Experimental investigation on mechanical properties of binary and ternary blended pervious concrete

Abstract

The purpose of the investigation was to study the effect of binary and ternary blends of cement on the mechanical properties of pervious concrete (PC) specimen through destructive (DT) and non-destructive testing (NDT). Various combinations of fly ash (FA), limestone powder (LP), metakaolin (MK), and silica fume (SF) as mineral admixtures have been investigated to partially replace the cement up to 30% by weight in PC. Standard cube specimens of size 150 mm × 150 mm × 150 mm of binary and ternary blends of mineral admixture of pervious concrete were prepared to conduct standard compressive strength test and split tensile test at 7 and 28 days of curing. The ultrasonic pulse velocity (UPV) test and Rebound Hammer test were used as a non-destructive testing tool to substantiate the robustness of PC and to determine the approximate mechanical properties where other destructive testing tools are not feasible in case of in-place pervious pavements. Overall the pervious concrete made with LP based ternary blends (PLM and PLS) were found to perform better than FA based ternary blends (PFM and PFS) and control mix (PC) in destructive and non-destructive testing.

Keywords

mineral admixture / ternary / compressive strength / split tensile strength / pervious concrete / ultrasonic pulse velocity

Introduction

Pervious concrete is a distinctive model of ecofriendly concrete material with ability to infiltrate water through interconnected system of pores making it suitable for pavement for surface runoff and other benefits. The constituents of pervious concrete are homogenous with normal concrete except the presence of little or no fine aggregates; although the network of pores in pervious concrete makes it structurally compromised. The optimal mix composition is prepared to impart ample coating around the aggregates to maintain network of pores. In accordance with ACI 522R, subtle properties like porosity (15%–35%) and permeability (0.4–1.7 cm/s) make it environmentally friendly providing numerous benefits like groundwater recharge, reducing pollutants from storm water, reduction of heat island in urban areas, etc. Pervious concrete having limited strength and being porous in nature has been restricted in its application to low-traffic roads such as parking lots, sidewalk and walkways, etc. [¹–³]. The biggest and well known use of pervious concrete has been in China in 2008. Pervious ground and walkways were laid in Beijing for Olympics [⁴].

Structural strength and porosity, both are essential for mix design of pervious concrete and are generally made with locally available materials. Nguyen et al. [⁵] proposed the mix design on the presumptions that the cement paste only provides sufficient coating around the aggregates and does not fulfill the voids and gives the strength of 28 MPa. Paste volume is considered as essential parameter in guiding the compressive strength; the mechanical properties are directly proportional to paste volume irrespective of aggregate size and if paste volume is constant than lower maximum size aggregate derive higher strength readings [⁶,⁷]. In accordance with ACI 522R the mechanical strength of pervious concrete to be utilized in pavements should be generally in the range of 2.8–28 MPa. Furthermore the addition of sand up to 5% is efficient in increasing the compressive strength [⁸,⁹]. Polymer modified mixes exhibited delayed curing but the mechanical strength improves considerably [¹⁰]. It is believed that the addition of polymer improves the intrinsic linkage and water holding with cement matrix and aggregates. It also enhances the binding among aggregate particles [¹¹].

In the binary blends, fly ash (FA) decrease heat of hydration and concrete cost but also decrease the compressive strength [¹²–¹³], silica fume (SF) increase the compressive strength but is expensive [¹⁴], limestone powder (LP) act as filler and increase early strength if used in small quantity but in large quantity, decrease the compressive strength [¹⁵,¹⁶]. Metakaolin (MK) increases the compressive strength but is costly [¹⁷]. Keeping these patterns of individual admixtures in mind, ternary blends are prepared applying principal of maximizing the advantages and minimizing the disadvantages. For example, FA at substitution level of 30% decrease the cost but also decrease the compressive strength, similarly MK at 30% replacement, increase the strength but makes the mix costly. Hence a ternary blend (PFM) is prepared where 20% cement is replaced by FA which makes it cost effective and 10% is replaced by MK thus increasing the compressive strength of mix, so in a single mix, it is possible to utilize the benefits of two different admixtures. Similarly other ternary mixes with other such combinations are also prepared in the anticipation of improved mechanical and physical properties. To combine material with slow and high reactivity like FA and SF/MK provides more benefit than using single one [¹⁸]. Several studies have reported that the combined use helped in achieving higher mechanical strength, pore size distribution and sulfate resistance [¹⁹–²²]. The reasonable prediction of compressive strength based on previous studies presents that it is of utmost importance to take account of the strength of pervious concrete in laboratory and in-place concrete. For in-place strength testing of concrete the non-destructive testing such as ultrasonic pulse velocity (UPV) and rebound hammer test (RHT) are inexpensive, easy, direct, and valuable techniques for the characterization of cement blended composites. NDT is very expedient and suitable for engineering structures like pavement as it can assess the material properties without damaging its practicality. Use of Non-destructive testing (NDT) allows the evaluation of aged and deteriorated structures and suitable for the quality control of new structures [²³–²⁵]; however, structure can be well analyzed by combining the NDT and laboratory evaluation [²⁶]. NDT of cementitiously stabilized materials (compressive strength) was studied through UPV found that the P-wave velocity is directly proportional to density, curing time and binder content [²⁷]. Quality and elastic properties of the material can be aptly established with ultra-sonic pulse velocity. In conventional concrete it is well utilized for different purposes over the ages [²⁸]. Evaluation of concrete strength can be easily determined by correlating UPV and strength of concrete and there by extending to test existing structures [²⁹,³⁰].

NDT through rebound hammer is inexpensive and convenient method to ascertain the approximate strength of concrete. Moderate rebound value will be obtained for concrete with low strength concrete and high rebound value for comparatively high strength concrete. RHT is non-invasive method useful in checking the uniformity of concrete, but it cannot be considered as substitute for standard compression test but give approximation of strength of concrete to be evaluated.

The literature review on pervious concrete infers that binary mixes using mineral admixture have been studied by various researchers but ternary blends have not been investigated. In this paper various binary and ternary blends were formulated by substituting ordinary Portland cement (OPC) with OPC FA, LP, MK, and SF in a fixed proportion.

The objective of present experimental investigation is determination of strength properties through destructive and non-destructive testing on binary and ternary blends of pervious concrete. Figure 1 shows the execution of the present research program. The effect of selected mineral admixtures as a limited supplantment of OPC was also studied. The research objectives were:

1)‚To prepare binary and ternary blended cementitious pervious concrete mixes.

2)‚To demonstrate the mechanical properties of pervious concrete mixtures through destructive testing techniques.

3)‚To figure out the mechanical properties of pervious concrete mixtures through non-destructive testing techniques.

Experimental program

Material and mix proportioning

Ordinary Portland cement (OPC-43 grade) satisfying the requirements of IS 8112 [³¹] and coarse aggregate conforming to Indian standard IS 2386 [³²] and IS: 383 [³³] are used in all the PC mix combinations. The coarse aggregates of size between 4.75 and 6.3 mm in the ratio (40:60) were used. The properties of aggregate used are provided in Table 1. In total 14 binary and ternary mixtures were produced by different combinations of mineral admixtures and the detail of all the mix proportioning is provided in Table 2. All the mixes having cm/a of 0.144 and 0.25 were designated as low paste and high paste, respectively. In addition to this sand (passing 2.36 mm sieve) was also added (5% of aggregate by weight) in high paste mixtures. The present work includes mineral admixture FA, LP, MK, and SF as supplementary cementitious material in partial replacement to cement. Similar combinations were also utilized in Ref. [³⁴]. Total 30% by weight of cement was replaced with mineral admixture in binary and ternary blends using 0.34 as water to cementitious material ratio. The chemical constitution and physical attributes of mineral admixture is provided in Tables 3 and 4. The particle size analysis of mineral admixture used in this study is shown in Fig. 2. All mix combinations were produced for a 1 m³ batch of concrete and the standard proctor hammer was used for standard compaction.

The microcracking starts at the interface between coarse aggregate and the surrounding cement paste. In pervious concrete, mortar is not present and only just sufficient cement paste is available which coats aggregates to bind them together. In single size coarse aggregates, the contact surface area reduces which increase porosity but decrease strength properties. In this study, aggregates of two sizes 4.75 and 6.3 mm were used in 40:60 ratios. In doing so the pores of 6.3 mm aggregates were not filled by 4.75 mm aggregates resulting into sufficient porosity and 4.75 mm aggregates increased the surface area and help in increasing the strength.

Sample preparation

The binary combinations of coarse aggregates of sizes 4.75 and 6.3 mm in proportion (40:60) were prepared through sieve analysis. All the mixture constituents (cement, mineral admixtures, and coarse aggregates) were then methodically mixed in dry condition approximately for one minute to get uniform mixture of the ingredients. Water was added along with super plasticizer. To get the required workability superplasiticizer (BASF master gleniumsky 8233) was used in the portion of 0.1% by weight of cement as suggested by the manufacturers in all mix combinations.

Standard cubic specimens (150 mm × 150 mm × 150 mm) were casted for compressive destructive and nondestructive testing and standard cylinders (100 mm (diameter) × 200 mm) were casted to calculate the unit weight. The specimens were compacted by the help of vibrating table for about 2–3 s and then standard proctor hammer was used to compact the specimen. Subsequently the specimens were deserted in the mold for a day in the laboratory and were covered with plastic sheets immediately after being casted. Exactly after one day, all the specimens were water cured for 7 and 28 days and left untouched under laboratory conditions before testing.

Testing of hardened properties

Unit weight and porosity

The unit weight of all the samples was determined before mechanical testing as per ASTM C 1688 [³⁵]. The unit weights were measured on standard cylinder 100 mm (diameter) × 200 mm in surface dry condition. All specimens were kept inside the laboratory for 24 h before the measurement. The weight was then calculated as mass divided by bulk volume.

Standard cylinder mold of 100 mm (diameter) × 200 mm was taken as specimen for porosity measurement. The Effective porosity of the concrete sample was calculated as per method described by Lian and Zhuge [³⁶] and used following Eq. (1).

(1)P=\left({\displaystyle \frac{{{\displaystyle V}}_{\mathrm{T}}-{{\displaystyle V}}_{\mathrm{c}}}{{{\displaystyle V}}_{\mathrm{c}}}}\right)\times \mathrm{100}\mathrm{\%},

where P is effective porosity (%), V_T is volume of specimen (mm³), V_T−V_C is the volume of void space (mm³).

Destructive testing

The compressive strength test was conducted as per IS 516-1959 [³⁷] specifications. 150 mm×150 mm×150 mm cubic specimens were used for destructive testing. Compressive strength of cube was computed by measuring highest load applied per unit area before breaking of cube specimen. The split tensile test was performed in accordance with IS: 5816 [³⁸] and conducted on 150 mm × 150 mm × 150 mm cubic specimens. The cube specimen was placed diagonally in the machine and load was applied. At a certain load, the cube splits diagonally. This peak load applied is taken as reading. Split tensile strength for cube was calculated by the method given in Ref. [³⁹] from the Eq. (2) given below:

(2){{\displaystyle f}}_{\mathrm{s}\mathrm{t}}=\mathrm{0.5187}{\displaystyle \frac{P}{B^2}},

where f_st is split tensile strength (MPa), P is the load at failure (kN), and B is the side of cube (150 mm). Average of three readings was taken as illustrative value of compressive strength and split tensile strength of each mix. Figure 3 shows the testing of specimen in compression and split tensile test and magnified image of failure surface.

Nondestructive testing

UPV testing

The ultra-sonic pulse velocity test is a non-destructive test to ascertain the characteristics of pervious concrete. The test is conducted in accordance with IS: 13311(part 1) [⁴⁰]. The concept of test is that the ultrasonic pulse is passed through the pervious concrete and time taken by the pulse to cross the entire cube specimen is recorded.

The schematic layout of UPV testing is shown in Fig. 4. Good quality pervious concrete having good density, uniformity, and homogeneity will give excellent velocity values. As shown in figure, a transducer which is in contact with cube specimen produces pulse of vibrations along the length of the standard 150 mm × 150 mm × 150 mm cube under test. A coupling material like gel was used between transducer and cube specimen. The pulse travels inside the pervious concrete and echoes through dissimilar margins of diverse material segments. Electronic timing circuits provide the value of the time T taken by the pulse to pass through cube length. The length L of two appropriate test points was marked and measured on cube specimen. The two most important factors of ultrasonic pulse velocity are density and modulus of elasticity. For each specimen the dynamic modulus of elasticity (E_d) was determined by the Eq. (3). In this study the possions ratio is taken as 0.25 [⁴¹]. Generally the value lies in the range 0.15–0.20 for conventional concrete.

(3){{\displaystyle E}}_{\mathrm{d}}={\displaystyle \frac{\left(\mathrm{1}+\mu \right)\left(\mathrm{1}-\mathrm{2}\mu \right)}{\left(\mathrm{1}-\mu \right)}}{\rho V}^{\mathrm{2}},

where m is Poisson ratio, r is unit weight (kg/m³), and V is ultrasonic pulse velocity (m/s).

RHT

RHT of all the mixes were determined using Rebound hammer (Photographic view shown in Fig. 5) following the procedure illustrated in IS 13311 (part 2) [⁴²]. RHT is influenced by the variation in material properties such as mix proportions, aggregate type, age, moisture content.

To conduct the RHT, the surface other than casting surface was used and before performing the test it was rubbed with pumice stone. This preparation is necessary as the pervious concrete surface is typically not uniform due to the void macrostructure. The rebound hammer was held straight perpendicular to the concrete surface for taking the measurements. Total of 12 readings were taken on each samples leaving the highest and lowest. The averages of 10 readings were taken as rebound number. Holding the hammer at intermediate angles can give different readings for the same concrete. The point of impact was approximately 20 mm away from the edge of sample. Then a correlation curve between rebound number and compressive strength was obtained by testing the concrete cubes in compression. Typically, rebound number increases as the strength increases. It can only judge the strength of the concrete mix. The internal flaws or diversity across the cross section are not suggested by rebound numbers. The RHT cannot replace compressive strength testing, however, it only gives rough assessment of concrete strength.

Reaction mechanism of mineral admixture

Cement additives modify the concrete properties through their physical and chemical activities. On addition of mineral admixtures, three mechanism broadly occur which include filler, heterogeneous nucleation, and pozzolanic reaction.

Pozzolanic material are mainly silicates or aluminous material which itself do not have cementitious property but in finely powder form, chemically react with Calcium hydroxide (CH) (product of hydration) to form compounds containing cementitious properties. Silicious or aluminous compounds react with CH to form highly stable complex compounds of water, calcium and silica. Amorphous silicates in pozzolonic materials are highly reactive in nature and reaction with CH in presence of moisture is referred as pozzolanic reaction [⁴³]. This reaction is initially slow, involves depletion of CH and no further production of CH. Reduction of CH increase the strength and durability of cement paste.

FA

FA particles are spherical in shape and much smaller in size than cement. The property help in improving the cement paste quality by pore refinement mechanism and pozzolanic activity.

FA as a partial replacement of cement increases the workability through ball bearing action of spherical particles. FA provide additional surface for hydration products to participate. The hydration of C₃S and C₂S produce C-S-H and calcium hydroxide. The CH is undesired product from strength and durability perspective. When Fly ash is present in the cement then pozzolanic reaction takes place and the CH react with pozzolana to form additional C-S-H gel [¹⁵,¹⁶]. The CH produced during hydration of cement is not sufficient for dissolution of FA due to this reason FA is regarded as inert material during hydration of cement but with time CH are depleted or utilized to form C-S-H. The depletion of CH indicates reactivity of FA. The strength development with addition of FA is slow but after 120 to 180 days of curing, strength development begins to increase.

SF

SF contains very high amorphous SiO₂ levels. The reaction mechanisms of SF as supplementary replacement of cement happen in three ways. First is pore size sophistication and matrix densification, second is reaction with available free lime, and third is improvement of cement paste aggregate interface. The peculiarity of transition zone between aggregates and cement paste is important in influencing cement aggregate bond in concrete. SF addition alters the thickness of transition phase in cement and change the degree of positioning of the CH crystals in it. After formation of enough CH in hydration process, C-S-H gel starts to form on SF particles. Half of SF particles react in one day and two third during initial three days [¹⁴]. The early hydration of cement is generally accelerated by the presence of SF and it depends on water powder ratio and substitution level.

LP

LP as a part replacement of cement, affect the mix through both physical and chemical means; although the chemical reactivity is not very intensive. Physically it augments the strength due to its smaller size compared to cement. Small particle size have better pore filling effect and refines the packing density and make the structure more compact. It abates the interstitial voids and brings down the entrapped water in the system. Chemically it affects the hydration process by supplying ions into the phase solution and modifying the hydration process and products [⁴⁴]. The LP reacts with tricalcium aluminate (C₃A) and form monocarboaluminate which participate in formation of ettringite partially and increase early strength. In the concrete without LP, the C₃A and C₄AF react slowly with calcium sulfate (CaSO₄) and form ettringite. When concentration of sulfur reduces, the remaining C₃A and C₄AF react with ettringites and form monosulphates. When LP is present in the phase as a supplement, it helps in formation of more stable monocarbonates instead of less stable monosulphates. The stable voluminous, water rich ettringite helps in overall increase in volume of hydration products.

MK

MK reacts with undesired CH from calcium silicate hydrates and create more C-S-H gels which function as secondary strength developer. The reaction depends on type of cement, ratio of AS₂ and CH, availability of free water and temperature. Along with C-S-H gel, several calcium aluminate products (C₄AH_13, C₃AH₆) and alumino silicate hydrates (C₂ASH₈) are also formed. During hydration, CH are quickly consumed and microstructure get richer by presence of C-S-H and stratlingite (C₂ASH₈) [⁴⁵]. Addition of MK significantly reduce CH at all replacement levels and at 20% substitution, entire CH is replaced.

Results and discussions

Influence on compressive strength

The Figs. 6(a) and 6(b) show the compressive strength (CS) of 7 and 28 days for low and high paste for all mixes respectively. The compressive strength of binary mix (PC3) containing LP is 11% higher than the mix containing FA (PC2) and marginally lesser than the control mix (PC1) at 7 days curing time, however, at 28 days the mix containing FA gives better results (higher by 14%) compared to LP but still lower than control mix. The probable reason could be LP having filler properties and thus increasing the early strength of the mix of pervious concrete. The pattern was similar in both high and low pastes. At 30% replacement of cement by limestone, the CS is lesser compared to control mix which signify that it does not have significant role in strength development of its own. Limestone powder is good filler and react swiftly hence compared to FA which is assumed to work slowly, gives higher early strength at 7 days. It can be observed that compared to control mix, both LP and FA give lower strength which signify that loss of strength due to cement replacement was higher compared to gain in strength due to replacement of admixture at high percentage of 30%. All ternary mixes (PC4 to PC7) containing different proportions of FA, SF, LP, and MK gave better results of CS for both 7 and 28 days compared to control mix (PC1) and binary mix (PC2 and PC3) for the paste with cm/a ratio 0.14. Similar results were obtained for the mixes with cm/a ratio 0.25. The strength lies in between (2.2–6.6 MPa) and (14.53–21.56 MPa) at 28 days for low and high paste, respectively. PC14 containing 20% LP and 10% MK gave maximum strength among all the ternary mixes for both 7 and 28 days. The effect on CS with porosity is shown in Fig. 7. The value of regression coefficient is 0.89 (R² = 0.89) which shows that they are positively correlated. The increase in paste volume (cm/a = 0.25) increase the CS and decrease the porosity. Significantly lower CS with higher porosity is obtained with decrease in paste volume (cm/a-0.14) for the mixes under investigation.

On comparing the compressive strength results of ternary blended PC with binary blended PC, it is found that the increase in compressive strength at 28 days for FA based ternary blend, i.e., PC11 was 18% higher than the FA based binary blended PC, i.e., PC9. On the other hand, the increase in strength of LP based ternary blended PC, i.e., PC13 was found to be 35% higher than the binary blended PC, i.e., PC10. The ternary blends PC11 and PC13 have higher compressive strength than binary blends of FA and LP (mix designated as PC9 and PC10). The ternary mix of LP and MK, i.e., PC14 had the maximum compressive strength of 21.56 MPa among all the PC mixes at 28 days. The results obtained are in sync with previous study with OPC only [³⁰].

In all the ternary blends (PC4 to PC7 and PC 11 to PC14), the 7 days CS was substantially higher compared to control mix for both the mixes at 7 days curing period. Early strength development can help in pavement laying since pavements are open for public use in 7 days [¹,⁸].

Influence of mineral admixture

It can be inferred that pervious concrete containing 10% MK gives higher CS compared to control mix for all curing periods. The addition of MK has valuable impact on the strength at 7 days and 28 days. It gives highly beneficial effect on the pervious concrete strength after 28 days with 10% substitution. Hence 10% replacement of MK was found to be best in strength development and cost effectiveness for pervious concrete under current study.

At higher level of FA substitution (30%), the strength development at early ages (7 and 28 days) was relatively slow compared to control mix. At higher curing periods above 90 days it can provide higher strength with proper water curing [¹⁶]. Replacement of MK alone at 20%–30% is not cost effective, FA is waste product and very cheap but its slow reactivity makes its application impractical when early strength development is necessary. The use of these two material together as ternary mix (20% FA+ 10% MK) which is designated as mix no PC5/12 in current study provide solution to disadvantages of individual SCMs which were not cost effectiveness and early strength development. Contrary to FA, SF reacts from early age and contributes to strength enhancement. These C-S-H compounds could efficaciously densify the microstructure of cement paste and improve its mechanical behavior. When FA is added along with SF as in ternary blend (20% FA+ 10% SF), early age and late age characteristics could be obtained. The heat development of SF concrete can be controlled by utilizing FA. The CH produced by early hydration of cement is consumed by highly pozzolanic SF and CH produced by later hydration of cement is consumed by less reactive FA in mix and provide further refinement of porosity and upgrade microstructure.

LP (as neither cementitious nor pozzolonic) substitution can provide beneficial effects of CS improvement only on early age because of its filling effect. Use of (30% LP) mentioned as PC3, PC10 in study was found to reduce CS. The mix had lower CS compared to control mix at both ages and this proves that it is not effective on CS with higher substitution of cement with LP [³⁰]. At 7 days the mix containing 30% LP showed higher CS compared to FA. Increase of strength could be due to pore filling effect of fine grounded LP and by providing suitable nucleus for hydration thus catalyzing the process of hydration at early ages. The ternary blends of LP (20% LP+ 10% SF and 20% LP+ 10% MK), mix designated as PC 6, PC7, PC13, and PC14, had higher compressive strength compared to ternary blends containing FA (PC4, PC5, PC11, and PC12).

Split tensile strength

The bar graph in Fig. 8(a) and 8(b) shows the results of split tensile strength of all mixtures (low and high paste) of pervious concrete at 28 days curing period. The STS varies in the range of 0.81 to 1.06 MPa and 2.56 to 2.98 MPa for low and high paste mixtures of pervious concrete. The splitting strength was higher for ternary blends as compared to binary blends and follows the same trend as in compressive strength. The partial replacement of cement in binary blends PC2 and PC3 decrease the STS by 6% and 10% respectively and increase in ternary blends by 1%, 9%, 16%, and 18%, respectively, for PC4, PC5, PC6, and PC7 compared to control mix PC1. Similarly, for High paste 30% replacement of cement in binary blends PC9 and PC10 decrease the STS by 3% in both and increase in ternary blends by 6%, 9%, 10%, and 12% for PC11, PC12, PC13, PC14 compared to control mix PC8. Similar results were also obtained by using OPC only in pervious concrete in the past studies [⁴⁶,⁴⁷].

Figure 9 shows the scatter plot between compressive strength and split tensile strength at 28 days. The trend obtained depicts that as the compressive strength increase, split tensile strength also increase and follows the same trend as compressive strength and are function of mix parameters.

Ultra sonic pulse velocity

The UPV values were utilized for evaluating the dynamic modulus of elasticity (E_d). Here poisson ratio (µ = 0.25) was taken which is considered as more appropriate for pervious concrete [⁴¹]. The dynamic moduli of elasticity (E_d) values are shown in (Figs. 10 and 11) for all mixture of pervious concrete. The UPV and E_d values of pervious concrete mixes were found to be in the range of 1605–1755 m/s and 3.9–6.5 GPa for low paste mixes respectively and 3531–4049 m/s and 16.6–23.9 GPa for high paste mixes respectively at 28 days of curing. The E_d values range depicts that the concrete quality is worthy in accordance with IS 13311-Part 1 [⁴⁰]. Figure 12 exhibits scatter plot of UPV versus compressive strength for high paste and low paste for 7 and 28 days curing period. The regression coefficient range from 0.92 to 0.97 demonstrates that both are strongly correlated.

Figure 13 shows relationship between E_d and UPV for low paste and high paste mixes for 7 and 28 days curing period. It is clearly evident that E_d increase as UPV increase in a linear relationship for all the mixes at all curing period.

The UPV measurements are believed to be depending on mix parameters like properties of paste coating around the aggregate and properties of aggregate. As the cm/a ratio increases from 0.144 in low paste to 0.250 in high paste, the thickness of paste around the aggregate increased which improved mechanical properties like compressive strength, dynamic modulus of elasticity (E_d) for all blends of pervious concrete at all curing days.

The linear increase in compressive strength with unit weight can also be seen from Fig. 14. The mixes with higher unit weight displayed increase in the compressive strength and the trend was common across both low and high paste.

Among the mixes, the ternary blends displayed 11% and 6% (7 days and 28 days curing period) higher UPV values compared to control mix (PC1 & PC8) but the difference among ternary blends was not found to be significantly high. The results obtained are in sync with previous literature of conventional concrete [⁴¹].

Rebound hammer

The average rebound number (RN) was obtained by striking the hammer against the prepared concrete surface. In this work, rebound number was obtained for all the mixtures after 28 days curing by the method described in IS 13311-Part2 [⁴²] and plotted against compressive strength for all combinations as shown in Fig. 15. Regression coefficient value R² = 0.96 (low paste) and R² = 0.78 (high paste) was obtained which show reasonable correlation with compressive strength. It can be observed that increase in cm/a ratio from 0.14 (low paste) to 0.25 (high paste) has direct influence on strength parameters. The strong paste content enabled and connected all the aggregates and displayed higher strength properties. The average RN obtained predicts compressive strength which is lower than the actual strength and it is prone to errors because of irregularity of the pervious concrete surface while taking measurements.

From the previous literature, it can be said that it is not considered as a reliable technique as the expected results have ±25% variation [⁴⁸] but in case of in place pervious concrete pavements, this NDT method can be considered as quick method for approximation of strength measurement.

Conclusions

The present experimental investigation was performed to inspect the mechanical properties through destructive testing and non-destructive testing of the pervious concrete mixtures using binary and ternary mixes of FA, SF, MK, and LP admixture as limited substitution (30% of cement in total). The present research program involved contemplation of compressive strength, split tensile strength, UP velocity, dynamic modulus of elasticity, and rebound hammer number of different binary as well as ternary blended PC mix combination using constant w/cm ratio. Based on various findings, the major conclusions of the study are listed as follows:

1)‚In the FA based ternary blends (PFS and PFM), the CS improved by 4%–7% and in LP based blends (PLS and PLM), it improved by 11%–22% compared to control mix at 28 days. The ternary mix PLM was top performer with CS of 21.6 MPa and it improved by 22% in comparison to control mix. It was also observed that the CS decreased in binary blends of FA by 11% and in LP based by 17% at 28 days in both having substitution of cement with mineral admixture at higher level (30%) compared to control mix.

2)‚The splitting tensile strength of range 2.65–2.98 MPa was achieved. It was observed that the trend in split tensile strength was same as in compressive strength with all ternary blends performing better and binary blends showing decrease in strength compared to control mix at 28 days.

3)‚Strength dynamic modulus of elasticity (E_d) which was derived from UPV technique was in the range 16.6–23.9 GPa for all blends of mixes for pervious concrete at 28 days. It was highest for LP based ternary blended PC and the trend was found to be similar to compressive strength measurement by destructive technique. From the results, it is evident that trend of increase in compressive strength is in direct relationship with unit weight, UPV, and E_d.

4)‚The value of regression coefficient of 0.78 indicates a satisfactory correlation between compressive strength and rebound hammer number. The increase in rebound number shows increase in compressive strength in all mix combinations under study.

It can be finally interpreted that addition of supplementary cementitious materials (SCMs) like FA, SF, LP, and MK in pervious concrete can improve early and late strength compared to pervious concrete with only cement. Hence such materials are of importance to concrete in terms of designing pervious concrete mixes having sufficient strength for low traffic usage. When used in pervious concrete, they have the potential to further increase the ecological benefits of utilizing waste products and providing environment sustainable solutions in terms of better ground water recharge, curbing pollution, stopping it from mixing in natural resources and excellent economic benefits.

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