3.1 Fresh Property
3.1.1 Slump and Fresh Density
Workability fresh concrete is a mix property which includes the different requirements of stability, mobility, compatibility, finish ability and placeability [43]. Workability of Concrete decreased as the proportion of RCA enhanced as compared to blank mix. Maximum slump was achieved at 0% substitution of RCA while maximum slump was obtained 60% Substitution of RCA as shown in Figure 3. It is due to physical features RCA i-e porous nature that absorb more water from concrete mix and hence less free water is available for lubrication to reduce the internal friction between concrete ingredients. It has been reported aggregate that the surface texture of RCA is more rough as compare to coarse which adverse effect on workability of concrete [44]. Furthermore, when GBBS was added to recycle aggregate concrete mix, workability of concrete increased as compared to without GBBS recycle aggregate concrete mix. This increased in workability is due to smooth and fine particle of GGBS [45]. The fine particle of GBBS fills the gap between coarse aggregate, RCA, sand and cement leading to more dense concrete having less voids and hence more paste is available which facilitate better flow of cement concrete.it has been also reported that, more voids is present in concrete, lesser will be workability because paste fills the void and no paste is available for lubrication [3].It well known that any kinds of fibers reduce workability of concrete due lager surface area of fibers that increased the internal friction between aggregate resulting less workable concrete [35, 46]. It has been also reported that, for higher dosages of steel fiber required higher dosage of super plasticizer [35]. Therefore, 1.0 % of superplasticizer was kept constant throughout studies. However, addition of GBBS and steel fiber required higher dosage of superplasticizer. It can be observed that workability of recycle aggregate concrete is still improved with incorporation of steel fiber and superplasticizer. Although it has been also reported that steel fibers decreased workability [35] while superplasticizer increased workability [47], however combined substitution ratio steel fibers and superplasticizer shows workability comparable to control [22].
Figure 4 shows fresh density of concrete with varying dosages. Fresh density is index that determine performance concrete i-e higher density results more dense concrete with less voids in harden concrete resulting more strength. Fresh density decreased with addition of RCA. Maximum fresh density was obtained at 0% substitution of RCA while minimum fresh density was obtained at 60% substitution of RCA. Negative effect of RCA cloud be attributed due to porous nature of RCA having large volume with less mass which results lower fresh density as compare to control mix ( concrete made with natural coarse aggregate [44]. When GGBS was added to recycle aggregate concrete, fresh density improves as the percentage of GGBS increased. Maximum fresh density was obtained at 20% substitution of GGBS to recycle aggregate concrete as in comparison to refence mix. The positive influence of GBBS on fresh density is ascribed to the pozzolanic reaction of Sio2 in GGBS with Ch of cement creating secondary cementitious compounds resulting to more density of concrete [45]. Also, GGBS fill the voids in RCA, which was in porous nature, giving more compact mass, leading to more fresh density. When Steel fiber was added to GGBS recycle aggregate concrete, fresh density of mix still improved. Although it has been reported the fresh density concrete decreased with incorporation of steel fiber, particular at higher dosage ( beyond 2.0% by weight of cement) due to lack of workability [35]. Therefore, super plasticizer was added for each dosage of steel fiber which results more dense concrete. When superplasticizer is added to concrete, create similar charge resulting repelling to each other, lead to more workable and dense concrete [47].
3.2.1 Compressive Strength
Compressive strength of concrete is one the most important property of concrete which can be define as the ability of cylindrical sample to resist stresses when it is subjected to compressive forced in compressive testing machine. A standard size cylinder of 300mm length and 150mm diameter were casted and tested to find compressive strength of concrete as per ASTM standard [48].
Figure 5 shows compressive strength of different dosages of RCA, GBBS and SF. It can be noticed that compressive strength decreases with incorporation of RCA having minimum strength at 60% of RCA as compared to the reference concrete. It is due physical nature of RCA which absorb more water, leading to porous concrete resulting less compressive strength.
It has been also reported that compressive strength of recycle aggregate concrete due unreactive cement [4]. RCA absorb water from concrete and no water is available for hydration process. When GGBS was added to recycle aggregate concrete, considerable improvement in compressive strength was observed. At 28 days curing, compressive strength of aggregate 60% recycle aggerate concrete is approximately comparable to control mix at 20% substitution of GGBS. The positive impact of GGBS on compressive strength is due to the pozzolanic reaction of Sio2 in GGBS with Ch of cement, producing additional cementitious compounds [3, 21]. The additional binder produced by the GGBS reaction with available lime allows concrete to continue to gain strength over time. However, at higher dosage of GGBS (beyond 20 % by weight of cement) strength reduce due dilution effect which leads to alkali silica reaction due higher quantity of unreactive silica available due high quantity of GGBS [4]. Also, GGBS fill the voids in recycle aggregate, giving more dense concrete which results improvement in compressive strength of concrete. when steel fiber was to GGBS recycle aggregate concrete, compressive strength increased as the dosage of steel fiber increase up to 2.0% and then reduced. All the fiber reinforced shows higher compressive strength as compared to control concrete having maximum compressive strength after 28 days curing as compared to control mix ta 2.0% substitution of steel fibers. The positive impact of steel fibers on compressive strength is due to confinement of steel fibers around the cylindrical specimens. Laterally expansion produce under the application of compressive load which resisted by fibers due to confinement and as result to increase compressive strength [35].
A relative is also carried out in which 28 days of control mix is considered reference mix, from which other mix is compared as shown in Figure 6. After 7 days curing, compressive strength is about 19% less than from reference mix at 60% substitution of RCA. When GBBS is added to recycle coarse aggregate concrete, at 20% substitution of GBBS and 40% RCA show 7.0% more than compressive strength from reference mix after 7 days of curing. When steel fiber is added, concrete of mix 40% RCA, 20%GBBS and 2.0% steel fibers shows compressive strength more than 14% from reference mix at 7 days of curing. After 28 days curing, compressive strength is about 13% less than from reference mix at 60% substitution of RCA. When GBBS is added to recycle coarse aggregate concrete, at 20% substitution of GBBS and 40% RCA show 12% more than compressive strength from reference mix after 28 days of curing. When steel fiber is added, concrete of mix 40% RCA, 20%GBBS and 2.0% steel fibers shows compressive strength more than 17% from reference mix at 28 days of curing. After 56 days curing, compressive strength is about 7.0% less than from reference mix at 60% substitution of RCA. When GBBS is added to recycle coarse aggregate concrete, at 20% substitution of GBBS and 40% RCA show 26% more than compressive strength from reference mix after 56 days of curing. It due to fact that pozzolanic reaction procced slowly. Some studies also reported that, early age strength reduced with addition of pozzolanic materials to concrete [21]. When steel fiber is added, concrete of mix 40% RCA, 20%GBBS and 2.0% steel fibers shows compressive strength more than 36% from reference mix at 56 days of curing.
3.2.2 Split Tensile Strength
The ability of cylindrical sample to resist the stresses when it is subject to tensile (Stretching or pulling) force are terms as tensile strength of that material. Direct tensile strength of concrete is not possible because of eccentricity and griping of sample. Therefore, indirect split tensile strength can be determined by placing cylindrical sample in compressive machine to split in vertical diameter. As per standard procedure define by ASTM as ASTM C39/C39M [48] was apply for split tensile strength test of cylindrical sample size 300mm length and 150mm diameter.
Figure 7 shows compressive strength of different dosages. Similar to compressive strength, split tensile strength decreases with incorporation of RCA having minimum strength at 60% of RCA as compared to reference concrete. It is because of physical nature of RCA which absorb more water, leading to porous concrete resulting lower compressive strength. it is worth to mention that split tensile more effectively decreased with incorporation of RCA than compressive strength. When GGBS was added to improve split tensile strength of recycle aggregate concrete. At 28 days curing, split tensile strength of aggregate 60% recycle aggerate concrete is about 10% more than to control mix at 20% substitution of GGBS.
The positive impact of GGBS on compressive strength is because of pozzolanic reaction of silica (Sio2) in GGBS with calcium hydrate (CH) of cement, giving supplementary cementitious gel i-e calcium silicate hydrate (C-S-H) [21]. According to past literature, split tensile strength mainly depend upon the binder strength. Aggregate trying move away from each other during tensile force. The additional binder produced by the GGBS result more resistance [3]. But, at higher proportion ratio of GGBS (beyond 20 %) split tensile strength decrease because of dilution effect which results to alkali silica reaction because of higher amount of unreactive silica (Sio2) accessible because of high quantity of GGBS [21]. Also, GGBS fill the voids in recycle aggregate, providing more compact concrete which results improvement in split tensile strength of concrete. when steel fiber was to GGBS recycle aggregate concrete, split tensile strength increase as the proportion of steel fiber (SF) enhance up to 2.0% and then decreased gradually. All the fiber reinforced shows higher strength as comparison to reference concrete having maximum split tensile strength after 28 days curing at 2.0% addition of steel fibers. It is wort to mention here that split tensile of concrete increased more effectively than compressive. According to the past researchers show that fiber improved split tensile strength more effectively than compressive strength [46]. Laterally expansion produce under the application of compressive load which resisted by fibers because of confinement and as result to increased split tensile strength [35].
3.3.0 Durability of concrete
3.3.1 Water absorption
Water absorption is one of the simple tests to detect durability concrete. Higher water absorption results lower durability of concrete. More water absorption also led to freezing and thawing action which results degradation of concrete. According to past literature, higher water absorption of concrete cause freezing and thawing of concrete particularly when concrete is to place in abruptly changing of temperature [22].
Figure 8 shows Water absorption of different dosages. Water absorption increased with incorporation of RCA having minimum Water absorption at 0% of RCA as compared to control mix. RCA are pore which absorb more water, leading to less Water absorption. When GGBS was added to recycle aggregate concrete, Water absorption decreased.
At 28 days curing, Water absorption at aggregate 60% recycle aggerate concrete is almost equal to control mix at 20% substitution of GGBS. The positive impact of GGBS on Water absorption is due to the pozzolanic reaction which produce more viscus binder surrounding to the aggregate which decreased water absorption [21]. Also, GGBS fill the void in recycle aggregate which result less water absorption. However, at higher substitution ratio of GGBS (beyond 20 %) Water absorption enhanced because of less workability of concrete which enhance compaction affords resulting more porous concrete [4]. When steel fiber (SF) was added to GGBS recycle aggregate concrete, Water absorption decreased as the substitution ratio of SF enhanced up to 2.0% and then decreased gradually. All the fiber reinforced shows lower Water absorption as compared to control having minimum water absorption after 28 days curing at 2.0% substitution ratio of steel fibers. According to past literature, fiber protect formation of shrinkage cracks due to which water cannot be easily penetrate [22].
3.3.2 Acid Resistance
Different aggressive acids available, like as HCL (hydrochloric acids), NHO3 (nitric acids) and H2SO4 (sulfuric acids) etc. In this study, H2SO4 (sulfuric acids) is considered as an acid attack, on the concrete specimens with different proportions of RCA, GBBS and SF.
Figure 9 shows Acid resistance of different dosages of RCA, GBBS and SF. Acid resistance decreased with incorporation of RCA having minimum acid resistance at 0% of RCA as compared to control mix. RCA are pore through which acid can easily penetrate through concrete, leading to less acid resistance. When GGBS was added to recycle aggregate concrete, acid resistance increased due filling voids and pozzolanic reaction leading to more dense concrete through which acid cannot easily penetrate. At 28 days curing, acid resistance at aggregate 60% recycle aggerate concrete is 5.0% more than control mix at 20% substitution of GGBS. However, at higher dosage of GGBS (beyond 20 % by weight of cement) acid resistance decreased due less workability of concrete which enhance compaction affords resulting more porous concrete [49]. When steel fiber was to GGBS recycle aggregate concrete, acid resistance increased as the dosage of steel fiber increase up to 2.0% and then reduced. All the fiber reinforced recycle aggregate concrete shows more acid resistance as compared to control having maximum acid resistance after 28 days curing at 2.0% substitution of steel fibers. According to past literature, fiber protect formation of shrinkage cracks due to which acid resistance cannot be easily penetrate [49].
3.3.3 Drying Shrinkage
Concrete durability can be detected through dry shrinkage. Water and chemical can easily inter onto the concrete body through small cracks on the surface of concrete causing deterioration of concrete due dry shrinkage of concrete which results less durable concrete. Drying shrinkage with respect to time for varying dosage of RCA, GGBS and SF is given in Figure 10. RCA increased dry shrinkage of concrete. Although it has been reported that dry shrinkage is basically movement of cement paste while aggerate restrict these movement [50]. According to past literature drying shrinkage is affected by the concrete porosity and stiffness [50]. RCA is basally porous nature which results pores in harden concrete. It has been also reported that dry shrinkage of concrete increased with addition of RCA. When GBBS is added to recycle aggregate concrete, dry shrinkage is considerably reduced. The formation C-S-H (calcium silicate hydrate gel) due to pozzolanic reaction cause more dense concrete which might be caused to decrease shrinkage. It has been also reported that fly ash considerably reduced drying shrinkage to filling micro pore in concrete which enhance internal compactness of concrete [21]. when SF is added to GBBS recycle aggerate concrete, dry is father reduced. SF restrict the formation of microcracks on the surface of concrete which restraint the movement of harmful elements in samples, which leads to minimalized cracks density and dimension and eliminates the detrimental effects of drying shrinkage. Addition of SF to GBBS recycle aggregate concrete, reduced the drying shrinkage due crack prevention by SF as well as pozzolanic reaction of GBBS.
3.4.0 Scan Electron Microscope (SEM) and X- Ray Diffraction (XRD)
SEM is used to evaluate the microstructure analysis of steel fiber, GBBS and recycle aggregate in concrete as per ASTM C1293 267 test [51], Scanning Electron Microscope (SEM) images of concrete samples and to X- ray diffraction analysis with presence and absence of steel fiber (SF), GBBS and recycle Coarse aggregate on concrete after curing of 28 days as show in Figure 11 and Figure 12 respectively. Figure 11 (a), (b), (c) and (d) shows that, Various pore and interfacial transition zone (ITZ) are appear as percentage of RCA increased particularly at 60% substitution which results lower strength. To analyze the measure of quartz and C-S-H (calcium silicate hydrate) gel in control and GBBS substituted batches. Peaks of C-S-H gel at 30° and 45° were selected for analysis. For control sample without GBBS, quartz is more as compared to C-S-H. During hydration of cement, C-S-H is formed due to chemical reaction quartz (Sio2) with lime (CH). The quantity of calcium hydrate (CH) is more than Sio2 which convert all Sio2 into C-S-H gel and hence no further Silica (Sio2) available for the reaction with CH to C-S-H gel which gives the binding properties. All Silica (Sio2) convert into C-S-H gel. CH remain unreactive forming weak pockets resulting less strength of concrete. It has been also reported that, CH reactive with other chemical ingredient present in concrete resulting less durable concrete [49]. Also reported that, Pozzolanic materials must be added to utilize CH which is by product form during hydration process of cement in order to obtain high strength durable concrete [49]. BGGS which rich in silica is added to neutralize CH. It can be seen from XRD analysis, that peak of quartz (Sio2) reduces while the peak of C-S-H is increased as the percentage of GBBS increased. Maximum C-S-H peaks was observed when the substitution of fly ash rate is 30%. It is due to the pozzolanic reaction which convert CH into C-S-H gel.