Statistical Analysis of Rebound Hammer Assessment on Reinforced Concrete Buildings

doi:10.21203/rs.3.rs-2839925/v1

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Statistical Analysis of Rebound Hammer Assessment on Reinforced Concrete Buildings

https://doi.org/10.21203/rs.3.rs-2839925/v1

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This research explores the use of non-destructive equipment (Schmidt Rebound Hammer) to investigate the state of reinforced concrete buildings. A total of 138 Slabs, 266 Beams and 210 Columns were assessed as structural elements spread across six (6) different buildings. The columns, beams and slabs for the residential buildings have overall mean rebound compressive strength of 26.15 N/mm², 20.18 N/mm² and 32.2 N/mm² respectively. For the institutional buildings, the overall average rebound compressive strength for the columns, beams and slabs is 25.71 N/mm², 27.22 N/mm² and 34.92 N/mm² respectively. The columns, beams and slabs for the industrial buildings have overall mean rebound compressive strength of 30.27 N/mm², 24.23 N/mm² and 39.87 N/mm² respectively. Rebound compressive strength values reduce as the height of the building increases. The standard deviation and coefficient of variance for a set of structural elements in the buildings showed high level of uncertainties and disparities which is an established factor in the use of the rebound hammer. This is a clear indication that a more robust method of analysis is needed for the interpretation and drawing of conclusions from the rebound hammer assessment.

Civil Engineering

Rebound hammer

Compressive strength

Reinforced Concrete Building

Failure

Structural Health

The Structural health of existing buildings poses much threat to our environment due to inadequate methods of acquiring data on the structures’ responses to loads, environmental impacts, and age. Building collapse which is an extreme case of building failure has been so rampant recently in the construction world with the ten (10) worst building collapse cases in the world occurring in Asian, North and South American Countries [1]. Nigeria has suffered and still suffering its share of building collapses with varying degrees of fatalities; statistics show that about 461 buildings collapsed in the last four decades [2]. In most cases, the causes of these collapses are not identified, a clear indication of failure to keep data on existing structures and limitations on building failure analysis.

The rate of structural failure and collapse of buildings has raised global concern and there is now a demand for a lasting solution [3]. The causes of these failures especially in developing countries such as Nigeria are attributed to poor structural designs by non-professionals (quacks), use of substandard materials [4], [5], negligence of the use of appropriate personnel in building construction supervision, poor workmanship/supervision and most importantly the ineffective use of structural health monitoring (SHM) data [6]. Inventions have provided a way out in non-destructive methods or techniques to assess existing buildings. Non-destructive inspections (NDI) are ways of assessing a structure working with attached or embedded equipment which does not in any way affect the state of the structure [7]. The process of making use of non-destructive techniques and equipment to observe and study a structure within a time frame is known as Structural Health Monitoring (SHM). Inspections in RCB are a periodic exercise, the transition from NDI to SHM is a transition from the traditional “time-based” to the emerging “condition-based” maintenance.

There have been extensive studies [8]–[13] with different approaches towards the common aim of improving reinforced concrete structures maintenance practices by increasing the accuracy of the predictions, improving the structural safety and reducing the life-cycle cost. The use of rebound hammer generally has been researched and numerous conclusions drawn [14]–[17]. [18] researched on Experimental Compressive strength test of concrete and rebound hammer check with result comparison using correlation and regression analysis, the performance of normal strength concrete with the use of rebound hammer was investigated and the use of a specific rebound hammer number was concluded not to be suitable for estimating the strength of concrete. [19] worked on a review of factors influencing the performance of rebound hammer used for non-destructive testing of concrete members, B-Proceq curve used to calculate the concrete strength are affected by so many factors like the w/c ratio, carbonation effect, exposure to fire, compaction, curing condition, and type of the cement used in concrete, etc. There is an urgent need to develop/modify the NDT test standards, considering the different factors affecting the rebound index. This research focuses on the use of rebound hammer on existing reinforced concrete buildings different to what previous research did on laboratory test samples.

Six (6) reinforced concrete buildings all located in Kwara State, Nigeria are assessed in this research. Buildings used are categorized into three (3) groups based on occupancy namely: Residential Buildings, Institutional Buildings and Industrial Buildings. The categorization follows the pattern of the various classes of buildings in the British standard code BS 6399 [20]. These categories listed above have the highest record of structural failure compared to other classes available in the code. Furthermore, their rate of usage in the environment, the uncertainty surrounding loadings and forces applied to them and the rate of failures associated with them in recent times are higher. Two (2) different buildings are assessed for each category of residential (RB1, RB2), institutional (EB1, EB2) and industrial (IB1, IB2). The buildings assessed differ in number of wings and floors which are made up of structural elements (Slab, Beam and Column). The number of structural elements assessed for each building is presented in Table 1.

Table 1

Number of Structural Elements of Buildings Assessed
S/No	Type of Building	Building Code	Total Number
S/No	Type of Building	Building Code	Slab	Beam	Column
1	Residential	RB1	24	36	36
2	Residential	RB2	63	140	84
3	Institutional	EB1	12	18	18
4	Institutional	EB2	33	54	54
5	Industrial	IB1	6	12	12
6	Industrial	IB2	0	6	6

Rebound values are obtained from the use of a rebound hammer on the different structural elements of RCBs as listed in Table 2. Rebound values for columns are taken with the device held horizontally to the surface and the rebound values for slabs and beams are taken with the device held vertically upward. The average pressure resistance (\(kg/{cm}^{2}\) ) given in (iii) above is converted to compressive strength in \(N/{mm}^{2}\) using Eq. 1.

Compressive Strength (\(N/{mm}^{2}\)) = Average Pressure Resistance × 0.0980665) (1)

Where 0.0980665 is the conversion factor

Structural elements (Slab, Beam and Column) in RCB were assessed in triplicates using the rebound hammer. Nine (9) points were assessed on each structural element and the average rebound value was used to determine the compressive strength of the structural element. The codes used in the result presentations are defined in Table 2.

Table 2

Result Codes Definition
Code	Representation
RB	Residential Building
EB	Institutional Building
IB	Industrial Building
1, 2	Building Number
A, B, C, D, E, F, G	Building Wing

Residential Buildings Rebound Assessment

The results of the average rebound compressive strength for residential buildings (RB1 and RB2) are presented in Figs. 1–7 and the wings' mean strength values for each element, standard deviation and coefficient of variance (COE) values are presented in Tables 3 and 4.

The columns in wing A (Fig. 1) of the residential building (RB1) showed the highest average compressive strength of 36.6 \(N/{mm}^{2}\) compared to other wings while wing D is showing the lowest average strength value of 21.6 \(N/{mm}^{2}\). It is also deduced that across floors, the columns on the first floor are low in strength compared to the ground floor and the second floor. The beams also followed the same pattern with wing A giving the highest average compressive strength of 31.4 \(N/{mm}^{2}\) and wing D has the lowest average compressive strength of 15 \(N/{mm}^{2}\) as shown in Fig. 2. The first floor for the beams showed better average strength compared to the ground floor and second floor for wings B and D. The slab elements in RB1 (Fig. 3) followed the same pattern as the column with wing A giving the highest average compressive strength and wing C having the lowest average compressive strength of 40.2\(N/{mm}^{2}\) and 19.6 \(N/{mm}^{2}\) respectively. The first-floor slab also exhibited the lowest average compressive strength compared to the ground floor and second floor for all the building wings. As shown in Table 3, the beams have the lowest average rebound compressive strength across the wings compared to the columns and slabs, the slabs also have the highest average rebound compressive strength except for wing C where the column has the highest.

Table 3

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in RB1
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
RB1A	Column	35.6	5.1	14.39
	Beam	22.7	4.8	21.04
	Slab	37.8	2.8	7.45
RB1B	Column	27.2	4.4	16.26
	Beam	22.7	4.8	21.04
	Slab	33.3	2.8	8.43
RB1C	Column	28.1	4.9	17.48
	Beam	25.2	6.9	27.24
	Slab	26.8	5.2	19.51
RB1D	Column	21.8	2.0	9.19
	Beam	16.7	4.4	26.45
	Slab	34.4	5.1	14.80

Generally in RB1, the values for the columns, beams and slabs are showing a high standard deviation which means the rebound values are spread out away from the mean values. Based on floors, the beams showed the highest level of dispersion from the mean having the highest COV for all the wings.

For the second residential building RB2, the relatively weak columns are located at wings A, C and F. The columns at the ground floor of wing D have the highest average rebound compressive strength of 37.9\(N/{mm}^{2}\) (Fig. 4). It is observed that the average rebound compressive strength reduces as the height of the building increases. The second floor and the last floor have the weakest set of columns. This is also the same for the beams with the lowest average rebound compressive strength of 10.5\(N/{mm}^{2}\) recorded at the suspended floors for the 1200mm set of beams (Fig. 5).

All the beams on the last floor have low average rebound compressive strength (Fig. 6). On the contrary, the first-floor slab recorded the lowest strength values compared to the ground floor and the second floor. Wing B slabs showed a significant decrease in strength as the building height increased with a massive 51.7% reduction from the ground-floor slab strength to the second-floor slab. The second-floor slabs at wings C and G have the highest average strength values of 39.6 \(N/{mm}^{2}\) and 38.6 \(N/{mm}^{2}\) respectively (Fig. 7). Table 4 reveals that the beams have the lowest average rebound compressive strength across the wings compared to the columns and slabs, the slabs also have the highest average rebound compressive strength following the results for the RB1. The average values for the columns across the wings range from 20.1–29.6\(N/{mm}^{2}\), for the beams 14.5–25.7 \(N/{mm}^{2}\) and for the slabs 29.2–34.5\(N/{mm}^{2}\).

Table 4

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in RB2
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
RB2A	Column	25.4	8.4	33.17
	Beam (1200mm)	14.5	2.7	18.39
	Beam (600mm)	20.2	6.6	32.79
	Slab	31.5	6.3	20.04
RB2B	Column	25.7	7.0	27.20
	Beam (1200mm)	12.2	2.2	18.15
	Beam (600mm)	17.7	3.5	19.84
	Slab	29.2	9.2	31.41
RB2C	Column	23.4	6.7	28.76
	Beam (1200mm)	21.5	6.3	29.30
	Beam (600mm)	20.5	4.6	22.27
	Slab	31.2	9.3	29.77
RB2D	Column	29.6	10.1	34.21
	Beam (1200mm)	18.5	4.2	22.92
	Beam (600mm)	17.9	3.8	21.18
	Slab	30.2	8.0	26.61
RB2E	Column	24.0	5.8	24.29
	Beam (1200mm)	24.5	4.3	17.36
	Beam (600mm)	17.2	5.7	33.20
	Slab	34.2	5.0	14.58
RB2F	Column	20.1	4.4	21.68
	Beam (1200mm)	20.9	5.6	26.68
	Beam (600mm)	19.9	1.5	7.30
	Slab	31.2	4.4	14.06
RB2G	Column	26.7	6.4	23.79
	Beam (1200mm)	25.7	5.9	23.03
	Beam (600mm)	24.8	7.6	30.72
	Slab	34.5	3.9	11.42

Generally in RB2, the values for the columns, beams and slabs are showing a high standard deviation which means the rebound values are spread out away from the mean values. Based on floors, the columns and slab showed a high level of dispersion from the mean compared to the beams which have low COV for most of the wings.

Institutional Buildings Rebound Assessment

The results of the average rebound compressive strength for institutional buildings (EB1 and EB2) are presented in Figs. 8–13 and the wings' mean strength values for each element, standard deviation and coefficient of variance (COE) values are presented in Tables 5 and 6.

The columns on this two-wing institutional building showed high average rebound compressive strength on the ground floor (Fig. 8). The columns on the first floor for wing A and the second floor for wing B recorded low compressive strengths of 28.8 \(N/{mm}^{2}\) and 26.5 \(N/{mm}^{2}\) respectively. The highest column average rebound compressive strength was recorded on the wing A ground floor columns. For the beams (Fig. 9), second-floor beams on both wings gave the highest average rebound compressive strength, the least beam average rebound compressive strength value was at the first floor of wing B with the value 30.1 \(N/{mm}^{2}\). As shown in Fig. 10, the slabs on the first floor for wing A experienced a 1.25% increase in strength compared to the ground floor while the slabs on the first floor for wing B had a 7.09% reduction in average strength.

Table 5

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in EB1
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
EB1A	Column	38.1	8.0	20.91
	Beam	32.4	4.3	13.25
	Slab	37.6	4.6	12.30
EB1B	Column	36.3	7.5	20.74
	Beam	33.6	7.4	22.03
	Slab	33.0	6.0	18.20

All structural elements gave mean rebound compressive strength ranging from 32.4–38.1 \(/{mm}^{2}\), the values for the columns, beams and slabs for the institutional building EB1 are showing a high standard deviation which means the rebound values are spread out away from the average values. Based on floors, the slabs showed the lowest level of dispersion from the mean compared to the columns and beams for both wings (Table 5).

The seven-wing institutional building assessed showed relatively low column strength with 70% of the columns having strengths below 24 \(N/{mm}^{2}\) (Fig. 11), wings A, C, D and G have columns with strengths below 20 \(N/{mm}^{2}\). For wings, E, F and G which have only one suspended floor, the columns experienced a reduction in average rebound compressive strength as the building increases from the ground floor to the first floor. Generally, wing F has the columns with the highest average strength of 34.7 \(N/{mm}^{2}\) and 28.1 \(N/{mm}^{2}\) for the ground floor and the first floor respectively.

The highest beam strength was recorded at the ground floor of wing D with a value of 35.6\(N/{mm}^{2}\) and the least beam strength at the last floor of wing C with the value 14.7\(N/{mm}^{2}\) (Fig. 12). For the slabs as shown in Fig. 13, the slabs at wing E and F gave the highest average rebound compressive strength of 43.8\(N/{mm}^{2}\) and 47.4\(N/{mm}^{2}\). The slabs in general gave a close range of strength values from 25.8\(N/{mm}^{2}\) to 33.7\(N/{mm}^{2}\).

As shown in Table 6, the slabs have the highest average strength across the wings compared to the columns and beams. Structural elements at wings E, F and G showed a low level of dispersion from the mean.

Table 6

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in EB2.
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
EB2A	Column	18.3	3.1	17.13
	Beam	23.6	5.2	21.98
	Slab	29.7	5.3	17.92
EB2B	Column	23.6	5.4	22.84
	Beam	24.7	3.4	13.71
	Slab	27.9	5.0	17.75
EB2C	Column	19.2	6.0	31.35
	Beam	22.4	6.2	27.82
	Slab	30.1	9.3	30.80
EB2D	Column	19.1	6.2	32.45
	Beam	28.1	5.8	20.74
	Slab	31.1	3.1	10.10
EB2E	Column	21.7	3.8	17.55
	Beam	25.3	1.2	4.70
	Slab	43.8	6.1	13.84
EB2F	Column	31.4	9.5	30.24
	Beam	30.7	5.2	16.85
	Slab	47.4	2.6	5.43
EB2G	Column	23.7	3.3	13.81
	Beam	24.2	3.4	13.98
	Slab	33.7	3.6	10.72

Industrial Buildings Rebound Assessment

The results of the average rebound compressive strength for industrial buildings (IB1 and IB2) are presented in Figs. 14–19 and the wings' mean strength values for each element, standard deviation and coefficient of variance (COE) values are presented in Tables 7 and 8.

Table 7

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in IB1.
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
IB1 Left & Right Wing	Column	42.3	2.1	4.90
	Beam	33.2	8.7	26.18
	Slab	42.5	7.2	16.94

The columns for the industrial building IB1 gave a high average rebound compressive strength of 43.5 \(N/{mm}^{2}\) for the left wing and 41.2\(N/{mm}^{2}\) for the right-wing (Fig. 14). The beams in the building have the lowest average rebound compressive strength of 33.2\(N/{mm}^{2}\) compared to the columns and slabs which have 42.3\(N/{mm}^{2}\) and 42.5\(N/{mm}^{2}\) respectively (Figs. 15 and 16). The values of the beam and slabs are spread away from the mean value while the values for the column strength are spread close to the mean value showing a standard deviation of 2.1 and a coefficient of variance of 4.9% (Table 9).

For Industrial Building IB2, the columns at the left wing reduced in strength as the building height increases by 23.7% while for the right-wing there was an increase in the average rebound strength by 6.9% as the building height increases (Fig. 17). The beams on the first floor gave low average rebound compressive strength compared to the ground floor (Fig. 18).

Table 8

Mean, Standard Deviation and Coefficient of Variance values of Structural Elements across floors in IB2.
Building Wing	Structural Element	Mean	Standard Deviation	Coefficient of Variance (%)
Left Wing	Column	24.5	3.7	14.97
	Beam	19.4	5.6	29.03
	Slab	37.9	2.0	5.31
Right Wing	Column	24.0	1.5	6.12
	Beam	20.1	3.7	18.41
	Slab	39.2	5.3	13.39

The slabs generally have higher average rebound compressive strength compared to the column and beams (Table 9). All structural element's strength values are widely spread away from the mean except for the slabs at the left wing and the columns at the right-wing given a standard deviation of 2.0 and 1.5 respectively.

Rebound Hammer assessment on the reinforced concrete buildings reveals that the structural elements on the ground floor gave higher and better strength values compared to structural elements on the suspended floors. However, reliable conclusions on the strength and structural health of the buildings can’t be inferred from the result generated due to uncertainties related to the equipment used, building age and detailed design information on the buildings. Generally, readings gotten showed very wide deviations from the mean which emphasis the unreliability nature of the rebound hammer as an effective way to assess reinforced concrete buildings. The wide range of values given is a clear indication that a more robust method of analysis like Bayesian analysis and reliability theory is needed for the interpretation and drawing of conclusions from the rebound hammer assessment.

Conflict of interest

The authors declare that there are no conflicts of interest.

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Statistical Analysis of Rebound Hammer Assessment on Reinforced Concrete Buildings

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Abstract

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I. Introduction

Ii. Methodology

Iii. Results

Iv. Conclusion

Declaration

References

Table

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