Scholarly article on topic 'Impact of recycled gravel obtained from low or medium concrete grade on concrete properties'

Impact of recycled gravel obtained from low or medium concrete grade on concrete properties Academic research paper on "Civil engineering"

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{"Recycled gravel" / Concrete / "Silica fume" / "Compressive strength" / "Mass transport"}

Abstract of research paper on Civil engineering, author of scientific article — Yasser Abdelghany Fawzy

Abstract This paper investigates the effect of recycled gravel obtained from low (Gl) or medium (Gm) concrete grade on fresh property of concrete (slump), mechanical properties (compressive-splitting tensile strength) and mass transport properties (ISAT-sorptivity) of concrete containing dolomite as a natural coarse aggregate. Concrete specimens were prepared with cement, water, sand and dolomite admixed with recycled gravel. The percentage of recycled gravel/dolomite was 0:100, 25:75, 50:50 and 75:25 at w/c=0.50, 0.55 and 0.60. The effect of silica fume and bonding admixture at w/c=0.55 on concrete properties were also considered. The results indicated that, increasing the percentage of recycled gravel/dolomite led to decreasing the slump. All mechanical properties of concrete discussed were inversely affected by increasing percentage of recycled gravel/dolomite from low and medium concrete. Adding 10% SF or bonding admixture increased the mechanical properties of concrete. Mass transport properties of concrete (ISAT-sorptivity) were enhanced by decreasing the percentage of recycled gravel/dolomite. The optimum percentage of recycled gravel/dolomite=25%.

Academic research paper on topic "Impact of recycled gravel obtained from low or medium concrete grade on concrete properties"

HBRC Journal (2016) xxx, xxx-xxx

Housing and Building National Research Center HBRC Journal

http://ees.elsevier.com/hbrcj

Impact of recycled gravel obtained from low or medium concrete grade on concrete properties

Yasser Abdelghany Fawzy *

Structural Engineering Department, Beni-Suef University, Egypt Received 22 September 2015; revised 9 April 2016; accepted 18 April 2016

KEYWORDS

Recycled gravel; Concrete; Silica fume; Compressive strength; Mass transport

Abstract This paper investigates the effect of recycled gravel obtained from low (Gl) or medium (Gm) concrete grade on fresh property of concrete (slump), mechanical properties (compressive-splitting tensile strength) and mass transport properties (ISAT-sorptivity) of concrete containing dolomite as a natural coarse aggregate. Concrete specimens were prepared with cement, water, sand and dolomite admixed with recycled gravel. The percentage of recycled gravel/dolomite was 0:100, 25:75, 50:50 and 75:25 at w/c = 0.50, 0.55 and 0.60. The effect of silica fume and bonding admixture at w/c = 0.55 on concrete properties were also considered. The results indicated that, increasing the percentage of recycled gravel/dolomite led to decreasing the slump. All mechanical properties of concrete discussed were inversely affected by increasing percentage of recycled gravel/ dolomite from low and medium concrete. Adding 10% SF or bonding admixture increased the mechanical properties of concrete. Mass transport properties of concrete (ISAT-sorptivity) were enhanced by decreasing the percentage of recycled gravel/dolomite. The optimum percentage of recycled gravel/dolomite = 25%.

© 2016 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/

by-nc-nd/4.0/).

Introduction

The term RAC refers to the recycled aggregate concrete and it is defined as the concrete prepared by using recycled aggregates or the combination of recycled aggregates and

* Coresponding author. Tel: +201224030928. E-mail address: yasser_gaeb@yahoo.com.

Peer review under responsibility of Housing and Building National Research Center.

natural aggregates. The recycled aggregates may be either fine recycled or coarse recycled aggregates which are obtained by crushing the waste/demolished concrete [1]. It is believed that RA has been used since 1945 in concrete production and started when World War II damaged large quantities of concrete structures and consequently, there was high demand of aggregates to rebuild the structures [2]. Waste arising from construction and demolition constitutes one of the largest waste streams within the Asian and many other countries. It is estimated that core waste amounts to around 180 million tons per year. The estimates for the UK are 30 million tons/year [3]. Re-use of waste concrete as RA in new concrete is beneficial from the view point of environmental protection [4]. Many studies [1,5,6]

http://dx.doi.org/10.1016/j.hbrcj.2016.04.003

1687-4048 © 2016 Production and hosting by Elsevier B.V. on behalf of Housing and Building National Research Center. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

investigated the influence of RAC on slump. They reported that, the slump of concrete decreased when increasing RA. Many researchers studied the impact of recycled aggregate concrete on concrete compressive strength. Concrete containing recycled aggregates provided lower compressive strength than that of control specimen [5]. The compressive strength of RAC is lowered when replacement percentage of RA increased [6,8,10,11]. Not much significant difference was observed in compressive strength of RAC up to 50% replacement of coarse aggregates [1,3,4,9]. Other research studies showed that, the compressive strength of concrete was not affected when using recycled aggregates up to 25% of the total aggregate [7]. The effect of RCA on splitting tensile strength of concrete was investigated [6-8,12]. It was concluded that increasing RA decreased splitting tensile strength. Adding 10% SF increased the compressive and splitting strength of concrete [12-14]. It is noted that most of the study on recycled aggregate in concrete has been focused on the effect of recycled aggregate on fresh and mechanical properties of concrete and there is a lack of information on the impact of recycled coarse aggregates on mass transport properties of concrete, as well as, determine the optimum percentage of recycled gravel obtained from low or medium concrete grade to dolomite. This study focuses on the impact of using gravel as recycled aggregates obtained from low or medium concrete grade with dolomite as natural coarse aggregates on concrete properties. These replacements were 0:100, 25:75, 50:50 and 75:25. The w/c used for all mixes were 0.50, 0.55 and 0.60. The effect of replacement percentages of recycled coarse aggregates on slump, compressive and splitting tensile strength and mass transport properties of concrete (ISAT-sorptivity) were investigated. Moreover, the influence of 10% SF and bonding admixture on slump and compressive strength of concrete at w/c = 0.55 was studied.

Experimental program

Preliminary work

Two groups of concrete specimens were used in this work, the first group represents the low concrete grade of characteristic strength 180 kg/cm2, whereas, the other group represents the medium concrete grade of 250 kg/cm2 characteristic strength. The constituents of the two groups are presented in Table 1. Concrete specimens representing the groups were cast in cubes 15 x 15 x 15 cm, cured and then tested at the age of 28 days for compression. They were crushed manually using steel hammer and sieved, and the gravel obtained from these concrete specimens with the attached mortar was used in main experimental program.

Table 2 Chemical and physical analysis of cement.

Property OPC

Chemical composition, %

SiO2 20.56

AhOs 5.59

Fe2O3 2.65

CaO 63.13

MgO 1.94

Na2O 0.22

K2O 0.6

SO3 2.61

Bogue compounds, %

C3S 61

C2S 12.7

C4AF 9.1

b - physical properties

Initial setting time, min 135

Final setting time, min 300

Specific surface, m2/kg 370

Soundness, mm 2

c - compressive strength, kg/cm2

3 days 250

7 days 350

Main experimental program Materials

Normal Portland cement was used in this investigation; it was delivered from ''Beni-Suef cement company", Type CEMI 42.5 N. Testing of cement was carried out per the Egyptian Standard Specifications ESS 2421/2005 [15]. The chemical and physical analysis of cement is presented in Table 2. Natural siliceous sand with fineness modulus 2.73 was used as fine aggregates. Dolomite with 25 mm maximum nominal size was used as natural coarse aggregates. Gravel obtained from low or medium concrete grade of 20 mm maximum nominal size was used as recycled coarse aggregates. Silica fume was used as a cement replacement material, bonding admixture according to ASTM C 631-2009 [16] of density 1.08 gm/cm3 was used and tap water was used for mixing and curing.

Mixture proportions

Forty mixtures were tested in this research, twenty mixtures for medium concrete grade (Gm) and the other for low concrete grade (Gl). These mixtures were made with percentage of gravel to dolomite = 0/100, 25/75, 50/50 and 75/25. Dolomite of 25 mm maximum nominal size was mixed with cement, sand, recycled gravel and water. The mix constituents were 350 kg cement, 1200 kg dolomite, and 600 kg sand per cubic meter of concrete at w/c = 0.50, 0.55 and 0.60. 10% SF was used in the mix at w/c = 0.55, and also bonding admixture of 1/7 mass of water was used at w/c = 0.55. These constituents of concrete were mixed in mixer for two minutes, and then placed in cube moulds 10 x 10 x 10 cm for ISAT, sorptivity, 15 x 15 x 15 cm for compressive strength testing, whereas, the specimens for splitting tensile testing were cylinders 15 x 30 cm. Table 3 presents the compositions of materials used (kg/m3) .Table 4 shows the tests that were conducted, sizes of specimens and testing ages.

Table 1 Composition of the concrete mixtures (kg/m3) of preliminary work.

Group Cement w/c Sand Gravel Water

Gl 250 0.60 600 1200 150

Gm 350 0.50 600 1200 175

Table 3 Composition of the concrete mixtures (kg/m3) of Gm or Gl.

Mix % w/c Cement Sand Total SF Bonding

no. RA coarse admix.

aggregate

1 0 0.50 350 600 1200 - -

5 0 0.55 350 600 1200 - -

9 0 0.60 350 600 1200 --

13 0 0.55 315 600 1200 35 -

17 0 0.55 350 600 1200 - 27.5

small pots in the same container beside the specimens. The test duration was two hours. The test was carried out according to BS 1881: part 208-1996 [18]. (d) Sorptivity test: The specimen preparation was carried out by the same method as that of ISAT. The test was carried out using a plastic container filled with water to a depth of 20 mm. Steel bars of 16 mm diameter were rested on the bottom of the container such that, the water was just above the top surface of the steel bars. The specimens were weighted using a digital electric balance of 0.05 g accuracy. All sides around the chosen side were then greased to about 2 cm. The weights of the specimens were measured after removing the surface water at periods of 15 min up to 2 h. The total amount of water absorbed was then monitored. The sorptivity of the tested specimens was calculated using the following equation [19]:

i = A + st0:5, (1)

A, is constant,

i, is the increase in mass in g/mm2,

t, is the time, measured at which the weight is determined, s, is the sorptivity in mm/s0.5.

Table 4 The specimens and age of testing.

Test Test specimens Age of

testing (days)

Compressive 150 mm cube 3, 7, 14 and

strength 28

Splitting tensile Cylinder of 150 mm diameter 28

strength and 300 mm height

ISAT 100 mm cube 28

Sorptivity 100 mm cube 28

Testing

In this research work, slump test was carried out on fresh concrete, whereas, the following tests on hardened concrete were carried out:

(a) Compressive strength: The compressive strength test was carried out according to the Egyptian Standard Specifications ESS 1658/2006 [17] at test ages of 3, 7, 14 and 28 days.

(b) Splitting tensile strength: The splitting tensile strength test was carried out at the age of 28 days according to the Egyptian Standard Specifications ESS1658/2006 [17].

(c) Initial surface absorption test (ISAT): was carried out at 28 days. The tested cubes were oven dried at 105 0C until reaching constant weight. Then, the dried specimens were left in a closed container for 24 h until full stabilization. To avoid the effect of moisture, silica gel was put in

Test results and discussions

The effect of percentage of gravel obtained from low or medium grade of concrete to dolomite on slump are presented in Figs 1 and 2. It is apparent from these figures that, increasing the percentage of recycled aggregate decreases the slump values, irrespective of the origin of recycled aggregates. These results may be attributed to the absorption percentage of recycled aggregates which increases by increasing its percentage. These results are in agreement with the result obtained from previous research works [1,5,6]. It is also obvious that, the slump of concrete containing recycled gravel obtained from medium concrete grade is higher than those slumps of concrete made using recycled gravel obtained from low concrete grade at all tested percentages of gravel/dolomite. These results may be attributed to the attached mortar surrounding the recycled gravel from low concrete grade which has an absorption percentage higher than that of medium concrete grade. Adding 10% SF at w/c = 0.55 affected the slump values of concrete, as slump decreased at the studied percentages of recycled aggregate. However, bonding admixture increased the slump when compared to mixtures without admixtures.

Mass transport properties

Initial surface absorption test (ISAT)

The effect of percentage of gravel to dolomite in Gm and Gl at w/c = 0.55 on water flow in concrete is presented in Fig. 3. It was observed from this figure that, for Gm or Gl concrete, the flow of water ml/m2 s increased by increasing the percentage of recycled coarse aggregate. The flow of water in Gl was higher than that of Gm.

25% 50%

Percentage of recycled aggregates

—♦—w/c = 0.5 at Gl —■—w/c = 0.55 at Gl —*—w/c = 0.6 at Gl ■■■♦■■■ w/c= 0.5 at Gm

......■......w/c 0.55 at Gm

---A-- - w/c=0.6 at Gm

Fig. 1 Effect of percentage of recycled aggregates on concrete slump at various w/c ratios for Gl and Gm.

0% 25% 50% 75%

Percentage of recycled aggregates

Control at Gl Control at Gm S.F at Gl S.F at Gm

Bonding admix.at Gl Bonding admix.at Gmt

Fig. 2 Effect of silica fume and bonding admixtures on slump of recycled concrete at 0.55 w/c for Gl and Gm.

Sorptivity

The effect of percentage of gravel to dolomite for Gm and Gl at w/c = 0.55 on sorptivity of concrete is presented in Fig. 4. It was observed from the figure that, for Gm or Gl concrete, the sorptivity increased by increasing the percentage of recycled coarse aggregates. The sorptivity of concrete in Gl was higher than that of Gm.

Mechanical properties Compressive strength

Figs. 5-12 represent the impact of percentage of recycled aggregates obtained from low or medium concrete grade at various w/c ratios on the compressive strength of concrete at different concrete ages. It is apparent from these figures that, the compressive strength of concrete increases by increasing

concrete age, while increasing w/c ratio led to a decrease in concrete compressive strength. For concrete containing recycled aggregate obtained from low concrete grade Gl or for concrete made with recycled aggregate derived from medium concrete grade Gm, increasing recycled aggregate led to decreasing the compressive strength at 28 days. These results agree with the results obtained from ISAT and sorptivity, as well as, agree with the results reported previously [6,8,10,11]. Compared to control specimens, recycled concrete resulted in lower compressive strength for Gl or Gm, and these reductions were 8%, 12% and 20% at 25%, 50% and 75% recycled aggregates for Gl, whereas, the corresponding reductions were 4%, 9% and 15%, respectively for Gm. These results agree with the findings obtained by previous studies [5]. Generally, introducing dolomite as a natural coarse aggregate in concrete made with recycled gravel led to increasing the compressive strength. Compared to compressive strength of Gm in

Percentage of recycled aggregates Fig. 3 Effect of percentage of recycled aggregates on water flow at 0.55 w/c for Gl and Gm.

-„ 2.5 n

<U Tfl

X 2H a

I 0.5-&

-♦--- Soiptivity at Gl ■— Sorptivity at Gm

0% 25% 50%

Percentage of recycled aggregates

Fig. 4 Effect of percentage of recycled aggregates on sorptivity of concrete at 0.55 w/c for Gl and Gm.

3 days 7 days 14 days

Age, days

28 days

Fig. 5 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.50 w/c for Gl.

3 days

7 days Age, days

28 days

Fig. 6 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.55 w/c for Gl.

3 days

7 days Age, days

28 days

Fig. 7 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.60 w/c for Gl.

3 days

7 days

14 days

28 days

Age, days

Fig. 8 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.50 w/c for Gm.

3 days

7 days Age, days

28 days

Fig. 9 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.55 w/c for Gm.

3 days

7 days Age, days

28 days

Fig. 10 Effect of percentage of recycled aggregates on concrete compressive strength at various ages at 0.60 w/c for Gm.

preliminary work, the increase in compressive strength were 34%, 30%, 28% and 16% for 0%, 25%, 50% and 75%, respectively, whereas, the corresponding increase were 39%, 28%, 22% and 17%, respectively, compared to compressive strength of Gl in preliminary work. Adding 10% SF in concrete mixture led to increasing the compressive strength by 11% and 20% for Gl and Gm, respectively. However, bonding admixture improved the concrete compressive strength by 17% and 25% for Gl and Gm, respectively.

Splitting tensile strength

It is apparent from Figs. 13 and 14 that for concrete made with Gm or Gl, the splitting tensile strength decreases by increasing the percentage of recycled coarse aggregates. This result is in agreement with previous findings [6-8,12]. The splitting tensile strength for Gm at the studied percentages of recycled aggregates, were higher than those for Gl. These results are in agreement with the results obtained from ISAT, sorptivity and compressive strength.

Control at Gl Control at Gm S.Fat Gl

S.F at G m

Bonding Bonding admix. atGl admix. atGm

Fig. 11 Effect of silica fume and bonding admixture on compressive strength of recycled concrete at 28 days at 0.55 w/c for Gl and Gm.

—♦—w/c= 0.5 atGl ------- w/c-0.5 atGm

—*—w/c= 0.55 atGl —A— w/c= 0.55 at Gm —■—w/c= 0.6 at Gl ------- w/c=0.6 atGm

Percentage of recycled aggregates

Fig. 12 Effect of percentage of recycled aggregates on concrete compressive strength at 28 days at various w/c for Gl and Gm.

- w/c= 0.5

- w/c= 0.55 -w/c = 0.6

25% 50%

Percentage of recycled aggregates

Fig. 13 Effect of percentage of recycled aggregates on concrete splitting tensile at strength at 28 days various w/c for Gl.

-w/c= 0.5 -w/c= 0.55 -w/c = 0.6

Percentage of recycled aggregates

Fig. 14 Effect of percentage of recycled aggregates on concrete splitting tensile at strength at 28 days various w/c for Gm.

Conclusions

From the experimental work carried out in this study, the following conclusions can be drawn:

- Increasing the percentage of recycled gravel obtained from low or medium concrete grade led to a decrease in the concrete slump.

- The slump of concrete containing gravel obtained from medium concrete grade was higher than that of slump of concrete made with gravel obtained from low concrete grade at all studied w/c ratio.

- Mass transport properties of concrete determined by ISAT and sorptivity tests were enhanced for Gm, when compared to Gl, and also were enhanced by decreasing recycled coarse aggregates in the two cases.

- The compressive strength of recycled concrete increased by increasing concrete age and decreasing w/c.

- Compressive and splitting tensile strengths of concrete are inversely affected by increasing recycled coarse aggregates in low or medium concrete grade.

- The optimum percentage of recycled coarse aggregate is 25% gravel obtained from Gl or Gm, as at this percentage introducing dolomite as a natural coarse aggregate led to increasing the concrete compressive strength by 30% when compared to compressive strength of the control mix.

- Using 10% SF or bonding admixture led to an increase in concrete compressive strength by 11% and 17%, respectively for Gl, whereas, the increase was 20% and 25%, respectively for Gm.

Conflict of interest

Author states that there is no conflict of interest.

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