Long-Term Drying Shrinkage Strain of Engineered Cementitious Composite Concrete Contains Polymeric Fibers

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Introduction
Polymers used in cement composite modifcation are widespread due to their contribution in enhancing the materials' properties.Topica et al. [1] declared that adding polyvinyl alcohol solution PVA into a cement paste can adjust the Silicon-Oxygen chains morphology and diminish the porosity.Under several weather conditions, the hydrothermal weather properties of the cementitious composite material containing polyvinyl alcohol fbers were studied by Li et [2].Te study exhibited that fbers are not markedly degraded when exposed to natural weathering, and the products of PVA fber-cement may considered durable.Te reason behind the engineered cementitious composite ECC drying shrinkage is the absenteeism of the coarse aggregates, which acts as a restraining factor for cement internal shrinkage.Tis fact was intensely investigated by Al-Attar et al. [3], by examining the efect of coarse aggregate characteristics (shape, type, moisture content, and surface texture) on unrestrained shrinkage of concrete especially in hot climates.Large diferences in relative shrinkage for diferent mixes at early ages, less than 28 days, were revealed by Al-Attar [4].Al-Rihimy et al. [5] declared that the Iraqi ambient temperature displays a large variation between day and night reaching 20 °C depending on the season, whether it is winter or summer.For this purpose, they studied the efect of these conditions on the drying shrinkage.Te drying shrinkage is measured for 6 months after 7 days in water curing, for outdoor ambient temperature varies from −4 to +39 °C and the relative humidity ranges from 15 to 60%; results were compared to those of specimens kept in the shrinkage chamber, under controlled temperature of 21 °C and relative humidity of 35%.It was concluded that, due to the irreversible nature of shrinkage strain, the drop in ambient temperature and the rise of atmosphere moisture or relative humidity would not reverse the shrinkage strain [5].Moreover, higher cement content in ECC results in a higher shrinkage magnitude.Experimental studies showed a high value of ECC drying shrinkage that may exceed that of normal concrete by two to three times [2].Numerous techniques may be implemented to overcome the shrinkage tendency.Zhang et al. [6] presented that replacing Portland Cement (Type 1) with lowshrinkage cement makes it probable to reduce the ECC drying shrinkage to a limit lower than that of normal concrete.Likewise, Yang et al. [7] described ECCs with a shrinkage near the normal concrete upper range, achieved by using a large fy ash amount.Sahmaran et al. [8] recommended the use of an internal curing agent to overcome early age drying shrinkage and autogenous shrinkage.Ikram et al. [9] reported that using bendable concrete produced from Portland limestone cement is conceivable and introduces sensible compressive and fexural strength results, in addition to a lower drying shrinkage strain when compared to Ordinary Portland cement.It is possible to use locally manufactured Portland limestone cement (IL) in making engineered cementitious composite concrete with adequate mechanical properties [10].Sara et al. [11] discovered a reduction in drying shrinkage strain when using Portland limestone cement (IL), in addition to the availability of PVA acetate, which improves the bonding strength.Also, the presence of PP fbers acts as a bridge between matrix, fber, and matrix/fber interface, making the concrete more durable and stronger.According to the previous clarifcation, the bonding strength and the microstructure of the concrete were enhanced leading to a reduction in the drying shrinkage compared to reference mixes.Adeyemi et al. [12] determined that the 2% PVA fber content is adequate to prevent any cracks resulting from shrinkage in ECC mixtures.Donatas et al. [13] revealed that PVA fbers that have high modulus of elasticity may decrease the crack width and consequently reduce the drying shrinkage.Sara Saed et al. [14] displayed that the use of two types of Portland limestone cement, Karasta (CK) and Tasluja (CT) PLC, leads to a reduction in drying shrinkage and creep due to the chemical composition of the two types of the cement and the calcium carbonate percent added to the clinker; therefore, less clinker/cement ratio gives lower creep and shrinkage.Te high cementitious material content in engineered cementitious composite concrete makes it sufer from drying shrinkage strains intensively.Many drying shrinkage types of research focus on the short-term drying shrinkage test while exploring the long-term drying shrinkage strain is still enigmatic.Tis research aims to investigate the long-term drying shrinkage strain in a controlled chamber under 21 °C temperature and 40% relative humidity.Six engineered cementitious composite concrete mixes with two 28-day-strength levels, 30 and 60 MPa, and with two types of polymeric fbers, polypropylene PP and polyvinyl alcohol PVA fbers, were tested for drying shrinkage till the age of 365 days.

Significance of the Study
Te signifcance and novelty of this research lie in the exploration of the long-term drying shrinkage behavior of engineered cementitious composite concrete (Bendable concrete) from one side and the role of the two diferent types of polymeric fbers, PP which is a hydrophobic fber and PVA which is a hydrophilic fber, in controlling the progress of the drying shrinkage with time from another side.Te results submitted in this paper are worth contemplating since they will provide new data of study for a recently invented concrete type (Bendable Concrete).

Materials and Methods
Engineered cementitious composite (Bendable) Concrete used in this research is composed of Type I cement (ordinary Portland cement OPC), ASTM C150 [15], with two cement contents: 275, and 450 kg/m 3 , for 28-day compressive strength of 30 MPa and 60 MPa, respectively.Polyvinyl alcohol (PVA) solution was used to reduce the porosity and enhance the engineered cementitious composite density [16].Fine aggregate (sand) with fneness modulus (2.7) conformed to the ASTM C33-08 [17].Silica fume with an activity index of 120% was adopted; it is prepared according to ASTM C-1240 [18].It was used as a partial replacement of cement by about 10% of cement weight.Te role of silica fume in the mix appears as it afects the arrangement of the cement paste microstructure and its interfacial transition zone [11,19]; its replacement with the cement may reduce the drying shrinkage strain.Te characteristics of polypropylene fbers PP and polyvinyl alcohol fbers PVA are shown in Table 1.No coarse aggregate was used in the production of engineered cementitious composite concrete because coarse aggregates tend to increase the crack width, which contradicts the property of bendable concrete [2].Te absence of coarse aggregate causes the engineered cementitious composite concrete to sufer from high drying shrinkage strain because the coarse aggregate is the element that restricts the drying shrinkage of concrete [20].Polyvinyl alcohol, PVA, solution was added to a cement matrix to enhance its performance since it can afect water retention and its workability in the fresh mixing state.Fibers are used in engineered cementitious composite concrete to restrict the propagation of microcracks and reduce the drying shrinkage strains.Terefore, making a comparison of the behavior of engineered cementitious composite concrete with two types of polymeric fbers (polypropylene fbers and polyvinyl alcohol fbers) is worth contemplating.Polypropylene fbers are hydrophobic materials, while polyvinyl alcohol fbers are hydrophilic materials; therefore, they are coated with an ionic chemical flm; this flm may react chemically with the cement matrix and provide additional bonding.A drying shrinkage test was performed on a prism 2 Journal of Engineering of 78 × 78 × 380 mm which was arranged according to ASTM C192/192M [21], and an average of three specimens for each mix was taken.For the long-term drying shrinkage (age of 360 days), the behavior of concrete was tested according to ASTM C 157/C157M [22].

Mix Proportions
Four ECC concrete mixes were used in this research, their proportions (by cement weight) are presented in Table 2, and the specifcations of the used chemical admixtures conformed to the ASTM C-494 [23].Te practice for making and curing concrete test specimens in the laboratory conformed to the ASTM C-192 [21].Table 3 reveals the symbols of the used mixes.

Efect of Fiber Type on Drying Shrinkage Behavior.
Te drying shrinkage strain of the ECC concrete mixes was measured under controlled temperature (21 °C) and relative humidity (32%).Te short-term drying shrinkage was recorded till 28 days, from which the mixes containing PVA fbers had lower drying shrinkage strains compared to the mixes containing PP fbers; both mixes showed lower drying shrinkage than their related reference mixes.Te reason behind this behavior is related to the properties of PVA fbers, which are a hydrophilic material that tends to retain the mixing water of the mix for a longer period than polypropylene fbers which are hydrophobic materials.Also, the ionic coating of the PVA fbers provided a good chemical bonding with the matrix compared to PP fbers; the scanning electron microscopy (SEM), Figures 1 and 2, support this conclusion.Another explanation for this behavior is that a higher PVA fber modulus of elasticity than that of PP fbers reduces the drying shrinkage strain.Tis behavior conforms with the results of Donatas et al. [13], who reported that PVA fbers with a high modulus of elasticity might reduce the crack width and reduce the drying shrinkage.Results displayed that a denser layer of hydration products was formed around the PVA fbers and the amount of the formed portlandite and C-S-H gel was higher around the PVA fbers, as shown in Figure 1.Te composite shrinkage is physically limited due to the adhesion between the matrix and PVA fbers.Fibers increase the composite's tensile strength, reducing the potential for cracking and shrinkage.

Short-and Long-Term Drying Shrinkage Results
. Te normal concrete drying shrinkage strain ranges between (200-600) * 10 −6 [2].Te short-term strain for ECC concrete mixes till the age of 28 days of testing is two to three times that of normal concrete.Te results of this research are represented in Table 4 and Figure 3. Te long-term drying shrinkage behavior till one year of results reading is introduced in Figure 4, from which it can be noticed that the rate of drying shrinkage strain after 28 days of testing becomes lower till the end of the testing period (360 days).Generally, after 28 days of testing, high-strength concrete (60 MPa) records a lower percent increment in drying shrinkage readings than normal-strength concrete (30 MPa) (Table 5); this behavior may be attributed to many reasons such as the higher density, the lower w/b ratio, and the higher PVA solution which increases the viscosity and reduces the porosity of the high strength mixes and also due to the higher fbers volume fraction in the high strength mixes.
Te error bars of the standard deviation of the drying shrinkage results with time are presented in Figure 5, from which it can be noticed that there is an overlap between the same mixes with time which means there are no huge diferences between them.When comparing diferent mixes, no overlaps can be noticed; this means that there is a difference in the results between the diferent mixes with time.Many researchers [2,6,13,14] studied the drying shrinkage behavior for up to 28 days; since the long-term drying shrinkage is worth contemplating, this research studied the drying shrinkage behavior after 28 days to 360 days, Figure 4. Te percentage of increment in drying shrinkage strain     4 Journal of Engineering compared to 28 days strain is reviewed in Figure 6.It can be noticed that the high-strength mixes gave a lower percentage increment in drying shrinkage (5.67%) than normal-strength mixes (6.879%).Furthermore, using PVA fbers in the engineered cementitious composites reduces the percentage of increment in drying shrinkage strain.Te reduction percentage in drying shrinkage strains, as introduced in Figure 7, was 1.5% for 30 MPa mixes and 2.7% for 60 MPa mixes at 28 days age of testing and reached 1.85% for 30 MPa mixes and 3.4% for 60 MPa mixes after 360 days of testing.Tis behavior is related to the PVA properties and the high-strength characteristics of the engineered cementitious composite.Te presence of PVA solution helps to increase the bond strength, and the existence of the polymeric fbers acts as a bridge between the matrix, fber, and matrix/fber interface, which results in stronger and more durable concrete [2].Te earlier explanation acknowledged that the microstructure and the bonding strength of the engineered cementitious composite concrete were enhanced when using PVA solution and polymeric fber.

Reduction in Drying Shrinkage Strain Using Polymer
Fibers.Te fear of drying shrinkage in ECC comes from the fact that ECC does not contain coarse aggregates which perform as internal restraints on cement shrinkage.In addition, a higher shrinkage magnitude may be expected for ECCs with higher cement content.Free drying shrinkage measurements are a function of drying time and relative humidity [2].Te use of polymeric fbers restrains the microcracks and reduces the tendency of ECC to sufer drying shrinkage strains.
Te maximum % reduction in drying shrinkage when using polymeric fbers compared to the reference mixes was about 18.75 and 16.16% for PVA and PP fbers, respectively, in normal-strength mixes (30 MPa), Figure 8 and Table 6.It was about 57 and 40.5% for PVA and PP fbers, respectively, in the high-strength mixes (60 MPa), Table 7 and Figure 9. Tis behavior is due to the high density, high fber volume fraction, high modulus of elasticity of the PVA fbers, high percentage of PVA solution, higher silica fume ratio, and the higher ability of PVA fbers to retain the water since they are a hydrophilic polymer.

Mechanical Tests and Results
. Te recent research conducted a fexural strength (center point loads) test, it was adopted on a prism of (100 × 10 × 400 mm), and the test was implemented according to ASTM C293/2002 [24].Tensile strain capacity for a prism of (100 × 10 × 400 mm) was also implemented in this research [25].When the tensile strain of the concrete exceeds its tensile strain capacity, cracking occurs.From Figure 10, tensile strain capacity increases by using 2% polymeric fbers; this performance may be related to the cement content, which delivers satisfactory coating to the fbers, hence increasing the bond with the fbers.Results discovered an enhancement in the mechanical properties when using polyvinyl alcohol (PVA) fbers since they make a robust and suitable bond with the cementitious matrix; this behavior could be due to their characteristics and hydrophilic nature.Moreover, the distribution of PVA fbers in the matrix was homogeneous due to their admirable dispersion.Tin flm surface coating of PVA fbers permits them to slip when loaded so they are not fractured, and that will prevent the rupturing of fber, which would cause widespread cracking, Figure 1.Consequently, bendable concrete deforms much more than normal concrete but without fracturing [26].From Figure 11, it can be observed that high-strength mixes showed an enhancement in fexural strength of about 33% and 50% for mixes 60 PP and 60 PVA in comparison to mixes 30 PP and 30 PVA, respectively.Tis performance may be due to the higher fber percentage (2%) compared to 1% fbers for normal-strength concrete (fber fractions were implemented to attain the best workability and fowability for the mixes).On the other hand, mixes with polyvinyl alcohol fbers (PVA) show better behavior in fexural strength with time compared with mixes produced with polypropylene fbers (PP), due to the ionic coating of the PVA fbers, which gives additional bonding to the matrix, enhancing the fexural behavior of the bendable concrete.Te fexural behavior represented in Figure 11 may be due to the efect of the fber's tensile strength, 600 MPa for PP fbers compared to 1620 MPa for PVA fbers.Te mechanical behavior of the ECC concrete mixes conformed with Al-Mulla et al. [27].It introduces a broad explanation for the drying shrinkage behavior of the mixes, as it can be seen that the mixes contain PVA fbers show lower drying shrinkage, due to their higher fexural strength and higher strain capacity.

Conclusions
By studying the drying shrinkage behavior of diferent engineered cementitious composite concrete mixes, the following conclusions can be deduced: (1) Mixes with PVA fbers show lower drying shrinkage strain than those containing PP fbers, and the use of polyvinyl alcohol solution enhances the mix viscosity and reduces its porosity, thus reducing the drying shrinkage strain.Tis behavior was supported through the SEM images.(2) Studying the long-term drying shrinkage behavior to 360 days of testing revealed that high-strength engineered cementitious composite shows a lower increase in drying shrinkage behavior.(3) Te maximum percentage of reduction in drying shrinkage behavior was 3.4% when using mixes of 60 MPa with PVA fbers compared to mixes with PP fbers and 1.85% when using mixes of 30 MPa with PVA fbers compared to mixes with PP fbers.(4) Te high PVA fber's tensile strength provides a high ECC concrete fexural strength and strain capacity and a reduction in the drying shrinkage strain, compared to that of mixes with PP fbers.

Figure 1 :
Figure 1: SEM for 30 V mix, an interface between PVA fbers and cement matrix at 1200x magnifcation.

Figure 2 :
Figure 2: SEM for 30 P mix, an interface between PP fbers and cement matrix at 1200x magnifcation.

Figure 3 :Figure 4 :
Figure 3: Drying shrinkage strain at the age of 28 days for ECC mixes.

Figure 5 :Figure 6 :Figure 7 :
Figure 5: Error bars of the standard deviation of drying shrinkage strain with time.

Figure 8 :
Figure 8: Percentage of reduction in drying shrinkage for 30 MPa mixes compared to the reference mix.

Figure 9 :Figure 10 :
Figure 9: Percentage of reduction in drying shrinkage for 60 MPa mixes compared to the reference mix.

Figure 11 :
Figure 11: Flexural strength of bendable concrete and the curing age.

Table 3 :
Details of the used mixes.

Table 4 :
Drying shrinkage strain for ECC concrete mixes.

Table 5 :
Percentage of increment in drying shrinkage strain after 28 days.

Table 6 :
Reduction percentage in drying shrinkage compared to the reference mix for 30 MPa.

Table 7 :
Reduction percentage in drying shrinkage compared to the reference mix for 60 MPa.