Destructive and non-destructive evaluation of concrete for optimum sand to aggregate volume ratio

Abstract

Aggregates are the biggest contributor to concrete volume and are a crucial parameter in dictating its mechanical properties. As such, a detailed experimental investigation was carried out to evaluate the effect of sand-to-aggregate volume ratio (s/a) on the mechanical properties of concrete utilizing both destructive and non-destructive testing (employing UPV (ultrasonic pulse velocity) measurements). For investigation, standard cylindrical concrete samples were made with different s/a (0.36, 0.40, 0.44, 0.48, 0.52, and 0.56), cement content (340 and 450 kg/m³), water-to-cement ratio (0.45 and 0.50), and maximum aggregate size (12 and 19 mm). The effect of these design parameters on the 7, 14, and 28 d compressive strength, tensile strength, elastic modulus, and UPV of concrete were assessed. The careful analysis demonstrates that aggregate proportions and size need to be optimized for formulating mix designs; optimum ratios of s/a were found to be 0.40 and 0.44 for the maximum aggregate size of 12 and 19 mm, respectively, irrespective of the W/C (water-to-cement) and cement content.

Graphical abstract

Keywords

aggregates / non-destructive testing / sand-to-aggregate volume ratio ( s/ a) / maximum aggregate size (MAS)

1 Introduction

Aggregates are the most significant contributor to concrete volume (60%–80%), and as such, mechanical and physical properties of concrete are greatly affected by the volume of aggregate [1,2], and eventually, it could impact the long-term durability properties. Based on the requirements of mechanical properties for a specific application, such as compressive strength, tensile strength, and modulus of elasticity and its freshness properties (such as workability), concrete components are proportioned accordingly [3]. During designing mixes, the proportioning of ingredients is done keeping in mind the target strength, durability, and workability, and in the process, sand-to-aggregate volume ratio (s/a) is considered as one of the key parameters. For any mix design, there must be an optimum s/a that provides the required workability and minimizes the water requirement leading to enhanced mechanical properties. Consequently, the mechanical properties of concrete need to be evaluated for a series of s/a to find the optimum s/a. According to JSCE guidelines [4], s/a of the concrete mix should be determined experimentally depending on the specified W/C (water-to-cement) ratio and the required slump. Although it recommends a specified range of s/a (0.39–0.48) for air-entrained concrete that varies with the MAS and admixture used in the mix design, it does not specify any optimum s/a value for normal concrete considering the mechanical properties.

To investigate the mechanical and fresh properties of concrete, a large number of investigations were carried out considering the maximum aggregate size (MAS), mechanical properties of aggregate, W/C, the surface texture of aggregate, fineness of cement, grading of aggregate, the shape of aggregates, types of cement [5–10]. However, relatively few numbers of literature were found where s/a is considered as a key parameter for designing concrete mixes. Deepa et al. [3] investigated s/a ratios of 0.40 and 0.45 and concluded that the compressive strength of concrete increases with the increase of s/a. Bashandy and Soliman [11] investigated fine to coarse aggregate MAS ratios of 0.0, 0.5, 1.0, 2.0, and 3.0 and reported 0.5 as the optimum. In another study, Okonkwo and Emmanuel [2] investigated concrete’s mechanical properties considering five different MASs ratios of fine aggregate to coarse aggregate (0.30, 0.40, 0.50, 0.60, and 0.70). Based on this study, the optimal ratio of fine to coarse aggregate was found at 0.40. They also proposed 0.50 as an optimal fine to coarse aggregate ratio based on the flexural strength of concrete. Lin [12] investigated the impact of the s/a ratios (0.51, 0.52, 0.53, 0.54, and 0.55) on the mechanical properties of concrete and concluded that the compressive strength of concrete decreases with the increase of the sand portion but did not mention any optimum ratio of s/a. Other researchers also investigated the effect of increasing coarse aggregate portion in the mix design of concrete and concluded that the compressive strength of concrete improves as the percentage of the coarse aggregate in concrete increases [13]. Based on the research findings, it is expected that there would be an optimum s/a depending on the MAS of coarse aggregate and a range of s/a needs to be investigated to obtain the optimum number. To that extent, this study investigates a wide range of s/a ratio (0.36 to 0.56) in mixes having varied W/C, cement content, and MAS.

Along with destructive tests to evaluate mechanical tests of concrete, non-destructive ultrasonic testing was also carried out. Ultrasonic pulse velocity (UPV) measurement is one of the most commonly used reliable non-destructive methods to assess concrete’s uniformity and other mechanical and durability properties [14,15]. The propagation of an ultrasonic wave through concrete depends on its microstructure, and with a denser packing, the propagation time decreases, and consequently, wave velocity increases. The literature also reports several relationships between UPV and the penetration of aggressive agents into concrete and void-orientation [16–18]. Several studies have been conducted to correlate UPV and mechanical properties of concrete with varying aggregate types [19–26], the maximum size of aggregate [27–30], concrete maturity [31,32], W/C ratio [33], etc. However, very few studies focused on the relationship between s/a and UPV. Mohammed and Rahman [34] investigated the effect on UPV of concrete for various coarse aggregates and considered three specific s/a ratios (0.36, 0.40, and 0.44) as factors behind their study, and concluded that the UPV of concrete dropped with the increase of s/a. Likewise, Lin [12] reported that the concrete of lower s/a had higher UPV values at 56 d. However, none of them recommended any optimum value of s/a.

With the above context, an extensive investigation was conducted by varying the s/a, cement content, W/C ratio, and MAS to establish the influence of s/a ratio on the properties of concrete utilizing both destructive and non-destructive means of testing.

2 Experimental methodology

2.1 Materials

Crushed granite stone and river sand (as shown in Fig. 1) were used as coarse (CA) and fine aggregate (FA), respectively. Two different MAS, 19 and 12 mm, were used. Gradation curves of the aggregates are shown in Fig. 2. The gradations of both fine and coarse aggregates satisfy ASTM C33 [35] recommendations.

As per ASTM standards, both aggregates were tested for specific gravity, absorption capacity, unit weight, and fineness modulus (FM). Additionally, the coarse aggregate was tested for abrasion resistance. The aggregate properties are summarized in Table 1, along with relevant ASTM standards. Portland composite cement, CEM Type II A-M (as per BDS EN 197–1: 2003 [36]), was used as the binder. The cement consists of 80%–94% clinker and 6%–20% of mineral admixtures. For the mixing of concrete, potable tap water was used.

2.2 Mixture proportions, batching and specimen preparation

Table 2 summarizes the 48 mixture proportions of concrete investigated in this study. The parameters taken into consideration for the mixes are varying s/a (0.36, 0.40, 0.44, 0.48, 0.52, and 0.56), W/C (0.45 and 0.50), cement content (340 and 450 kg/m³), and MAS (12 and 19 mm). Using these mixture proportions, 576 cylindrical concrete samples (height of 200 mm and diameter of 100 mm) were made for 48 different cases as per ASTM C192 [42]. No chemical admixture was used in this study.

A traditional mixing procedure was followed for batching. All aggregates were brought to saturated surface dry (SSD) condition based on the water absorption summarized in Table 1 before batching. The cement was sandwiched between two layers of SSD sand in a portable concrete mixer and dry mixed for about one minute to achieve a homogeneous mix. The mix-water was then carefully added to the mixer, and the mortar was further mixed for about two minutes before introducing the SSD coarse aggregate to the mixer. The fresh concrete was mixed for another three minutes. For all cases, the wet mixing was continued for at least four minutes. Cylindrical specimens of diameter 100 mm and height 200 mm were cast right after mixing following ASTM C192 [42] guidelines and well compacted with a standard tamping rod. The freshly made specimens were then covered with a plastic sheet to limit water evaporation and left undisturbed. After 24 h, the specimens were de-molded and cured in water as per ASTM C192 [42] guidelines till the age of testing.

2.3 Testing procedure

2.3.1 Mechanical tests of concrete

At 7, 14, and 28 d, concrete specimens were tested under uniaxial compression using a 2000 kN compression machine as per ASTM C39 [43] specifications. A compressometer (mounted with two dial gauges) was attached to the specimen to measure deflection over a gauge length of 100 mm. At regular intervals of loading, the deformations were recorded. Based on these data, stress-strain curves were plotted, the stress of concrete at a strain level of 0.0005 was determined, and the modulus of elasticity of concrete was calculated by dividing the stress by the corresponding strain of 0.0005. The splitting tensile strength of concrete was determined at 28 d as per ASTM C496 [44] recommendations. For all mechanical tests, at least three specimens were tested, and the data presented in the paper are an average of three.

2.3.2 Ultrasonic pulse velocity

The UPV through SSD concrete cylinders was measured using a Portable Ultrasonic Non-destructive Digital Indicating Tester (PUNDIT) as per procedures outlined in ASTM C597-16 [45]. The PUNDIT is equipped with two transducers able to transmit and receive ultrasonic pulses, as illustrated in the schematic diagram in Fig. 3. To perform the UPV test, the direct transmission method was followed as it is more sensitive than the indirect and semi-direct transmission of the pulse [46]. Castrol Pyroplex Blue grease was used as a couplet agent to maintain maximum contact between the transducer and concrete specimen surfaces during testing. An ultrasonic pulse is emitted from the transmitting transducer at a nominal frequency of 54 kHz and received by the receiving transducer and the time required for the pulse to travel through the sample is recorded in the pulse meter. The pulse travel time on each specimen was measured at least 3 times, and the average value of the three results was recorded and is presented in results and discussions. Prior to each measurement, the PUNDIT was calibrated on a standard calibration rod provided by the manufacturer.

3 Experimental results and discussions

3.1 Influence of s/a on mechanical properties of concrete

3.1.1 Compressive strength

The variation of 28 d compressive strength of concrete (ranging from 16–36 MPa) with respect to s/a (0.36, 0.40, 0.44, 0.48, 0.52, and 0.56) is illustrated in Fig. 4 for all the cases investigated in this study. Two important trends can be observed in Fig. 4; the maximum compressive strengths for concrete made with 12 and 19 mm MAS resulted at s/a of 0.40 and 0.44 respectively, with a general decreasing trend for s/a beyond 0.40 (12 mm MAS) and 0.44 (19 mm MAS). Regardless of the variation in cement content and W/C, a statistical analysis was conducted using the concept of 95% confidence levels (95% CL) on the compressive strength data at 28 d considering the variation of s/a which is illustrated in Fig. 5. The 95% CL was computed with the “confidence interval formula” using a z-score of 1.96, the mean, standard error, and a sample size of three cylinders for each case. It is observed that concrete made with MAS of 12 mm and s/a of 0.40 shows 9.9%, 11.8%, 12.3%, 14.1%, and 15.5% more mean compressive strength than that for s/a of 0.36, 0.44, 0.48, 0.52 and 0.56, respectively. Similarly, concrete made with MAS of 19 mm and s/a of 0.44 shows 11.66%, 9.10%, 8.96%, 17.63%, and 22.87% higher mean compressive strength than that for s/a of 0.36, 0.40, 0.48, 0.52, and 0.56, respectively. The experimental data reveal different optimum s/a for different MAS, highlighting aggregate size’s influence on its mechanical properties.

Mohammed and Rahman [34] reported a similar trend for MAS of 19 mm; the compressive strength increases with the increase of s/a from 0.36 to 0.44. However, the study was conducted for s/a from 0.36 to 0.44, and the effect of s/a beyond 0.44 was not captured. For MAS of 19 mm, a compact aggregate mix is expected to be produced when s/a is 0.44. This ratio becomes 0.40 for MAS of 12 mm. Beyond these optimum s/a ratios, the sand content in the mixes increases with a relative drop in the coarse aggregate content under the absolute volume condition of 1 m³. As the coarse aggregate provides strength against compressive stresses [12], the reduced coarse aggregate content decreases compressive strength. Also, with an increase in the s/a, the number of fine aggregate particles and, consequently, the interfacial transition zone (ITZ) relatively increases, offering greater fracture planes for failure under uniaxial loading. However, a reduction in compressive strength for a lower s/a from the optimum indicates the insufficiency of the volume of mortar required to bind the coarse aggregates in the concrete matrix. From the experimental results, it is also understood that the optimum value of s/a would differ with MAS changes. The optimum ratio of s/a is reduced for a lower value of MAS. For this reason, for manufacturing high-strength concrete (usually utilizing MAS lower than 10 mm), the s/a ratio is reduced [47]. A general trend in increased compressive strength at relatively lower W/C and higher cement content has also been observed in Fig. 4.

3.1.2 Splitting tensile strength and modulus of elasticity

The influence of s/a on the tensile strength of concrete is illustrated in Fig. 6. Same as the compressive strength, it is also found that the optimum ratios of s/a to achieve a maximum tensile strength of concrete are 0.40 and 0.44 for MAS of 12 and 19 mm, respectively. The trend of changes in tensile strength of concrete with varying s/a is the same as the compressive strength. A similar trend has also been observed for modulus of elasticity, as illustrated in Fig. 7. Regardless of the W/C and cement content, the optimum s/a for the maximum elastic modulus of concrete is observed at 0.40 and 0.44 for MAS of 12 and 19 mm, respectively. From these observations, it is evident that the compressive strength of concrete is to be correlated to its elastic modulus considering the s/a ratio as a significant parameter. However, this is not reflected in the design guidelines, such as ACI 318-19 [48].

3.1.3 Correlation between modulus of elasticity and compressive strength of concrete with the variation of s/a

As elastic modulus of concrete varies with the s/a ratio, therefore, an attempt has been made to correlate elastic modulus with its compressive strength by introducing a factor to reflect the influence of the s/a ratio. Figure 8 demonstrates the relationship between elastic modulus and compressive strength of concrete made with different s/a ratios. By incorporating a factor related to s/a, the elastic modulus can be correlated with concrete compressive strength and unit weight by the following equation:

(1)E_{\text{c}}^{\mathrm{\prime }}=\lambda \times w_{\text{c}}^{1.5}\times 0.043\sqrt{f_{\text{c}}^{\mathrm{\prime }}},

where E_{\text{c}}^{\prime } denotes the elastic modulus of concrete in MPa, w_{\text{c}} denotes the bulk density of concrete in kg/m³, f_{\text{c}}^{\prime } denotes the compressive strength in MPa, and \lambda is a factor corresponding to s/a ratio. The values of \lambda are found to be 0.95, 1.05, 1.02, 0.99, 0.92 and 0.92 for s/a of 0.36, 0.40, 0.44, 0.48, 0.52 and 0.56, respectively with a reasonably strong R² values. According to ACI 318-19 [48], the value of \lambda is 1.0 without considering the influence of s/a.

3.1.4 Correlation between bulk density and elastic modulus with the variation of the volume fraction of coarse aggregate in concrete

Figure 9(a) demonstrates the relationship between the bulk density and volume percentage of coarse aggregate (volume % of CA) in concrete. From this figure, it is found that with the increase in volume % of CA in concrete, the bulk density increases as the specific gravity of coarse aggregate used in this study is about 14% higher than that of the fine aggregate used.

Figure 9(b) illustrates the relationship between elastic modulus and the volume percentage of coarse aggregate (volume % of CA) of concrete. The results follow a similar trend as observed for the bulk density of concrete. It is understood that for improving elastic modulus, it is necessary to increase the volume percentage of coarse aggregate in concrete.

3.2 Influence of s/a on ultrasonic pulse velocity of concrete

3.2.1 Effect of s/a on ultrasonic pulse velocity of concrete

The changes in UPV with varying s/a ratios are illustrated in Fig. 10. It is observed that the UPV of a concrete specimen is dependent on different parameters such as s/a, W/C, MAS, and cement content. The UPV decreases marginally with the increase of W/C of concrete due to the relatively porous nature of the microstructure in high W/C systems [28,49]. Concrete specimens made with a cement content of 450 kg/m³ and W/C of 0.50 showed a comparatively lower UPV value. A simultaneous increase in W/C and cement content of concrete increases the paste volume portion and may influence the microstructural properties. Regardless of the W/C and cement content, the highest values of UPV were found for the s/a ratios of 0.40 and 0.44 for concretes made with MAS of 12 and 19 mm, respectively, consistent with the trends observed in mechanical properties. It is expected that at these ratios, the aggregates in concrete are compactly distributed.

Statistical analysis was done using the experimental data and using the concept of 95% confidence levels (95% CL) as described in the previous sections. The results are shown in Fig. 11. The mean values and the 95% upper and lower confidence limits follow the same trend as observed for other mechanical properties of concrete, such as compressive strength, tensile strength, and elastic modulus. Concrete manufactured with MAS of 12 mm and s/a of 0.40 resulted in 1.5, 2.9, 4.3, 4.9, and 5.3 percent increase in UPV compared to s/a of 0.36, 0.44, 0.48, 0.52, and 0.56, respectively. Similarly, concrete made with MAS of 19 mm and s/a of 0.44 resulted in 3.9, 1.8, 3.1, 4.9, and 6.0 percent higher mean value of UPV than concrete made with s/a of 0.36, 0.40, 0.48, 0.52, and 0.56, respectively. It is further noted that for the same mix proportion, relatively higher UPV is found in concrete made with the MAS of 19 mm (ranging from 4150 to 4710 m/s) compared to the MAS of 12 mm (4300 to 4660 m/s). In a separate study, Mohammed and Mahmood [27] also stated that UPV through concrete increases with the increase of MAS due to the decrease in the perimeter of the ITZ around aggregates, and this trend is valid until the MAS reaches 37.5 mm.

3.2.2 Understanding variability between predicted data and experimental data

Several exponential relationships are proposed in the literature to predict the compressive strength of concrete by taking less time-consuming and less labor-intensive UPV testing. These equations are developed from a pool of experimental data on concrete manufactured with several varying parameters like aggregate type, cement content, MAS, etc. [21,26,34,50,51]. Statistical analysis was carried out to evaluate the coefficient of variance (COV), mean absolute percentage error (MAPE) and, integral absolute error (IAE) between the compressive strength predicted from the proposed equations of other studies and the compressive strength which were found by conducting experiments on concrete specimens made with different s/a ratios and are reported in Table 3 and Table 4. COV, MAPE, and IAE were evaluated using Eqs. (2)−(4).

(2)COV=\frac{\sqrt{{\displaystyle \frac{1}{n-1}}{\displaystyle \sum _{i=1}^n{\left(f_{\text{CP}}^{\mathrm{\prime }}-f_{\text{cE}i}^{\mathrm{\prime }}\right)}^2}}}{\mu },

(3)MAPE=\frac{{\displaystyle \sum _{i=1}^n\mid \frac{f_{\text{cP}i}^{\mathrm{\prime }}-f_{\text{cE}i}^{\mathrm{\prime }}\mid }{f_{\text{cE}i}^{\mathrm{\prime }}\mid }}}{n}\times 100,

(4)IAE=\frac{{\displaystyle \sum _{i=1}^n{f}_{\text{CP}i}^{\mathrm{\prime }}-f_{\text{CE}i}^{\mathrm{\prime }}}}{{\displaystyle \sum _{i=1}^n{f}_{\text{CE}i}^{\mathrm{\prime }}}}\times 100,

where n is the total number of samples, μ is the average experimental compressive strength of concrete, f_{\text{cE}i}^{\prime } is the experimental compressive strength of concrete for the ith data point, f_{\text{cP}i}^{\prime } is the predicted compressive strength for the ith data point. Hedjazi and Castillo [52] also carried out a similar analysis in their study to understand the variability between the measured compressive strength of various fiber reinforced concrete and the predicted results from different equations using the UPV values. Mohammed and Rahman [34] proposed Eqs. (5)−(7) fors/a of 0.36, 0.40, and 0.44 respectively as listed in Table 3.

Table 4 summarizes the COV, MAPE, and IAE between the actual experimental compressive strength and the predicted compressive strength of concrete using the equations proposed by several researchers without considering the effect of s/a. From these data (Tables 3 and 4), it is observed that the values of COV, MAPE, IAE become higher for the equations (Table 4), which were developed without considering the effect of s/a. On the other hand, these values are lower for the equations developed with the consideration of s/a (Table 3). It indicates that the equations proposed by Mohammed and Rahman [34] can provide a reasonable prediction of compressive strength based on UPV for the s/a of 0.36, 0.40, and 0.44, respectively. Therefore, in this study, the relationships between UPV and compressive strength of concrete are developed considering the effect of s/a. These equations are explained in the next section.

3.2.3 Correlation of compressive strength with ultrasonic pulse velocity of concrete for different s/a

Variation of UPV with compressive strength of concrete for different s/a ratios, such as, 0.36, 0.40, 0.44, 0.48, 0.52, and 0.56 is shown in Fig. 12. It is observed that the compressive strength of concrete can be reasonably correlated to UPV measurements with exponential relationships. The correlations are summarized in Table 5 for individual s/a considered in this study (where f_{\text{c}}^{\prime } is the compressive strength in MPa and UPV is ultrasonic pulse velocity in km/s). Similar exponential relationships are also proposed in the literature [26,27,34,50,54–56] except for a few proposed linear relationships [57,58].

Based on the aforementioned discussions, the correlations between the compressive strength and UPV of concrete can be expressed as an exponential equation of the form f_c^{\prime }= Ae ^B(UPV), where A and B are parameters dependent on the material properties. Although equations proposed by Mohammed and Rahman [34] summarized in Table 3 indicate that with an increase in s/a, the numerical value of A decreases and that of B increases, no similar trend can be observed in the findings of literature as compiled in Table 4. Similarly, no such trend has been observed in this research, as summarized in Table 5. However, these correlations can still be useful in predicting the compressive strength of concrete from non-destructive UPV testing for a given s/a ratio.

3.2.4 Effect of concrete maturity on UPV for different s/a

Figure 13 illustrates the variation of UPV with concrete maturity for MAS of 12 and 19 mm for different s/a ratios. As a general trend, the UPV increases with time due to the continuous hydration of cement [34]. However, the rate of increase of UPV with the age of concrete is not the same for all s/a. The highest rate of increase of UPV for MAS of 12 mm is found for s/a 0.40 and the UPV increase is about 8.8% from 7 to 28 d. For MAS of 19 mm, the maximum rate of increase of UPV with age is observed for s/a of 0.44, and the increase in UPV is about 7.9% from 7 to 28 d. This indicates a more homogeneous and compact internal structure of concrete at the optimum s/a ratios resulting in superior mechanical strengths as discussed in earlier sections.

3.2.5 Correlation of ultrasonic pulse velocity with bulk density of concrete and volume fraction of coarse aggregate

The variation of UPV with the bulk density of concrete is shown in Fig. 14(a). Despite having a moderately low R² value, it is reasonably observed that regardless of MAS, W/C, s/a, and cement content, with the increase in the bulk density of concrete, there is a rise in UPV measurements which is in line with findings in the literature [46,59]. The UPV assesses the uniformity of the concrete, and faster velocity is expected in a dense matrix. And thus, bulk density, with some reasonable confidence, can be directly proportioned to the packing ability or compactness of concrete providing higher UPV values for denser concrete [60]. Therefore, it can be understood that the uniformity of concrete is also proportional to its packing ability.

On the other hand, a better correlation of UPV can be observed with the volume fraction of the coarse aggregate (volume % of CA) in concrete, as illustrated in Fig. 14(b). It is observed that there is an optimum value of the volume fraction of CA for which maximum UPV is observed. The optimum volume fraction of coarse aggregate is found around 40% of the total volume of the concrete regardless of MAS, W/C, and cement content.

4 Conclusions

The following conclusions are drawn based on the experimental results of this study.

1) Based on the measured compressive strength, tensile strength, elastic modulus, and UPV of concrete, the optimum ratios of s/a are 0.40 and 0.44 for MAS of 12 and 19 mm, respectively, irrespective of changes in the W/C and cement content.

2) The s/a ratio has a significant influence on the mechanical properties of concrete. Therefore, a factor related to s/a has been incorporated to correlate the modulus of elasticity of concrete with its corresponding compressive strength. Also, UPV of concrete has been correlated to compressive strength with due consideration of s/a.

3) The volume fraction of coarse aggregate has a significant influence on bulk density and elastic modulus of concrete.

4) The rate of development of UPV with the age of concrete depends on both s/a and MAS. The highest rate of development of UPV for MAS of 12 mm is observed for s/a of 0.40, and for MAS of 19 mm, a similar outcome is evident for s/a of 0.44.

5) UPV and bulk density of concrete have linear relationships between them, and the uniformity of concrete is closely associated with the packing ability of concrete.

6) The optimum value of the volume fraction of coarse aggregate is 40% for maximum UPV.

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