
Parameters That Influence The Results of NonDestructive Test Methods for Concrete Strength
Ana Catarina Evangelista, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Ibrahim Shehata, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Lidia Shehata, Universidade Federal Fluminense, Rio de Janeiro, Brazil
ABSTRACT
The Nondestructive Test methods (NDT) have been used to evaluate the concrete strength in many countries, and experimental studies have investigated which method is more reliable and practical. The concrete strength is estimated using curves that correlate the NDT measurements with the compressive strength of concrete established by a laboratory testing program. Usually the parameters that affect these curves are the water/cement ratio, the aggregate type, the maximum aggregate size and the cement type of the concrete. This work presents a study on the influence of the mentioned parameters when the ultrasonic pulse velocity, probe penetration and rebound hammer methods are used.
Introduction
Control tests for the compressive strength at the age of 28 days are usually made in standard cylinders (or cubes) for evaluating the quality of the concrete used in the structures. However, the specimens are not truly representative of the concrete in the structure, which had different conditions of placing, compacting and curing. In the last decades some attempts have been made to develop nondestructive test methods for obtaining information about the strength and other properties of the concrete as it exists in the structure.
For an adequate use of the nondestructive tests, it is necessary to know the factors that influence the obtained measurements, and the correlation between the compressive strength and these measurements for the type of concrete under investigation. This work presents the results of a study on the influence of the type and maximum size of the coarse aggregate, and the type of cement on the correlation curves for concretes made with materials available in Rio de Janeiro having a 28 days compressive strength varying from 16 MPa to 53 MPa.
Experimental Programme
Materials, Mixes and Specimens
Four different types of coarse aggregates were used; three crushed natural aggregates and one lightweight artificial aggregate: gneiss with two different grading, trachyte, and expanded clay. Characteristics of these aggregates are given in table 1. The fine aggregate was natural sand with fineness modulus of 2.71 and specific gravity of 2.62 kg/m^{3}. Normal or rapid hardening Portland cement and chemical admixture (from 0.5% to 0.8% of the cement mass) were used. The volumes of coarse aggregate and water were the same in all concretes, in each series of concretes (M1, M2, M3, M4, M5) the water/cement ratios were 0.65, 0.60, 0.55, 0.50, 0.45 and 0.40, and the slump varied from 80 mm to 120 mm. The concrete mixes proportions are in table 2.
Characteristics
 Gneiss
 Trachyte
 Gneiss
 Expanded Clay

Maximum size(mm)  19  19  9.5  19

Specific gravity(kg/dm^{3})  2.72  2.65  2.70  1.28*

Fineness modulus  6.85  6.96  5.59  6.99

Table 1: Coarse aggregates characteristics.

*Satured surface dry
Materials
 M1
 M2
 M3
 M4
 M5

Coarse aggregate (kg)  Gneiss D_{max}=19mm  GneissD_{max}=9.5mm  Trachyte
D_{max}=19mm  Gneiss D_{max}=19mm  Expanded Clay D_{max}=19mm

1075  1070  1050  1075  505

Fine aggregate (kg)  830 to 680  830 to 680  830 to 680  830 to 680 
830 to 680

Cement (kg)  277 to 450  277 to 450  277 to 450  277 to 450*  277 to 450

Water(l)  180  180  180  180  180

w/c  0.65 to 0.40  0.65 to 0.40  0.65 to 0.40  0.65 to 0.40  0.65 to 0.40

Table 2: Concrete mixes proportions.

* rapid hardening cement
D_{max} maximum size
The compression and nondestructive tests of the concretes were carried out at the ages of 3, 7, 14, 28 and 90 days.
Cylindrical specimens (150 mm x 300 mm) were used for the compression, rebound hammer (ProceqDigi Schimdt equipment) and ultrasonic pulse (Pundit Equipment  54 kHz) tests; for the probe penetration (Walsyva gun) tests, the specimens were prismatic (200 mm x 200 mm x 600 mm).
The cylindrical specimens were cured in water up to 48 hours before the date of the test, in view of the influence of humidity on the rebound hammer measurements. The prismatic specimens were immersed in water until the date of the probe penetration tests.
Results
The expressions proposed by other authors for correlating the compressive strength (f_{c}) with the measurements of pulse velocity (V), rebound hammer index (R) and probe exposed length (L_{p}) are presented in tables 3, 4 and 5, respectively. Table 5 includes only the authors that used the same kind of gun used in this work. These expressions have been based on the results of concretes with various types of materials and mix proportions and, because of this, they represent quite different correlation curves. This is shown in figures 1, 2 and 3.
In this study, the standard curve fitting procedure considered different types of correlation relationship: linear, power, exponential, logarithmic and polynomial.
The best fit for the correlations between f_{c} and V were the exponential curves shown in figure 1. A statistic analysis showed that, from the varied factors, the ones that had more influence on the correlation between f_{c} and V were the density of the coarse aggregate and the type of cement.
Fig 1: Comparison between curves that correlate f_{c} with V.

For the correlation between f_{c} and R, the best fit was given by the power curves seen in figure 2. It was verified that the factors that have a significant influence on these curves are also the density of the coarse aggregate and the type of cement.
Fig 2: Comparison between curves that correlate f_{c} with R.

In figure 2 it can be observed that the curve proposed by the manufacturer of hammer (ProceqDigi Schimdt) gives lower compressive strength than the ones obtained in this work, except for the case of lightweight concrete (M5) when R is greater than about 30. The curves calibrated for concretes with higher strength differ considerably from the others proposed.
Linear correlations between f_{c} and L_{p} were obtained for the concretes with crushed rock aggregate. The probe penetration test, with the probe (55 mm length and 6.3 mm diameter) and gun power load used, was not suitable for the lightweight aggregate concretes, since in most cases full penetration of the probe occurred. It is seen in figure 3 that the correlation curve for the concrete with rapid hardening cement in noticeably different from the ones for the concretes with normal hardening cement.
Fig 3: Comparison between curves that correlate f_{c} with L_{p}.

Analysis of the variance (ANOVA) at a 5 % significance level, considering the data of series M1 concretes as the reference, showed which of the varied parameters had significant influence on the values of f_{c}, V, R and L_{p}. The results of this analysis are in table 6. It is worthwhile pointing out that the coarse aggregate volume, that could have relevant influence on the results, was the same for the analyzed concretes.
Conclusions
The analysis of this work showed that parameters that significantly influence the concrete strength may not influence the nondestructive test results in the same way and vice versa.
For the concretes studied, it would be possible to use the same correlation curves for the concretes with rock crushed coarse aggregates and normal hardening cement. Different curves, however, would be necessary for the concretes with rapid hardening cement and lightweight aggregate.
The correlation curves given by different authors show that reliable estimate of insitu strength can only be obtained if the correlation between compressive strength and the nondestructive test measurement for the same kind of concrete is properly established.
Author
 Expression*
 f_{c} (MPa)
 Specimen
 Type of aggregate
 Note

Ravindrajah et al (1988)  f_{c} = 0,060e^{1,44V}  15,0 to 75,0  Cube 100mm  granite (D_{max}=20mm) 

Almeida (1993)  f_{c} = 0,0133V^{5543} f_{c} = 0,011V^{5654}  40,1 to 120,3  Cube 150mm  granite (D_{max}=25mm) 
1^{st} and 2^{nd} test series

Gonçalves** (1995)  f_{c } = 0,02V  65,4  18,0 to 42,0  Core70mmx70mm    28 days to 3 months

Qasrawi (2000)  f_{c}= 36,72V  129,077  6,0 to 42,0  Cube 150mm  Variable  Air cure

Soshiroda and Voraputhaporn (1999)  f_{c 28} = 44,52V_{1}  126,83 f_{c 28} = 54,18V_{28}  206,27  20,0 to 65,0  Cube 150mm 
Gravel  V1  1 day V28  28 days

Phoon et al (1999)  f_{c } = 124,4V  587,0 + e  35,0 , 55,0 and 75,0  Cube 150mm  granite (D_{max}=20mm)  28 days

Pascale et al** (2000)  f_{c} = 10^{28}V^{8.1272}  30,0 to 150,0  Cube 150mm  limestone(D_{max}=15mm) 

Elvery and Ibrahim(1976)  f_{c} = 0,012e^{2,27V}±6,4  15,0 to 60,0  Cube 100 mm  gravel (D_{max}=19mm) 

Teodoru (1988)  f_{c} = 0,0259e^{1,612V}  2,0 to 24,0      28 days

Yun et al (1988)  f_{c} = 0,329V  1065  5,0 to 30,0  Core150mmx300mm  gravel
(D_{max}=25mm and D_{max}=40mm) 

Table 3: Expressions of other authors for correlating f_{c} with V.

* f_{c} in MPa and V in km/s , ** f_{c} in MPa and V in m/s
Author
 Expression*
 f_{c} (MPa)
 Specimen
 Type of aggregate
 Note

Ravindrajah et al (1988)  f_{c} = 7,25e^{0,08R}  15,0 to 75,0  Cube 100mm  granite (D_{max}=20mm) 

Almeida (1993)  f_{c} = 1,0407R^{1,155} f_{c} = 1,041R^{1,155}  40,1 to 120,3  Cube 150mm  granite (D_{max}=25mm)
 1st and 2nd test series

Gonçalves (1995)  f_{c} = 1,73R  34,3  18,0 to 42,0  Core70mmx70mm    28 days to 3 months

Pascale et al (2000)  f_{c} = 0,000135R^{3,4424}  30,0 to 150,0  Cube 150mm  Limestone(D_{max}=15mm) 

Qasrawi (2000)  f_{c} = 1,353R  17,393  6,0 to 42,0  Cube 150mm  variable 

Soshiroda and Voraputhaporn (1999)  f_{c 28} = 161R_{3}  137 f_{c 28} = 147R_{28}  16,85  20,0 to 65,0  Cube 150mm 
gravel  R_{3}  3 daysR_{28} 28days

ProceqDigi Schimdt  f_{c 7} = 1,4553R_{7}  22,817 f_{14 56} = 1,398R_{1456}  2017  25,1 to 33,1  Cube 200mm  gravel(D_{máx}=32mm)  7 days14 days to 56 days

Lima and Silva (2000)  f_{c} = 0,0501R^{1,8428}  25,1 to 33,1  Cylinder  

Table 4: Expressions of other authors for correlating f_{c} with R.

*f_{c} in MPa
Author
 Expression*
 f_{c} (MPa)
 Specimen
 Type of aggregate

Vieira (1978)  f_{c} = 0,7294 L_{p} + 41,231  7,0 to 38,5  Cylinder  

Danielleto (1986)  f_{c} = 0,08L_{p}^{2}  780L_{p} + 187,53  14,8 to 53,1  Cylinder  gneiss

Table 5: Expressions of other authors for correlating f_{c} with L_{p}.

*f_{c} in MPa and L_{p} in mm
Varied parameter
 f_{c}
 V
 f_{c}V
 R
 f_{c}  R
 Lp
 f_{c}  Lp

Type of crushed rock aggregate     X 
 

Lightweight aggregate  X  X  X  X  X  *  X

D_{max}   X     X 

Cement type  X   X  X  X  X  X

Table 6: Varied parameters that had significant influence on the results of f_{c}, V, R and L_{p}, and on the correlation curves, considering concrete series M1 as a reference.

* probe penetration test results not considered
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