casting dry / semi-dry concretes onto the floor and forming them by application of pressure. This method is designed to provide an early setting (due to the semi-dry concrete), quick drying and development of compressive strength. The pressure compaction is designed to provide proper form and sharp edges to the floor.
However, this method of screeding has its limitations. Concrete / mortar used to make floor screeds has different properties. Semi-dry concrete with lower water content is by nature difficult to compact. In addition, products must be free of efflorescence. A particular challenge in the line is the achievement of sufficient early strength to enable the rapid use in terms of walkability. Only a perfectly matched combination of admixtures and casting techniques is able to produce the desired results. Figure 1 shows the application of such a levelling screed.
To countermand these limitations and meet requirements, a series of additives were developed, which on one hand modify the rheology of the semi-dry mix by lubricating it and densifies the mix on the other hand to provide optimum compaction and hydrophobicity. In the case of use without additives, a similar mix will set and dry after approximately 42 days. A standard cement screed of 5 cm thickness needs approximately 6 weeks until it is ready to be overlaid. Additional moisture is often added by using watery materials, for example during subsequent paint or render works, which results in even longer curing periods.
Using a compaction aid / densifying additive reduces the amount of water that can be added while improving compatibility of the concrete by lubricating the mix as well as the interface between the mix and the mould. This additive also makes the mix more robust by reducing the sensitivity of the mix to added water, which allows the finisher to produce a floor area with much more efficiency. In short adding a compacting aid expands the limit of water addition in the mix from a narrow point to a range of values. The use of this additive will ensure that the screed will be ready for overtreatment in 4 to 7 days depending on temperature. Lesser water means better green strengths of the concrete, better stability, quicker drying and better compaction and sharp edges in the floor.
Advantages of Using Compaction Aids
– Shortening of drying time
– Hydrophobising Effect (No efflorescence or water ingress)
– Excellent plasticizing and application (earth moist mixtures can be pumped) (Figure 2)
– Optimisation of screed properties in application
– Ready for overlay after 4 to 7 days (even with resin based coatings)
– Improvement in properties far superior to common accelerators
– Screeds are easier to draw out, apply level and compact
– Homogenous screeds with even surfaces
– Cement screeds with high mechanical properties (up to CT-C40-F6)
– Economic solution for large-scale projects
– Time-saving through becoming ready to be covered early
– Touch up becomes unnecessary because crack building is prevented
– Beautiful and even surfaces without cavities
– Covers can be installed without need for extensive surface preparation
– Good compaction right down to the substrate
– Unsusceptible to backward moisture
– Suitable for basements, garages and wet rooms
– No need to sand surface
– No segregation as in case of flowable screeds / Economy
– Bonded screeds, – Screeds on separation layers
– Screeds on insulation layers, – Heat resistant screeds
An example of a mix design for a 30 to 50 mm levelling screed is given below.
1 bag (50 kg) 43 or 53 Grade OPC, 4 bags (200 kg) Sand up to 4 mm, 10 to 12 kg Water, 1.5 kg Screed Additive
This material will achieve a compressive strength of nearly 35 MPa with no cracking, efflorescence and prevention of any water ingress. All in all, this can help make floors more durable, aesthetically pleasing and easily overworked. Figure 3 shows examples of finished floor screed.
In a bid to reduce dead load on the flooring by using a tiling system in common areas, another option available is to do the screed for levelling and use a new generation floor coating system. This rapid roller coating for interior and exterior surfaces provides an aesthetic look that is durable and lasts long. The coating provides a safe and long lasting finish for balconies, pergolas, terraces and garages even in adverse weather conditions. Irrespective of the season whether damp and cold or during the hot summer months – screed and concrete substrates can be finished with an aesthetically pleasing and resistant exterior surfaces.
Some of the benefits of using the screed and coating system include:
– Scratch-resistant surfaces
– Can be applied at ambient temperatures between 2 °C to 35 °C
– Easy to apply – UV-resistant
– Project execution even under bad weather conditions
– Short reworking times and quickly ready for use
– Early water resistance, rainproof after 30 minutes, can be used for balconies and terraces
– Anti-slip structures after half a day
Some of the floor finishes that can be achieved are shown in Figure 4.
Considering the large amount of finishing works to be done, using the screed and coating system, can be an effective way to:
1. Reduce Dead Load in Common Areas (flooring finishing)
2. Provide a low maintenance surface that is tough, scratch resistant and waterproof
3. Provide beautiful surfaces / aesthetic surfaces that are easy to clean
4. Can be applied under any weather conditions, which means it can speed up works
5. Systems resistant to backward moisture and therefore debonding
The strength and durability of the concrete structures do not depend only on correct composition and placing of concrete, but also on correct curing practices. Yet this aspect is the least understood or practiced part of the concrete construction. Water curing, the most popular form of curing is also improperly employed at the construction sites thereby losing the advantages of the practice. Proper curing particularly in the early age of concrete up to seven days can only lead to durable concretes with good surfaces.
As per classical definition adopted by the ACI committee on curing, “Curing is the process of maintaining a satisfactory moisture content and favorable temperature in the concrete during hydration of the cementitious materials, so that the desired properties of the concrete are developed.” The principle of curing, therefore, is to prevent the evaporation of water in the concrete so that sufficient water is available for complete hydration of the cement in the concrete.
The schematic diagram of the water curing is shown in Figure 5. The aim of curing is specifically to keep the concrete saturated until the water-filled spaces in the fresh cement paste are filled to the desired extent by the products of the hydration. The main factors that have to be considered while selecting any mode of curing are:
a. The water loss should be prevented, and
b. The temperature gradient should be kept minimum for dissipation of heat of hydration
The major disadvantages of improper curing are as under:
– Lowering of compressive and flexural strengths
– Sanding and dusting of surfaces and lowering abrasion resistance
– Higher permeability
– Cracking due to plastic shrinkage, drying shrinkage and thermal cracking.
– Increased rate of carbonation
– Lower weathering and frost resistance
– Higher ingress of chlorides and atmospheric chemicals
The curing of concrete is the prevention of the water evaporation from the concrete surface. The evaporation of water from the concrete surface depend on four basic factors:
a. Air temperature, b. Relative humidity
c. Fresh concrete temperature, d. Wind velocity
Out of the four factors above; concrete technologists can only monitor the fresh concrete temperature while the other factors are environmental influences. Classic charts are presented by LERCH, (Figure 6) from which it is possible to calculate the evaporation of water per square meter per hour and quantities are alarming enough to trouble any quality conscious civil engineer. For e.g. based on the LERCH Chart, at conditions of:
– Air Temperature: 27°C, – RH of 50%, – Concrete Temperature: 32°C, – Wind Velocities of 15kmph – Water Loss is 1.1 kg / sqm. / Hr
Traditional Curing compounds are based on wax or hydrocarbon emulsions in water or solvents. The wax or the hydrocarbons are the film-forming elements for this type of curing compound, while the water or solvents form the medium or vehicle for dispersion and evaporate when the curing compound is sprayed or applied. The advantages of these curing compounds include:
– Early Beginning, – One Time application, – No entry of water into the concrete, – Reduction of concrete temperature
– No wetting and drying problem for the young concrete
Its limitations include:
– These curing compounds need complete degradation or manual /mechanical removal of the wax or resins (these act like bond breaking materials between concrete and subsequent layers of glue or coatings) before subsequent layers of treatment
– If any part of the curing compounds remains within surface pores of the concrete, it may affect bonding of subsequent layers.
– Therefore the applications of these curing compounds can be limited to roads, open decks, external facades in buildings, etc.
New Generation curing compounds are based on a blend of various acrylic dispersions in an aqueous medium. These curing compounds can be universally used. Apart from the advantages offered by the group of curing compounds mentioned above, these curing compounds have the following additional advantages:
– Healing of surface Microcracks in concrete
– Seals Concrete Safely, protects from water ingress
– UV Stable, – Solar reflectivity, – It has no need for removal
– Less Water Used in Curing and removal, – Green Solution
– No mess on the site
Application for this type of curing compound includes surfaces to be plastered or coated, precast segments for tunnels, bridge elements, STPs, etc.
There is also a special class of water-based epoxies that can be used for curing of concrete floors. This material is especially of interest when the finished concrete floor is to be coated with a resin-based material. It is better to cure the concrete floor by application of this curing compound and delay the coating upto the time the moisture content in the flooring falls to levels acceptable to the resin-based coating. Some of these types of curing compounds and their bases, mediums and areas of application are shown in Table 1.
Curing of concrete by proper methods ensures the durability of concrete. Since the actual site curing practices as well the quality controls installed on the construction sites are inadequate, the technical advantages of membrane fanning curing compounds outweigh the shortfalls. The shortage of water in summer months, non-availability of curing water, problems associated with curing of vertical surfaces and overhead repairs can be simply overcome by usage of curing compounds. In the case of slip form construction practices of tall structures, curing using membrane- forming curing compounds seems to be the most practical solution considering the inaccessibility and limitations of daily water curing and quality control methods.
The advent of new generation curing compounds based on acrylic resins protect the concrete from the environmental effects in the early life and also act as long-term protection against the atmospheric pollution, acting as carbonation resistant coating. This property can be of tremendous advantage to tall structures like chimneys, cooling towers etc. especially for these structures built in aggressive atmospheric conditions.
Waterproofing has been a part of construction in India for a long time. However, these conventional systems had a lot of limitations and often did not / do not perform as needed. This gave rise to the concept of waterproofing guarantees, which was more psychological as opposed to providing a real safeguard against water ingress into structures. The success of waterproofing system depends not only on the materials alone, but also more on application and understanding limitations of the materials in question. Rather than asking for Guarantees from applicators, which has not stopped failures, the adherence to Quality Assurance systems should be reverted to. Guarantees can only be asked from bonafide, qualified and authorized applicators. This phase becomes very critical especially in the case of tall structures, where rework and maintenance become difficult.
Some of the conventional waterproofing systems include:
– Brickbat Coba, – Bituminous membranes – APP/SBS
– Acrylic modified cement coatings, – Metal deck
– Asbestos sheets, – China Mosaic Tiled roof
Some of the shortcomings of these systems are:
– High Dead Load on structure, – No engineered detailing
– Shorter durability under critical exposure, – Fire issues (mainly with bitumen), – Leaking through Joints (tiles, metal deck)
– Lack of additional benefits like energy smart, – Very costly maintenance and repair, – No insulation, – Not a green / sustainable system
Advanced Surface Barrier Coatings
There are many types of high-quality surface barrier coatings available as per current technology. These includes:
1. Crystalline Based Cementitious Coatings
2. 2K Polymer Modified Highly Flexible Mineral based coatings
3. Epoxy or Polyurethane Resin based coatings
4. Polyurea-based coatings
5. 1K Highly flexible Acrylic Coatings
These systems need to be evaluated for use. For e.g. Acrylic and Acrylic Modified Cementitious coatings are flexible and breathable are lower in abrasion resistance. Epoxy, PU or Polyurea are better in abrasion and chemical resistant but not breathable. Each coating can easily be modified for use with supplementary systems for use in diverse applications. However, the field of advanced barrier coatings is fast advancing. Some of the latest advancements in this line are:
High-Performance bitumen based coatings: These coatings are state-of-the-art in bituminous coatings. These coatings are resin-free, liquid applied bitumen coatings, which can easily be applied to damp surfaces, are relatively breathable and have excellent resistance to underground chemical compositions. Figure 7 shows these coatings. Some advantages of these systems are:
– Highly flexible and crack bridging, – Simple and economic application, – Applied by spray or trowel, – Cold applied – even on vertical surfaces, – Solvent-free – non-toxic, non-polluting
– Impervious to pressurized water, – Bonds to damp surfaces
Flexible Underwater Mineral Coatings: Concrete surfaces subject to permanent water loading require particular protection – especially when water and pollutants are able to penetrate through cracks into the building structure. The self-cross-linking and crack bridging new generation a mineral coating offers a waterproofing that fulfills these requirements.
This system can protect concrete surfaces in fire-water basins, service water tanks, clarifiers, process water plants, collection basins, and comparable structures from damaging substances with lasting effect. The revolutionary mode of action of this material is founded on the BaseCoat. The rolled on base coat doesn’t just lie on the concrete surface, but is additionally absorbed by the concrete’s capillary pore system where it crystallizes. The capillary pore system thus seals itself; this greatly reduces water transport and osmosis, thereby waterproofing the structure. Figure 8 shows the working mechanism of this coating.
EHS Based Green Waterproof Coating: Bitumen is valued for its plasticity and flexibility. These properties are vital for perfect waterproofing. But there’s also a “green” solution: The experts in innovative waterproofing systems have developed a solution that works without bitumen. Furthermore, the performance of the new “green” generation actually exceeds the requirements for the waterproofing of building structures by a wide margin. These Elastomer hybrid skin (EHS) based coating covers the building like a skin: It is highly flexible, high yielding and extremely stable under water pressure. However, this product is capable of more: It is suitable for multipurpose applications whilst being hygienic and ecologically safe – it is ideal for use where there is contact with groundwater. This is shown in Figure 9.
– Environmentally friendly because it contains no solvents
– Bitumen-free, – Highly flexible, – Multipurpose applications
– Hygienic and ecologically safe, even in contact with groundwater
Corrosion Protection by use of Anti-Carbonation Coatings
To prevent corrosion, we need to remedy the concrete cover both in terms of quality and quantity. Concrete Protection can be carried out based on two principles:
Principle 1: – Set up an additional concrete cover that is sufficient in density and thickness (like jacketing) OR
Principle 2: – Leave the cover as it is or set up a minimum required cover and apply a surface protection system.
Figure 10 shows both the principles. The application of the protective coating is an attempt to increase the effective cover of concrete in terms of both quality and quantity. By virtue of their formulations, anti-carbonation protective coatings provide the protection quotient of meters of concrete cover in a very thin layer. Figure 11, shows this principle with available materials. Therefore the principle of an equivalent cover is very valuable both in repair strategies and even to safeguard new structures that have a very long design life. Specialized protective coatings can be of many types:
1. Thick Coatings (1-2mm): Like Different Breathable waterproof cementitious or mineral based polymer modified coatings
2. Breathable, Elastic Elastomeric Crack bridging coatings (200-300 micron thickness)
Several benefits can be obtained by incorporating the surface treatments in a well-designed manner right from conception of the project. Although the main function of any surface protectant is moisture ingress control either by physical barrier concept or conversion of capillaries to hydrophobic, the coatings can also be designed for resisting chemicals. Chloride ingress can be effectively controlled by surface protectants. Further, the diffusivity of carbon dioxide, sulphur dioxide and oxygen can be lowered for corrosion control. Root and vegetation growth can be prevented in concretes under damp conditions.
The challenge here is to choose the right coating for the right application. For e.g. Marine Structures, Wastewater Treatment Structures, Wet areas in bathrooms and building construction, terraces, Interior Walls of water tanks, etc. call for thick protective coatings. Exposed Concrete Structures like bridges, power plants, fair face finished building walls, external faces of water tanks, etc. call for special anti-carbonation coatings.
Properties Needed in Coatings for Concrete in General Applications
The coatings to be applied on concrete should not be selected solely for aesthetics but also on protection criteria. It is possible to have both aesthetic and protection judiciously combined in a single well-designed coating. Figure 12 shows the properties and requirements of general protective coatings for concrete and cover the concrete protection solutions in the section above. According to international literature the following properties, are most-essential for an excellent concrete protection system.
It is that property which enables the water vapor to move in and more importantly out of the concrete with the fluctuation of temperature and humidity. The water in concrete (or for that matter any porous media) tends to go from wetter to drier sections. Breathability is essential because, moisture movement in concrete may lead to osmotic pressure building up under non-breathable coatings and eventual debonding of the protective coating. Breathability also enhances drying of the internal concrete, arresting corrosion. Mineral based coatings or high- performance acrylic polymer modified anti-carbonation elastic elastomeric coatings, provide much higher breathing capacity than most resin-based coatings, and thus can protect structures longer.
Impermeability to Water, CO2 and other gases
Protective coatings should be permeable to water vapor, but impermeable to water. Though these properties look contradictory they must exist complimentarily. Most coatings do possess waterproofing characteristics but are not breathable. This is the balancing point separating protective coatings from normal paints. In addition to water, it prevents ingress of corrosion-causing gases into the concrete. Overall breathability and impermeability combined should form a protective coating not very unlike human skin. Good protective coatings provide a sound physical and chemical protection barrier to prevent corrosion in concrete, ensuring durability. These properties can be verified by adequate testing to ensure the quality of coatings.
This is an important property as the coating should be able to negotiate expansions and contractions in the structure due to temperature or dynamic loads. It should also effectively seal and bridge over cracks, to ensure no water or gases enter concrete through them. Even after extensive weathering, protective coatings should provide excellent crack bridging capacities in a variety of static or dynamic loading conditions, in varying exposure conditions and even on weathered concrete surfaces.
Ultraviolet (UV) rays resistance
For exterior application, it is mandatory that the coatings should be UV stable. The effect of sunlight can cause certain polymers to become brittle and lose their physical and chemical properties. Exposure to the sun can also lead to fading of pigments and differential coloring marring the beauty of the structures. Correctly designed protective coatings remain flexible even after exposure to sunlight without degrading.
Specifying the Protective Coating
Rather than specifying coatings by generic names and film thickness, it is preferable to specify them by performance requirements. Some performance properties that can be normally specified are:
1. Diffusion resistance of CO2
2. Diffusion resistance against H2O vapour, when designated as breathable coatings.
3. Resistance against water penetration
4. Resistance to chlorides (ponding), 5. UV Resistance
6. Crack Bridging ability under weathered conditions
Overall these protective coatings are very important to maintain the longevity of tall buildings and structures such as buildings, water tanks, chimneys, etc. They protect the concrete by providing equivalent cover. In practice use of these coatings have shown a lifespan in excess of 15 years. So for exterior areas, parking garage walls, etc. these coatings can really help improve the lifespan of the structure as a whole and reduce maintenance routines. These should be incorporated into modern tall structure construction.
The review paper above delves into the working mechanisms and applications of the latest construction chemical technologies in an improvement of concrete for tall structures. In addition, the industry offers a variety of solutions including, but not limited to:
– Mould Release Agents for improved surface finish
– Concrete Cosmetics to mend broken edges, minor blemishes, blowholes, etc.
– Non-shrink grouts and micro-concretes to join precast elements on site
– Impregnations and clear coatings to impart hydrophobicity and carbonation resistance to precast elements to improve durability
Of course, improved mix design techniques and placing techniques too play a very important role in the production of high quality, durable, concrete elements. Construction chemicals do however, significantly broaden the range of possibilities of tall building construction to achieve different aims such as chemical resistance, improved durability, surface finish, faster cycle times in casting and adding robustness to the concrete in terms of production. Having more durable elements for tall buildings, would also mean lower life-cycle costs, reduced energy costs, reduced maintenance and of course benefits for the contractor as well. Correct usage of construction chemicals would be vital in all areas of tall building construction to enhance benefits. Therefore, it is imperative that Construction Chemicals be a vital component of tall structures to promote sustainable construction.