**3. Results**

### *3.1. Test Results*

The method of assessing the quality of early-age concrete must enable identification of internal defects, regardless of the components used for its manufacture or the conditions under which the structure of hardened concrete forms. The potential of the IADP method was analyzed in this context. Damage processes were identified based on the assessment of signal classes recorded during the test.

The AE signals were recorded for 12 h on days: 1–8, 12, 16, 20, 24, 28, 38, 46, and 57 using MISTRAS software. Then the proposed IADP method was used to analyze the signals (hits). The signals from the tests were compared by 12 AE parameters with signals from the database and assigned to particular classes. The reference database was first developed using K-means clustering and then verified in [43].

An analysis was performed on averaged results of the number of recorded signals (hits) with respect to destructive processes assigned to them. Concrete of each series was investigated by 6 sensors (two sensors were attached to one side of each of the three samples (A, B, and C) in a given series). All signals recorded by these 6 sensors capturing the processes were averaged for each series. The number of signals recorded on average by one sensor was analyzed.

The development of damage processes in series W3, W4, W6, and W7 and B2, B3, and B4 is shown in Figures 5, 7, 9, 10, 12, and 13. Corresponding images of side surface cracks (for samples A, B, and C) are presented in Figures 6, 8, and 11, except W6 and B2-B4 samples, because no cracks were observed on their surfaces.

#### 3.1.1. Concrete W3—Curing in Water, Variable Hardening Temperature (+42 to −5 ◦C)

Figure 5 shows the values of destructive processes I–III captured during 56 days in samples W3. Throughout the test, 5195 signals (hits) were assigned to the initiation of internal microcracks (damage process I) and 94 AE signals were assigned to damage process II.

**Figure 5.** Number of damage processes I–III obtained from W3 samples.

Twenty-three hits assigned to surface microcrack formation (destructive process III) were captured in samples A, B, and C (Figure 6).

**Figure 6.** Surface microcrack distribution on sides of the W3 samples. Linear location of destructive processes III (AE signal class 3) in W3 samples obtained by the AE (Acoustic Emission) method is marked in red.

3.1.2. Reinforced Concrete W4—Curing in Water, Variable Hardening Temperature (+42 to −5 ◦C)

Figure 7 shows the values of destructive processes I–III captured during 56 days in samples W4. Most processes I and II were recorded during 20 days, later their number decreased. Throughout the test, 8608 hits were assigned to the initiation of internal microcracks (damage process I) and 90 AE signals were assigned to damage process II.

**Figure 7.** Number of damage processes I–III obtained from W4 samples.

Ten signals assigned to destructive process III (surface microcracks formation) were detected in samples A, B, and C (Figure 8) until day 46.

**Figure 8.** Surface microcracks distribution on sides of the W4 samples. Linear location of destructive processes III (AE signal class 3) in W4 samples obtained by the AE method is marked in red.

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3.1.3. Concrete W6—Curing in Water, Constant Hardening Temperature of 22 ◦C
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Figure 9 shows the values of destructive processes I and II recorded during 56 days in samples W6. Most processes were recorded during the first week of the test, then their number decreased. Throughout the test, 2560 hits were assigned to the initiation of internal microcracks (damage process I) and 94 AE signals were assigned to damage process II but not on every measuring day.

**Figure 9.** Number of damage processes I–III obtained from W6 samples.

Class III signals were not captured in the test and no microcracks at the surface of the samples were observed.

3.1.4. Concrete W7—Curing in Water, Constant Hardening Temperature of 22 ◦C

Figure 10 shows the values of destructive processes I-III captured during 56 days in samples W7. Most processes denoted as I were recorded during 20 days, later their number decreased. Throughout the test, 2795 hits were assigned to the initiation of internal microcracks (damage process I) and six AE signals were assigned to damage process II until day 24.

A few signals Class III assigned to the formation of surface microcracks were recorded in W7 samples. A single surface microcracks were detected on the sides of the samples (Figure 11).

3.1.5. Concrete B2 (with Admixtures)—Curing in Water, Constant Hardening Temperature of 22 ◦C

Figure 12 shows the values of destructive processes I and II recorded during 56 days in samples B2. Most processes were recorded during the initial days of the test, then their number decreased. Throughout the test, 4519 hits assigned to the initiation of internal microcracks (damage process I) were recorded together with 52 AE signals assigned to damage process II, which practically faded out after day 24 of the test.

Class III signals were not captured in the test and no microcracks at the surface of the samples were observed.

**Figure 10.** Number of damage processes I–III obtained from W7 samples.

**Figure 11.** Surface microcracks distribution on sides of the W7 samples.

3.1.6. Concrete B4—Curing in Water, Constant Hardening Temperature of 22 ◦C

Figure 13 shows the values of damage processes I and II captured during 56 days in samples B4. Throughout the test, 2886 hits assigned to the initiation of internal microcracks (damage process I) and 19 AE signals assigned to damage process II were recorded.

Class III signals were not captured in the test and no microcracks at the surface of the samples were observed.

**Figure 12.** Number of damage processes I–III obtained from B2 samples.

**Figure 13.** Number of damage processes I–III obtained from B4 samples.

### *3.2. Destructive Processes Analysis*

The analysis of the destructive processes confirmed that the internal structure of the concrete was a ffected by a range of factors such as aggregate and cement type, curing conditions (especially moisture and ambient temperature) that influence cement hydration, dimensions of the tested element as well as reinforcing bars embedded in the samples.

The identified processes, number of signals and curing conditions are shown in Table 1.

Analysis of the results shows that Class 1 signals were recorded most often in the test. These signals correspond to internal microcrack formation. Most damage processes I were observed during the first week. Their number decreased over time but they did not fade out during 56 days.

The number of Class 2 signals assigned to damage process II (internal microcracks development) was almost an order of magnitude smaller.

Class 3 signals were not recorded in (W2, W6, and B2–B4) concrete samples cured after demolding during 10 days and then hardened at constant temperature (22 ± 2 ◦C). This indicates that destructive processes III (surface microcrack formation) did not occur, which was confirmed by the observation of the sample.

In addition to tracking the growth of individual destructive processes in time, the number of destructive processes recorded at any given time interval can be also analyzed. Several analyses based on this precise information about damage and damage development in early age concrete were performed.

The results selected for W3 and W4 concrete samples (Figure 14) show that in the case when reinforcement was used, the number of damage processes I (internal microcrack formation) increased (in the analyzed concrete samples by about 65%) due to the occurrence of additional interfacial transition zones (ITZ) between reinforcing bars and cement paste. Damage processes II (propagation of internal microcracks) in the analyzed samples slightly decreased and damage processes III (formation of microcracks on the surface of concrete) were limited by embedded reinforcement.

In Table 5 shown the testing conditions of the samples with the results of number of AE signals and destructive processes in non-loaded concrete obtained by modified IADP acoustic emission method.

Figure 15 shows an influence of variable temperature on the number of damage processes in concrete hardening without initial curing. In the samples hardened at variable temperatures (−5 to +42 ◦C), far more destructive processes (I–III) were recorded compared to constant temperature conditions. This indicates a significant impact of the heating and cooling cycles on damage processes development in the early-age concrete, which may influence the strength of hardened concrete.


**Table 5.** Sample parameters, testing conditions, results, and detected damage.

1 The tests and analysis of W5 and W8 samples results were described in [45], 2 the test results of the B2 and B3 samples were described in [44].

**Figure 15.** Number of hits accompanying processes I–III recorded in W7 and W8 samples (without curing and constant and varied temperature).
