Concrete in the times of the Romans
Concrete technology has come a long way since the Romans discovered the material, with a number of ingredients, which include a host of mineral and chemical admixtures, besides of course, the Portland cement, aggregates (coarse and fine), and water. These ingredients should be precisely determined, properly mixed, carefully placed, vibrated (not required in self compacting concretes), and cured properly so that the desired structural properties are obtained; they should also be inspected at regular intervals and maintained adequately till their intended life.
Though Roman concrete bears little resemblance to modern Portland cement concrete, several concrete structures by built Romans are still in existence, the famous being the Pantheon, built in AD 126 (Subramanian, 2019). Similarly, some old concrete structures in India (for example, the Gateway of India, Marine Drive, Central Railway station building in Mumbai; Dum Dum and Coronation Bridges in West Bengal, Power Station in Ahmadabad) are in good condition even after a long life span of more than 70-100 years.
However, several structures constructed recently have shown severe signs of premature deterioration. In some of the industrially developed countries, over 40% of total resources of the building industries are currently spent on repairs and maintenance. It is because of the non-consideration of durability considerations (Subramanian and Kulkarni, 2021). Hence, in order to reduce the consumption of concrete (especially in columns of tall buildings and in bridges) and to increase its durability, higher and higher strength of concrete and steel are being specified and used.
High Strength Concrete
Concrete is the world’s most consumed material after water. According to the Global Cement and Concrete Association (GCCA), during 2020, about 14 billion m3 of concrete and 4.1 billion tonnes of cement were consumed. Although the basic ingredients of concrete, i.e. cement, aggregates and water, are still used, a number of other waste and industrial byproducts such as fly ash, GGBS, calcined clay, silica fume, etc., and a host of chemical admixtures are now employed. In addition, the concrete used 50 years ago with strength of 15 MPa is not used now. The minimum strength of concrete to be used in reinforced concrete structures has increased, depending on its exposure to the environment. This was due to the deterioration of concrete structures all over the world. Moreover, the construction of super tall buildings, with heights reaching about 1 km, and large span structures with spans exceeding 15 m, necessitated using high-strength concrete.
These developments resulted in high-strength and high-performance concrete having strength up to 100 MPa. High-strength deformed rebars of strength up to 827 MPa have also been developed and approved for use by ACI 318-19. Higher strength of concrete is always associated with reduced ductility and tensile strength. Hence, a new class of cementitious material known as ultra-high-performance concrete (UHPC) has been developed, which offers superior tensile and durability characteristics compared to other fiber-reinforced composites. Devoid of coarse aggregates and composed of a dense cementitious matrix with a disconnected pore network, and reinforced with high-strength steel fibers, UHPC is characterized by very low permeability, high compressive strength (exceeding 120 MPa), and sustained tensile resistance. UHPC also results in sustainable constructions.
Strength of Concrete During 1970s
For several years, the concrete industry used the basic recipe: cement, aggregates (coarse and fine aggregates) and water. A few additives and admixtures were used to improve the properties of concrete. The first use of a chemical admixture in concrete occurred in 1934, when a road was built using blended cement containing about 85% of Portland cement and 15% of Rosendale natural cement. This was due to the efforts of the engineer Bertrand H. Wait of USA. In 1937, New York State made this blended cement as the standard for highway construction, which was followed soon by other states in the USA (Jackson and Timms, 1954). Over the decades, admixtures have evolved from primitive organic ingredients into synthetic lab developed compounds that have more consistent and controllable properties.
The minimum grade of concrete to be used in construction during this time was M15. This was achieved with OPC 33 grade cement with a water-cement ratio of 0.50 to 0.55. Mostly nominal mixes were only used (For example, cement: sand: aggregate ratio of 1:2:4 was used for M15 and 1:1.5:3 for M20 concrete). It was not difficult to get the specified compressive strength using these nominal mixes. The 1978 version of IS 456 specified grades from M10 to M40 only. Of course for durability, the concrete should be properly mixed, placed and compacted. Plain mild-steel rods of strength 250 MPa were used along with the M15 Concrete, although high-strength deformed bars were introduced during 1967 and mostly replaced the mild steel bars by 1975. Several buildings built with this combination of concrete and steel are still performing well. Examples of two buildings, one in Chennai and the other in Mumbai are shown in Fig.1. These two were considered as the tallest buildings in India when they were built.
The relationship between the water-cement ratio and the compressive strength of concrete was realized by Duff Abrams as early as 1919, who proposed the following equation:
fc = A / Bw/c (1)
Where fc is the strength of concrete and w/c is the water cement ratio by volume. For the 28 day strength, the constants are A and B are 124.45 MPa and 14.36 (ACI 211.1-91).
Strength of Concrete After 2000
In the 2000 version of IS 456, durability of concrete, in addition to its strength was given importance. As per Table 5 of IS 456:2000, the minimum grade of concrete to be adopted in reinforced concrete structures is M20, which has to be increased to M25 for moderate exposure, to M30 for severe exposure, to M35 for very severe exposure and to M40 for extreme exposure. Depending on the exposure condition, the maximum water-cement ratio (varying from 0.40 to 0.55) and minimum cement content (varying from 300 kg/m3 to 360 kg/m3) are also specified in Table 5. This version of IS 456 specified grades from M10 to M100, designating M10-M20 concrete as ordinary concrete, M25-M60 as standard concrete, and M60-M100 as high-strength concrete.
However, it has to be noted that the provisions in this version of the code are based on experiments conducted on concrete up to M40 and hence strictly applicable up to M40 concrete only. IS 456:2000 allows the use of different types and grades of cement such as 33 grade, 43 grade, and 53 grade. It may be of interest to note that the cement manufacturers are making available only the 53 grade cement. Though grade 53 cement offers higher strength at early ages, it may result in higher heat of hydration and hence the concrete should be properly cured. It is also advisable to use fly ash or GGBS in the mix.
Mix Design of Different Concrete Strengths
The first concrete mix proportioning code, IS 10262, was released only in 1982. A graph of water-cement ratio verses 28-day compressive strength was provided in the 1982 edition of this code. Another graph was also provided which showed the relationship between free water-cement ratio and compressive strength for 6 different cement strengths. A mix design procedure was provided for concrete up to M60 strength.
The first revision of the code IS 10262 occurred in 2009. In this revision, the requirements for selection of water-cement ratio, water content, and estimation of coarse aggregate and fine aggregate content were modified, as well as the requirements for trial mixes and illustrative examples. The applicability was still for ordinary and standard concretes only. This Standard was again revised in 2019, incorporating the mix proportioning for high-strength concrete for M65 or above (up to target strength of M 100).
As the shortage of natural fine and coarse aggregates was felt in many parts of India, it also included aggregates from other than natural sources like manufactured sand, iron/slag aggregates, recycled aggregates, etc., as per IS 383:2016. A graph of water-cement ratio versus 28 day strength of concrete has been introduced for different grades and types of cement. Examples of mix proportioning for PPC, OPC with Fly ash, OPC with GGBS, high-strength concrete and self-compacting concrete have been provided.
High-Strength Steel Reinforcement
Deformed high-strength rebars were becoming popular throughout the world by the end of the Second World War. Cold twisted and hot rolled deformed rebars with proof strength of about 420 N/mm2 were starting to be widely used. In India, Tor Isteg Steel Corporation of Luxembourg introduced ‘cold twisted deformed circular rebars’ of proof stress 420 N/mm2, during 1967, which became popular as Tor 40 rebars. By about 1970, these Tor 40 rebars (grade Fe 415) replaced about 50 % of plain mild steel rebars in India (Viswanatha, 2004). Tor 50 and Tiscon 50 (grade Fe 500) were introduced around 1978. Tata Steel introduced TMT bars, produced using through Tempcore technology during 1992. It was followed by SAIL through Thermex technology, and several others also started producing these TMT rebars. Torkari rebars, conforming to IS 1786, with strength of 550 N/mm2 were introduced in India by Torsteel Research Foundation in India (TRFI) around 1987.
It has to be noted that the grade Fe 415 steel rebars are not readily available in India at present, and only higher strength bars are available. IS 1786:2008 also included specifications for Fe 415D, Fe 500D, Fe 550D and Fe 600 rebars (the letter D denoting ductile). High-strength MMFX (also known as ChromX) steel bars, conforming to ASTM A1035, have been introduced in the USA recently, which are also corrosion resistant, similar to TMT CRS bars. They are available in two strength grades (yield strength of 689 MPa and 827 MPa) and three chromium levels (Type CL, CM, and CS-with 2, 4 and 9 per cent chromium). The CM grade bars could guarantee 40 to 60 years and the CS grade up to 100 years of service life (www.cmc.com). They have been included in clause 126.96.36.199 of ACI 318-19.
High-Strength/High Performance Concrete (HPC)
High-strength concrete (HSC), with strengths up to 100 MPa, is being increasingly used in India and many parts of the world such as the USA, Canada, Europe, and Australia, especially for the columns of high-rise buildings and in long-span bridge structures. The use of high-strength concrete in columns results not only in economy but also in reduced column sections and consequent increase in carpet area of buildings. As per Table 2 of IS 456:2000, any concrete that has a characteristic (28th day) compressive strength of 65 to 100 MPa (Grade M65-M100) is considered as high-strength concrete (HSC).
In HSC, the aggregate plays an important role on the strength of concrete. HSC is often produced with high-quality Portland cement (with cement content in the range of 400 to 450 kg/m3), good quality aggregates (the maximum size of coarse aggregate restricted to about 12.5mm and with fine aggregates having fineness modulus of about 3.0), restricting the w/cm ratio (preferably less than 0.3), using chemical admixtures, such as super-plasticizer (to obtain the required workability), and strengthening transition zone between aggregates and cement paste by the partial replacement of cement with mineral admixtures, such as GGBS, fly ash, or silica fume.
High performance concrete (HPC) may be defined as any concrete that provides enhanced performance characteristics for a given application. It is difficult to provide a unique definition of HPC, without considering the performance requirements of the intended use. American Concrete Association (ACI) has adopted the following broad definition of HPC:‘A concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices.
The requirements may involve enhancements of characteristics such as easy placement, compaction without segregation, long-term mechanical properties, early-age strength, permeability, density, heat of hydration, toughness, volume stability, and long service life in severe environments’ (ACI 363R-10). Table 1 lists a few of these characteristics. Concrete possessing many of these characteristics often achieve higher strength (HPC have usually strengths greater than 50 to 60 MPa). Therefore, HPC will often have high-strength, but a HSC need not necessarily be called as HPC.
|Table 1 Desired characteristics of HPC|
|Property||Criteria that may be specified|
|High-strength||70 to 140 MPa at 28 to 91 days|
|High-early compressive strength||20 to 28 MPa at 3 to 12 h or 1 to 3 days|
|High-early flexural strength||2 to 4 MPa at 3 to 12 h or 1 to 3 days|
|High modulus of elasticity||More than 40 GPa|
|Abrasion resistance||0 to 1 mm depth of wear|
|Low permeability||500 to 2000 coulombs|
|Chloride penetration||Less than 0.07% Cl at 6 months|
|Sulphate attack||0.10% /0.5 % max. expansion at 6 months for moderate/severe sulphate exposures|
|Low absorption||2 to 5%|
|Low diffusion coefficient||1000 × 10–14 m/s|
|Resistance to chemical attack||No deterioration after 1 year|
|Low shrinkage||Shrinkage strain less than 0.04% in 90 days|
|Low creep||Less than normal concrete|
Super-plasticizers are usually used to make these concrete fluid and workable. Note that without super-plasticizers, the w/cm cannot be reduced below a value of about 0.40. Typically, 5 to 15 L/m3 of super-plasticizers can effectively replace 45 to 75 L/m3 of water (Aϊtcin and Neville, 1993).This drastic reduction in mixing water reduces the distance between cement particles, resulting in much denser cement matrix than normal strength concrete (NSC).
With the currently available cement and super-plasticizers, and the present-day mixing, placing, and curing practices, the optimum value of w/cm is about 0.22; values lower than this are harmful, because an adequately high density of the cement matrix cannot be achieved (Aϊtcin, 1998). Since mixing water reacts chemically with cement and is lost by self-desiccation, the resulting cement paste in HPC has a very low porosity. This high-density matrix, in addition to the chemical bonds created by the hydrates (which also exist in NSC), results in high compressive strengths (Aϊtcin and Neville, 1993).
Studies have shown that 35% of cement paste by volume in HPC represents an optimum solution in balancing the conflicting requirements of strength, workability, and dimensional stability. Note that despite the use of low w/cm, HPC may require air entrainment for protection against repeated cycles of freezing and thawing. Though some cement mixes of HPC may have silica fume, which is about 10 times costlier than cement, it is not an essential ingredient of HPC, though it makes it easier to attain high-strengths above 60 MPa (Neville, 2012). The quantity of silica fume necessary to achieve very high-strength is about 10% of the volume of Portland cement. The optimum particle-packing mixture design approach may be used to develop a workable, and highly durable design mixture (with cement content less than 300 kg/m3), having compressive strength of 70–80 MPa (Kumar and Santhanam, 2004).
In the design of HPC mixes, one needs to consider the cement-super-plasticizer compatibility, as their physical and chemical interaction is very complex and may result in rapid slump loss. The controlling factor is the solubility of SO3 (which has a maximum content of 3.0 to 3.5% depending on the content of C3A) in the given cement. Ideal cement for HPC from rheological point of view may be the one which is not too fine, has a low C3A content, rich in belite(C2S) content, moderate alite (C3S) content, low alkali content (<0.6%), and with as little interstitial phase content as possible (i.e., C3A + C4AF less than 10%, with C3A < 3% and C4AF < 7%), and should have high-strength– OPC 53 grade is commonly chosen (Rheology is the science of the deformation and flow of materials).
The behaviour of super-plasticizer is a function of its structure and the degree of polymerization. Polycarboxylates and acrylic copolymers are the most effective of all the chemicals used in concrete (IS 9103:1999). An ideal super-plasticizer should consist of rather long molecular chains. In the absence of guidelines, the final selection of the cement and the super-plasticizer should be made based on trial concrete mixes. The cement substitutes, such as fly ash, slag, and silica fume, refine the pore structure and block the capillaries due to ‘secondary hydration’ process (Mullick, 2005).
As the crushing process takes place along any potential zones of weakness within the parent rock, and thus removes them, smaller particles of coarse aggregates are likely to be stronger than the large ones. Hence, for strengths in excess of 100 MPa, the maximum size of aggregates should be limited to 10 to 12 mm; for lesser strengths, 20 mm aggregates can be used (Aϊtcin and Neville, 1993). Strong and clean crushed aggregates from fine-grained rocks, mostly cubic in shape, with a minimum of flaky and elongated shapes, are suitable for HPC.
In order to have good packing of the fine particles in the mixture, as the cement content increases, the fine aggregates should be coarsely graded and have fineness modulus of 2.7 to 3.0. The development of ready-mixed concrete (RMC) plants in India has ensured better quality of high-strength/performance concrete, since it is properly designed and mixed in these plants with strict computerized controls on the entire production process. The RMC industry in India is reportedly growing at the rate of about 20% and has a market share of about 15-20%. In developed countries like the USA, however, the market share of RMC is around 75% (Subramanian and Kulkarni, 2021).
As the HPC has very low water content, there will not be any bleeding, leading to plastic shrinkage cracking (However, the drying shrinkage will be minimum due to the low w/cm ratio). Note that hydration of cement in HPC is very rapid. The silica fume, if used, also reacts very early, rapidly using up the available water and contributing to self-desiccation. This causes autogenous shrinkage, resulting in micro-cracking throughout the concrete mass. Hence, it is important to effectively cure HPC, as early as possible. Membrane curing is not suitable for HPC, and hence fogging or wet curing should be adopted to control plastic and autogenous shrinkage cracking.
High-performance concrete has been primarily used in tunnels, bridges, pipes carrying sewage, offshore structures, tall buildings, chimneys, and foundations, and piles in aggressive environments for its strength, durability, and high modulus of elasticity. It has also been used in shotcrete repair, poles, parking garages, and agricultural applications. Note that in severe fires, HPC results in bursting of the cement paste and spalling of concrete. HPC, with strength exceeding 100 MPa, has been used in the 451.9 m tall, 88-storey Petronas twin towers in Kuala Lumpur, Malaysia.
In the world’s tallest (828 m tall) Burj Khalifa, in Dubai, United Arab Emirates, the walls and columns were made of C80 to C60 cube strength concrete using Portland cement, fly ash, and local aggregates. In the Rs. 16 billion ($19.2 million) prestressed concrete Bandra–Worli Sea link project, M60 concrete was used. In many tall buildings in India concrete of grade up to M80 has been used in the columns. However, it is not beneficial to use HSC in the slabs of these high-rise buildings, as normal strength concrete (NSC), with strengths in the range of 20-30 MPa are adequate to meet the flexural requirements imposed in the codes of practices. Moreover, it is extremely uneconomical to use HSC in slabs. Hence, NSC or even lightweight aggregate concrete (LWAC) slabs are usually employed in these high-rise buildings.
Ultra-High-Performance Concrete (UHPC)
Ultra-high-performance concrete (UHPC) is a high-strength, high-stiffness, self-consolidating, and ductile material formulated by combining Portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water, and steel or organic fibres. Originally, it was developed by the Laboratoire Central des Pontset Chaussées (LCPC), France, containing a mixture of short and long metal fibres, and known as Multi-scale FRC (Rossi, 2001). Typical composition of UPHC is shown in Table 2 (note that there are no coarse aggregates). A low w/cm ratio of about 0.2 is used in UHPC compared to about 0.4 to 0.5 in NSC.
The material provides compressive strengths of 120 to 240 MPa, flexural strengths of 15 to 50 MPa, post-cracking tensile strength of 7.0 to 10.3 MPa, and has a modulus of elasticity from 45 to 59 GPa. Ductal® (Lafarge, France), CoreTUFF® (US Army Corps of Engineers), BSI®, Densit® (Denmark), Ceracem® (France and Switzerland), are some of the examples of commercial products. The enhanced strength and durability properties of UHPC are mainly due to optimized particle gradation that produces a very tightly packed mix, use of steel fibres, and extremely low water-to-powder ratio.
|Table 2 Typical composition of UHPC (Extracted from Graybeal and Davis, 2008)|
|Material||Amount (kg/m3)||% by weight|
|Super-plasticizer (High range water reducing admixture)||30||1.2|
The 15 m span Shepherds Creek Road Bridge, NSW, Australia, built in 2005, is the world’s first UHPC Bridge for normal highway traffic. Since then, a number of bridges, and other structures have been built utilizing UHPC all over the world (FHWA Report FHWA-HRT-13-060, 2013 provides details of many such structures.). Fig. 2 shows the first UHPC Bridge constructed in the United States, at Wapello County, Iowa, and the Sakata-Mirai Bridge, Sakata, Japan. The bridge at Wapello County, Iowa is a simple single-span bridge having three 33.5 m long precast, prestressed concrete, 1.14 m deep modified Iowa bulb-tee beams topped with a cast-in-place concrete bridge deck. Each beam contained forty-seven 15.2 mm diameter, low-relaxation prestressing strands and no shear reinforcement (FHWA Report FHWA-HRT-13-060, 2013).
The materials for UHPC are usually supplied by the manufacturers in a three-component premix: powders (Portland cement, silica fume, quartz flour, and fine silica sand) pre-blended in bulk-bags; super-plasticizers; and organic fibres. Care should be exercised during mixing, placing, and curing. The ductile nature of this material makes concrete to deform and support flexural and tensile loads, even after initial cracking. The superior durability characteristics are due to the combination of fine powders with a small grain size (maximum 600 µm) and chemical reactivity. The net effect is a maximum compactness and a small, disconnected pore structure.
The use of this material for construction is simplified by the elimination of reinforcing steel and its ability to be virtually self-placing. More details about UHPC may be found in Schmidt et al. (2004), Fehling et al. (2008), and Schmidt et al. (2012). A comparison of stress-strain curves concrete is provided in Fig. 3, and another comparison based on strength, is provided in Table 3.
|Table 3 Comparison of concrete based on strength (Farny and Panarese, 1994)|
|Parameter||Conventional concrete||High-strength concrete||Very-high-strength concrete||Ultra-high-strength concrete|
|Mineral admixtures||Not-necessary||Flyash||Silica fume2||Silica fume2|
|Freeze-thaw protection||Needs air entrainment||Needs air entrainment||Needs air entrainment||No freezable water|
1WRA= Water reducing admixture, HRWR- High range water reducer
2May also contain fly ash
Sustainability of UHPC
The UHPC is a very durable product; hence, the structures that use it will have a longer service life and require less maintenance than structures built with conventional concrete. In addition, UHPC will have greater frost and deicing salt resistance, a lower rate of carbonation, and greater chloride resistance than conventional concrete. Although the UHPC may have higher initial costs, life cycle cost analysis conducted by researchers in Germany, on two replacement methods for the Eder Bridge in Felsberg, found that the life cycle cost over 100 years would be less for the UHPC Bridge (FHWA-HRT-13–060, 2013).
Another study in Germany concluded that the environmental impact of structures made with state-of-the-art UHPC may be up to 2.5 times greater than with conventional concrete. However, it was felt that the environmental impact could be decreased by reducing the amount of Portland cement, steel fibres, and high-range water-reducing admixtures in the UHPC.
Summary and Conclusion
Traditionally, the concrete industry used the same basic recipe: water, aggregates and cement. In the early days, minimum the specified strength was only 15 N/mm2, and hence the water-cement ratio was not given any importance. The deterioration of RC structures all over the world resulted in the codes increasing the minimum concrete strength, depending on the environment in which the structure is built. Moreover, the construction of tall buildings and large span structures and bridges, necessitated the use of still higher strength concrete.
In order to have better performance, specifications were developed for high-performance concrete, which is often a high-strength concrete. In order to have more strength and at the same time more ductility and tensile strength, ultra-high strength concrete has been invented and used in several applications. The use of HPC and UHPC also supports sustainability.
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