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Concrete and its Myriad Forms

Besides construction speeds and durability, there is now a third driving force, namely the environmental friendliness of construction materials, which is becoming increasingly important in technology assessment for the future. In this perspective, Built Expressions aims to capture myriad forms of concrete in brief with a little background of various technologies that matter in terms of cost of materials and construction, durability, and environmental friendliness of concrete.

Innovation in concrete is endless. Among the recent advancements, most noteworthy have been in the areas of high fluidity, low porosity is generally characterized by high strength and high durability. Macro-defect-free cements and chemically bonded ceramics are examples of alternative technological approaches to obtain low-porosity, high-strength products. For the specific purpose of enhancement of service life of reinforced concrete structures exposed to corrosive environments, the use of corrosion-inhibiting admixtures and epoxy-coated reinforced steel are among the better known technological advancements.

Self Compacting Concrete¹

Self-compacting concrete can be considered as one of the greatest technical advancements and most revolutionary development in concrete technology over the years at least from 1980. The concrete of the future, as it is known, will replace the normal concrete, because of many advantages it offers. The need for development of such concrete arose from scarcity of skilled manpower in Japan during 80s. A concrete that is capable of self compacting (self consolidating), occupies all the space in the form without any external efforts can be termed as Self Compacting Concrete. The guiding principle behind the self-compacting is that the sedimentation velocity of a particle is inversely proportional to the viscosity of the floating medium in which the particle exists. For a concrete to be self compacting, to occupy the full space, flowing through the congestion in the form without any external efforts, it has to have an acceptable level of passing ability, filling ability, flow-ability and stability.

Self-Compactability of Fresh Concrete

Mechanism for achieving self-compactability involves not only high deformability of paste or mortar, but also resistance to segregation between coarse aggregate and mortar when the concrete flows through the confined zone of reinforcing bars. The frequency of collision and contact between aggregate particles can increase as the relative distance between the particles decreases and then internal stress can increase when concrete is deformed, particularly near obstacles.

Research has found that the energy required for flowing is consumed by the increased internal stress, resulting in blockage of aggregate particles. Limiting the coarse aggregate content, whose energy consumption is particularly intense, to a level lower than normal is effective in avoiding this kind of blockage. Highly viscous paste is also required to avoid the blockage of coarse aggregate when concrete flows through obstacles. When concrete is deformed, paste with a high viscosity also prevents localized increases in internal stress due to the approach of coarse aggregate particles. High deformability can be achieved only by the employment of a superplasticizer, keeping the water-cement ratio to a very low value.

The degree of packing of fine aggregate in SCC mortar is approximately 60% so that shear deformability when the concrete deforms may be limited. On the other hand, the viscosity of the paste in SCC is the highest among the various types of concrete due to its lowest water-powder ratio. This characteristic is effective in inhibiting segregation. Three purposes for self-compactability tests relating to practical purposes:

• To check whether or not the concrete is self-compactable for the structure
• To adjust the mix proportion when self-compactability is not sufficient
• To characterize materials

Factors of Self-Compactability

The factors making up self-compactability were described in terms of the test results for fresh concrete and mortar: Influence of coarse aggregate depending on spacing size-It is not always possible to predict the degree of compaction into a structure by using the test result on the degree of compaction of the concrete into another structure, since the maximum size of coarse aggregate is close to the minimum spacing between the reinforcing bars of the structure.

Role of mortar as fluid in flowability of fresh concrete-Sufficient deformability of the mortar phase in concrete is required so that concrete can be compacted into structures by its self-weight without need for vibrating compaction. In addition, moderate viscosity as well as deformability of the mortar phase is required so that the relative displacement between coarse aggregate particles in front of obstacles when concrete is to flow around such obstacles can be reduced and then segregation between coarse aggregate and mortar can be inhibited. Role of mortar as solid particles-In addition to its role as a liquid, mortar also plays a role as solid particles. This property is so-called "pressure transferability", which can be apparent when the coarse aggregate particles approach each other and mortar in between coarse aggregate particles is subjected to normal stress.

Influence of coarse aggregate -Content, shape and grading- The influence of coarse aggregate on the self-compactability of fresh concrete, especially flowability through obstacles, can be equal despite the shape of the coarse aggregate particles' shape as long as the ratio of coarse aggregate content to its solid volume in concrete is the same. However, the influence of the grading of coarse aggregate has also to be considered if the spacing of the obstacles is very close to the maximum size coarse aggregate.

Acceptance Test at Job Site

Since the degree of compaction in a structure mainly depends on the self-compactability of concrete, and poor self-compactability cannot be compensated by the construction work, self-compactability must be checked for the whole amount of concrete just before casting at the job site. However, conventional testing methods for self-compactability require sampling and this can be extremely labourious if the self-compactability acceptance test is to be carried out for the whole amount of concrete.

Role of Superplasticizer

There is more room for improvement for admixtures such as superplasticizer suitable for self-compacting concrete. In order to achieve this purpose, characterization of materials is indispensable. The requirements for superplasticizer in self-compacting concrete:

• High dispersing effect for low water/powder (cement) ratio: less than approx. 100% by volume
• Maintenance of the dispersing effect for at least two hours after mixing
• Less sensitivity to temperature changes

Segregation-Inhibiting Agent

It has been found that it is possible to manufacture self-compacting concrete with constant quality, especially self-compactability. However, any variation in material characteristics can affect self-compactability. The most influential variant is the water content of fine aggregate, which results in variations in the water content of the concrete itself. To solve this problem, some general construction companies employ a segregation-inhibiting agent. This type of agent is effective in making self-compactability less sensitive to the variation of the water content in the concrete.

Large Scale Construction

Self-compacting concrete is currently being employed in various practical structures in order to shorten the construction period of large-scale constructions. Self-compacting concrete is often employed in concrete products to eliminate vibration noise. This improves the working environment at plants and makes the location of concrete products plants in urban areas possible. In addition, the use of self-compacting concrete extends the lifetime of mould for concrete products. The production of concrete products using self-compacting concrete has been gradually increasing.

Design and Construction Systems

Using self-compacting concrete saves the cost of vibrating compaction and ensures the compaction of the concrete in the structure. However, total construction cost cannot always be reduced, except in large-scale constructions. This is because conventional construction systems are essentially designed based on the assumption that vibrating compaction of concrete is necessary. Self-compacting concrete can greatly improve construction systems previously based on conventional concrete that required vibrating compaction. This sort of compaction, which can easily cause segregation, has been an obstacle to the rationalization of construction work. Once this obstacle is eliminated, concrete construction can be rationalized and a new construction system, including formwork, reinforcement, support and structural design, can be developed.

Since a rational mix-design method and an appropriate acceptance testing method at the job site have both largely been established for self-compacting concrete, the main obstacles for the wide use of self-compacting concrete can be considered to have been solved. The next task is to promote the rapid diffusion of the techniques for the production of self-compacting concrete and its use in construction.

High Density Concrete as a shield²

Any sound concrete, in sections of sufficient thickness, can be used to construct a satisfactory biological shield, and where space is not a consideration, conventional concrete still appears to be the most satisfactory shielding material from the standpoint of cost. However, in many applications, space is a definite consideration. It is for these applications that research on the development and use of high-density concrete has been done. In such cases it is not possible to place the desired amount of normal weight concrete in the given space, hence, to provide adequate shielding, high density concrete is used. Since the ability of a radiation shield to attenuate alpha, beta, gamma, neutron, and proton particles is almost directly proportional to the density of the shield, attempts have been made to develop a concrete of maximum density using various heavy aggregates. High density aggregates are the key ingredient in high density concrete. Heavy density concrete or High Density concrete is concrete that has a density greater than 2400 Kg/cum up to a density of 5600 Kg/cum.

High-density concrete is used extensively in nuclear power plants for radiation shielding against biological hazards. Apart from the basic physical properties (i.e., compressive strength, density, and absorption), knowledge of thermal properties of such concrete is required to access its performance under service conditions. It also forms a good shielding material against both neutrons and gamma radiation due to high density and availability of bound water. Therefore, it is universally used shield material in stationary power reactors. The density of high density concrete is based on the specific gravity of the aggregate and properties of the other components of concrete. The aggregates and other components are based upon the exact application of the high density concrete. Some of the natural minerals used as aggregates in high density concrete are hematite, magnetite, limonite, barite and some of the artificial aggregates include materials like steel punchings & shot. Minerals like bauxite, hydrous iron ore or serpentine, all slightly heavier than normal weight concrete can be used in case high fixed water content is required. It is essential that heavy weight aggregates are inert with respect to alkalis and free of oil and foreign coatings which may have undesired effects on bonding of the paste to the aggregate particles or on cement hydration.

There can be a broad variation in the components used in High Density concrete based on the application. During batching and mixing it is important to avoid over-mixing due to the fragility of certain aggregates. Contamination of the heavy weight aggregates with normal weight aggregates should be avoided by purging all aggregate handling and batching equipment, including pre-mixers and truck mixers.

Accuracy and condition of conveying and scale equipment, aggregate storage and concrete batching bins is especially important, since it is necessary to weigh the aggregates accurately to maintain water cement ratio and check fresh density frequently. Due to the high loads on the mixing equipment it is advisable to avoid overloading the mixers and conveying equipment; and starting and stopping while loading mixers.Forms and pumps must be designed keeping in mind the higher than normal densities of the aggregates and consequently the concrete.

To prevent segregation of coarse aggregates the slump should be kept low and over-vibration avoided. Mortar may be placed in layers of specific thickness over which a fixed quantity of coarse aggregate may be placed and vibrated into the mortar. This is known as puddling. There are no major differences between curing and protection procedures for normal weight and high density concrete.

Characteristics of HD Concrete

It has very good fluidity and pumpability while still maintaining properties of cohesion, strength, density, stability and hence can be poured from inconvenient locations such as heights, small gaps, crevices, ceilings, etc. In the hardened state, it forms a non shrink material and hence crack-free which is very important for radiation proof concrete. It has good bond strength, is compact and very dense and hence the formation of honeycombs is almost next to impossible. This makes it an excellent choice for applications like radiation proof concrete as the formation of honeycombs would render the walls susceptible to leakage of radiation.

Translucent Concrete³

Today we are living in a world where energy expenditure and environmental problems have escalated to global scale. In today’s developed world our built environment takes energy; energy to make the materials that go into the buildings, energy to construct them (Embodied energy) and energy to heat, cool and light them (Operating energy).

Translucent concrete can reduce this operating energy by exploiting vast amount of potential energy in the form of sunlight. Another additional feature is its pleasing aesthetics that can change the image of the concrete which is generally perceived as dull, pale, opaque grey material.

The concrete generally used in construction generally consist of at least cement water and aggregates (fine or coarse). As is known, traditional concrete has a grayish colour, and it's high density prevents the passage of light through it, which means that it is also impossible to distinguish bodies, colours and shapes through it. As can be imagined, concrete with the characteristic of being translucent will permit a better interaction between the construction and its environment, thereby creating ambiences that are better and more naturally lit, at the same time as significantly reducing the expenses of laying and maintenance of the concrete.

With the aim of eliminating these and other drawbacks, thought has been given to the development of a translucent concrete, which concerns a formulation of concrete which, as well as permitting the passage of light through it, also works more efficiently in the mechanical sense than traditional concrete.

The optical fibres used in the formulation of this concrete are basically fine glass or plastic threads that guide the light. The communication system arises from the union between the light sources that are sufficiently pure for not being altered. The types of fibres used are monomode and virgin fibres, in other words, those in the pure state and without any coatings, the aim of which is so that the light can pass through the concrete. Used as additives are: pigments; bridging agents for favouring the attachment to the matrix, giving resistance and protection against aging; lubricant agents for giving surface protection and filmogenic gluing agents for giving integrity, rigidity, protection and impregnation, metal salts, thixotropic agents (flakes of inorganic materials, glass microspheres, calcium carbonates, silicon dioxide, etc.), flame retardant agents (elements containing chlorine, bromine, phosphorus and so on) and UV protection agents (stabilisers). Silica sol, also known as silica hydrosol, is a colloidal solution with a high molecular hydration of silica particles dispersed in water. It can be used as a binding agent. Silica of between 0.5 and 10% by weight of resin has to be used so that, once set, the silica used provides greater resistance and hardness to the concrete. According to the study the mechanical characteristics such as compressive resistance of a translucent concrete with epoxy matrix is up to 220 Mpa.

Characteristics of Translucent Concrete

The mechanical characteristics such as compressive resistance of a translucent concrete with polycarbonate matrix is up to 202 MPa, as well as allowing light to pass through without any distortion at all. The good dispersion of the aggregates, additives and, above all, of the matrix, can be appreciated. The direction of the layers is parallel to the direction of the moulding. It has a laminar drying in the same direction in which it is cast. It displays good crystallisation in the highest parts, and decreases a little when approaching the lower end. The manufacturing process of this concrete consists of the mixture of two processes, one where the cement is mixed with water, and the other where the matrices are mixed.

Permeability Test

For the concrete cubes, the interfacial bonding of the POFs and concrete is a crucial factor in determining ultimate impermeability properties. The chloride diffusion coefficient method (or electric flux method) is used to test the impermeability property of translucent concrete, which can rapidly evaluate the permeability of concrete by measuring the electric energy through concrete.

Applications and Advantages

Thanks to new features this material presents innovative technical solutions, semi-natural and ecological, for the traditional construction problems allowing a wide area of applications in construction, architecture, decoration and even furniture. Some of the possible applications for this new material are spread over several areas creating new possibilities to various products such as:

• Translucent concrete blocks suitable for floors, pavements and load-bearing walls.
• Facades, interior wall cladding and dividing walls based on thin panels.
• Partitions wall and it can be used where the sunlight does not reach properly.
• In furniture for the decorative and aesthetic purpose.
• Light fixtures.
• Light sidewalks at night.
• Increasing visibility in dark subway stations.
• Lighting indoor fire escapes in the event of a power failure.
• Illuminating speed bumps on roadways at night.

The main advantage of these products is that on large scale objects the texture is still visible - while the texture of finer translucent concrete becomes indistinct at distance. When a solid wall is imbued with the ability to transmit light, it means that a home can use fewer lights in their house during daylight hours.

With its good aesthetic enhancing property it can be a good energy saving option in terms of providing lighting to inside areas of buildings and habitat spaces. The main disadvantage is this concrete being its high cost factor due to the presence of fibres and casting of concrete requires extra skilled labours..

A novel architectural material called translucent concrete can be developed by adding optical fibre or large diameter glass fibre in the concrete mixture. The translucent concrete has good light guiding property and the ratio of optical fibre volume to concrete is proportion to transmission. The translucent concrete not loses the strength parameter when compared to regular concrete and also it has very vital property for the aesthetical point of viewIt can be used for the best architectural appearance of the building.

Also used where the light cannot reach with appropriate intensity. This new kind of building material can integrate the concept of green energy saving with the usage self-sensing properties of functional materials.

Ultra High Performance Concrete in Infrastructure⁴

Ultra-high performance concrete (UHPC) is an advanced construction concrete that affords new opportunities for the future of the highway infrastructure. The term UHPC refers to a relatively new class of advanced cementitious composite materials whose mechanical and durability properties far surpass those of conventional concrete. UHPC formulations often consist of a combination of portland cement, fine sand, silica fume, high-range water-reducing admixture (HRWR), fibres (usually steel), and water. Small aggregates are sometimes used, as well as a variety of chemical admixtures. Different combinations of these materials may be used, depending on the application and supplier.

Mechanical Properties of UHPC

The application of heat curing has a significant and immediate impact on the mechanical properties of UHPC. It increases the compressive strength, tensile cracking strength, and modulus of elasticity. It decreases creep and virtually eliminates subsequent shrinkage. These beneficial properties can also be achieved without heat curing. However, the effect is reduced, and it takes a longer time to achieve the beneficial properties.Sufficient information has been published about the mechanical properties of UHPC to establish a range of properties to consider in structural design.

Structural Design and Durability of UHPC

Limited testing under flexural or axial loads indicates that the flexural and axial strengths of UHPC members can be calculated with reasonable accuracy if the stress-strain relationships of UHPC are included in the analyses. However, the calculations are more complex than using the simplified approach of a rectangular compressive stress block and zero tensile strength.

The dense matrix of UHPC prevents deleterious solutions from penetrating into the matrix, and so the mechanisms that can cause conventional concrete to deteriorate are not present. Consequently, durability properties, as measured by permeability tests, freeze-thaw tests, scaling tests, abrasion tests, resistance to ASR, and carbonation, are significantly better than those of conventional concrete. For fire resistance, it appears that a special formulation may be necessary. Primary Characteristics of UHPC that distinguish it from conventional concrete:

• Higher compressive strength.
• Higher tensile strength with ductility.
• Increased durability.
• Higher initial unit cost.

The compressive strength of UHPC makes it an ideal material for use in applications in which compressive stress is the predominant design factor. The ductility in tension allows the tensile strength of UHPC to be considered in both service and strength design for flexure, shear, and torsion. The durability of UHPC makes it an ideal material for use in an outdoor or severe exposure environment. The higher initial unit cost means that its use needs to be optimized for the intended application and that greater attention should be given to life-cycle costs. In addition, specifiers should consider all costs associated with the use of UHPC on a project, not just the material unit cost. In many cases, the use of UHPC may allow a redesign of the structure thus affecting many aspects of the total cost of deploying the structure. For example, the ability to omit shear reinforcement in a beam can result in a savings of both materials and labour that must be considered alongside the increased material costs.

High Performance Concrete⁵

The High Performance Concrete (HPC), which ensures long - time durability in structures exposed to aggressive environments. Durability of concrete is its ability to resist weathering action, chemical attack, abrasion and all other deterioration processes. Weathering includes environmental effects such as exposure to cycles of wetting and drying, heating and cooling, as also freezing and thawing. Chemical deterioration process includes acid attack, expansive chemical attack due to sulphate reaction. Alkali aggregate reaction and corrosion of steel in concrete are due to moisture and chloride ingress. 

HPC has been redefined by American concrete Institute as “Concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices".

The philosophy of HPC concrete design should be strength through durability rather than durability through strength. HPC is required as a construction material in structures constructed in very severe environment. The structures like tunnels in sea beds, tunnels and pipes carrying sewage, offshore piers and platforms, confinement structures for solid and liquid wastes containing toxic chemicals and radioactive elements, jetties and ports, sea link bridge piers and superstructures and high rise buildings, chimneys and towers, foundations and piles in aggressive environment.

Role of Mineral Additives

It is a known fact that given high-quality aggregates the strength and permeability of all concrete mixtures will depend on the physical and chemical properties of cement, the type and dosage of chemical admixtures and mineral additives, the original water- cement ratio and the degree of hydration. Minimum water demand for a given consistency of the cement paste is a primary consideration in the selection of cement for making HPC.

In order to enhance the properties of HPC, the mineral additives should be properly investigated for their performance. The essential factors being fineness, particle size, pozzolanic and or cementitious characteristics, degree of uniform dispersion and curing conditions.

Role of Chemical Admixtures

HPC in addition to Portland cement also consists of mineral additives which are very fine and need to be uniformly dispersed in the concrete mix. Chemical admixtures therefore are mainly used for dispersion of all fine particles in the mix, reduction of water content in HPC while maintaining the desired workability. The other objectives for use being improving consistency,  controlling time of set and providing protection against deterioration by freezing and thawing cycles.

Conclusion

HPC mixes need not be used in all structures. HPC mixes require superior quality materials, methods and supervision besides being costlier at first instance. In important structures HPC mixes are the best answer to improve performance and reduce the maintenance cost.

However, there is no answer which gives clearly a demarcating point as to where do normality of concrete ends and high performance starts. Countries to country even normal concretes are defined differently. From time to time even the definition of normal concrete keeps changing in the same country. It is likely that concrete regarded as HPC today will in future be considered as normal and even HPC will be redefined in future.

Fibre Reinforced Concrete⁶

The usefulness of fibre reinforced concrete (FRC) in various civil engineering applications is indisputable. Fibre reinforced concrete has so far been successfully used in slabs on grade, shotcrete, architectural panels, precast products, offshore structures, structures in seismic regions, thin and thick repairs, crash barriers, footings, hydraulic structures and many other applications. The concerns with the inferior fracture toughness of concrete are alleviated to a large extent by reinforcing it with fibres of various materials. The resulting material with a random distribution of short, discontinuous fibres is termed fibre reinforced concrete (FRC) and is slowly becoming a well accepted mainstream construction material. Significant progress has been made in the last thirty years towards understanding the short and long-term performances of fibre reinforced cementitious materials, and this has resulted in a number of novel and innovative applications.

The characteristics of Fibre reinforced concrete are changed by the alteration of quantities of concretes, fibre substance, geometric configuration especially length, diameter and type of anchorage, dispersion, direction and concentration. Portland cement concrete is believed to be a comparatively brittle substance. When un- reinforced concrete is exposed to tensile stresses, it is likely to fracture and fail. The failure mode is moreover very brittle. After the reinforcement of concrete by steel; it becomes a composite group in which the steel endures the tensile stresses. When concrete is reinforced by using fibre in the mixture, it further increases the tensile strength of composite system. Research has revealed that the ductility of concrete can be improved tremendously by the addition of fibre reinforcement and after cracking of concrete there is post crack tensile capacity available which is not in case of un- reinforced concrete. Since the stretching ability under load of reinforcing fibre is greater than concrete, initially the composite system will function as un- reinforced concrete. However, with additional loading the fibre reinforcement will be activated to hold the concrete mix together.

Generally, there are currently 200,000 metric tons of fibres used for concrete reinforcement. Steel fibre remains the most used fibre of all (50% of total tonnage used) followed by polypropylene (20%), glass (5%) and other fibres (25%)

Performance Characteristics

Concrete carries flaws and micro-cracks both in the material and at the interfaces even before an external load is applied. These defects and micro-cracks emanate from excess water, bleeding, plastic settlement, thermal and shrinkage strains and stress concentrations imposed by external restraints. Under an applied load, distributed micro-cracks propagate coalesce and align themselves to produce macro-cracks. When loads are further increased, conditions of critical crack growth are attained at the tips of the macro-cracks and unstable and catastrophic failure is precipitated.

In FRCs with low to medium volume fraction of fibres, fibres do not enhance the tensile/flexural strength of the composite and benefits of fibre reinforcement are limited to energy absorption or toughness’ enhancement in the post-cracking regime only. For high performance fibre reinforced composites, on the other hand, with a high fibre dosage, benefits of fibre reinforcement are noted in an increased tensile strength, strain-hardening response before localization and enhanced toughness’ beyond crack localization.

Fibre-Matrix Bond

As in any fibre reinforced composite, fibre-matrix bond in FRC is of critical importance. However, unlike fibre reinforced polymers (FRPs) used in aerospace and automobile industries where fibres are employed to enhance strength and elastic modulus, in FRCs, toughness‘ or energy absorption capability is of primary interest. Therefore, inelastic bond failure mechanisms such as interfacial crack growth, crack tortuousity and fibre slip are of greater relevance.

Fibre reinforced concrete is a promising material to be used for sustainable and long-lasting concrete structures. Its performance has already been proven in other hot and arid climates and in other chemically deleterious environments.

Light Weight Concrete

Lightweight concrete can be defined as a type of concrete which includes a expanding agent which increases the volume mixture while giving additional qualities such as nail- ability and lessens the dead weight. It is lighter than the conventional concrete. The use of lightweight concrete has been widely spread across various types of constructions. The main characteristics of lightweight concrete are its low density and low thermal conductivity.

Its advantages are that there is a reduction of dead load, faster building rates in construction and lower haulage and handling costs.

Lightweight concrete maintains its large voids and does not form laitance layers or cement films when placed. However, proper water cement ratio is vital to produce adequate cohesion between cement and water. Insufficient water can cause lack of cohesion between particles, thus loss in strength of concrete. Likewise, too much water can cause cement to run off aggregate to from laitance layers, subsequently weakening in strength.

Concrete that is made with conventional hard rock natural aggregates has a narrow range of density because the specific gravity of most rocks varies a little. In practice, Structural LWC has a density (unit weight) in the order of 1440 to 1840 kg/cum compared to normal weight concrete a density in the range of 2200 to 2600 kg/cum. For structural applications the concrete strength should be greater than 17.0 MPa. Use of LWC with lower density permits construction on ground with a low load-bearing capacity and results in significant benefits in terms of load-bearing elements of smaller cross section and a corresponding reduction in the size of foundations. Furthermore, with lighter concrete, the formwork need withstand a lower pressure than would be the case with normal weight concrete, and also the total mass of materials to be handled is reduced with a consequent increase in productivity.

Light weight concrete can be produced using one of the following three methods:

• By injecting air/gases in its composition using an appropriate equipment or admixture. In this case it is known as - Aerated Concrete
• By omitting the finer aggregates. In this case it is known as No-Fines Concrete and/or
• By replacing normal aggregates with highly porous natural or manufactured aggregates, known as  Light Weight Aggregate Concrete.

Advantages of LWC

• Decreased Dead Load- Less mass is required to support additional weight, Structural reinforcement can be less demanding.
• Higher Seismic (Earthquake) Resistance- In lower densities concrete can actually absorb shock. LWC is often used in ballistic tests because of this ability. Hammer blows can be absorbed without fracturing the concrete.
• More Sound Absorption- The transmission of sound is inversely related to the number of air/solid interfaces. LWC has a high number of these interfaces, thus more sound is absorbed.
• Greater Insulation- Enhanced R-values, especially in the lower density range. Again, this is due to the    increased number of air/solid interfaces.
• Increased Fire Resistance- Greatly improved due to lower thermal conductivity. Spalling (scaling or flake chipping from heat) is reduced or eliminated.
• Adaptability- Lighter weight increases options for onsite casting. Forming can be swifter and easier due to less supported weight.
• Simplicity-Ordinary tools can be used for alterations. It can be easily sawn and sculpted, and nailed or screwed without pre-drilling.
• Handling capabilities are vastly improved- Concrete does not need to be cold, damp, dense, and hard to work with.

LWC Applications

• Screeds and thickening for general purposes especially when such screeds or thickening and weight to floors roofs and other structural members.
• Screeds and walls where timber has to be attached by nailing.
• Casting structural steel to protect it against fire and corrosion or as a covering for architectural   purposes.
• Heat insulation on roofs and screed
• Insulating water pipes.
• Construction of partition walls and panel walls in frame structures.
• Fixing bricks to receive nails from joinery, principally in domestic or domestic type construction.
• Elements with insulation properties-Thermal and sound.

With the advancement of technology and manufacturing abilities, LWC has expanded its uses. Researches in respect of High Performance Cellular Concrete (HPCC) have shown to reach strengths up to 55 MPa. Higher strengths can be produced with the addition of supplementary cementitious materials. Density and strengths can be controlled to meet specific structural and non structural design requirements. There are several benefits for the use of lightweight concrete on roof applications.

Self Healing Concrete

The concerns related to civil infrastructure deterioration are not limited to the economic cost of repair and rehabilitation, but extend to social and environmental costs. While there is little documentation and quantification of the social and environmental costs, it is generally agreed that repeated repairs of civil infrastructure over their service life is decidedly unsustainable.

Over the last decade, the concept of concrete infrastructure able to repair itself without human intervention has emerged as a possible cure for overcoming civil infrastructure deterioration. While the idea remains a novelty in practice, it has attracted a significant amount of attention in the research community.

Many different approaches to functionalizing concrete to possess self-healing ability have been investigated. Given that damage in concrete is dominated by cracks, much attention has been given to self-repair of cracks. A concrete having at least six robustness criteria of long shelf life, pervasive, quality, reliable, versatile, and repeatable can be termed as Self-healing concrete. It is mainly suitable for sustainable infrastructure through reduction of maintenance and repair in the use phase. Five broad categories of self-healing approaches, namely chemical encapsulation, bacterial encapsulation, mineral admixtures, chemical in glass tubing, and intrinsic healing with self-controlled tight crack width, are evaluated against the robustness criteria.

It is suggested that while significant progress has been made over the last decade in laboratory studies, important knowledge gaps must be filled in all categories of self-healing approaches to attain the goal of smart sustainable infrastructures that possess self-repair capability in the field.

Infrastructure Life Cycle Cost

Some direct benefits of concrete self-healing include the reduction of the rate of deterioration, extension of service life, and reduction of repair frequency and cost over the life cycle of a concrete infrastructure. These direct benefits may be expected to lead to enhanced environmental sustainability since fewer repairs implies lower rate of material resource usage and reduction in energy consumption and pollutant emission in material production and transport, as well as that associated with traffic alterations in transportation infrastructure during repair/reconstruction events.

Self-Healing Approaches

Self-healing approaches may be broadly grouped into five categories – chemical encapsulation, bacterial encapsulation, mineral admixtures, chemical in glass tubing, and intrinsic self-healing with self-controlled tight crack width. Related approaches that require human intervention, such as applying heat, are not considered here.

Chemical encapsulation includes all approaches that utilize self-healing chemical agents contained in microcapsules that are dispersed uniformly in the concrete.  The bacteria additive self-healing approach utilizes bacteria that induce precipitation of calcium carbonate as a result of carbonate generation by bacteria metabolism in a high calcium environment. The specific bacteria chosen must be able to withstand the high alkalinity of cement and the internal compressive pressure as microstructure continuously densifies with cement hydration. A nutrient must also be available to feed the bacteria. Mineral admixtures have been deployed as an approach for self-sealing of concrete cracks by reducing the water permeability after concrete damage.

The use of glass tubing for self-healing is based on the concept of self-sensing and actuation when a concrete crack is intercepted by the glass tubing which reacts by fracturing and releasing a repair chemical. Indeed the glass tubing approach may be considered a variant of chemical encapsulation as an alternative form of healing agent delivery approach, with the advantage of potentially carrying a larger amount of healing agent compared with microcapsules.

Conclusion

Self-healing concrete is an area of research that should receive much more attention in the future, if the objective is to realize extended service life and to reduce economic, social and environmental life-cycle costs for civil infrastructure. Given the trajectory of research and progress made over the last decade, self-healing sustainable concrete infrastructure appears to be a realistic expectation in the recent future provided that the six self-healing robustness criteria are met with additional research and validated under field conditions.

Geopolymer Concrete⁷

It is quite widely acknowledged that conventional Portland Cement Concrete, in spite of being the most extensively used construction material, suffers from certain serious property limitations, such as low flexural strength, low failure, low resistance to aggressive chemicals caused by the presence of voids and pores in the structure. These drawbacks are well recognized by the design and construction engineers, who in most situations adopt practical means to ensure the durability of the concrete structure for a given exposure condition.

The development of alkali activated binders with superior engineering properties and longer durability has emerged as an alternative to Portaland Cement. In the near future, Geopolymer or alkali-activated cementitious materials as anew construction material will be to use high performance materials of low environmental impact that are produced at reasonable cost. These materials are inorganic polymers based on alumina and silica units, they are synthesized from a wide range of de-hydroxylated alumina-silicate powders condensed with alkaline silicate in a highly alkaline environment. Using lesser amounts of calcium-based raw materials, lower manufacturing temperature and lower amounts of fuel, result in reduced carbon emissions for Geopolymer cement manufacture up to 22 percent to 72 percent in comparison with Portland cement.

Geopolymer was invented by Devidovits in 1979 as a 3 dimensional alumina silicates while the idea came out of the Great Pyramid Mystery. Davidovits states that supplementary cementing materials which are coal and lignite fly ash, rice husk ash, palm oil fuel ash, other ashes, blast furnace and steel slag, silica fume, limestone, metakaolin, natural pozzolan can produce geo-polymer with mineral and metal resources.

Geopolymeric materials are synthesized by activation of alumina silicate materials with alkaline silicates or hydroxides at ambition to maximum 120°C temperatures. Geopolymers have amorphous to semi crystalline structures with nano particle size depend on curing temperatures. Alumino silicates are used in Geopolymer synthesis are kaolin, metakaolin, fly ash and metallurgic slag such as blast furnace slag and activators solutions are sodium and potassium silicates which can be used with sodium and potassium hydroxides.

Geopolymer Concrete Mix Design

In Geopolymer concrete, aggregates are bound by binder which is composed from two parts including aluminasilicates and alkali solution and named Geopolymer binder. Mix proportioning is based on determining the quantities of the ingredients, when mixed together and cured properly will produce workable concrete that achieves the desired strength and durability when hardened. Therefore different variables including desired workability measured by slump, water to binder ratio, binder content and aggregate proportions should be considered in the mix design procedure.

Casting and Curing

The equipments needed for Geopolymer concrete production are the same as OPC concrete. Usually for casting this type of concrete, fundamental materials such as fly ash and aggregates are mixed and alkaline solution with additives are added to it. Curing at elevated temperature helps the reaction of the paste in Geopolymer concrete.

Carbon Footprint

Two potential advantages of concrete made with alkali activated alumina silicate compared with other binders are its carbon footprint and cost. Increased pressure to improve sustainability within the concrete industry makes these factors very important. The relation between CO2 footprint and cost of Geopolymer concrete and its compositions in comparison with Portland-based cements is roughly quantified.

Environmental Benefits

De-carbonation of lime and calcination of cement clinker release CO2 as a reaction product in OPC concrete while the use of an alkaline hydroxide or silicate activating solution rather than water for cement hydration does reintroduce some CO2. Production of these activators needs temperature similar to de-carbonation of lime in OPC manufacture. The CO2 emission of Geopolymer concrete can be quantified in terms of its compositions.

Conclusion

Geopolymer concretes develop moderate to high mechanical strength with a high modulus of elasticity and shrinkage much lower than with OPC Geopolymer concrete manufacture is liable to reduce CO2 emission from 22.5% to 72.5% compared to OPC production. Geo-polymer concrete can be produced with the same cost of OPC concrete and comparable properties.

Hot Weather Concrete⁸

Using and placing concrete during the hot summer months present far different challenges than during cold weather. The summer month effects of temperature, wind, and air humidity can all have a negative impact on the performance of concrete. For purposes of concrete use and placement, "hot weather" can be defined as any period of high temperature during which special precautions need to be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but critical drying factors such as high winds and dry air can occur at any time, especially in arid or tropical climates.

Higher temperatures cause water to evaporate from the surface of the concrete at a much faster rate and cement hydration occurs more quickly, causing the concrete to stiffen earlier and improving the chances of plastic cracking occurring. Concrete cracking may result from rapid drops in the temperature of the concrete. This occurs when a concrete slab or wall is placed on a very hot day and which is immediately followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for cracking in massive concrete structures. Higher relative humidity tends to reduce the effects of high temperature.

Other hot weather problems include increased water demand, which raises the water-cement ratio and yield lower potential strength, accelerated slump loss that can cause loss of entrained air, fast setting times requiring more rapid finishing or just lost productivity

Related Issues

Increased Water Demand As the ambient temperature increase, so does the concrete temperature. Higher the concrete temperature, higher is the amount of water content required to produce the same slump. Thus the increase in water content will either increase the cement content in the mix in order to maintain the required water binder ratio or increase the dosage of admixture to achieve the desired strength and workability.

Increased Rate of Slump Loss-Higher concrete temperature accelerates the rate of hydration of cement thereby increasing the rate of slump loss. In order to achieve the desired slump, either sufficient retarder should be added at the batching plant or split dosing of admixture is to be done at the placing point. It is reported the approximately 25mm decrease in slump results for each 11 degree Celsius increase in temperature.

Increased Rate of Setting-Higher concrete temperature also increases the rate of setting time. It may be noted that the amount of set retarders required during peak summer will vary against the required during winter time to achieve a certain setting time because of the effect of concrete temperature and suitable adjustment must be made in the dosage of plasticizer and set retarder.

Production of concrete in hot weather

The key to successful hot weather concreting is the recognition of the factors that affect concrete; and planning to minimize their effects. In addition, use proven local recommendations for adjusting concrete proportions, such as the use of water reducing and set retarding admixtures. Modifying the mixture to reduce the heat generated by cement hydration, such as the use of ASTM Type II moderate heat cement and the use of pozzolans and slag can reduce potential problems with high concrete temperature. Advance timing and scheduling to avoid delays in delivery, placing and finishing is essential.

In the case of extreme temperature conditions or with mass concrete, the concrete temperature can be lowered by using chilled water or ice as part of the mixing water. The ready mixed concrete producer uses other measures, such as sprinkling and shading the aggregate prior to mixing, to help lower the temperature of the concrete. If low humidity and high winds are predicted, windbreaks, sunscreens, mist fogging, or evaporation retardants may be needed to avoid plastic shrinkage cracking in slabs.

Placing and Curing

Placing and curing must be carefully planned if good results are to be obtained in hot weather. Planning should be aimed at transporting, placing, consolidating and finishing the concrete at the fastest possible rate. The planning should also take into account that the more rapid rate of slump loss in hot weather places greater strain on vibrating equipment and hence breakdowns may be more frequent. Provision should be made for sufficient number of standby vibrators.

Curing is very important to avoid rapid loss of moisture from the surface. Curing compound is used to protect the concrete from drying till it has reached final set. Wet curing either by making bunds or with hessian cloth or by sprinkler should start immediately after concrete has attained final set. It is a common practice to continue wet curing for at least 10 days under hot arid conditions.

Rules for Hot Weather Concrete

• Modify concrete mix designs as appropriate. Retarders, moderate heat of hydration cement, pozzolanic materials, slag, or other proven local solutions may be used. Reduce the cement content of the mixture as much as possible, while ensuring the concrete strength.
• Have adequate manpower to quickly place, finish and cure the concrete.
• Limit the addition of water at the job site add water only on arrival at the job site to adjust the slump. Water addition should not exceed about 2 to 21 gallons per cubic yard (10 to 12 L/cum). Adding water to concrete that is more than 11 hours old should be avoided.
• Slabs on grade should not be placed directly on polyethylene sheeting or other vapor retarders. Cover the vapor retarder with a minimum 4-inch (100 mm) layer of compactable, easy-to-trim, granular fill material.
• On dry and/or hot days, when conditions are conducive for plastic shrinkage cracking, dampen the subgrade, forms and reinforcement prior to placing concrete, but do not allow excessive water to pond.
• Begin final finishing operations as soon as the water sheen has left the surface; start curing as soon as finishing is completed. Continue curing for at least 3 days; cover the concrete with wet burlap and plastic sheeting to prevent evaporation or use a liquid membrane curing compound, or cure slabs with water. Using white pigmented membrane curing compounds will help by indicating proper coverage and reflecting heat away from the concrete surface.
• Protect test cylinders at the jobsite by shading and preventing evaporation. Field curing boxes with ice or refrigeration may be used to ensure maintaining the required 60 to 80°F (17 to 27°C) for initial curing of cylinders.
• Do not use accelerators unless it is common practice to avoid plastic shrinkage cracking and expedite finishing operations.

Cold Weather Concrete⁹

Concrete can be successfully placed, finished, and cured in cold weather or during the winter, but it requires an understanding of the impact of cold weather on the process of creating long-lasting concrete. Fresh and newly - hardened concrete both lose moisture and heat rapidly in cold-weather conditions. You must protect cold weather concrete against early freezing to ensure the development of the proper strength and how that may impact other construction projects that are waiting for the concrete to set and cure.

Cold weather concreting brings special planning. To achieve a long life concrete product during placed during cold weather the production of aggregates, proper design of mixes, proper mixing and transporting, proper placing and finishing practices and special care in protection must be observed. Concrete can be placed, finished, and cured to its proper strength in cold weather conditions if sufficient planning and care are taken.

Cold Weather Concreting: ACI 306

ACI 306 defines cold weather as "a period when, for more than 3 consecutive days, the following conditions exist:

• The average daily air temperature is less than 40ºF
• The air temperature is not greater than 50ºF for more than one-half of any 24-hour period."

The Cold Facts

Depending on the type of admixture used and the mix design proportion, the freezing temperature of concrete is approximately 28ºF (-2º C). Strength gain is a function of time and temperature. The rate of hydration at 50ºF is approximately one half the rate of concrete with a concrete temperature of 70ºF. As the temperature of concrete approaches 40ºF, the rate of hydration slows almost to the point of stopping.

Some admixtures accelerate the rate of setting but not the rate of strength gain. Others accelerate the rate of setting and the rate of strength gain. The setting time relates to the finishing properties of the concrete while the rate of strength relates to the length of time the concrete must be protected from low temperatures and freezing. All concrete subjected to a wet environment and freezing temperature should be air entrained and should be protected from freezing and thawing until the compressive strength has reached at least 2500 psi. Air entrained concrete that is low in compressive strength can be damaged by exposure to repeated wet freezing and thawing cycles.

Cold Weather Precaution

Concrete should not be placed over frozen substrate. If outdoors, it has been found that covering the substrate with black polyethylene sheets can draw solar heat and thaw the substrate. If indoors and the substrate is heated using fossil fuel heaters, a heat exchanger must be used. The heat exchanger and flue gases must be vented to the outdoors. If the flue gases are allowed to come in contact with freshly placed concrete, carbonation damage will occur. If carbonation damage occurs, the surface of the concrete will be covered with a white powder.

This white powder is chemically identified as calcium carbonate. Severe carbonation can destroy the surface of a concrete floor to a degree requiring the concrete floor to be replaced. In less severe cases of carbonation, the concrete surface may be restored to its intended service life by treating the surface with a chemical hardener or densifier. Concrete should never be placed over frozen reinforcing bars or other metallic objects. This can result in local freezing of the concrete at the interface.

Acid Resistant Concrete¹⁰

Concrete's relative low cost, ease of application and relative long term service life compared to other materials is the main cause of its popularity in the infrastructure sector. The disadvantage of using concrete is that the micro structure of concrete allows the penetration of water and other destructive species that will cause premature failing of the concrete surface.

A permeable concrete will allow infiltration of aggressive agents (chlorides, carbon dioxide and acids) to the steel reinforcement bars causing complete failure of the structure. In general, concrete has a low resistance to chemical attack. The common forms of chemical attack on concrete and the embedded reinforcement in reinforced concrete are chloride attack, sulphate attack, Carbonation due to Carbon dioxide, Alkali-aggregate reactions and acid attack.

The growing threat to concrete structures by acidic attack is a worrying topic of increasing significance all over the world. The sources of acidic media are mainly natural and industrial waters, acidic rains and silage effluents. The acidic vulnerability of the cement-based materials is their universal alkalinity, which makes them ever-willing partners for the interaction with an acidic medium. Therefore, no material based on the current mineral binder can be acid resistant. Therefore, ensuring the desirable life service of concrete structures in acidic media represents a worldwide crucial problem. In general, concrete is susceptible to damage by action of acids. The degradation mechanism involves dissolution of soluble constituents of cement paste destroying its permeability and the concentration and type of acids. Ordinary Portland cement based concretes are more vulnerable to attacks on account of high quantity of calcium hydroxide during hydration of Calcium Silicates. It should be noted that the Binder is damaged by acid attack.

The decomposition of the concrete depends on the porosity of the cement paste, on the concentration of the acid, the solubility of the acid calcium salts (CaX2) and on the fluid transport through the concrete. Insoluble calcium salts may precipitate in the voids and can slow down the attack. Acids such as nitric acid, hydrochloric acid and acetic acid are very aggressive as their calcium salts are readily soluble and removed from the attack front. Other acids such as phosphoric acid and humic acid are less harmful as their calcium salt, due to their low solubility, inhibits the attack by blocking the pathways within the concrete such as interconnected cracks, voids and porosity. Sulphuric acid is very damaging to concrete as it combines an acid attack and a sulphate attack.

Special Concretes For Acid Attack Prevention

The risk of acid attack on concrete can be minimized by providing due consideration to concrete porosity. Lesser the porosity, lesser will be the chances of acid attack on concrete. In general it is said that all high performance concrete mixtures show a better resistance against acid attack than the reference ordinary type concrete. By High performance concrete is meant a concrete with superior qualities including low permeability and low diffusion. In another way, the concrete resistance to acids can also be provided by giving its surface an acid resistant coating.

Silica Concrete

Using fly ash in combination with micro silica fume, results in an improved resistance against acid attack. Fly ash produces a densely packed mixture of cement paste and aggregates, while silica fume reacts pozzolanically and transforms the calcium hydroxide into Calcium-Silicate-Hydrate (CSH) gel in accordance with the equation

SiO2 (solid) + Ca2+ + 2OH- → CaO.SiO2.H2O (CSH gel)

The CSH gel is the source of strength in concrete. On one hand it increases the bond between the cement paste and aggregates and on the other hand, it increases the compressive strength and chemical resistance of the concrete. The additional CSH produced by silica fume is more resistant to attack from aggressive chemicals than the weaker CH.

Air entraining concrete-Air entrainment is the process of incorporating minute air bubbles into concrete. Although, air entrainment is mainly practiced to increase freeze-thaw resistance of the concrete, it can also be used as a means of acid resistance. The air entrainment increases the acid resistance because the air voids block micro capillaries and prevent the acid from invading the concrete through these canals. Air-entraining agents are available as additives as well as admixtures.

High Performance Concrete

High performance concrete (HPC) is a concrete having properties much superior than an ordinary concrete. Along with other properties like strength and durability, it is supposed to have higher resistance against chemical attack. High-performance concretes are made with carefully selected high-quality ingredients and optimized mix design. The concrete is batched, mixed, placed, compacted and cured to the highest standards. Owing to lesser porosity, HPCs also offer significant acid resistance.

Use of Nanotechnology For Acid Attack Prevention

Nanotechnology is a new branch of material sciences that promotes the use of nano particles in different domains. On the other hand, nanotechnology offers quite interestingly certain materials with higher strength while possessing low to medium densities. The examples are carbon nanotubes (CNTs) and carbon nanofibers (CNFs).As an example, it is believed that the design life of a concrete bridge can be increased from 30 to 45-50 years, using nanotechnology.

It is a well-established fact that finer particles (nano) having more specific surface area can fill more effectively the pores in cement matrix, might lead to superior strength due to faster chemical reactions with water (hydration reactions). Or in other words nano materials might enhance the chemical resistance of concrete against acid attack.

With the help of nanotechnology, coatings with a molecular structure which simply rejects adhesion by foreign bodies have been created. Many nano-coatings are in the market, which effectively provide effective resistance against acid attack.

Pervious Concrete¹¹

Concern has been growing in recent years towards reducing the pollutants in water supplies and the environment. In the 1960s, engineers realized that runoff from developed real estate had the potential to pollute surface and groundwater supplies. Further, as land is developed, runoff leaves the site in higher rates and volumes, leading to downstream flooding and bank erosion. In this scenario, Pervious Concrete comes as a solution to reduce the impact of development by reducing runoff rates and protecting water supplies. Pervious concrete is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass through it, thereby reducing the runoff from a site and recharging ground water levels.

The high porosity is attained by a highly interconnected void content. Typically pervious concrete has little to no-fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids. Pervious concrete is traditionally used in parking areas, areas with light traffic, pedestrian walkways, and greenhouses. It is an important application for sustainable construction.

Pervious concrete also contributes to enhanced air quality by lowering atmospheric heating through lighter color and lower density, decreasing the impact of heat island effects. The heat island effect occurs when tree-covered areas are replaced with dark pavement surfaces, and is characterized by up to a 12-degree average temperature increase between an urban area and its surrounding countryside. This heat island effect increases ground level ozone production by as much as 30%.

Pervious concrete also naturally filters storm water and can reduce pollutant loads entering into streams, ponds and rivers. Pervious concrete functions like a storm water retention basin and allows the storm water to infiltrate the soil over a large area, thus facilitating recharge of precious groundwater supplies locally. All of these benefits lead to more effective land use.

Pervious concrete can also reduce the impact of development on trees. A pervious concrete pavement allows the transfer of both water and air to root systems allowing trees to flourish even in highly developed areas.

Pervious concrete also know as porous, gap-graded, permeable  or enhanced porosity concrete, mainly consists of normal Portland cement, uniform-sized coarse aggregate and water. This combination forms an agglomeration of coarse aggregates surrounded by a thin layer of hardened cement paste at their points of contact. This configuration produces large voids between the coarse aggregate, which allows water to permeate at a much higher rate than conventional concrete.

Percolation Rate

One of the most important features of pervious concrete is its ability to percolate water through the matrix. The percolation rate of pervious concrete is directly related to the air void content. Test have shown that a minimum air void content. Tests have shown that a minimum air void content of approximately 15 percent is required to achieve significant percolation. Because the percolation rate increases as air void content increases and consequently, compressive strength decreases, the challenge in pervious concrete mixture proportioning is achieving a balance between an acceptable percolation rate and an acceptable compressive strength.

Test and Inspection

Pervious concrete can be designed to attain a compressive strength ranging from 400 psi to 4000 psi (2.8 to 28 MPa) though strengths of 600 psi to 1500 psi (2.8 to 10 MPa) are more common. Pervious concrete, however, is not specified or accepted based on strength. More important to the success of a pervious pavement is the void content. Acceptance is typically based on the density (unit weight) of the in-place pavement.

Pervious concrete has been successfully used for low volume streets, driveways, sidewalks, golf cart paths, retaining walls, slope protection, and French drains. Pervious concrete can be utilized in a variety of paving applications to provide hardscape without altering hydrology of the land.

Benefits of Pervious Concrete

• Reduces storm water runoff
• Eliminates the need for detention ponds and other costly storm water management practices
• Replenishes water tables and aquifers
• Allows for more efficient land development
• Minimizes flash flooding and standing water
• Prevents warm and polluted water from entering our streams
• Mitigates surface pollutants

Recycled Aggregate Concrete¹²

Demolition of old and deteriorated buildings and traffic infrastructure, and their substitution with new ones, is a frequent phenomenon today in a large part of the world. The main reasons for this situation are changes of purpose, structural deterioration, rearrangement of a city, expansion of traffic directions and increasing traffic load, natural disasters (earthquake, fire and flood), etc.

The most common method of managing this material has been through its disposal in landfills. In this way, huge deposits of construction waste are created, consequently becoming a special problem of human environment pollution. On the other hand, production and utilization of concrete is rapidly increasing, which results in increased consumption of natural aggregate as the largest concrete component.

A possible solution to these problems is to recycle demolished concrete and produce an alternative aggregate for structural concrete in this way. The reuse of hardened concrete as aggregate is a proven technology - it can be crushed and reused as a partial replacement for natural aggregate in new concrete construction. The hardened concrete can be sourced either from the demolition of concrete structures at the end of their life – recycled concrete aggregate, or from leftover fresh concrete which is purposefully left to harden – leftover concrete aggregate.

Alternatively fresh concrete which is leftover or surplus to site requirements can be recovered by separating out the wet fines fraction and the coarse aggregate for reuse in concrete manufacture – recovered concrete aggregate. Additionally, waste materials from other industries such as crushed glass can be used as secondary aggregates in concrete. All these processes avoid dumping to landfill whilst conserving natural aggregate resources, and are a better environmental option.

Recycled concrete aggregate (RCA) is generally produced by two-stage crushing of demolished concrete, and screening and removal of contaminants such as reinforcement, paper, wood, plastics and gypsum. Concrete made with such recycled concrete aggregate is called recycled aggregate concrete (RAC). Through recycling, the waste concrete can be converted into a resource. Aggregates obtained by recycling of demolished concrete RAC are mainly used as aggregates in granular base or sub-base applications, for embankment construction and earth construction works.

Recycling or recovering concrete materials has two main advantages - it conserves the use of natural aggregate and the associated environmental costs of exploitation and transportation, and it preserves the use of landfill for materials which cannot be recycled. At the same time, crushed concrete can be used as a sub-base material for pavements and civil engineering projects.

Technology of RAC production is different from the production procedure for concrete with natural aggregate. Because of the attached mortar, recycled aggregate has significantly higher water absorption than natural aggregate. Therefore, to obtain the desired workability of RAC it is necessary to add a certain amount of water to saturate recycled aggregate before or during mixing, if no water-reducing admixture is applied. One option is to first saturate recycled aggregate to the condition ?water saturated surface dry?, and the other is to use dried recycled aggregate and to add the additional water quantity during mixing. The additional water quantity is calculated on the basis of recycled aggregate water absorption in prescribed time.

Recycled Aggregate

The use of crushed aggregate from either demolition concrete or from hardened leftover concrete can be regarded as an alternative coarse aggregate, typically blended with natural coarse aggregate for use in new concrete. The use of 100% recycled coarse aggregate in concrete, unless carefully managed and controlled, is likely to have a negative influence on most concrete properties – compressive strength, modulus of elasticity, shrinkage and creep, particularly for higher strength concrete. Also the use of fine recycled aggregate below 2 mm is uncommon in recycled aggregate concrete because of the high water demand of the fine material smaller than 150 µm, which lowers the strength and increases the concrete shrinkage significantly. Nevertheless, recycled aggregate concrete can be manufactured using recycled aggregate at 100% coarse aggregate replacement where the parent concrete, the processing of the recycled aggregate and the manufacture of the recycled aggregate concrete are all closely controlled. However as target strengths increase, the recycled aggregate can limit the strength, requiring a reduction in recycled aggregate replacement.

Processing Hardened Concrete

Processing of hardened concrete refers both to crushing demolition concrete and to crushing leftover concrete. However, recycled concrete aggregate is different from leftover concrete aggregate, as demolition concrete tends to have a higher level of contamination (chlorides, oils etc.). Moreover leftover concrete will generally be crushed at an earlier age so leftover concrete aggregate will have less adhered mortar than recycled concrete aggregate. The presence of adhered mortar on the surface of crushed concrete aggregate generally degrades the quality of the recycled aggregate and consequently the fresh and hardened properties of concrete made from it.

Recycled fine aggregate is rarely used in recycled aggregate concrete because of the increase in water demand and effect on compressive strength and shrinkage. Control of the amount of material passing the 150 µm sieve is an issue. Also it is difficult to accurately determine the absorption, saturated surface dry (SSD) condition and free water content of such fine material. Depending on the age of crushing, recycled fine aggregate retains some cementitious capacity, which can be desirable in low strength concrete applications such as trench fill.

Characteristics of RA

The variability in the physical properties of recycled aggregate will depend on the variability of the parent concrete. Recycled concrete aggregate which has been sourced from a number of demolition concretes will have greater variability than recycled concrete aggregate from one demolition concrete source, and this is likely to have an effect on the uniformity of the recycled aggregate concrete.

Chemical Properties: Establishing chemical properties of recycled concrete aggregate is important because the history of the demolition concrete is unlikely to be known. For leftover concrete, because the properties of the parent concrete correspond to the properties of natural aggregate processed by a particular ready mixed concrete plant, there is less uncertainty about the chlorides, sulphates and alkali present than for recycled aggregate concrete. Therefore contaminants are not the issue for leftover concrete aggregate that they are for recycled concrete aggregate.

Chlorides: The recycled concrete aggregate is a potential source of chloride which must be added to other chloride sources when checking for compliance requirements for the maximum allowable chloride levels in concrete.

Workability of RAC

Recycled aggregate concrete made from crushed leftover concrete may in general require a small cement adjustment to compensate for increase in water demand. Concrete made from demolition concrete, which generally has a harsher texture from the increased adhered mortar will have an even higher water demand. 

Increased cement contents are more likely to be necessary for higher percentages of recycled aggregate replacement and for higher specified strengths of the recycled aggregate concrete. Adjustment to air entraining and plasticizing admixtures will assist in minimizing any increase in cement content.

-Vinutha

Acknowledgement: This article is not based on any original research. The author has compiled information from various sources besides narration of experiences by experts of the industry. The material compiled from various references is duly acknowledged.

References

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3. Translucent Concrete by Soumyajit Paul, Avik Dutta Department of Civil Engineering, SRM University, Kattankulathur Chennai, Tamil Nadu
4. Ultra-High Performance Concrete: A State-of-the-Art Report for the Bridge Community Research, Development, and Technology Turner-Fairbank Highway Research Centre 6300 Georgetown Pike McLean, VA 22101-2296
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