Scholarly article on topic 'Microstructure and properties of concrete using bottom ash and waste foundry sand as partial replacement of fine aggregates'

Microstructure and properties of concrete using bottom ash and waste foundry sand as partial replacement of fine aggregates Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Yogesh Aggarwal, Rafat Siddique

Abstract The possibility of substituting natural fine aggregate with industrial by-products such as waste foundry sand and bottom ash offers technical, economic and environmental advantages which are of great importance in the present context of sustainability in the construction sector. The study investigated the effect of waste foundry sand and bottom ash in equal quantities as partial replacement of fine aggregates in various percentages (0–60%), on concrete properties such as mechanical (compressive strength, splitting tensile strength and flexural strength) and durability characteristics (rapid chloride penetration and deicing salt surface scaling) of the concrete along with microstructural analysis with XRD and SEM. The results showed that the water content increased gradually from 175kg/m3 in control mix (CM) to 238.63kg/m3 in FB60 mix to maintain the workability and the mechanical behavior of the concrete with fine aggregate replacements was comparable to that of conventional concrete except for FB60 mix. The compressive strength was observed to be in the range of 29–32MPa, splitting tensile strength in the range of 1.8–2.46MPa, and flexural strength in the range of 3.95–4.10MPa on the replacement of fine aggregates from 10% to 50% at the interval of 10%. Furthermore, it was observed that the greatest increase in compressive, splitting tensile strength, and flexural strength compared to that of the conventional concrete was achieved by substituting 30% of the natural fine aggregates with industrial by-product aggregates. The inclusion of waste foundry sand and bottom ash as fine aggregate does not affect the strength properties negatively as the strength remains within limits except for 60% replacement. The morphology of the formations arising as a result of the hydration process was not observed to change in the concrete with varying percentages of waste foundry sand and bottom ash.

Academic research paper on topic "Microstructure and properties of concrete using bottom ash and waste foundry sand as partial replacement of fine aggregates"

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Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

Microstructure and properties of concrete using bottom ash and waste foundry sand as partial replacement of fine aggregates

Yogesh Aggarwala'*, Rafat Siddique b

a Civil Engineering Department, National Institute of Technology, Kurukshetra, India b Civil Engineering Department, Thapar University, Patiala 147004, India

HIGHLIGHTS

• Use of industrial by-products i.e. bottom ash and waste foundry sand in concrete. . Microstructure analysis of concrete using XRD and SEM.

• Mechanical and durability properties of concrete using industrial by-products.

ARTICLE INFO

ABSTRACT

Article history:

Received 19 September 2013

Received in revised form 6 December 2013

Accepted 16 December 2013

Keywords:

Industrial by-products Waste foundry sand Bottom ash Mechanical properties Rapid chloride penetration Deicing salt surface scaling XRD SEM

The possibility of substituting natural fine aggregate with industrial by-products such as waste foundry sand and bottom ash offers technical, economic and environmental advantages which are of great importance in the present context of sustainability in the construction sector. The study investigated the effect of waste foundry sand and bottom ash in equal quantities as partial replacement of fine aggregates in various percentages (0-60%), on concrete properties such as mechanical (compressive strength, splitting tensile strength and flexural strength) and durability characteristics (rapid chloride penetration and deicing salt surface scaling) of the concrete along with microstructural analysis with XRD and SEM. The results showed that the water content increased gradually from 175kg/m3 in control mix (CM) to 238.63 kg/m3 in FB60 mix to maintain the workability and the mechanical behavior of the concrete with fine aggregate replacements was comparable to that of conventional concrete except for FB60 mix. The compressive strength was observed to be in the range of 29-32 MPa, splitting tensile strength in the range of 1.8-2.46 MPa, and flexural strength in the range of 3.95-4.10 MPa on the replacement of fine aggregates from 10% to 50% at the interval of 10%. Furthermore, it was observed that the greatest increase in compressive, splitting tensile strength, and flexural strength compared to that of the conventional concrete was achieved by substituting 30% of the natural fine aggregates with industrial by-product aggregates. The inclusion of waste foundry sand and bottom ash as fine aggregate does not affect the strength properties negatively as the strength remains within limits except for 60% replacement. The morphology of the formations arising as a result of the hydration process was not observed to change in the concrete with varying percentages of waste foundry sand and bottom ash.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

High consumption of natural sources, high amount production of industrial wastes and environmental pollution are some of the factors which are responsible for obtaining new solutions for a sustainable development. A sustainable development can be achieved only if the resource efficiency increases. The resource efficiency increment is possible by the reduction in use of energy

* Corresponding author. Tel.: +91 1744 233361; fax: +91 1744 233050. E-mail addresses: yogesh.24@rediffmail.com (Y. Aggarwal), siddique_66@ yahoo.com (R. Siddique).

and materials. Thus, solution is utilization of industrial by-products or solid wastes such as fly ash (FA), bottom ash (BA), waste foundry sand (FS), slag, silica fume, and waste glass in producing concrete. These concrete technologies reduce the negative effects on economical and environmental problems of concrete industry by having low costs, high durability properties and environmental friendliness.

When coal is burned in a coal fired boiler, it leaves behind ash, some of which is removed from the bottom of the furnace known as bottom ash, and some of which is carried upward by the hot combustion gases of the furnace, and removed by collection devices (fly ash). Worldwide, coal-fired power generation presently accounts for roughly 38% of total electricity production. Coal use

0950-0618/$ - see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.12.051

in some of the more developed countries is static or is in decline. Significant increases in coal-fired generation capacity are taking place in many of the developing nations and large capacity increases are planned. During coal-fired electric power generation three types of coal combustion products (CCPs) are obtained. These by-products; fly ash, bottom ash and boiler slag are the largest sources of industrial waste. Utilization of CCPs in construction industry is an important issue involving reduction in technical and economical problems of plants, besides reducing the amount of solid wastes, greenhouse gas emissions and conserving existing natural resources. Some authors have reported the use of bottom ash in concrete as partial replacement of portland cement [1-3] or as a partial replacement of fine aggregates [4-9].

A foundry produces metal castings by pouring molten metal into a preformed mold to yield the resulting hardened cast. The metal casts include iron and steel from the ferrous family and aluminum, copper, brass and bronze from non-ferrous family. Waste foundry sand is high quality silica sand with uniform physical characteristics. It is a by-product of ferrous and non-ferrous metal casting industries, where sand has been used for centuries as a molding material because of its thermal conductivity. Foundries successfully recycle and reuse the sand many times. When the sand can no longer be reused in the foundry, it is removed from the foundry and is termed as waste foundry sand. Several authors have reported the use of used-waste foundry sand in various civil engineering applications such as highway applications [10-17], leaching aspect of usage of foundry sand [18-21], controlled low strength materials [22-24], concrete and concrete related products like bricks, blocks and paving stones [25-29], asphalt concrete [30].

Coal-combustion bottom ash and used foundry sand are abundant by-products which appear to possess the potential, to partially replace regular sand as a fine aggregate in concrete mixtures, providing a recycling opportunity for them. If these recycled materials can be substituted for part of the cementitious and virgin aggregate materials in concrete mixtures without sacrificing, or even improving strength and durability, there are clear economic and environmental gains. One of the primary impediments to beneficial reuse of industrial by-products such as waste foundry sands and bottom ash is a lack of engineering data that designers can use to evaluate the efficacy and economy of using the by-product in place of the natural sand. The engineering properties and behavior of sands can be readily estimated from the literature for use in preliminary design. In contrast, there is a dearth of similar information for industrial by-products and there are insufficient data to confirm that industrial by-products, which appear similar to sands, also have comparable engineering properties. With emphasis now being placed on engineering for sustainable development, there is a pressing need to provide this practical information to designers. Fulfilling this need is the primary purpose of this study. The objective is to provide practical information, regarding the strength, durability and micro-structural properties of bottom ash and waste foundry sand as replacement of fine aggregates in concrete. Both waste foundry sand and bottom ash have been studied as aggregate replacement, separately. The value of the current research is the use of both together. The present experimental study was conceived following the general purpose of testing new sustainable building processes and modern production systems, aimed not only at saving natural raw materials and reducing energy consumption, but also to recycle industrial by-products. The objectives of this study are to investigate the effect of use of bottom ash and waste foundry sand in equal quantities as partial replacement of fine aggregates in various percentages (0-60%), on concrete properties such as mechanical and durability characteristics of the concrete along with micro-structural analysis with XRD and SEM.

2. Experimental program

The effect of using various percentages of bottom ash and waste foundry sand as partial replacement of the fine aggregate in concrete was investigated. Also, the effect of incorporating waste foundry sand and bottom ash, in concrete on the mechanical, durability properties and microstructure were evaluated.

2.1. Materials and mix proportions

Portland Pozzolana Cement (53 MPa) conforming to Indian standard specifications IS:1489-1991 [31] with consistency as 27%, specific gravity as 3.56 and fineness as 5%, was used. Locally available natural sand with 4.75 mm maximum size was used as fine aggregate, fulfilling the requirements of ASTM C 33-02a [32] and IS:383-1970 [33] along with crushed stone of 20 mm maximum size used as coarse aggregate. Locally available waste foundry sand was used as partial replacement of fine aggregates (regular sand). The waste foundry sand showed lower fineness modulus and bulk density than the regular sand. As per the particle size distribution of the waste foundry sand, the size corresponding to 50% of passing (d50) was around 33 im and average diameter of waste foundry sand particle was observed to be 28.8 im. Coal bottom ash obtained from Panipat Thermal Power Station, Panipat, Haryana, was also used as partial replacement of fine aggregates. The properties of coal bottom ash conformed to IS:3812-2003. The particle size distribution of bottom ash was measured, which showed that, of the particles 100% were smaller than 56 im and 38% were smaller than 31.3 im with average diameter of the particle size distribution was 33.4 im with standard mean deviation of 8.1 im for bottom ash. Table 1 gives the chemical composition of waste foundry sand and bottom ash while Table 2 gives the physical properties of the aggregates used. A polycarb-oxylic ether based superplasticizer of CICO brand complying with ASTM C-494 type F [34], IS:9103-1999 [35] and IS:2645-2003 [36] was used.

Seven mix proportions were prepared. First was control mix (without bottom ash and waste foundry sand), and the other six mixes contained bottom ash and waste foundry sand in equal proportions. Fine aggregate (sand) was replaced with bottom ash and waste foundry sand by weight. The proportions of fine aggregate replaced ranged from 10% to 60% at the increment of 10%. Mix proportions are as given in Table 3. The control mix without waste foundry sand and bottom ash was proportioned as per Indian standard specifications IS:10262-1982 [37], to obtain a 28-day cube compressive strength of 36 MPa.

For these mix proportions, required quantities of materials were weighed. The mixing procedure adopted was as follows: First, the cement, waste foundry sand, and coal bottom ash were dry mixed till a uniform color was obtained without any clusters of cement, waste foundry sand and bottom ash particles. Weighed quantities of coarse aggregates and sand were then mixed in dry state, thoroughly until a homogeneous mix was obtained. Water was then added in three stages as 50% of total water to the dry mix of concrete in first stage; 40% of water and superplasticizer to the wet mix; Remaining 10% of water was sprinkled on the above mix and it was thoroughly mixed. All the moulds were properly oiled before casting the specimens. The casting immediately followed mixing, after carrying out the tests for fresh properties. The top surface of the specimens was scraped to remove excess material and achieve smooth finish. The specimens were removed from moulds after 24 h and cured in water till testing or as per requirement of the test.

2.2. Testing procedure

Fresh concrete properties such as slump flow, compaction factor, vee-bee con-sistometer were determined according to an Indian Standard specification IS:1199-1959 [38]. The results are presented in Table 3. The 150 mm concrete cubes were cast for compressive strength, cylinders of size 150 mm x 300 mm for splitting tensile strength and beams of size 100 x 100 x 500 mm for flexural strength. After required period of curing, the specimens were taken out of the curing tank and their surfaces were wiped off. The various tests performed were compressive strength test of cubes (150 mm side), splitting tensile strength of cylinders (150 mm x 300 mm), at 7, 28, 90, and 365 days and flexural strength of beams (100 x 100 x 500 mm) at 28, 90, and 365 days, as per IS:516-1959 [39].

The cylinders (100 mm x 200 mm) were cast for rapid chloride penetration resistance test and were sliced 51-mm (2-in.) thick of 102-mm (4-in.) nominal diameter. Rapid chloride penetration resistance test (according to ASTM C 1202-97 [40] covered the determination of the electrical conductance of concrete to provide a rapid indication of its resistance to the penetration of chloride ions. The test method consisted of monitoring the amount of electrical current passed through 51-mm (2-in.) thick slices of 102-mm (4-in.) nominal diameter cores or cylinders for a 6-h period. A potential difference of 60 V dc was maintained across the ends of the specimen, one of which was immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, was related to the resistance of the specimen to chloride ion penetration.

The test method (according to ASTM C 672 [41]) covers the determination of the resistance to scaling of a horizontal concrete surface exposed to freezing and

Table 1

Chemical properties of coal bottom ash and waste foundry sand.

Constituents Coal bottom ash Waste foundry sand

Percent by weight Codal requirement Percent by weight Requirements (American foundry men's society, 1991)

Silica (SiO2) 57.76 35%(min) 78.81 87.9%

Iron oxide (Fe2O3) 8.56 70%(min)Si02 + Fe2O3 + Al203 4.83 0.94%

Alumina (Al2O3) 21.58 70%(min)Si02 + Fe203 + Al203 6.32 4.70%

Calcium oxide (CaO) 1.58 - 1.88 0.14%(min)

Magnesium oxide (MgO) 1.19 5%(max) 1.95 0.3%

Total sulphur (SO3) 0.02 3%(max) 0.05 0.09%

Alkalies (a) sodium oxide (Na2O) 0.14 1.5%(max) -

(b) Potassium oxide (K2O) 1.08

Chloride 0.01 0.05%(max) 0.04

Loss on ignition 5.80 5%(max) 2.15 5.15%(max)

Table 2

Physical properties of aggregates.

Aggregates Specific gravity Unit weight (kg/m3) Fineness modulus

Sand 2.63 1890 3.03

Waste foundry sand 2.61 1638 1.78

Bottom ash 1.93 948 1.60

Coarse aggregates 2.77 1650 6.74

Table 3

Mix proportions of concrete mixes containing bottom ash & waste foundry sand.

Mix no. CM FB10 FB20 FB30 FB40 FB50 FB60

Cement (kg/m3) 350 350 350 350 350 350 350

Foundry Sand (%) 0 5 10 15 20 25 30

Foundry Sand (kg/m3) 0 30.25 60.50 90.75 121.00 151.25 181.50

Bottom ash (%) 0 5 10 15 20 25 30

Bottom ash (kg/m3) 0 30.25 60.50 90.75 121.00 151.25 181.50

Water (kg/m3) 175 180.30 185.60 190.90 201.50 212.12 238.63

W/C 0.5 0.52 0.53 0.55 0.58 0.61 0.68

Sand SSD (kg/m3) 605 544.5 484.0 423.5 363.0 302.5 242.0

Fine aggregate (kg/m3) 605 605 605 605 605 605 605

Coarse aggregate (kg/m3) 1260 1260 1260 1260 1260 1260 1260

Superplasticizer (kg/m3) 1.75 1.75 1.75 1.75 1.75 1.75 1.75

Slump (mm) 30 30 30 30 30 30 30

Compaction factor 0.83 0.81 0.78 0.81 0.78 0.78 0.81

Vee-bee consistometer (sec) 5.98 5.20 6.42 5.54 6.44 6.68 5.26

Air temperature (°C) 23 25 24 26 25 25 34

Concrete temperature (°C) 25 25 25 26 25 27 28

Air content (%) 2.1 2.6 2.6 2.7 2.7 2.9 3.4

Fresh concrete density (kg/m3) 2392 2397 2402 2408 2418 2428.87 2455.38

thawing cycles in the presence of deicing chemicals. It is intended for the use in evaluating surface resistance qualitatively by visual examination. Specimens of size 225 x 225 x 25 mm were prepared for all mixes. A dike of 25 mm wide and 20 mm high was placed along the perimeter of the top surface of the specimens. The specimens were removed from the moulds after 24 h and then cured. Since, the concretes with strength at different ages are to be compared, the specimen were cured till that age. At the desired age, the specimens were removed from moist storage and stored in air for 14 days. After completion of moist and air curing, the flat surface of the specimen was covered with 6 mm thick layer solution of calcium chloride solution and water. 100 mL of solution contains 4g of anhydrous calcium chloride. The specimens were placed in freezing environment for 16-18 h. The specimens were removed from freezer and placed in air for 6-8 h. Water was added at each cycle, as necessary, to maintain proper depth of solution. Cycle was repeated after every 24 h, flushing the surface at the end of each 5 cycles. After making the visual examination, the solution was replaced and the test continued for 50 cycles. Visual rating of the surface on the basis of the scale given in Table 4 was carried out.

X-ray diffraction analysis (XRD) was done on Philips PW 1140/09. Diffractom-eter operated at 35 KV, using Cu Ka radiation and Ni filler (k = 1.5418 A). The samples for X-ray diffraction analysis were prepared in powdered form. The concrete sample was taken from the inner core of the matrix. X-ray diffraction is based on the fact that, in a mixture, the measured intensity of a diffraction peak is directly proportional to the content of the substance producing it (Soroka [42]). Since 2d is a known constant, the 28 setting of each peak corresponds to a certain wave length.

Table 4

Rating for deicing salt surface scaling (ASTM C 672).

Rating Condition of surface

0 No scaling

1 Very slight scaling (3 mm depth, max., no coarse aggregates visible)

2 Slight to moderate scaling

3 Moderate scaling (some coarse aggregate visible)

4 Moderate to severe scaling

5 Severe scaling (coarse aggregate visible over entire surface)

The type, amount, size, shape, and distribution of phases present in a solid constitute its microstructure. It is the application of transmission and scanning electron microscopy techniques which has made it possible to resolve the microstructure of the materials to a fraction of one micrometer. Although, concrete is the most widely used structural material, its microstructure is heterogeneous and highly complex. Also, the microstructure-property relationships in concrete are not fully developed.

At the macroscopic level, concrete may be considered as a two-phase material, consisting of aggregate particles dispersed in a matrix of cement paste. At the microscopic level, complexities of the concrete microstructure are evident that two phases are neither homogeneously distributed with respect to each other, nor are they themselves homogeneous (Lea's [43]).

Table 5

Various strengths of CM and FB mixes.

Age 7-day (MPa) 28-day (MPa) 90-day (MPa) 365-day (MPa)

Mix Compressive Splitting Compressive Splitting Flexural Compressive Splitting Flexural Compressive Splitting Flexural

strength Tensile strength Tensile strength strength Tensile strength strength Tensile strength

strength strength strength strength

CM 25.58 1.30 36.27 2.08 4.44 43.91 2.66 5.03 44.42 2.97 5.37

FB10 17.67 1.34 29.02 1.80 4.10 33.47 2.04 4.90 40.59 2.12 5.24

FB20 18.25 1.39 29.63 2.05 4.00 34.69 2.45 4.80 41.44 2.78 5.05

FB30 19.36 1.54 31.81 2.46 4.34 37.37 2.83 4.97 42.69 3.22 5.30

FB40 18.42 1.46 29.95 2.35 3.87 35.85 2.72 4.61 42.03 2.98 4.83

FB50 18.91 1.41 30.53 2.25 3.95 36.86 2.58 4.71 42.27 2.86 4.93

FB60 13.45 0.84 21.08 1.45 3.60 24.03 1.52 4.46 28.22 1.53 4.55

Table 6

Decrease in compressive strength at various ages in comparison to reference mix (CM).

Mix 7-day(%) 28-day(%) 90-day(%) 365-day(%)

FB10 30.92 19.99 23.78 8.62

FB20 28.66 18.31 21.00 6.71

FB30 24.32 12.30 14.89 3.89

FB40 27.99 17.42 18.36 5.38

FB50 26.08 15.83 16.06 4.84

FB60 47.42 41.88 45.27 36.47

Original microstructure and morphology of the hydrate mixes were observed on fractured surfaces. Fractured small samples were mounted on the SEM stubs with gold coating. The scanning electron microscopic studies of various concrete samples and constituent materials were carried out using Philips XL20 Scanning Electron Microscope. The concrete specimens were first cured in water for 365 days and then oven dried at 105 "C for 24 h.

3. Results and discussions

3.1. Fresh concrete properties

The workability of fresh concrete is a composite property which includes the diverse requirements of stability, mobility, compacti-bility, placeability, and finishability. Slump is a measure indicating the consistency or workability of concrete. Slump for control mix CM and FB mixes was observed to be 30 mm. The compaction factor values for control mix, and FB mixes corresponded to the slump flow values as per Table 3. The presence of finer waste foundry sand particles and bottom ash, in concrete lead to the increase in the water demand, as compared to the regular sand particles. Thus,

to maintain the workability within specified range (slump has been kept constant at 30 mm), the water content was gradually increased with increase in replacement of sand with waste foundry sand and bottom ash, which gave an idea about the increase in water demand due to increase in replacement of sand with waste foundry sand and bottom ash. The water content increased gradually from 175 kg/m3 in control mix (CM) to 238.63 kg/m3 in FB60 mix. It was observed that for initial replacements of 10%, 20% and 30%, the increase in water content was constant and thereafter for 40% and 50%, again it remained constant but almost twice the value of initial replacements. For FB60 mix, the water content increased drastically which reflected on various strengths.

3.2. Compressive strength

Compressive strength results of FB mixes made with waste foundry sand and bottom ash in equal percentages are as given in Table 5. There is a decrease in the compressive strength of concrete mixes with the inclusion of waste foundry sand and bottom ash as replacement of regular sand. The percentage decrease in comparison to reference mix at various ages is as shown in Table 6. It was observed that the mixes with replaced fine aggregates had less difference from the reference mix as age increased to 365 days; the difference between CM and FB mixes left between 3% and 9% for all mixes (except FB60 mix which has showed higher decrease at all ages, but that also has decreased with increase in age).

The strength variation was observed to be marginal in the replaced mixes i.e. FB mixes, as the waste foundry sand tends to increase the strength as observed in the waste foundry sand mixes

Fig. 1. Variation of cube compressive strength with age for CM & FB mixes.

Fig. 2. Variation of splitting tensile strength with age for CM & FB mixes.

Table 7

Ratio of splitting tensile strength and cube compressive strength for FB mixes.

Mix 7-day 28-day 90-day 365-day

FB10 0.076 0.062 0.061 0.052

FB20 0.076 0.069 0.071 0.067

FB30 0.080 0.077 0.076 0.075

FB40 0.079 0.078 0.076 0.071

FB50 0.075 0.074 0.070 0.068

FB60 0.062 0.069 0.063 0.054

For FB mixes - The ratio of splitting tensile strength to cube compressive strength of concrete varies from 5% to 8%.

For Normal strength mixes - The ratio of splitting tensile strength to cube compressive strength of concrete is in the range of 7-9% [45].

given elsewhere [44] and inclusion of same proportion of bottom ash tends to decrease the strength [4]. Thus, not much difference in strength was observed from 10% to 50% replacement of sand with equal percentages of waste foundry sand and bottom ash. Also, the strength observed at FB30 was highest as compared to the other FB mixes, but less than that of reference mix. The maximum strength was obtained at replacement of 30% (15% waste foundry sand and 15% bottom ash) in the replaced mixes, which can be adjudged as optimum mix.

Compressive strength of all FB mixes increased with age. At 7 days, all FB mixes also showed the strength lower than CM mix but as the age increases to 365 days gradually attained strengths marginally lower than the reference mix CM as shown in Fig. 1.

The difference in the strengths of various mixes from reference mix was observed to decrease for various FB mixes as given in Table 6 with increase in age from 7 to 365 days. The reference mix gained increase in strength from 7 to 28 days of 41.79%, from 28 to 90 days of 21.06% and from 28 to 365 days of 1.16% which is characteristic of normal concrete. The FB mixes attained a relatively constant strength of 56-64% from 7 to 28 days, 14-19% from 28 to 90 days, and 14-21% from 90 to 365 days.

The FB mixes showed 7-day strength between 60% and 63% of 28-day strength; 90-day strength between 114% and 120%; and 365-day strength 133-140% of the 28-day strength. It was observed that at 7, and 90 days the gain of strength was less than that of the CM mix but at 365 days the strength gain for FB mixes, exceeded the strength gain of CM mix.

3.3. Splitting tensile strength

The results of splitting tensile strength of FB mixes are indicated in Table 5. The increase of 3.1%, 6.92%, 18.46%, 12.31% and 8.46% was observed for the mixes FB10-FB50 and decrease of 35.38% was observed for the mix FB60 at 7 days with regards to reference mix. The decrease of 13.46%, 1.44% and 30.29% for mixes FB10, FB20 and FB60 was observed at 28 days, with increase of 18.27%, 12.98%, and 8.17% for the mixes FB30, FB40 and FB50 in comparison to the reference mix CM. Similarly, at 90 days, decrease of 23.31%, 7.89%, 3.01% and 42.86% for mixes FB10, FB20, FB50, and

Fig. 3. Variation of flexural strength with age for CM & FB mixes.

Table 8

Ratio of flexural strength and cube compressive strength for CM & FB mixes.

Mix 28-day 90-day 365-day

CM 0.122 0.115 0.121

FB10 0.141 0.146 0.129

FB20 0.135 0.138 0.122

FB30 0.136 0.133 0.124

FB40 0.129 0.129 0.115

FB50 0.129 0.128 0.117

FB60 0.171 0.186 0.161

For normal concrete - The ratio of flexural strength to cube compressive strength of concrete is nearly 0.11 to 0.18 [45].

Table 9

Ratio of flexural strength and splitting tensile strength for CM & FB mixes.

Mix 28-day 90-day 365-day

CM 2.135 1.891 1.808

FB10 2.278 2.402 2.472

FB20 1.951 1.959 1.817

FB30 1.764 1.756 1.646

FB40 1.647 1.695 1.621

FB50 1.756 1.826 1.724

FB60 2.483 2.934 2.974

For normal concrete - Flexural strength is about 1.5-2 times the splitting tensile strength of concrete [45,61,62].

FB60 was observed, with increase of 6.39% and 2.26% for the mixes FB30 and FB40 in comparison to the reference mix CM. The trend observed was same at 365 days as indicated in Fig. 2.

At 7 days, all FB mixes showed the strength higher than CM mix except FB60, but as the age increases to 365 days all mixes showed almost comparable strengths to that of reference mix CM.

Table 7 gives the ratio of splitting tensile strength to the cube compressive strength of FB mixes. The results indicate that splitting tensile vary from 0.062 to 0.080; 0.062 to 0.078; 0.061 to 0.076 and 0.052 to 0.075 times the compressive strength, at ages of 7, 28, 90 and 365 days, respectively.

3.4. Flexural strength

The flexural strength of all FB mixes was observed to be less than the strength of reference mix CM and FB30 attained maximum strength i.e. more than all other FB mixes at all the ages. The flexural strength results of concrete mixes are shown in Table 5. Like compressive and splitting tensile strength, flexural strength of concrete mixes varied marginally with the increase in waste foundry sand and bottom ash content. The 28-day flexural strength of CM mix was observed as 4.44 MPa, whereas mixes FB10, FB20, FB30, FB40, FB50, and FB60 showed a decrease of 7.66%, 9.91%, 2.25%, 12.84%, 11.04%, and 18.92%. At 90 days, a decrease of 2.58%, 4.57%, 1.19%, 8.35%, 6.36% and 11.33% was observed for mixes FB10, FB20, FB30, FB40, FB50 and FB60, in comparison to the reference mixture CM. The same trend was observed at the age of 365 days with FB10, FB20, FB30, FB40, FB50, and FB60 showing decrease of 2.42%, 5.96%, 1.3%, 10.06%, 8.19%, and 15.27%.

From the results given in Fig. 3, it is also evident that flexural strength of FB mixes increased with the age. The FB mixes showed decrease in the strengths in comparison to reference mix as 7.662.42%, 9.91-5.96%, 2.25-1.30%, 12.84-10.06%, 11.04-8.19%, and 18.92-15.27% for mixes FB10, FB20, FB30, FB40, FB50, and FB60 with increase in age from 28 to 365 days. Also, an increase in strength from 28 to 90 days was observed to be 13.29% for CM mix whereas FB mixes showed increase in strength from 14.52% to 23.89%, from 28 to 90 days. Between 90 and 365 days, an increase in strength for CM mix was 6.76% and the FB mixes showed an increase of 2.02-6.94%.

3.5. Relationship of flexural strength to compressive and splitting tensile strength

The flexural strength as observed from Table 8 was 12.2%, 11.5% and 12.1% of the cube compressive strength at the age of 28 days, 90 days, and 365 days, respectively of the reference mix CM. The FB mixes showed the variation of 12.9-17.1% at 28 days, 12.8-18.6% at 90 days and 11.5-16.1% at 365 days of the cube compressive strength as has also been observed by Mehta and Monterio [45] and Price [46].

Table 10

Comparison of experimental values of flexural strength (fr) with the theoretical values predicted by other researchers.

28 days cube compressive strength, fck (N/mm2)

Flexural Strength, Fr (N/mm2)

Theoretical values as per references

Ratios based on experimental values fr/pfck

committee

IS:456-2000

CM 36.27 4.44 4.19 4.22 0.7372

FB10 29.02 4.10 3.55 3.77 0.7611

FB20 29.63 4.00 3.60 3.81 0.7348

FB30 31.81 4.34 3.75 3.95 0.7695

FB40 29.95 3.87 3.64 3.83 0.7072

FB50 30.53 3.95 3.70 3.87 0.7149

FB60 21.08 3.60 3.01 3.21 0.7841

Average value 0.744

The flexural strength was observed to be 2.13, 1.89 and 1.81 times of the splitting tensile strength at the age of 28 days, 90 days and 365 days, respectively of the reference mix CM. The FB mixes showed the variation of 1.64-2.4 times at 28 days, 1.69-2.9 times at 90 days and 1.6-2.9 times at 365 days of the splitting tensile strength as given in Table 9.

Table 10 shows the computations of ratios of flexural strength (fr) to the square root of the cube compressive strength (^fck) of experimental values of present investigation and the theoretical values of flexural strength based on expressions proposed by earlier investigators (ACI Committee [47], IS:456-2000 [48]). An average value so obtained for different concrete mixes has been found to be 0.744. This, in the general form, would give an expression as

Fr = 0.744^/fck

Fig. 4. Comparison of experimental values of flexural strength (Fr) with the theoretical values predicted by other researchers.

Further, a comparison of the experimental results has been made with those of other authors and shown in Fig. 4. The results of the flexural strength of present study are less than the values of the flexural strength as reported by Siddique et al. [25] and higher than those reported by ACI Committee 363 and slightly higher than IS:456-2000 which gives the average value as 0.7. It was observed that the concrete mixes containing waste foundry sand and bottom ash behave in similar manner to that as plain concrete.

Theoretical expressions for flexural strength:

• ACI Committee 363, proposed the expression for flexural strength as:

Fr = 0.94p/cc in MPa for 21 MPa < fcc 6 83 MPa

• IS 456-2000, proposed the expression for flexural strength for NSC as:

Fr = 0.7 pfck in MPa for fck 6 60 MPa

Compressive strength is assumed as an adequate index for all types of strength, and therefore a direct relationship ought to exist between the compressive and tensile or flexural strength of a given concrete. It has been observed that relationship among various types of strength is influenced by factors like the methods by which the tensile strength is measured (i.e., direct tension test, splitting test, or flexure test), the quality of concrete (i.e., low, moderate or high-strength), the aggregate characteristics (e.g., surface texture and mineralogy), and admixtures (e.g., air-entraining and mineral admixtures) [45].

Bottom ash when used as aggregate replacement from 20% to 50% showed decrease in compressive strength, from control mix which was designed for almost comparable strength as in the present research [49]. Waste foundry sand when used as aggregate replacement from 10% to 60% showed increase in strength, from the control mix which was same as in the present research, with

Table 11

Charge passed and rating for FB mixes.

Mix Charge passed in Charge passed in Chloride ion

coulombs (90-day) coulombs (365-day) penetrability

CM 578 323 Very low

FB10 628 357 Very low

FB20 616 306 Very low

FB30 600 321 Very low

FB40 664 383 Very low

FB50 652 377 Very low

FB60 741 486 Very low

strength at 30% replacement of fine aggregates, being highest in the replaced mixes and even higher than control mix [44]. The use of waste foundry sand and bottom ash together compensate for the increase and decrease of strength due to replacement with waste foundry sand and bottom ash, respectively. It provides an opportunity to use two by-products together and achieve strength comparable to that of reference mix.

4. Durability properties

Durability, and more specifically, resistance to chloride ion penetration and deicing salt surfacing are of major importance for reinforced concrete structures. Before using any industrial byproduct such as waste foundry sand and bottom ash, these behavior needs to be investigated to study the effect of use of waste foundry sand and bottom ash on concrete.

4.1. Resistance to rapid chloride penetration

The ability of concrete to resist the penetration of chloride ions is a critical parameter in determining the service life of steel-reinforced concrete structures exposed to deicing salts or marine environments. The effect of fly ash on the mass transfer properties of concrete has been well documented; however, no documentation of waste foundry sand and bottom ash together, as replacement of fine aggregates in concrete mixes is available. The measurement concerns the chloride ions that come into concrete and also those flowing through the samples.

The RCPT values of FB mixes, at the age of 90 and 365 days are given in Table 11. It can be observed that the RCPT value decreases with increase in age. For the FB mixes, RCPT values were found to be more than reference mix CM with maximum value observed for FB60 mix. Results reported [50-53] for the normal concrete and concrete with various additives also indicate decrease of RCPT values with increase in age.

The RCPT values in coulombs, from literatures available, were observed to be very high for normal concretes as shown in Table 12, mostly above 1500 coulombs for most of the mixes, such as 7890 coulombs [54]; 2766 coulombs [55], 1802 coulombs [56], 2869 coulombs [57], 5250 coulombs [58] at 28 days, 1725 coulombs [51] and 2971 coulombs [52] at 90 days; 3767 coulombs at 180 days [50]. The concrete with other additions like slag and rice husk ash also showed higher RCPT values at 28 days [53,59]. As compared to these concretes, it was observed that all FB mixes in this study showed very low RCPT values, less than 750 coulombs at 90 days and 500 coulombs at 365 days on the

Table 12

Charge passed at various ages for various types of concretes.

Author Type of concrete Fly ash content (%) RCPT Values (Coulombs) 28 56 90 180

Ramezanianpour and Malhotra [50] Normal 0 4251 3767

Oh et al. [55] Normal 0 2766

Naiket al. [51] Normal 0 3150 1725

Mackechnie and Alexander [56] Normal 0 1802

Feng et al. [57] Normal 0 2869

Yang and Chiang [54] Normal 0 7890

Guneyisi [52] Normal 4093 2971

Gu et al. [58] Normal 5250

Gastaldini et al. [53] Rice husk ash 0 3166 2136

20 1557 692

Cho and Chiang [59] Slag 0 9639

20 6355

40 2709

50 2148

70 1350

Table 13

Weight loss and visual rating ASTM C 672 for FB mixes.

Mix no.

90-Day

Weight loss (kg/m2)

365-Day

Weight loss (%)

Visual rating

Weight loss (kg/m2)

Weight loss (%)

Visual rating

0.8099 0.3160 0.4148 0.6123 0.4741 0.4938 0.7901

0.61 0.31 0.36 0.49 0.43 0.40 0.66

1.7975 1.0074 1.4222 2.0741 1.8765 1.6198 2.2716

1.36 0.92 1.17 1.67 1.62 1.30 1.87

Fig. 6. X-ray diffraction pattern of reference (CM) mix.

addition of waste foundry sand and bottom ash, as is evident from Table 11, which comes under very low category as per ASTM-1202C.

It is observed that cement type, w/c ratio, curing condition, and testing age have effect on chloride permeability of concrete. The normal concretes or the concrete with various additives could vary in above parameters, thereby effecting RCPT values. The FB mixes in the present study showed very less RCPT value thereby indicating good resistance to permeability on addition of waste foundry sand and bottom ash in concrete.

4.2. Relation between compressive strength and resistance to chloride ion penetration

The fundamental destructive effect of chlorides is their influence upon the reinforcement corrosion process, is primarily due to their capacity to negate the corrosion inhibiting properties of the alkaline cement paste pore solution. This risk increases with increasing concentration of free chlorides in the pore solution. It is generally believed that there is a threshold concentration of the chloride ions, which must be exceeded before corrosion oc-

Fig. 7a. X-ray diffraction pattern of FB10 mix.

Fig. 7b. X-ray diffraction pattern of FB20 mix.

Fig. 7c. X-ray diffraction pattern of FB30 mix.

curs. The threshold concentration may depend on the concrete that due to carbonation. In extreme cases, the corrosion rate in composition and on environmental parameters. The corrosion real structures can be 5 mm/year compared to 0.05 mm/year due to chloride ingress progresses at a much higher rate than for carbonation-induced corrosion. The correlation as obtained

Fig. 7d. X-ray diffraction pattern of FB40 mix.

Fig. 7e. X-ray diffraction pattern of FB50 mix.

from Fig. 5 shows a good relation between compressive strength and chloride permeability (R2 as 0.80 and 0.78 for 90 days and 365 days). A decrease in RCPT values with increase in strength of FB mixes is also evident from Fig. 5.

Fig. 7f. X-ray diffraction pattern of FB60 mix.

4.3. Deicing salt surface scaling

For the average cumulative mass of scaled-off material obtained after 50 freezing-thawing cycles along with average visual surface ratings determined as per the ASTM C672, results are given in Ta-

ble 13. For reference mix, weight loss was observed as 0.81 kg/m2. All FB mixes showed weight loss lower than the reference mix which was observed as maximum of 0.79 kg/m2 for FB60 mix at 90 days. Except for the FB mixes FB30 and FB60, all other mixes showed weight loss lower than reference mix at 365 days. The mass loss was observed to be between 0.31% and 0.66% at 90 days and between 0.92% and 1.87% at 365 days for the FB mixes. In the present study, the visual rating as per ASTM C 672, for most of the mixes was between 0 and 1, and never exceeded 2 as given in Table 13.

Wang et al. [63] reported mass loss of the normal concrete specimens immersed in CaCl2 deicing solution, as also used in the present study, about 2% of the total weight of the sample at 20 wetting dry cycles. For the present study, the mass loss was observed to be 0.61% and 1.36% at 90 and 365 days for the CM mix.

4.4. X-ray diffraction (XRD)

XRD technique was conducted to analyze the components of concrete mixes and the results are shown in Figs. 6 and 7. The X-ray diffraction pattern and analysis of the concrete mixes i.e. reference mix, and FB mixes was carried out at age of 365 days. One of the major problems encountered in the qualitative and quantitative analysis of cement is that there are strong overlapping of major diffraction peaks of all the main phases of cement components in the angular range of 20 values from 30° to 35° making the identification of the individual components extremely difficult. In all the mixes, C2S, C3S, and C4AF peaks are not visible indicating that they may be totally consumed or overlapping of the peaks of unhy-drated cement by that of Si may have occurred as all analyzed mixes were concrete specimens with large number of aggregate particles containing quartz which resulted in intensive Si peaks. Hence, as shown, SiO2 peak indicating free silica, in CM mix was observed at 1800. The X-ray diffraction pattern observed in FB10 mix was similar to CM mix as the overall replacement of the sand was only 10%, with waste foundry sand and bottom ash as 5% and 5%, respectively. The FB20 to FB50 mix showed SiO2 peak between 4000 and 4500. The strength variation in all the FB mixes was comparatively less, thus the FB40 and FB50 mixes show almost same intensity of SiO2 peak at 4200. FB60 gave the SiO2 peak at 3100. Phase determination could not be carried out as the mixes are complex and XRD analysis is done for single crystalline and poly-crystalline (two) for phase determination. Using the software library, the analysis for various mixtures was carried out which showed that the main component consisted of highly crystalline quartz (compounds shown in the graphs were obtained at various 20 values, from the standard library of the software itself). Since, in the material with crystalline structure, X-rays scattered by ordered features will be scattered coherently "in-phase" in certain directions meeting the criterion for constructive interference. The conditions required for constructive interference are determined by Bragg's Law.

4.5. Scanning electron microscope (SEM) analysis

It is well known that, the calcium-silica-hydrate (C-S-H) is major phase present. The factors that influence the mechanical behavior of C-S-H phases are: size and shape of the particles, distribution of particles, particle concentration, particle orientation, topology of the mixture, composition of the dispersed/continuous phases and the pore structure. Considering various scanning electron microscope images, the phases were indicated studying the literature available [Lea's [41], Yazici [60]]. It was assumed that the bright and dark matter in the images stands for C-S-H gel/ paste and inert aggregates respectively, after having some idea about the presence of C-S-H gel/paste and inert aggregates respec-

tively, then further, referring to the above literatures, the differentiation in various particles of inert aggregates, was tried to be carried out as the medium dark particles considered as waste foundry sand particles while the spherical like particles considered as bottom-ash particles. The assumptions regarding presence of particles is based on the facts that these medium dark particles are seen in almost every sample indicating waste foundry sand particles except the reference mix CM (every sample except the reference mix CM contains waste foundry sand and bottom ash), while the spherical like particles are visible indicating bottom-ash [60]. These assumptions can be justified based on the fact that

Fig. 8. Micrograph of reference (CM) mix.

Fig. 9a. Micrograph of FB10 mix.

Fig. 9b. Micrograph of FB20 mix.

Fig. 9c. Micrograph of FB30 mix.

Fig. 9d. Micrograph of FB40 mix.

Fig. 9e. Micrograph of FB50 mix.

the basic structure of the concrete in all the samples is the same i.e. the mix designed for the reference mix has been kept constant in all the samples changing only the waste foundry sand and the bottom ash part in these mixes.

Fig. 8 is micrograph of reference mix i.e. the SEM image at 1.5 KX magnifications. It shows the formation of proper and clear C-S-H gel in various stages. The encircled portions represent the voids while rest of the picture consists of C-S-H gel and inert aggregates (both fine and coarse). The important point to be noted in the micrograph is that the C-S-H gel i.e. the bright masses with nodules and big chalky gel parts are spread over the entire micro-

Fig. 9f. Micrograph of FB60 mix.

graph, as it is evident from various literatures, the C-S-H gel gets spread over the aggregates thus acting as binders for the paste.

Fig. 9a, micrograph of FB10 mix shows two major features. Firstly, the number of voids in the mix has significantly reduced and secondly, the C-S-H gel paste is not as widely spread as it was in the reference mix, showing some aversion to the binder paste but more importantly, the effect of waste foundry sand and bottom ash has been negative on the strength because of lesser quantity of waste foundry sand and bottom ash, clearly evident as the strength of the mix has deteriorated significantly. The microstructure also shows the presence of waste foundry sand and bottom ash particles of various sizes at various places. The decrease in strength could be attributed to the non formation of proper C-S-H gel as compared to CM mix microstructure, although, at few places the formation of C-S-H gel could be detected as the percentage of the waste foundry sand and bottom ash, added was only 10%.

Figs. 9b-9e, the micrographs of FB20, FB30, FB40 and FB50 show the presence of needle like structures around waste foundry sand and bottom ash particles at various places. The reaction or the formation of C-S-H gel is better, thereby indicating comparative densification of the mixes till 50% replacement.

Fig. 9f, micrograph of FB60 mix shows that the mix has crumbled with coming out of waste foundry sand and bottom ash particles from the mix. The C-S-H gel could not be seen at many places in the micrograph. The most important inference from the image is that the paste is crumbling, as the amount of replacement goes so high in this sample that the equilibrium falls and leads to lower strength.

The micrographs from Figs. 9b-9e show similarity in the pattern formation of C-S-H gel in these mixes with all of them nearly having same strength except for the mix with 60% replacement.

Fig. 9f, micrograph of FB60 mix shows that the mix has crumbled with coming out of waste foundry sand and bottom ash particles from the mix. The C-S-H gel could not be seen at many places in the micrograph. The most important inference from the image is that the paste is crumbling, as the amount of replacement goes so high in this sample that the equilibrium falls and leads to lower strength.

In fact, in the present study the mixes with amount of replacement of sand more than 50% with waste foundry sand and bottom ash, also lead to crumbling at the time of curing done experimentally. These results simply imply that more than 50% replacement of sand with waste foundry sand and bottom ash leads to flaws in concrete, but the best mixture in any case is inarguably the 30% replacement mix. Further, FB30 mix showed large formation of C-S-H gel thus, development of dense microstructure. The fibrous C-S-H formation acts as a thick impermeable membrane for the ingress of chloride ions into concrete. This makes the con-

crete more resistant to aggressive environment as observed from RCPT values.

5. Conclusions

The following conclusions could be arrived at from the study:

1. The studies carried out indicate the viability of using waste from the foundry industry and bottom ash from electrostatic precipitators as recycled fine aggregates in the production of concrete for structural purposes.

2. As, it was observed that for initial replacements of 10%, 20% and 30%, the increase in water content was constant and thereafter for 40% and 50%, again it remained constant but almost double the value of initial replacements. The mixes can be developed by varying the water content at constant rate as specified in the study till 30% and thereafter till 50% replacement of fine aggregates. The mix FB60 is not recommended as the water content of this mix is high which also reflects on various strengths.

3. The mechanical behavior of the concrete with waste foundry sand and bottom ash showed strengths comparable to that of conventional concrete except for FB60 mix, at the age of 365 days. Furthermore, it was observed that the greatest increase in compressive, splitting tensile strength and flexural strength was achieved by substituting 30% of the natural fine aggregate with industrial by-product aggregate in replaced mixes. Also, the maximum replacement could be taken as 50%.

4. The splitting tensile strength for FB30 mix was observed to be more than the control mix at all ages.

5. An increase in strength from 28 to 90 days was observed to be 13.29% for CM mix whereas FB mixes showed increase in strength from 14.52% to 23.89%. Between 90 and 365 days, an increase in strength for CM mix was 6.76% and the FB mixes showed an increase of 2.02-6.94%.

6. The inclusion of waste foundry sand and bottom ash as fine aggregate does not affect the strength properties negatively as the strength remains within limits. The concrete was endowed with comparable mechanical properties and greater resistance to aggressive agents (chemical, physical and environmental).

7. The morphology of the formations arising as a result of the hydration process was not observed to change in the concrete with varying percentages of waste foundry sand and bottom ash except in FB60.

8. The possibility of substituting natural fine aggregate with industrial by-product aggregate such as waste foundry sand and bottom ash offers technical, economic and environmental advantages which are of great importance in the present context of sustainability in the construction sector.

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