LITERATURE REVIEW
2.0 INTRODUCTION
This chapter presents the review of related literature on making high concrete strength from granulated calcined clay. Views and opinions of other authors will be presented as follows.
2.1 CALCINED CLAY
The main source of such material is gravel which is obtained from river beds. Moreover, the extraction of gravel from river beds has caused siltation of rivers, a negative environmental impact. In order to find a more economically viable and environmental friendly material for coarse aggregate in concrete, it is worthwhile to study the viability of synthetic aggregates such as, calcined clay.
The term synthetic aggregate or Calcined Clay, refers to a material obtained from the processing of a soil or clayey material with satisfactory mechanical strength for a particular purpose. These characteristics are usually obtained by heating the ceramic body at high temperatures, above 760°C. The quality depends crucially on the ceramic raw material, firing temperature, and process of ceramic mass conformation. In some cases, certain properties of ceramic products can be improved by using soils with a higher percentage of flux elements.
Calcined clays consisting of single clay minerals e.g. kaolinite, montmorillonite and illite have been in focus of many investigations as they exhibit pozzolanic reactivity and may serve as type II additions to concrete. Most researchers used powder samples of more or less pure clays for their studies. Normally, the powder samples were calcined carefully in a laboratory furnace at well-defined temperature levels, which were maintained for a sufficient time span in order to ensure completion of the calcination. Subsequently the samples were allowed to cool down naturally to ambient temperature. The investigations reveal a clear ranking with respect to pozzolanic reactivity starting with Ca-montmorillonite and metakaolin followed by mixed-layer mica/smectite, Na-montmorillonite, sepiolite and finally illite. This ranking holds for the individual optimum calcination temperature. Thus, the second most important parameter influencing pozzolanic reactivity of calcined clay is its calcination temperature that is somewhere in the range from 760 °C [1400 °F] for 23 kaolinite and 930 °C [1706 °F] for illite and mixed-layer mica/smectite.
The efficiency of calcined clay depends on physical and chemical properties of the final product. Particle size distribution is an important physical effect since fine dispersed calcined clay may serve as nucleus thus enhancing early hydration of cementitious systems. Additionally, silica and alumina ions dissolve from the clay layers into the pore solution the easier the smaller the particles are. These ions are the source of the pozzolanic reactivity of calcined clays. The rate of the chemical interaction between the calcined clay and the pore solution is indicated by the change in the concentration of calcium hydroxide (CH) in the 6 pore solution. The reactivity of the calcined clays was studied either in combination with ordinary Portland cement (CEM I) or Ca(OH)2. Usually, other cements were not considered.
Belver C, Miguel A B Munoz , and M A Vicente; Chemical Activation of a Kaolinite under Acid and Alkaline Conditions; Chem. Mater, 2002, VOL-14 (5), pp 2033–2043 did a study on the synthesis of the metakaolins which was carried out by the calcination of kaolin. The calcination was carried out at four different temperatures. The XRD patterns of the calcined items were studied which indicated various pattern examples. The pattern of kaolinite disappeared but the pattern of mica remained. Acid activation of the sample was carried out with HCl of concentration of 6M at room temperature. The temperature was maintained at 90°C and reflux conditions were provided using a reflux tube. The times of treatment varied from 6h to 24h. No change in the properties or in the structure or of the metakaolins were found. The treatment which was done under reflux conditions for 6h helped in the removal of a large portion of the Al3+ cations which were octahedral in shape, and a formless silica was also developed.
Müller (2005) investigated the interaction between the non-calcined clay minerals (kaolinite, illite and montmorillonite) with lime from a soil stabilization perspective. However, in addition to simple ion-exchange between alkalis and calcium, he also observed a real pozzolanic reaction forming new hydration products after about a month.
Mixed clay (Mielenz et al, 1945) was affected slightly by calcination to 980°C. Before calcination non of the materials in this group caused final set of lime-pozzolan paste at ages less than 1,000 hours, and some of them caused no final set of paste even after calcination at 535°C. The compressive strength of Portland cement-pozzolan mortar at age 90 days ranged from 50 to 91 % of that the control mortar, reaching a maximum after calcination at 980°C.
2.2 FLY ASH
Fly ash is a predominantly inorganic residue obtained from the flue gases of furnaces at pulverized coal power plants. When coal is burnt in pulverized coal boilers, the minerals, entrained in the coal, are thermally transformed into chemical species that are reactive or could be chemically activated, for example by the addition of calcium hydroxide.3 The finely divided glass phase, the predominant phase in fly ash, reacts as a pozzolan, defined by Manz as "...a siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
Scheetz and Earle12 comment on the use of fly ash in America. Only 27.4 % of the ash produced in 1996 was used in non-landfill applications (confirmed by Hower and others7). Scheetz and Earle12 challenge researchers with the following remark: ...[Fly ash] was imparted with an excess energy, either chemical or stored surface energy, which can be utilized to participate in chemical reactions, if properly activated. The challenge for the scientific community is to exploit these resources, as low tech materials, to solve large-volume societal-environmental needs.12 Malhotra and others15 report on the use of fly ash in America in 2002. They estimate that only 30 % of the fly ash produced is used. Two thirds is used in the concrete industry, which has reached a maximum consumption figure. Malhotra and others15 challenge researchers to find low cost but high volume applications of fly ash, and to convert ashes into value- added products.15 Foner and others2 emphasize the role of developing new applications of fly ash in 1999, by pointing out that Israel would produce 1.3 megatonnes of coal ash per annum by 2001 and that only 0.6 megatonnes could be used by the cement industry.2 Nathan and others16 estimated the figures as 1.2 megatonnes and 0.8 megatonnes respectively by 2000.
Chatterjee (2011), reported that about 50 % of fly ash generated is utilised with present efforts. He also reported that, one may achieve up to 70% replacement of cement with fly ash when high strength cement and very high reactive fly ash is used along with the sulphonated naphthalene formaldehyde superplasticizer. He reported improvement in fly ash property could be achieved by grinding and getting particles in submicrocrystalline range. Bhanumathidas, &Kalidas, (2002) with their research on Indian fly ashes reported that the increase in ground fineness by 52% could increase the strength by 13%. Whereas, with the increase in native fineness by 64% the strength was reported to increase by 77%. Looking in to the results it was proposed that no considerable improvement of reactivity could be achieved on grinding a coarse fly ash. Authors also uphold that the study on lime reactivity strength had more relevance when fly ash is used in association with lime but preferred pozzolanic activity index in case of blending with cement. Subramaniam, Gromotka, Shah, Obla& Hill, (2005) investigated the influence of ultrafine fly ash on the early age property development, shrinkage and shrinkage cracking potential of concrete. In addition, the performance of ultrafine fly ash as cement replacement was compared with that of silica fume. The mechanisms responsible for an increase of the early age stress due to restrained shrinkage were assessed; free shrinkage and elastic modulus were measured from an early age. In addition, the materials resistance to tensile fracture and increase in strength were also determined as a function of age. Comparing all the test results authors indicated the benefits of using ultrafine fly ash in reducing shrinkage strains and decreasing the potential for restrained shrinkage cracking.
2.3 GRANULATED BLAST FURNANCE
Concrete is probably the most extensively used construction material in the world with about six billion tones being produced every year. It is only next to water in terms of precipitate consumption. However, environmental sustainability is at stake both in terms of damage caused by the extraction of raw material and CO2 emission during cement manufacture. This brought pressures on researchers for the reduction of cement consumption by partial replacement of cement by supplementary materials. These materials may be naturally occurring, industrial wastes or by-products that are less energy intensive. These materials (called pozzalonas) when combined with calcium hydroxide, exhibits cementetious properties. Most commonly used pozzalonas are fly ash, silica fume, metakaolin, ground granulated blast furnace slag (GGBS). This needs to examine the admixtures performance when blended with concrete so as to ensure a reduced life cycle cost. There are competing reasons, in the long term, to extend the practice of partially replacing cement with waste by products and processed materials possessing pozzolanic properties. Lately some attention has been given to the use of natural pozzolans like GGBS as a possible partial replacement for cement. Amongst the various methods used to improve the durability of concrete, and to achieve high performance concrete, the use of GGBS is a relatively new approach; the chief problem is with its extreme finesse and high water requirement when mixed with Ordinary Portland cement. The present paper focuses on investigating characteristics of concrete with partial replacement of cement with GGBS. The Blast-Furnace slag is a by-product of the iron manufacturing industry. Iron ore, coke and limestone are fed into the furnace and the resulting molten slag floats above the molten iron at a temperature of about 1500 0C to 1600 0C. The molten slag has a composition of about 30% to 40% SiO2 and about 40% CaO, which is nearly similar to the chemical composition of Portland cement. After the molten iron is tapped off, the remaining molten slag, which consists of mainly siliceous and aluminous residue is then water-quenched and cooled rapidly, resulting in the formation of a glassy crystalline granulates. This glassy granulates are dried and Pulverized which is known as ground granulated blast-furnace slag (GGBS).The production of GGBS requires less additional energy as compared with the energy needed for the production of Portland cement. The replacement of Portland cement with GGBS will lead to significant reduction of carbon dioxide gas emission. GGBS is therefore an environmental friendly construction material. It can be used to replace as much as 80% of the Portland cement used in concrete.
Shariq et al. (2008) studied the effect of curing procedure on the compressive strength development of cement mortar and concrete incorporating ground granulated blast furnace slag. The compressive strength development of cement mortar incorporating 20, 40 and 60 percent replacement of GGBFS for different types of sand and strength development of concrete with 20, 40 and 60 percent replacement of GGBFS on two grades of concrete are investigated. Tests results show that the incorporating 20% and 40% GGBFS is highly significant to increase the compressive strength of mortar after 28 days and 150 days, respectively.
Peter et al. (2010) studied the BS 15167-1 which requires that the minimum specific surface area of GGBS shall be 2750 cm2/g (BS 15167-1:2006). In China, GGBS is classified into three grades; namely S75, S95 and S105. The GB/T18046 requires a minimum surface area of 3000 cm2/g for grade S75 GGBS, 4000 cm2/g for grade S95 and 5000 cm2/g for grade S105, which are higher than the BS EN’s requirements (GB/T18046-2008). It was reported that slag with a specific surface area between 4000 cm2/g and 6000 cm2/g would significantly improve the performance of GGBS concretes.
Mojtaba Valinejad Shoubi et al. (2013) reviewed in their research the specifications, production method and degree of effectiveness of some industrial byproducts such as GGBS, Silica Fume and PFA as cement replacement to achieve high performance and sustainable concrete which can lead not only to improving the performance of the concrete but also to the reduction of ECO2 by reducing the amount of PC showing how they affect economic, environmental and social aspects positively.
Aveline Darquennes et al. (2011) determined the slag effect on cracking. Their study focuses on the autogenously deformation evolution of concretes characterized by different percentages of slag (0 and 42% of the binder mass) under free and restraint conditions by means of the TSTM device (Temperature Stress Testing Machine).
Elsayed (2011) investigated experimentally in his study the effects of mineral admixtures on water permeability and compressive strength of concretes containing silica fume (SF) and fly ash (FA). The results were compared to the control concrete, ordinary Portland cement concrete without admixtures. The optimum cement replacement by FA and SF in this experiment was 10%. The strength and permeability of concrete containing silica fume, fly ash and high slag cement could be beneficial in the utilization of these waste materials in concrete work, especially in terms of durability.
Reginald Kogbara et al. (2011) investigated the potential of GGBS activated by cement and lime for stabilization/solidification (S/S) treatment of a mixed contaminated soil. The results showed that GGBS activated by cement and lime would be effective in reducing the leachability of contaminants in contaminated soils. Martin et al. (2012) studied the influence of pH and acid type in the concrete. The conclusions were that concrete tested cannot adequately address the durability threat to all parts of wastewater infrastructure over a significant life span due to the extraordinarily harsh nature of this form of attack.
Wang Ling et al. (2004) analyzed the performance of GGBS and the effect of GGBS on fresh concrete and hardened concrete. GGBS concrete is characterized by high strength, lower heat of hydration and resistance to chemical corrosion.
Atul Dubey, Dr. R. Chandak and Prof. R.K. Yadav, “Effect of Blast Furnace Slag Powder on Compressive Strength of Concrete”, International Journal of Scientific & Engineering Research(IJSER), Vol. 3, Issue 8, Aug. 2012, ISSN: 2229-5518, examined the effects of partial replacement of cement with 5 to 30 % of BFS on compressive strength of concrete. The test was conducted at 7, 14 and 28 days on cubes made of standard size of 150 mm x 150 mm x 150 mm. He concluded that as the percentage of BFS increases, the strength tends to decrease. On replacement of OPC with 15% blast furnace slag powder, the depreciation in 28 days compressive strength is being near about only 5 %.
Oner and S. Akyuz, “An Experimental Study on Optimum Usage of GGBS for the Compressive Strength of Concrete”, ELSEVIER (Cement and Concrete Composites), Vol. 29, Jan. 2007, pp. 505-514, doi:10.1016/j.cemconcomp.2007.01.001, conducted a study in which he replaced cement partially with GGBS in various percentages from 15% - 110% by weight. Compressive strength test was conducted on test specimens cured at 7, 14, 28, 63, 119, 180 and 365 days and it was found that early age strength values of GGBS concrete mix are lower than control mixtures but as the curing period is extended the strength values increases. This is because the pozzolanic reaction is slow and depends on the calcium hydroxide availability so the strength gain takes longer time for the GGBS concrete. It was also observed that as the percentage of GGBS is increased, the strength gain increases. The optimum level of GGBS content for maximizing strength was found out about 55% – 59% by Bolomey and Feret strength equation. He also found out that as the GGBS content increases, the water/binder ratio decreases for the same workability and thus, the GGBS has positive effects on the workability.
Sabeer Alavi.C, I. Baskar and Dr. R. Venkatasubramani, “Strength and Durability Characteristics of GGBFS Based SCC”, International Journal of Emerging trends in Engineering and Development (IJETED), Vol. 2, Issue 3, Mar. 2013, pp. 510-519, ISSN: 2249-6149 studied the effects of partial replacement of cement with 10 - 50% of GGBFS and found that 30% GGBFS replacement is good as beyond that the compressive strength starts decreasing. He also concluded that the split tensile strength and flexural strength conducted at 7 and 28 days increases with increase in GGBFS content. It was also found that the workability increases with the increase in percentage of GGBFS.
Yogendra O. Patil, Prof. P.N. Patil and Dr. Arun Kumar Dwivedi, “GGBS as Partial Replacement of OPC in Cement Concrete – An Experimental Study”, International Journal of Scientific Research (IJSR), Vol. 2, Issue 11, Nov. 2013, pp. 189-191, ISSN: 2277-8179, researched on the effects on compressive strength and flexural strength of concrete with partial replacement of cement with various percentages of GGBS. The tests were conducted at 7, 28 and 90 days with replacement ranging from 10 % to 40 %. It was observed that the strength of concrete is inversely proportional to the percentage of replacement of cement with GGBS. The replacement of OPC by GGBS up to 20% shows the marginal reduction of 4 – 6 % in compressive and flexural strength for 90 days curing and beyond that of more than 15%. He concluded that, GGBS as replacement of OPC by 20% results in reduction in cost of concrete at the current market rate by 14%.
2.4 SUPERPLASTICIZERS
Superplasticizers are widely used in concrete processing to increase the rheological properties of hardened pastes. Super plasticizers are chemical admixtures which can maintain an adequate workability of fresh concrete at low water/cement ratio for a reasonable period of time, without affecting the setting and hardening behaviour of the cementitious system. Superplasticizers are introduced in concrete like many other admixtures to perform a particular function, consequently they are frequently described according to their functional properties. Super plasticizers have been classified as high range water reducers (HRWR) to distinguish them from other categories of less effective water reducers.
Franklin (1976) stated that, super plasticizers are organic polyelectrolytes, which belong to the category of polymeric dispersants. The performance of super plasticizers in cementitious system is known to depend on cement fineness, cement composition mode of introduction to the mixture etc., as well as on the chemical composition of super plasticizers.
For many years, it was not possible to reduce water/cement ratio of concrete below 0.40 till the advent of super plasticizers. The super plasticizers were first used in concrete in 1960s and their introduction occurred simultaneously in Germany and Japan.
(Meyer and Hottori, 1981). At first, the super plasticizers were used as fluidizers than water reducing agents. By using large enough super plasticizer, it was found possible to lower the water/binder ratio of concrete down to 0.30 and still get an initial slump of 200mm. Reducing the water/binder ratio below 0.30 was a taboo until Bache reported that using a very high dosage of super plasticizers and silica fume, water binder ratio can be reduced to 0.16 to reach a compressive strength of 280MPa (Bache, 1981).
Aitcin et al. (1991) reported, that by choosing carefully, the combination of Portland cement and superplasticizer, it was possible to make a 0.17 water/binder ratio concrete with 230mm slump after an hour of mixing which gave a compressive strength of 73.1MPa at 24 hours but failed to increase more than 125MPa after long term wet curing.
During 1980s, by increasing the dosage of super plasticizers little by little over the range specified by the manufacturers, it is realized that super plasticizers can be used as high range water reducers (Ronneberg and Sandvik, 1990).
Super plasticizers can be used for three different purposes or a combination of these
( To increase workability without changing the composition of the mix.
( To reduce the mixing water and the water/cement ratio in order to increase strength and improve durability.
( To reduce both water and cement in order to reduce creep, shrinkage and thermal strains caused by heat of cement hydration.
However, there are two main practical limits in using these chemical admixtures.
( The method of addition affects the slump increase effect.
( Slump loss may reduce the beneficial effect at the time of placing (Cellopardi, 1994).
Ozkul and Dogan (1999) studied the effect of an N-vinyl copolymer super plasticizer on the properties of fresh and hardened concretes. Workability of concrete was measured by slump flow test and in situ tests were undertaken to find out the pumping ability of super plasticized concrete. The coarse aggregate was crushed stone with the maximum size of 25 mm. By using this chemical admixture, which was a little bit different from the conventional ones, the ability of water reduction was increased along with the retention of high workability for a longer time.
In situ test results obtained by Ozkul and Dogan (1999) demonstrated that the superplasticized concrete could be pumped easily from a height of about 13 m and the filling capacity was greater than 85%. The pumping pressure was the same as for normal pumpable concrete and no segregation was observed. For mixtures with watercement ratios between 0.3 and 0.45, the slump diameters were between 500 mm and 740 mm and the compressive strength varied between 53 MPa and 68 MPa at 28 days of age. In their work, Roncero (1999) et al. evaluated the influence of two super plasticizers (a conventional melamine based product and a new-generation comb-type polymer) on the shrinkage of concrete exposed to wet and dry conditions. Tests of cylinders with embedded extensometers have been used to measure deformations over a period of more than 250 days after casting. In general, it was observed that the incorporation of super plasticizers increased the drying shrinkage of concretes when compared to conventional concretes, whereas it did not have any significant influence on the swelling and autogenous shrinkage under wet conditions. The melamine-based product led to slightly higher shrinkage than the comb-type polymer.
It must be realized that the introduction of super plasticizer in concrete involves a new chemical component in a complex hydraulic binder system, which already contain several added chemicals. Due to variety of admixture formulations, it is difficult to provide the concrete industry with simple rules specifying proper use of super plasticizers in the presence of other admixtures. However the Marsh Cone test is popularly used to evaluate the characteristics of different pastes, in order to select the optimum dosage of super plasticizers (Giaccio 2002).
2.5 METAKAOLIN
Metakaolin is manufactured from pure raw materials to strict quality standards. It is not a by-product. Other pozzolanic materials are currently available, but many are by-products, which are available in various chemical compositions. They may also contain active components (such as sulphur compound, alkalis, carbon, reactive silica) which can undergo delayed reactions within the concrete and cause problems over long time periods. Metakaolin is obtained by calcinations of pure or refined kaolintic clay at a temperature between 650°c and 850°c, followed by grinding to achieve a fineness of 700 to 900m2 /kg. The resulting materials have high pozzolanicity.
Metakaolin is a high quality Pozzolanic material, which is blended with Portland cement in order to improve the durability of concrete and mortars; it removes chemically reactive calcium hydroxide from the hardened cement paste. Metakaolin reduces the porosity, densifies, thickness of interfacial zone, this improving the adhesion between the hardened cement paste and particles of sand or aggregates.
Zhang and Malhotra (1995) also noted an increased demand for air-entraining admixture comparable to a silica fume concrete. Metakaolin is beneficial in reducing drying shrinkage when compared to silica fume concrete. Optimum ranges for metakaolin addition depend upon desired properties. The optimum dosage was found out to be 15 to 25% for compressive strength.
Khatib and Wild (1996) reported that the large pores in the pates decrease with increase in metakaolin content. Wild et al. (1996) presented the mechanical properties of super plasticized metakaolin concrete. Khatib and Wild (1998) studied the improved sulphate resistance of metakaolin mortar. Curcio et al. (1998) presented the utility of metakaolin as micro filler in the production of high performance mortars.
Palomo et al. (1999) studied the chemical stability of metakaolin based cement composites. Frias and Cabrera (2000) investigated the relationship between the pore size distribution and degree of hydration of metakaolin based cement pastes.
High-reactivity metakaolin (HRM) is a more recently developed supplementary cementitious material. It is a reactive aluminosilicate pozzolan formed by calcining purified kaolinite at a specific temperature change. Chemically, HRM combines with calcium hydroxide to form calcium silicate and calcium aluminate hydrates. It has been shown that HRM in powder form is a quality-enhancing mineral admixture that exhibits enhanced engineering properties comparable to silica fume slurry (Caldarone et al., 1994; Khatib and Wild, 1996; Khatib and Wild, 1998; Curcio et al., 1998; Frias and Cabrera, 2000).
Brooks et al. (2000) studied about the effect of silica fume, Metakaolin, fly ash and ground granulated blast furnace slag on the setting times of high strength concrete. They observed that the general effect of silicon, metakaolin, fine aggregate and GGBS is to retard the setting time of high strength concrete. In high strength concrete containing metakaolin there was increase in the retarding effect up to 10% replacement level and at higher replacement level of 15%, the retarding effect appears to reduce.
Quian and Zongjinli (2001) presented the stress-strain relationships (tension and compression) for concrete containing 0%, 5%, 10%, and 15% of metakaolin. The results indicated that the tensile, flexure and compressive strengths of concrete increase with the increasing metakaolin content. The compressive elasticity modulous of concrete showed only small increase with the increase in metakaolin content.
Poon et al. (2001) investigated about the rate of pozzolanic reaction of metakaolin in High-Performance-Concrete. Hydration progress in metakaolin blended high performance concrete with age was studied from the compressive strength, porosity and pore size distribution properties. The results were compared with concretes containing silicafume, flyash and Portland cement. They reported the rates of pozzolanic reaction and calcium hydroxide consumption in the metakaolin blended cement concretes. The higher pozzolanic activity results in a higher rate of strength development and pore structure reinforcement for the cement concrete at earlier ages.
Roy et al. (2001) studied about the effect of Metakaolin, silica fume and fly ash on chemical resistance of concrete. Mortars were prepared with various proportions of OPC, silica fume and Metakaolin/low calcium fly ash (0-30% weight replacement). Chemical resistance was found to increase in the order of SF to Metakaolin to Fly Ash and decreased as the replacement level is increased from 0- 10% weight replacement to 15 – 30% weight level. They finally concluded that it is important to evaluate a particular concrete formulation before predicting its performance in a special acid environment.
Poon et al. (2001) studied about the rate of Pozzolanic reaction of metakaolin in high performance cement mortars. Hydration progress in metakaolin blended high performance cement paste with age was studied from the compressive strength, porosity and pore size distribution properties. The results were compared with pastes containing silica fume, fly ash and Portland cement. They reported the rates of Pozzolanic reaction and CH (calcium hydroxide) consumption in the metakaolin blended cement pastes are higher than that in the silica fume or fly ash blended cement pastes. The higher Pozzolanic activity results in a higher rate of strength development and pore structure reinforcement for the cement pastes at earlier ages.
Gruber et al. (2001) investigated metakaolin and PFA mortars for heat of hydration. In this study 5-15% metakaolin was replaced with Portland cement and investigated the replacement effect on heat of hydration. The result showed that heat of hydration was higher in metakaolin-portland cement mortars when compared to reference Portland cement mortar. The increased heat of hydration was attributed to combined effect of Portland cement hydration and metakaolin pozzolanic reaction. Further, the study showed that heat of hydration of PC (Portland cement) - PFA (pulverised fuel ash) mortars was lower than that in equivalent Portland cement mortars. The decreased heat of hydration in PC-PFA was explained that dilution of Portland cement with PFA and negligible pozzolanic activity of PFA in the initial hours.
2.6 COMPREHENSIVE STRENGTH
The common design compressive strengths required by the construction industry for cast-in-place, precast, and prestressed structures range from 3000 to 8000 psi. These design strengths are economically met with the use of lightweight aggregate. Some lightweight aggregate concretes can obtain strengths above 8000 psi; however, not all lightweight aggregates are capable of obtaining these strengths.
A common concept used to indicate the maximum compressive and/or splitting tensile strengths of concretes using lightweight aggregate is a “strength ceiling.” A mixture reaches its strength ceiling when, using the same aggregate, it possesses only slightly higher strength with higher cement content. This property is predominantly influenced by the coarse aggregate fraction of the mixture. The strength ceiling can be increased by reducing the maximum size of the coarse aggregate. As with normal weight concrete, water reducing and mineral admixtures can be used with lightweight concrete to improve the workability, placing, and finishing.
Kahn et al. (2004) investigated the development of 8,000 psi, 10,000 psi, and 12,000 psi compressive strengths for high-performance lightweight concretes for precast, prestressed bridge girders. A strength ceiling of about 11,600 psi was found using a ½-in. expanded slate aggregate and normal weight natural sand. Laboratory and field mixtures were developed that met the 8,000 psi and 10,000 psi design strength, with the field mixtures attaining higher strengths.
Ozyildirim et al. (2005) investigated 8,000 psi and 4,000 psi design strengths for lightweight concretes used for beams and decks, respectively. Test beams were prepared and tested for material properties. A test mixture was designed for normal weight and lightweight high-performance concretes. The average 28-day compressive strength for the normal weight mixture was close to the 8,000 psi design strength, however, the average 28-day compressive strength for the lightweight mixture was below the 8,000 psi design strength. After 1 year, the average compressive strength for the normal weight mixture was above the 8,000 psi design strength, and the average compressive strength for the lightweight mixture was still below the design strength. The low compressive strength was attributed to excess water in the mixture. Therefore, it was determined that better water control was needed during mixture production. Testing was also performed on the actual mixtures used for the bridge beams and deck. For the bridge beams the average 28-day compressive strength was at or near the target value of 8,000 psi. The average 28-day compressive strength for the deck was above the specified 4,000 psi design strength. From these results, the importance of water control in mixture production is apparent.
Zhou et al. (1998) found that for the same mixtures using either normal weight aggregate or lightweight aggregate, there was a 22% reduction in the compressive strength for the lightweight aggregate concrete mixtures compared to the normal weight concrete. Reaching the 8,000 psi to 10,000 psi 28 day compressive strength threshold consistently with lightweight concrete (which is necessary for lightweight concrete to be competitive with high performance, normal weight concrete) is presently a challenge. Better understanding and further development of high performance, lightweight concrete mixtures is necessary so that this level of compressive strength can be reached at precasting plants on a routine basis with adequate workability.