TWO STAGE MIXING APPROACH

    1.   INTRODUCTION

1.1 GENERAL

Thorough mixing of concrete ingredients is essential to producing a homogeneous mixture that permits uniform particle distribution and hydration of cement particles in the concrete system. As fine cementitious materials, low water-tobinder ratios, and high binder content are increasingly common in modern concrete, the agglomeration of fine cementitious particles often occurs, which not only impairs the uniformity of hydration of cementitious materials, but also reduces the workability of concrete. Also important is the proper coating of the aggregates in the mixing operation with proper paste that eliminates undissolved cement particles. Hence, optimal quality mixing becomes increasingly important and a method or technique is needed to help ensure the optimum quality of cement paste and coating of the aggregates. A two- stage mixing process for concrete involves mixing a slurry of cementitious materials and water ,then adding the slurry to coarse and fine aggregate to form concrete. Research has indicated that this process might facilitate dispersion of cementitious materials and improve cement hydration, the characteristics of the Interfacial Transition Zone (ITZ) between aggregate and paste, and concrete homogeneity. The two -stage mixing process divides the mixing operation to improve both the paste uniformity and the aggregate coating . The first stage prepares the paste by premixing water, cement, and additives to create a slurry of cementitious materials. The second stage is then adds the slurry of fine and coarse aggregate and water coats the aggregate with the paste.

Recycled aggregates are broken concrete, bricks and broken pavement generally RA are produced in the process of construction of various types new concrete structures and demolition old concrete structures [buildings, bridges supports, airport runways, concrete roadbeds etc.], by manually and time to time of Natural disasters such as Wenchuan earthquake in 2008, Yashu earthquake in 2010 and Yuman earthquake in 2011 in China, Bhuj earth quake in 2001 in India and recently in Nepal 2015. According to the statistics data 15.5 million tons of C&D waste produced every years in China, 180 million tons in Europe [laurite report 2004], 27.7 million tons C&D waste produced as per report of Dubai waste management department [2007] and Shrisvastava et al [2009] likewise in India 1.46 million tons C&D waste generated according to 11th five plan [2007]. Initially C&D waste were used for filling material on low land Area and base material of road but large production C&D materials waste disposal possess serious problems in the world as well as negative effect of environment. So therefore the many country attention towards reuse this C&D waste to overcome environmental problems and preservation of natural resources and increasing of sustainable development. The most of authors reported that fresh and hardened properties of different mix grade RAC are lower than the Normal aggregate concrete such as Katz et al.(2003) concluded that the Concrete made with 100% recycled aggregates was weaker than concrete made with natural aggregates at the same w/c ratio. Tam et al. (2005) experimentally shows that the two-stage mixing approach can provide an effective method for enhancing harden properties [compressive strength, splitting tensile strength and flexural strength] and durability properties [creep, shrinkage, permeability, chloride penetration] and other mechanical performance. Xiao et al. (2005) reported that the compressive strengths including the prism and the cube compressive strengths of RAC generally decreases with increasing RAC contents. Etxeberria et al. (2007) concluded that the concrete made with 100% of recycled coarse aggregate has 20 to 25% less compressive strength than conventional concrete at 28 days, Rahul et al (2007) also reported that the 28 days target compressive strength for all five mixes of RCA (20, 25, 30, 40 and 50MPa) were achieved except for the 40 and 50MPa where the observed strength was slightly lower than the target strength. ChakhradharaRao et al. (2011) observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios.

Material for the development is concrete, which forms the indispensable material for construction, can be considered as the second most highly used item in the world after water. The basic constituents of concrete are the natural resources i.e., stone, aggregate, sand and water, suggesting this industry has degrading impacts on these environmental assets. In addition, the quarrying and transportation of aggregates further lead to ecological imbalance and pollution. Not only this, the disposal of the debris of the demolished concrete structures has also become a big problem in various cities due to paucity of landfill sites. These environmental problems are a driving force in developing an urgent and thoughtful sustainable approach towards our natural resources to which the recycling of the aggregates seems to be a allowable remedy. The paper presents a comparison of the compressive strength of the concrete made through NMA and TSMA. Concept of use of recycled aggregate in concrete is not new, researches have been carried out on recycled aggregate all over the world. However, use of Recycled Aggregate in high strength concrete production could not become popular in India. M C Limbachiya, indicating the inferiority of recycled aggregate concrete, reported that often this concrete is used in as road construction, backfill for retaining walls, low grade concrete production, drainage and brick work and block work for low cost housing.

1.2 RECYCLED AGGREGATE CONCRETE

Until recently almost all demolished concrete was thrown away and there is a shortage of landfill areas. Reducing the waste generation is a pressing issue all over the world. Concrete is such an essential, mass produced material, such as steel in the construction industry, and much effort should be made to recycle and conserve these resources. Recycling of concrete and other building materials during the building process of new buildings and at the end of the life cycle is usually very inadequately arranged. Concrete that is suitable for complete recycling, will allow repeated recycling, as is the case for steel and aluminium. Since concrete is composed only of cementitious materials, even the powders generated during the production of RA can be reprocessed as cement resources. This enables concrete to be recycled in a fully closed system, thus enhancing the benefit to the environment. Recycling of concrete demolition waste can provide opportunities for saving resources, energy, time and money. Furthermore, recycling and controlled management of concrete demolition waste will save the use of land and create better opportunities for handling other kinds of wastes. There are a number of opportunities for utilising concrete demolition waste apart from dumping. Recycling of concrete can be accomplished by the reuse of concrete products, processing into secondary raw materials for use as fill, road bases and sub-bases or aggregates for the production of new concrete. Economic benefits, reducing environmental impacts and saving resources can be gained in adopting RA.6–10 Although there are environmental and cost benefits in using RA, the current legislative regulations and experience are not enough to support and encourage recycling demolished concrete for higher-grade applications. These technical problems include weak interfacial transition zones, porous and transverse cracks on demolished concrete, high levels of sulphate and chloride contents, impurity, cement remains, poor grading, lower quality and higher variation in quality.

From the research in, three types of RA are classified: Types C1, C2 and C3 for recycled coarse aggregate and Types F1 and F2 for recycled fine aggregate (see Table 1). For the recycled coarse aggregate, Type C1 has the best quality with the lowest water absorption rate of 3% or less and sulfate soundness of 12% or less, while recycled coarse aggregate Type C3 is designed to have 7% or less of water absorption. For the recycled fine aggregate, Types F1 and F2 are designed to have 5% or less and 10% or less of water absorption, respectively. From these three types of recycled coarse aggregate, C1,C2 and C3, and two types of recycled fine aggregate, F1 and F2, four types of suggested RAC applications are recommended for civil and building works, as tabulated in Table 2. Among the three types of civil engineering applications of RAC, namely, CI, CII and CIII with respect to different combinations of recycled coarse aggregate and recycled fine aggregate (see Table 1), Type CI RAC can be designed up to 18 to 24 MPa, thus suitable for reinforced and plain concrete, lower structure of bridges, tunnel lining and retaining walls. The four types of building work applications of RAC, namely, BI, BII, BIII and BIV (see Table 1), can all be designed with a strength of 18 MPa or higher for various types of application including ordinary reinforced concrete building structures, foundations, foundation slabs and backfilling concrete, respectively. Several potential areas in the application of recycled material are tabulated in Table 3.

Table 1 Quality standard of recycled aggregate concrete for public work

Table 2 Types of recycled aggregate and suggested uses in civil and building works

 Table 3 Potential areas of application of recycled materials

1.3 TWO-STAGE MIXING APPROACH

The TSMA was developed by Tam et al. for improving the quality of RAC, in which the mixing process is divided into two parts and the required water is proportionally split into two, which is added at different timing. First, fine and coarse aggregates are mixed for 60 s before half of the water required is added and mixed for another 60 s; then cement is added and mixed for 30 s before the remaining half of water is added and mixed for 120 s. Improvement of strength is achieved up to 21.19% for 20% of RA used under 28-day curing conditions under TSMA. During the first stage of mixing, half of the required water is used for mixing leading to the formation of a thin layer of cement slurry on the surface of RA, which will permeate into the porous old cement mortar, filling up the old cracks and voids. At the second stage of mixing, the remaining water is added to complete the concrete mixing process.

Research has indicated that this process might facilitate dispersion of cementitious materials and improve cement hydration, the characteristics of the Interfacial Transition Zone (ITZ) between aggregate and paste, and concrete homogeneity. The two -stage mixing process divides the mixing operation to improve both the paste uniformity and the aggregate coating. Traditional concrete mixing practice today is regulated by a specific mixing time required to achieve specified performance of the fresh and hardened concrete. The mixing time is based on a long experience of developed correlations between the mixing process and the concrete performance, and is generally detailed in specifications. The effects of the mixing procedure on the materials in concrete is an important area of research. A complete understanding of these procedures on a step-by-step basis, in theory and through empirical relationships, will lead to increases in the efficiency of the mixing process and improvements in concrete properties.

2. OBJECTIVES

The goal of the study was to find optimal mixing procedures for production of a homogeneous and workable mixture and quality concrete using a two-stage mixing operation. This was to be accomplished by characterizing the mixing process and mixing order to produce a homogeneous cementitious material to improve production rate while maintaining durability and quality of recycled aggregate. The specific objectives of the study are as follows:

1. To achieve optimal mixing energy and time for a homogeneous cementitious material

2. To characterize the homogeneity and flow property of the pastes

 3. To investigate effective methods for coating aggregate particles with cement slurry

 4. To study the effect of the two-stage mixing procedure on concrete properties

 5. To obtain the improved production rates. 6. Experimenting the TSMA and assessing the benefits possibly gained with recycled aggregates.

3. MATERIALS AND METHOD

Since there are many unsolved problems encountered in controlling the quality of RAC, which include low compressive strength, wide variability of quality, high drying shrinkage, large creep and low elastic modulus, applications of RAC are hampered. These problems are mainly resulted from the following two reasons:       

 
· Concrete wastes are always contaminated with foreign materials; and    
· Recycled aggregate particles are always attached with substantial amount of relatively soft cement mortar paste, making these aggregates more porous and less resistant to mechanical attacks.

Under normal situations, some modifications to the mix proportion are needed in the production of RAC, which can then be produced with the same production procedure as the conventional concrete does. However, such an approach will produce concrete with poorer quality, depending directly on the proportion of RA added. Hence, most studies recommend a limit of 30% of RA. Many researchers have successfully applied RA on pavement and roadwork or simple structures, underground structures, foundations, piles and mass concrete . However, its application to higher grade concrete is not common. These weaknesses of RA, including high porosity, high amount of cracks, high level of sulphate and chloride contents, high level of impurity and high cement mortar remains, will affect the mechanical performance of RAC. The prerequisite in applying RA to high-grade concrete is to overcome these weaknesses. A new mixing approach, two-stage mixing approach (TSMA), is proposed. For NMA, the mixer is firstly charged with about one-half of coarse aggregate, then with fine aggregate, then with cement and finally with the remaining coarse aggregate. Water is then added immediately before the rotation of the drum or starting the pan, while TSMA divides the mixing process into two parts and proportionally splits the required water into two which are added at different timing. Fig. 1 illustrates the TSMA mixing procedure, while Table 4 shows the symbols used.

Table 4 Symbols used for representing various materials

Fig 3.1 mixing procedure of (i) Normal mixing approach (ii) Two stage mixing approach

4. EQUIPMENTS USED  

4.1 LABORATORY EQUIPMENTS        
4.1.1 Concrete Mixers

There are three main types of concrete mixers: drum mixers, pan mixers (which are considered batch mixers) and continuous mixers . Drum mixers have a large rotating drum with blades attached to the inner sides of the drum. The concrete materials are mixed by the blades lifting the materials while rotating and then dropping the materials back into the center of the drum. The rotational speed of the drum is controlled to ensure proper mixing for the mix design and batch size.

           
4.1.2 Drum Mixers

Variations of drum mixers include a reversing drum (where the rotational direction can be reversed and the concrete constituents are loaded from one end and discharged from the other) and tilting angle drums (where the centerline axis of the drum can be increased from horizontal, forcing the concrete to mix in the bottom portion of the drum). Tilting axis drum mixers are loaded and discharged from the same end. Drum mixers that operate at a zero degree angle, completely horizontal, provide more energy to the concrete mixing process, because the concrete materials are lifted to the largest height by the blades before being dropped. Typical concrete truck mixers fall into the tilting drum category, rotating at 2 rpm for premixed concrete and 15 rpm when all the separate concrete ingredients are added and mixed in the truck mixer. Concrete truck mixers typically have a 15 degree angle of tilt. Figure 4.1 below shows a typical cross-section of a drum mixer.

Fig 4.1 crossection of a drum mixer.

4.1.3 Pan Mixers

Pan mixers employ a flat cylindrical pan to hold the concrete constituents. The pan is either stationary or rotating and has mixing blades separate from the pan that rotate inside it. If the pan is rotating, the blades rotate in the opposite direction. A separate blade is fixed against the inside edge of the pan and scrapes the material off the side, moving it towards the center where the mixing blades are rotating. Figure 4.2 shows the various configurations of blades. Note that the configurations of the blades may vary, but all have the same effect. Large pan mixers—those greater than 0.2 m3 (0.26 yd3 )—typically discharge from a door in the bottom of the pan. Small pan mixers— those less than 0.2 m3 (0.26 yd3)—discharge by removing the material from the top of the pan.

4.1.4 Continuous Mixers

Continuous mixers load material at the same rate that it is discharged. They are usually non-tilting drum mixers with a screw-type blade configuration that mixes the material as it is pushed through the mixer. These mixers are used for situations that require a short mixing time, have small batches, or are located in remote sites not convenient for ready-mix truck delivery. Portable batch plant mixers that produce low slump concrete are often continuous mixers.

Fig 4.2 Various blade configurations of a pan mixer.

4.2 FIELD EQUIPMENTS 
4.2.1 Hydromix

A photograph of the Hydromix apparatus, manufactured by Hydromix Inc., is shown in Figure 4.3 . The dry cementitious materials are fed directly into the top and mix water is injected through the black injection nozzles. The Hydromix mixing process is designed to be added to a conventional batch plant underneath the cementitious materials storage bins. The Hydromix mixing process is a continuous mixing process. Once the cementitious materials are being batched and ribbon fed (slurry), the aggregate belt starts to charge the awaiting ready mix truck with the previously batched coarse and fine aggregate. A 10–12 yd
3 batch of concrete can be produced in about 60–90 seconds. Admixtures such as air entraining agent and water reducer are sprayed onto the aggregate as it is being charged into the ready mix truck.

Fig 4.3 A Hydromix apparatus

           
4.2.2 Countercurrent Concrete Mixers

A countercurrent concrete mixer is one in which four concrete mixing arms rotate around a center vertical axis. This delivers intense countercurrent mixing action and is reported to transfer all the mixing energy directly into the concrete. This mixing procedure could be used to produce large quantities of slurry in a two-stage mixing operation. Figure 4.4 shows a countercurrent concrete mixer. Output capacities range from 1/3 yd3 to 4 yd3 of fresh concrete. Using this as a sole provider of fresh slurry, one could easily batch a 10 yd
3 batch of concrete using this in a two-stage mixing operation. If a greater output is desired, two countercurrent concrete mixers could work in tandem.

Fig 4.4 Counter current concrete mixer

4.2.3 High Shear Mixer

High shear mixers may be used in a conventional batch plant if placed appropriately. High shear mixers can be continuous, with material output ranging from 50–500 gallons of slurry per minute. This mixing method is very efficient and may be a viable alternative for field retrofit. Figure 4.5 shows a powder-liquid high shear mixer and shows a schematic of how the powder-liquid high shear mixer works. Note that a significant amount of effort will be required to adapt this equipment to a conventional ready-mix concrete plant for production of portland cement concrete.

Fig 4.5 Powder-liquid high shear mixer

4.2.4 Water Jet Mixers

A water jet mixer by name uses water jetting action to fully mix a dry powder with water, with no mechanical mixing needed. Figure 4.6 shows a schematic of a water jet mixer. The pressurized water stream is converted from pressure-energy to high velocity as the fluid enters the nozzle. The issuing high velocity jet stream produces a strong suction in the mixing chamber, causing a powder, granular material or secondary fluid to be drawn through a suction port into the mixing chamber. An exchange of momentum occurs when the powder intersects with the motive fluid. The dynamic turbulence between the two components produces a uniformly mixed stream traveling at a velocity intermediate between the motive and suction velocities through a constant diameter throat where mixing is completed. The diffuser is shaped to reduce the velocity gradually and convert velocity energy back to pressure at the discharge with a minimum loss of energy.

Fig 4.6 water jet mixer

5. EXPERIMENTAL STUDY 

   
5.1 COMPARISON OF COMPRESSIVE STRENGTH OF CONCRETE MADE BY TWO STAGE MIXING APPROACH USING FLY ASH AND NOMINAL CONCRETE MADE BY NORMAL MIXING APPROACH

Tam V.W.Y et al(2005), proposed the technique of modified mixing of concrete. They concluded that the higher water absorption and higher porosity results in poor quality of Recycled Aggregate Concrete (RAC). The weaker interfacial transition zone (ITZ) between Recycled Aggregates(RA) and cement mortar limits the application of RAC in higher grade applications. In the study, the TSMA provides strength to the weak link of RAC, which is located at the (ITZ) of the RA. In TSMA , cement slurry formed gels up with RA providing a stronger ITZ by filling up the cracks and pores within RA. Concrete made through TSMA shows improved compressive strength when tested in laboratory. This approach provides an effective method for enhancing the strength characteristics and other mechanical properties of RAC, and thus, opens a wider scope of applications

5.2 MATERIALS USED

A. Cement: Ordinary Portland cement of 43 grade satisfying the requirements of IS: 8112-1989[14]. The specific gravity of cement was found to be 3.005.
16 B. Fine aggregates: The sand generally collected from Haryana. Sand is the main component grading zone-I of IS: 383-1978[13] was used with specific gravity of 2.62 and water absorption of 1 % at 24 hours. C. Coarse aggregates: Mechanically crushed stone from a quarry situated in Haryana with 20 mm maximum size, satisfying to IS: 383-1978[13] was used. The specific gravity was found to be 2.63 and water absorption is 0.5 % at 24 hours. D. Recycled coarse aggregates: Aggregates obtained by the processing of construction and demolition waste are known as recycled aggregates. RCA for the experimental analysis was procured from the C & D waste plant in Delhi which is in collaboration with Municipal Corporation Of Delhi. E. Fly Ash: Fly ash is used as partial replacement of cement which replaces 10% of total cementitious material in all the cases of the experiments. Class F fly ash is used from Haryana having specific gravity as 2.4 and satisfying IS 3812-1999.

Fig 5.1 Coarse aggregate

5.3 METHODOLOGY

NMA follows the following steps:

1) First, coarse and fine aggregate are mixed.

2) Second, water and cementitious materials are added and mixed.

However,

TSMA follows different steps:

1) First, coarse and fine aggregates are mixed for 60 seconds and then half of water for the specimen is added and mixed for another 60 seconds.

2) Second, cementitious material is added and mixed for 30 seconds.

3) Thirdly, the rest of water is added and mixed for 120 seconds. 

The specific procedure of TSMA creates a thin layer of cement slurry on the surface of RA which is expected to get into the porous old mortar and fill the old cracks and voids. Using recycled concrete as the base material for roadways reduces the pollution involved in trucking material.

5.4 EXPERIMENTAL OBSERVATIONS 

Following table shows the experimental observations of the test samples made from TSMA and nominal mix by NMA.

A. M-25(10-25) signifies the specimen mix having 10% fly ash and 25% RCA content.

B. M-25(10-50) signifies the specimen mix having 10% fly ash and 50% RCA content.

C. M-25(10-25) signifies the specimen mix having 10% fly ash and 75% RCA content.

D. M-25(10-25) signifies the specimen mix having 10% fly ash and 100% RCA content.

Table 5 Experimental Observation

5.5 RESULTS AND DISCUSSION

5.5.1 Results:

The above experimental analysis provides us with the following results:

(1) The compressive strength of M-25 grade nominal concrete made by NMA gives 7 day and 28 day strengths as 17.84 MPa and 31.7 MPa respectively .

(2) Using TSMA, addition of 10% fly ash, the specimen made by 25% RCA gives 7 day and 28 day strengths as 18.81 MPa and 33.77 MPa respectively.

(3) Using TSMA, addition of 10% fly ash, the specimen made by 50% RCA gives 7 day and 28 day strengths as 20.21 MPa and 32.88 MPa respectively.

(4) Using TSMA, addition of 10% fly ash, the specimen made by 75% RCA gives 7 day and 28 day strengths as 22.51 MPa and 32.88 MPa respectively.

(5) Using TSMA, addition of 10% fly ash, the specimen made by 100% RCA gives 7 day and 28 day strengths as 17.10 MPa and 27.99 MPa respectively.

5.5.2 Discussion:

The specimen mix M-25(10-25) shows an increase of 5.46% in 7 day compressive strength and 6.52% in 28 day strength , whereas, specimen mix M-25(10-50) shows an increase of 13.32% in 7 day compressive strength and 3.72% in 28 day strength with respect to nominal mix specimen . The specimen mix M-25(10-75) shows an increase of 26.17% in 7 day compressive strength and 3.72% in 28 day strength , however, specimen mix M-25(10-100) shows decrease of 15.10% in 7 day compressive strength and 11.70% in 28 day strength with respect to nominal mix specimen. From 28 day strength point of view, specimen M-25(10-25) shows optimum increase in strength i.e 6.52% with respect to nominal mix specimen.

Fig 5.2 7th day strength 

Fig 5.3 28th day strength

5.5.3 Summary

Following can be concluded from the experimental analysis that concrete made by replacement of 25% and 50% RCA and addition of 10% fly ash using TSMA gives more compressive strength for both 7 day and 28 day strength than the referred nominal concrete specimen made by NMA. However on using 75% and 100% RCA and addition of 10% fly ash using TSMA, the concrete shows decrease in compressive strength than the Nominal concrete. Maximum 28 day strength is obtained by concrete made by using TSMA involving replacement of 25% RCA and addition of 10% fly ash. This concrete so made will be cost effective, strong as well as durable.

6. IMPROVEMENT IN QUALITY OF CONCRETE STRUCTURES BY TWO STAGE MIXING METHOD

6.1 TWO-STAGE MIXING METHOD AND OPTIMUM Wi/C RATIO

Fig 6.1 This is a manufacturing technique in which primary water is first added to set up a suitable surface moisture content of aggregates, and primary mixing is performed together with cement, followed by introduction of the remaining secondary water for secondary mixing. The essential point of this method lies in coating the surface of aggregates, especially of fine aggregate of large total surface area, with cement paste of low water-cement ratio which is in a capillary state). 
Fig.6.2 shows the influence of the ratio by weight Wj/C of primary water and cement on the rate of bleeding of mortar indicated with the fine aggregate cement ratio by weight S/C as the parameter. The bleeding ratio of mortar mixed in two stages with Wi/C made extremely low becomes higher compared with the conventional simultaneous mixing method. However, the bleeding ratio declines with increase in Wi/C and becomes extremely low compared with the case of conventional simultaneous mixing. And, when a certain value of Wi/C is exceeded, the bleeding ratio increases again. In this way, there exists an Optimum Wi/C at which bleeding ratio becomes a minimum. Such a condition in which the bleeding ratio becomes a minimum is prominent with a rich mortar of low s/c. This condition is alleviated with a lean mortar of high S/C, while the Optimum Wi/C is in a wide range, and moreover, the value of Wi/C itself becomes large as shown in Fig.6.2 In this way, establishment of Wi/C, the ratio by weight of primary water to cement in. primary mixing, is important when adopting the two-stage mixing method, the quality of concrete differs greatly depending on the value set.

Fig 6.1 two stage mixing method

Fig 6.2 Primary water-cement ratio and bleeding ratio of mortar

Fig 6.3 Primary water-cement ratio and qualities of concrete

6.2. QUALITY OF CONCRETE

The results of an experiment on concrete are shown in Fig.6.3 This was a case of the final water-cement ratio W/C being 0.50 and slump l8 cm. Two-stage mixing was performed and especially in the ränge of Wi/C of 0.l6 to 0.2U, bleeding was reduced drastically to an extent that hardly any bleeding water could be detected. Practically no change was seen in slump even when two-stage mixing was performed. As a result of testing the amount of dewatering when a pressure of 3.43 MPa was applied to investigate the pumpability of concrete, it was found that concrete made by two-stage mixing was 20 to 60 percent smaller in the amount of dewatering for both the initial and final stages of pressurizing. This trend was more prominent the lower the slump and the lower the water-cement ratio. It is clear from Fig 6.3 that compressive strength of concrete is increased by two-stage mixing. Fig 6.4 shows cases of slump maintained constant at approximately 18 cm and with water-cement ratio varied between 0.30 and 0.60. When using the two-stage mixing method compressive strengths and Splitting tensile strengths are 10 to 20 percent higher compared with concrete made by the conventional simultaneous introduction method.           

6.3. IMPROVEMENT IN QUALITY OF STRUCTURE

It can hardly be said that concrete structures have always been entirely of uniform quality In the paste, qualities differing between upper and lower positions and parts of the structure, and depending on the conditions when executing work, Particularly, with walls and columns taller than 3m, wet consistency concrete of slump higher than 15 cm is often used, and with such members the strength of concrete and bond strength with reinforcing steel are lower at the upper parts and these become weak points of the structure. Concretes made by the two-stage mixing method and by the conventional simultaneous mixing method were placed in reinforced concrete wall panels of 3-m height, 0.8-m width, and 15-cm thickness, and the compressive strengths of the concretes in the vertical directions of the panels and bond strength distributions of reinforcing bars were compared. The results are given in Fig. 6.5.The compressive strengths and bond strengths according to core samples from various heights are shown. Concrete slump was 18 cm, and water-cement ratio 0.50.

Whereas bond strengths at a height of 2.7 m were 40 to 60 percent lower compared with the bottom parts of the wall panels when using concrete mixed by the conventional method, the decrease in case of the two-stage mixing method was limited to a maximum of 25 percent. Bond strengths per se were higher with the twostage mixing, and the degree of increase was greater the higher the location in the wall panel, Although not as prominent as with bond strengths, the distributions of compressive strengths showed the effectiveness of two-stage mixing. That is, compressive strengths at various locations in the wall panels were increased 10 to 20 percent over the conventionally-mixed method, and strength reductions did not occur even at a height of 2.5m Such an effect of the two-stage mixing method was confirmed with a reinforced concrete wall panel 8 m in height, 1 m in width, and 40 cm in thickness. In essence, compared with the bottom part of the wall panel, the reductions at a height of 7.5 m were held to 25 percent for bond strength of reinforcing steel and 15 percent for compressive strength, so that the strength reductions were smaller. That it is possible to reduce Variation in quality at various locations in a concrete structure in this way is because with the two-stage mixing method a concrete with extremely little segregation in the forms of bleeding and settling of aggregates is successfully made.

Fig 6.4 Compressive strength and tensile strength concrete

Fig 6.5 Distribution of compressive strength of concrete and bond strength of reinforcing steel in wall panels

6.4 CASES OF PRACTICAL USE

Concrete made by the two-stage mixing method was used in large quantities of tunnels such as Seikan Tunnel, Subsequently, it was also used in offshore concrete. Application to buildings and dams lagged behind slightly, this concrete came to be adopted as the excellent uniformity and stability of quality and the good workability received high regard.

7. ADVANTAGES

· Based on this research, two-stage mixing can significantly improve concrete
uniformity.
· Due to increased mixing time (from the time the cement contacts water to the end of
mixing), two-stage mixed concrete generally shows a reduced slump.
· Two-stage mixing may increase concrete strength 5%–10% over conventionally
mixed concrete. Laboratory results show an 8%–10% increase, field results show a
5%–10% increase.
· This method improves the quality of recycled aggregate effectively.
· Due to use of recycled aggregate no dumbing of debris is required, so the landfills are
saved.
· Using recycled materials as gravel reduces the need for gravel mining.
· Using recycled concrete as the base material for roadways reduces the pollution
involved in trucking material.

8. DISADVANTAGES

· Not prove cost effective in practice due to additional cost of recycling of aggregate
· Recycled aggregate has less density result in higher porosity. Hence more quantity of water needed.

9. CONCLUSION

Concrete ingredients by two stage mixing forms a homogeneous mixture with sufficiently hydrated and uniformly distributed cement particles. Moreover, the mixing operation properly coats the aggregates with paste to eliminate undissolved cement particles. Eventhough, modern concrete increasingly contains fine cementitious materials, low water to-binder ratios, and high binder contents, the two stage approach prevents cementitious particles to agglomerate, helps cementitious materials for hydrating uniformly, and improve concrete workability. The poor quality of RAC resulted from the higher water absorption, higher porosity, weaker ITZ between RA and new cement mortar hampers the application of RAC for higher grade applications. In this study, the two-stage mixing approach is proposed to strengthen the weak link of RAC, which is located at the interfacial transition zone (ITZ) of the RA. The two-stage mixing approach gives way for the cement slurry to gel up the RA, providing a stronger ITZ by filling up the cracks and pores within RA. From the laboratory experiments, the compressive strengths have been improved. This two-stage mixing approach can provide an effective method for enhancing the compressive strength and other mechanical performance of RAC, and thus, the approach opens up a wider scope of RAC applications.

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