Asian Journal of Civil Engineering https://doi.org/10.1007/s42107-018-0050-3
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ORIGINAL PAPER
Correlation between compressive strength and other properties of engineered cementitious composites with high-volume natural pozzolana Said Choucha1 • Amar Benyahia1 • Mohamed Ghrici1 • Mohamed Said Mansour1 Received: 8 February 2018 / Accepted: 7 May 2018 Springer International Publishing AG, part of Springer Nature 2018
Abstract This paper aimed to analyze the relationship between flexural strength, elastic modulus, capillary absorption, bond strength, and compressive strength of engineered cementitious composite (ECC). For this purpose, sufficient data on the mechanical properties and durability characteristics of ECC produced by a high volume of natural pozzolana (NP) were developed. The experimental results showed that the incorporation of high volume of NP worsened the mechanical properties and durability characteristics of ECC mixtures as well as the bond strength between ECC overlay and concrete substrate. The results also show a strong correlations between compressive strength and the selected properties. This means that the selected properties could be predicted from the compressive strength values. Keywords Natural pozzolana Engineered cementitious composites Mechanical strengths Elastic modulus Capillary absorption Bond strength Correlation
Introduction Fiber-reinforced cementitious composites are one of the most economical and effective materials used in different civil engineering applications in the structure industry. Therefore, engineered cementitious composite (ECC) has been invented in the early 1990s at Michigan University (Zhou 2011; Li 2009). It is microstructurally tailored based on the micromechanics design theory (Li and Li 2011a). ECC is a special type of ultra-ductile fiber-reinforced cementitious composite. Unlike common cementitious materials, ECC is characterized by high ductility and strain-hardening behavior with strain capacity of 3–7% ¨ zbay et al. 2012; Said et al. 2015; Choucha (Fig. 1) (O 2017). This strain-hardening behavior is reached by the formation of multiple cracking with tight crack width lower than 100 lm. Especially, ECC is generally produced by polyvinyl alcohol (PVA) fibers with fiber volume fraction (typically 2%). & Mohamed Ghrici
[email protected] 1
Geomaterials Laboratory, Civil Engineering Department, University of Chlef, BP 151, 02000 Chlef, Algeria
Due to its higher ductility and better durability characteristics, ECC has been found more efficient for the use in different applications and suitable for repair of concrete structures (Yıldırım et al. 2015). Earlier researchers (Zhu et al. 2009, 2012; Yang et al. 2007; Wang and Li 2007; Sahmaran et al. 2013; Choucha et al. 2017) have investigated the properties of ECC incorporating different mineral additions such as fly ash (FA), silica fume (SF), metakaolin (MK), natural pozzolan (NP), and ground granulated blast furnace slag (GGBFS) into ECC design. The majority of experimental investigations evaluated the effect of FA on the mechanical properties and durability characteristics of ECC (Yang et al. 2007; Zhu et al. 2012). Compressive strength is generally the most important criterion for approving a concrete mixture by the structural engineering companies because its determination is easy, inexpensive, unburdensome, and rapid compared to assessing other mechanical properties, durability characteristics, and bond strength tests which are hardly evaluated. Therefore, several design codes of many countries have been established for the estimation of flexural strength (modulus of rupture) and elastic modulus from the compressive strength value (ACI 318 2008; Eurocode 2002).
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Fig. 1 Typical tensile stress-strain curve of ECC (Yang et al. 2007)
Moreover, it has been found that the most published empirical models in earlier researches were for normalstrength concrete (Yıldırım and Sengul 2011; Han and Kim 2004; Perumal 2014). However, a few researchers have developed experimental relationships between compressive strength and other properties of ECC. Zhou et al. (2010) investigated the effect of limestone powder (LP) and slag (S) on the properties of ECC and analyzed the relationships between its tensile and flexural properties. They noted that the uses of LP and S improve significantly the deflection and the strain capacity of ECC which ranged from 2.9 to 3.9 mm and 1.7 to 3.3%, respectively. They also found an excellent relationship between the flexural and tensile properties. Yıldırım et al. (2015) investigated the bond performance of high-early-strength ECC. They indicated that the compressive strength significantly affects the bond strength results. Additionally, they found a good relationship between the compressive strength and bond strength results in slant shear. They also concluded that shrinkage significantly affects pull-off test results, but it is not as influential on slant shear test results. It was observed from the literature that there is currently no information available about the effect of the NP replacement rate on the properties of ECC. Hence, the aim of the current analysis is to produce a data inventory of mechanical properties, capillary absorption, and bond strength of ECC produced by high volume of natural pozzolana to investigate their relationship with compressive strength.
Experimental investigation Material and mix proportions The ECC mixtures were produced using cement, natural pozzolana, silica sand, and high-range water-reducing
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admixture (HRWRA). CEM I 42.5R Portland cement of the cement plant of Zahana (Algeria) (PC) was used. The natural pozzolana used in this work was extracted from Bouhamidi deposit located at Beni Saf (in the south of Algeria). The physical properties and chemical composition of PC and NP are provided in Table 1. Fine silica sand with an average particle size and modulus of 250 lm and 2.01, respectively, was used in this study. A polyvinyl alcohol (PVA) fiber has been used at a fiber volume fraction of 2% in all ECC mixtures; its mechanical and geometrical properties are given in Table 2. To develop sufficient data on strength and durability characteristics, five ECCs were elaborated with different content of NP (NP/PC range from 1.2 to 3.2) in this study as shown in Table 3. The water–cementitious material ratio (w/cm) was kept at 0.29 for various ECC mixtures. Slight adjustments were conducted in the amount of HRWRA in each mixture to achieve consistent rheological properties for better fiber distribution and workability.
Specimen preparation and measurement In this study, a mortar mixer was used for the production of all ECC mixtures in laboratory temperature at 23 ± 2 C and 50 ± 5% RH. Three prism specimens (40 9 40 9 160 mm3) were prepared for the compressive and the flexural strength tests for each mixture. The compressive and flexural strengths tests were conducted at 3, 7, 28, and 90 days in accordance with the EN 12190-6. All samples have been stored in a curing chamber and immersed in water until the test ages. Three cylinders measuring 50 mm in diameter and 100 mm in length were
Table 1 Chemical and physical properties of cement and natural pozzolana Portland cement
Natural pozzolana
Chemical composition (%) CaO
64.0
10.5
SiO2
19.9
46.4
Al2O3
5.6
17.5
Fe2O3
2.5
10.5
MgO
1.8
3.8
SO3
3.1
0.4
K2O Na2O
0.7 0.1
1.5 3.4
Physical properties SSB (cm2/g)
3100
4100
Glass content
/
[ 15%
Asian Journal of Civil Engineering Table 2 Properties of the PVA fibers Length (mm)
Diameter (lm)
Elastic Modulus (GPa)
Elongation (%)
Tensile strength (GPa)
Density (g/cm3)
8
40
41
6.5
1.6
1.3
Table 3 Mix proportion of ECCs with different replacement level (kg/m3)
Mixture ID
PC
NP
Sand
w/cm
NP/C
HRWRA
Fiber (%)
ECC_1.2
568
682
450
0.29
1.2
12
2
ECC_1.7
463
787
450
0.29
1.7
13
2
ECC_2.2 ECC_2.7
391 338
859 912
450 450
0.29 0.29
2.2 2.7
14.5 14.5
2 2
ECC_3.2
298
952
450
0.29
3.2
15
2
prepared for the determination of the elastic modulus from each ECC mixture at the ages of 28 and 90 days. Halfprism specimens 40 9 40 9 80 mm3 were prepared to determine the capillary absorption coefficient (K) at 28 and 90 days in accordance with EN 1015-18 (2002).
Specimen preparation and testing for bond strength The slant shear test, as defined by ASTM C882, is one of the most widely used test methods to assess bond strength between repair material and substrate (Julio et al. 2006; Yıldırım et al. 2015; Benyahia et al. 2017a). Cylinders measuring 75 mm in diameter and 150 mm in length were prepared for the bond strength test. The ECC mixtures were cast to substrate with an elliptical slanted surface inclined at an angle of 30 to create composite specimens Ø75 9 150 mm and were then demolded after 24 h. The composite samples were stored in plastic bags at 23 ± 2 C and 95 ± 5% RH, until the day of testing. The substrate is a normal-strength concrete generally used in the reinforced concrete construction, with a minimum compressive strength of 30 MPa and a flexural strength of 4.5 MPa. Table 4 provides the mixture proportions of the concrete substrate. However, a clean and rough surface is required to obtain good bond strength results. Sandblasting is the appropriate method for surface treatment (Julio et al. 2004; Benyahia et al. 2017b). Therefore, the sandblasting method was used in this study to increase the roughness of the substrate’s surface, which results in high bond strengths.
Table 4 Proportion of concrete substrate
Results and discussion Relationship between flexural strength and compressive strength Experimental results of compressive strength, flexural strength, capillary absorption, elastic modulus, and bond strength are presented in Table 5. The relationship between the compressive strength and the flexural strength is represented using the following regression equation proposed by several researchers (Neville 1996): fr ¼ kfca ;
ð1Þ
where fc is the compressive strength in MPa and fr is the flexural strength in MPa. The flexural strength is plotted against compressive strength in Fig. 2 to analyze the relationship between these properties of ECC. The experimentally obtained relationship between compressive strength and flexural strength (Eq. 2) is given as: fr ¼ 1:4fc0:7 :
ð2Þ
A strong correlation has been found between fc and fr with a correlation coefficient of 0.94. However, as compressive strength increases, flexural strength increases as shown in Fig. 2. It is clear that when the compressive strength is greater than 55 MPa, the increase in flexural strength seemed to become less significant. This can be attributed to the flexural strength which mainly depends on the properties of the PVA fibers when the matrix is denser (Zhu et al. 2014). These results agree with those reported by Zhu et al. (2014) when they investigated the effect of
Cement (kg/m3)
Sand (kg/m3)
Coarse aggregate (kg/m3)
HRWRA (kg/m3)
Water (kg/m3)
445
611
806
1.5
222
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Asian Journal of Civil Engineering Table 5 Mechanical properties, capillary absorption, and bond strength of ECC mixtures Mixture ID
Compressive strength (MPa)
Flexural strength (MPa)
Elastic modulus (E) (GPa)
Capillary absorption coefficient (K) (kg/m2 s1/2)
Bond strength (MPa)
3 days
7 days
28 days
90 days
3 days
7 days
28 days
90 days
28 days
90 days
28 days
90 days
7 days
28 days
ECC_1.2
12.0
13.8
23.1
23.8
20.2
40.0
ECC_1.7
10.9
12.8
21.6
23.4
15.5
28.0
56.0
70.3
25.0
28.6
0.42
0.06
10.3
15.7
46.0
57.4
23.8
28.0
0.48
0.17
10.2
ECC_2.2
8.6
12.1
16.6
23.2
14.2
14.8
22.2
34.0
44.3
22.9
25.4
0.66
0.24
9.6
13.8
ECC_2.7
7.2
10.8
15.8
20.5
ECC_3.2
6.6
10.0
14.7
18.9
10.1
18.6
32.0
41.2
21.2
23.8
0.88
0.32
8.5
12.2
11.0
17.0
31.0
40.2
21.1
23.3
0.98
0.41
7.6
9.4
30
Flexural strength (MPa)
25
R² = 0.9379
20
15
3d 7d
10
28 d 90 d
5
Eq. 2 0 0
10
20
30
40
50
60
70
80
Compressive strength (MPa)
Fig. 2 Relationship between flexural strength and compressive strength of ECC mixtures
the binary and ternary using different mineral admixtures (FA, SL, and silica fume) on the ductility and the mechanical properties of ECC. The good correlation between fr and fc obtained in this study is attributed to the similar effect of high volume of NP on both compressive strength and flexural strength. It can be seen from Table 5 that increasing the NP content weakens the microstructure and hence, decreases significantly the strength of the ECCs mixtures. Regardless of the curing age, the flexural strength of ECC can be predicted from its compressive strength value. Several empirical relationships have been proposed by different concrete institutes of many countries for the estimation of the flexural strength as shown in Table 6. It should be noted that the ACI 318-08 (2008) (American Institute of Concrete) code defines the correlation between fr and fc by the equation fr ¼ 0:56fc0:5 and also recommends
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the correlation between fr and fc by the equation fr ¼ 0:62fc0:5 . A comparison of the proposed model-I (Eq. 2) of the current study with the prediction models given in ACI 318-08 (2008), Eurocode-02 (2004), and BS-8110 (1985) is shown in Fig. 3. It can be clearly seen from Table 5 that the results of the flexural strength obtained in this research are very higher than those calculated using different prediction models. The three models of concrete institutes underestimate the flexural strengths of ECC produced in this study as shown in Fig. 3. For example, at a compression strength of 40 MPa which corresponds a flexural strength of 8 MPa and 4 MPa according to EC-02 and ACI, the underestimation is approximately 40 and 70%, respectively. A slight underestimation was observed when compressive strength is lower than 20 MPa. This higher underestimation can be attributed to the strain-hardening behavior and the highdeformation capacity of ECC. According to Qian and Li (2007) and Li (2009), the higher flexural strength of the ECC is attributed to its strain-hardening behavior and high ductility. A flexural strength (modulus of rupture) of 10–15 MPa is easily achievable and is accompanied by a significant deflection-hardening regime (Woodson 2009). Therefore, it was indispensable to develop a correlation model between fr and fc of ECC. The proposed model developed in this study (Eq. 2) to estimate the flexural strength of ECC from the value of its compressive strength would be very useful for researchers and for several ECC applications.
Relationship between elastic modulus and compressive strength Compressive strength is an important property in the design and construction of concrete structures. Using compressive strength, several models have been
Asian Journal of Civil Engineering Table 6 Estimating equations in different codes and researchers
Code of practice/researcher
Country
Relation Flexural strength
Elastic modulus
ACI 318-08 (2008)
USA
fr ¼ 0:62fc0:5
pffiffiffiffi Ec ¼ 4:7 fc
BS-8110 (1985)
Grande-Bretagne
fr ¼ 0:60fc0:5
–
Eurocode-02 (2004)
Europe
fr ¼ 0:201fc
Ec ¼ 22ð0:1fc Þ0:3
Kim et al. (2002)
–
–
Yıldırım and Sengul (2011)
–
–
Ec ¼ 5:25ðfc Þ0:46 pffiffiffiffi Ec ¼ 6:6 fc 2:7
Ec : GPa; fc and fr : MPa
30
35 Eq. 2 ACI 318-08 Eurocode BS-8110
25
R² = 0.8894 Elastic modulus (GPa)
Flexural strength (MPa)
30
20 15 10
25
20
28 d
5
90 d
0
Eq. 3
0
20
40 60 80 Compressive strength (MPa)
100
120
20
Fig. 3 Comparison of the calculated values and the experimental data of ECC mixtures
established by different concrete institutes for the estimation of various material properties such as tensile strength, creep, elastic modulus, shrinkage, etc. (Han and Kim 2004). Therefore, it is useful to study the relationship between the compressive strength and the elastic modulus. Several researchers (Kim et al. 2002; Yıldırım and Sengul 2011) and design codes (ACI 318-08; Eurocode-02) used the equation Ec ¼ af bc to analyze the relationship between elastic modulus and compressive strength. The relationship between f c and Ec by age is shown in Fig. 4. The experimental relationship obtained in this study between the modulus and the compressive strength was obtained as: Ec ¼ 6:2fc0:36 ;
15
ð3Þ
where fc is the compressive strength in MPa and Ec is the elastic modulus in GPa. A correlation coefficient greater than R2 = 0.89 was obtained which indicates an excellent correlation between the compressive strength and the elastic modulus. As seen in Fig. 4, as the compressive strength of ECC increases, the elastic modulus increases. This is attributed to the higher matrix densification of ECC mixtures due to the increase in
30
40
50
60
70
80
Compressive strength (MPa)
Fig. 4 Relationship between elastic modulus and compressive strength of ECC mixtures
compressive strength, which leads to higher elastic modulus results (Sengul et al. 2002; Neville 1997). It should be noted that the elastic modulus could be predicted from its value of compressive strength using Eq. 3 developed in this study. Various design codes from many countries have been proposed for the estimation of the elastic modulus, and several empirical relations have been developed by several researchers (Yıldırım and Sengul 2011; Kim et al. 2002); the relationship between Ec and fc of concrete is given in Table 6. The models obtained by Yıldırım and Sengul (2011) and Kim et al. (2002) were also used for comparison with Eq. 3. A comparison of the proposed equation (Eq. 3) in this investigation with the calculated values by the equations in different codes proposed by earlier researchers is shown in Fig. 5. The different curves drawn in Fig. 5 were estimated based on the experimental data of the compressive strength of ECC mixtures. As seen in Fig. 5, all models overestimate the elastic modulus of ECC. For example, this overestimation is 50 and 30% for EC-02 and ACI, respectively, for a concrete having a compressive strength of 30 MPa
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65
Elastic modulus (GPa)
55
45
capillary absorption coefficient (kg/m2 s1/2)
28 j 90 j Eq. 3 ACI 318-08 EC-02 Kim et al. (2002) Yıldırım et al. (2011)
35
25
28 d 90 d Eq. 5 Eq. 6 0.80
R² = 0,95
0.40
R² = 0,97 0.00 25
15 20
30
40
50
60
70
35
80
Compressive strength (MPa)
Fig. 5 Comparison of the calculated values and the experimental data of ECC mixtures
which is the regular strength grade for concrete in different civil engineering application. However, as the compressive strength increases, the overestimation by all models increases significantly. For example, for a concrete having a compressive strength of 70 MPa, the overestimation is 44% for ACI, while it is 30% for a concrete having a compressive strength of 30 MPa. However, all the empirical relations used for the comparison have been developed for the prediction of the elastic modulus of normal-strength concrete, which is generally influenced by aggregate type. It should be noted that ECC is a unique type of ultraductile fiber-reinforced cementitious composites (Yang et al. 2009; Zhou et al. 2010), composed generally of cement, mineral additives, superplasticizer (SP), and silica sand with an average particle size of 300 lm (Yang et al. 2009; Zhou et al. 2010). Therefore, there is no aggregate greater than 0.3 mm (300 lm) in its composition. Additionally, the use of large aggregates in the concrete composition prevents the good dispersion of the fibers and more clumping occurs (S¸ ahmaran and li 2009). Hence, the main reason for this significant overestimation is the absence of coarse aggregates in the composition of ECC. These lower elastic modulus values obtained in this experimental investigation are in agreement with those reported by Li and Li (2011b) and Li and Li (2009). According to Li et al. (2011), a lower elastic modulus is desirable for concrete repair, because it prevents the internal stresses induced by restrained shrinkage.
Relationship between capillary absorption and compressive strength The relationship between compressive strength and capillary absorption for various ages is shown in Fig. 6. It can
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45
55
65
75
85
Compressive strength (MPa)
Fig. 6 Relationship between capillary absorption and compressive strength
be clearly seen from the figure that the capillary absorption coefficient (kÞ is inversely proportional to the compressive strength. The relationship between fc and k is represented using the non-linear function. The equation can be written as follows: k ¼ afcb :
ð4Þ
The experimentally obtained relationship between the capillary absorption coefficient and the compressive strength can be expressed as: k28 ¼ 92; 6fcð1:36Þ ;
ð5Þ
k90 ¼ 33063fcð3:08Þ ;
ð6Þ
where f c is the compressive strength in (MPa) and k28 and k90 are the capillary absorption coefficients at 28 and 90 days, respectively, in (kg/m2 s1/2). As shown in Fig. 6, regardless of the curing age, correlation coefficient greater than R2 = 0.95 was obtained, indicating an excellent correlation between compressive strength and capillary absorption coefficient. This means that the capillary absorption coefficient could be predicted from the value of its compressive strength. It can be clearly seen that the capillary absorption decreases with the increase in compressive strength as expected as shown in Fig. 6. It is generally recognized that the capillary absorption is directly related to the internal structure of the pores. The main reason for the excellent inversely proportional correlation between the capillary absorption and the compressive strength is the significant improvement in matrix densification due to the reduction of the permeable voids and porosity when compressive strength increases. In one word, the capillary pores are reduced by the formation of secondary C–S–H gel due to the hydration of cement at an
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early age and pozzolanic reaction at long-term which produce a denser microstructure and higher densification, and hence, the reduction in the capillary water absorption of ECC.
Relationship between bond strength and compressive strength Figure 7 shows the relationship between bond strength and compressive strength for different ages. The relationship between bond strength and compressive strength can be expressed by Eq. (7): fb ¼ 1:6fc0:5 ;
ð7Þ
where fc is the compressive strength in MPa and fb is the bond strength in MPa. As indicated in Fig. 7, regardless of curing age, there was a good correlation between compressive strength and bond strength with a correlation coefficient of 0.86. It can be seen from the figure that the bond strength is directly proportional to the compressive strength. It can also be clearly seen that as the compressive strength increases, the bond strength increases, but at a smaller rate. For example, when compressive strength increases from 20 to 60 MPa, bond strength increase from 10 to 15 MPa. The main reason for the good non-linear correlation between fb and fc is the enhancement of the interface zone between substrate concrete and ECC when the compressive strength increases. It should also be noted that the microstructure of the interface surface improves with age due to the pozzolanic activity which produces a secondary C–S–H gels, resulting in a denser interface and improved bonding properties in the interface surface between ECC and the substrate with an improved durability (Sahmaran et al. 2013). Moreover, the compressive strength of repair
material influences significantly on the interface zone between overlay and substrate. It can be concluded that ECC produced with high volume of NP is potentially an effective repair material due to its appropriate properties in terms of mechanical properties, durability characteristic with the substrate concrete. To the best of the authors’ knowledge; however, no code relationships between compressive strength and other properties of ECC produced with natural pozzolana are currently available. Therefore, a comparison between the proposed model (Eq. 7) of the current study and the experimental value obtained by Yıldırım et al. (2015) and Sahmaran et al. (2013) is provided in Fig. 7. It is clear that the results reported by Yıldırım et al. (2015) and Sahmaran et al. (2013) are in agreement with the experimental results of bond strength of the present investigation. This indicates the higher accuracy and reliability of the proposed equation (Eq. 7). It can be concluded that the developed model (Eq. 7) in this research is very useful to predict the bond strength of ECC from the value of its compressive strength.
Conclusion According to the results obtained in the present investigation, the following conclusions can be derived: •
•
•
•
• Fig. 7 Comparison of result’s value obtained by other researchers and the experimental data of ECC mixtures
A very strong relationship was obtained between flexural strength and compressive strength of ECC. It was found that all the three codes underestimate the flexural strength. This higher underestimation of flexural strength is attributed to the strain-hardening behavior of ECC. An excellent correlation was obtained between compressive strength and the elastic modulus. Thus, the elastic modulus of ECC can be predicted from the values of its compressive strength. However, all models overestimate the elastic modulus of ECC due to the absence of coarse aggregates in its composition. It was observed that the capillary absorption decreases with the increase of compressive strength. Therefore, an excellent relationship between compressive strength and capillary absorption coefficient correlation was found with a coefficient greater than 0.95. Increasing the content of natural pozzolana in the ECCs mixtures decreases the bond strength between ECC overlay and concrete substrate. However, all mixtures meet the requirement specified by the ACI 546R-04 (2004) at 28 days. In addition, a good correlation between the compressive strength and the bond strength was obtained. According to the excellent relationships obtained in this investigation, experimental equations are proposed for
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the prediction of the selected properties of ECC from the compressive strength value.
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