Assessment of concrete compressive strength by ultrasonic pulse velocity test

One of the most popular non- destructive techniques is ultrasonic pulse velocity (UPV) which used in assessment of concrete properties. A statistical experimental program was carried out in the present study to establish an accurate relation between the UPV and the concrete compressive strength. The program involved testing of concrete cubes cast with specified test variables. The variables are the age and density of concrete. In this research, all the samples were tested by direct ultrasonic pulse velocity (DUPV) and surface ultrasonic pulse velocity (SUPV) to measure the wave velocity in concrete and the compressive strength for each sample. An experimental study was conducted to compare between the velocities of ultrasonic waves that transmitted along the two paths; direct and indirect. A total of more than 150 cubes having dimensions of 150 mm side were prepared to conduct both non-destructive and the compressive strength (destructive testing). The results from experimental program were used as input data in a statistical program (SPSS) to predict the best equation, which can represent the relation between the UPV (direct, indirect), and compressive strength, a linear equation is proposed for this purpose. The UPV measurement and compressive strength tests were carried out at the concrete age of 7, 28, 56 days. A relationship curves were drawn between DUPV, SUPV, compressive strength and density. The mixes composition in this study consists of ordinary Portland cement, fine sand, gravel, super-plasticizer, and water. All the specimens were under (20) Cº. The statistical analysis revealed that the possibility in evaluating the properties of the concrete by using direct and indirect wave velocities


Introduction
Today, great amounts of engineering projects have used concrete as a basic material, which is primarily composed of cement, water, and aggregate. Generally, the aggregate consists of fine aggregate such as sand and coarse aggregate such as gravel or crushed rocks like granite [1]. Ultrasonic Pulse Velocity (UPV) test is a non-destructive test, which is performed by sending high-frequency wave (over 20 kHz) through the media. By following the principle that a wave travels faster in denser media than in provide information on the uniformity of concrete, cavities, cracks, and defects. The pulse velocity in material depends on the density and the elastic properties for material, which in turn are the looser one, the engineer, can determine the quality of material from the velocity of the wave this can be applied to several types of materials such as concrete, wood, etc [2]. Concrete is a material with a very heterogeneous composition. This heterogeneousness is linked up both to the nature of its constituents (cement, sand, gravel, reinforcement) and their dimensions, geometry or/and distribution. Thus, highly possible that defects and damaging should be exist. Non-destructive testing evaluation for these materials have motivated a lot of research work and several syntheses have been proposed [3]. UPV test of concrete is based on the pulse velocity method to provide information on the uniformity of concrete, cavities, cracks, and defects. The pulse velocity in material depends on its density and its elastic properties, which is turn, are related to the quality and the compressive strength of the concrete. It is therefore possible to obtain information about the properties of components by sonic investigations [3.] These methods are used to check variations in structure, changes in surface, presence of cracks, or other physical discontinuities, to determine the characteristics of industrial products materials. Nondestructive test (NDT) determination of materials properties is becoming increasingly important in design and life assessment consideration of components and systems . [4] NDT is conducted by measuring the time taken by ultrasonic wave to pass through concrete. Higher velocities indicate that the quality and continuity of the material is good, while slower velocities indicate that the concrete with many voids and cracks. [5] Direct transmission is defined as the spread of ultrasonic waves along a straight-line path between the two opposite surfaces of the specimen. Indirect transmission is defined as the spread of ultrasonic waves between points that are located on the same surface of the material.
ASTM C 597 stated that the indirect measurements indicative of properties for the layers that are near the surface, there for this measurements are not recommended except when only one surface of a material is accessible.
Presently, ASTM standards do not exist for indirect UPV measurements. In general, ASTM standards reported that, the direct wave velocities were higher than the indirect wave velocities. To explore the relation between concrete compressive strength and concrete qualities the UPV can be used for main idea that UPV waves are a function of the material density, which are correlated with the compressive strength . [6] BS (1881): Part 203 (BS, 1881, 1986) test describes the time need for pulse to travel through concrete. Then, the Pulse velocity is measured by a simple formula: - [7] Width of structure Pulse velocity = Time taken by pulse to go through V= L/T [4] Where:-V = velocity of pulse (m/s). L = length of path (m). T =effective time (s).
Here the time is equal to the measured time minus the zero-time correction. The zero-time correction is equal to the time takes for pulse travel between the transmitting and receiving transducers when they are pressed firmly together.
The quality of concrete in terms of uniformity, incidence or absence of internal flaws, cracks, segregation, etc., indicative of the level of workmanship employed, can thus be assessed using the guidelines given below, which have been evolved for characterizing the quality of concrete in structures in terms of the ultrasonic pulse velocity.

Methods of testing:-
Depending upon the placement of the transducers, there are three methods of testing the concrete:

Direct Method:-
In this method, the receiver is placed at one end and the transmitter at the other end throughout the member. (Fig -1-a).

Indirect method:-
In this method, the receiver and transmitter are place on the same surface of concrete. (Fig -1-b).

Semi-direct method:-
This method is used for corners member of concrete (Fig 1-c).  The influence of path length will be negligible provided it is not less than 100 mm, when 20 mm size aggregate is used or less than 150mm for 40 mm size aggregate. Pulse velocity will not be influenced by the shape of the specimen. Generally, the velocity of pulses in steel bar is higher than concrete, therefore, pulse velocity measurements may be high and is not representative of the concrete in the vicinity of reinforcing steel.
The influence of the reinforcement small if the bars run at right angles in a direction to the pulse path and the quantity of steel is small to the path length. The influence of the moisture content of the concrete can have a small but significant on the pulse velocity. Generally, when moisture content is increased the velocity is increased, and for lower quality concrete, the influence is more marked.

2-Background
In civil engineering, the NDT application has becoming an interesting testing in various countries. A great number of researches are used, because these tests do not affect the performance and the appearance of the structures analysed. This method can be done at the  [9] UPV methods can be considered as one of the most promising methods among the available methods of NDT for evaluating the concrete structures. It makes possible an examination of the homogeneity of material.. It makes possible to get a total control of a structure, using the properties variations with the time. It is possible to detect heterogeneous regions in the concrete or verify the capacity by using the analysis of the propagation variations of the ultrasonic velocity wave. [10] Many studies have been conducted to determine direct pulse velocity and the factors that affect it. ASTM 1999a, b; RILEM 1972; BS 1997) standards are available for measuring direct transmission velocity. There is less information available on indirect transmission. British Standards (BS 1881) made comparison between direct velocity and indirect velocity transmission and state that the indirect velocity is about 5 to 20% lower than the direct velocity . [11] Generally, indirect pulse velocity transmission can be used when only one face of the concrete structure is accessible as stated in ASTM C 597 (ASTM 1999a; RILEM 1972; BS 1997) stated that indirect measurements are not reliable.

Pulse Velocity (km/sec) Concrete Quality
A relationship between pulse velocity of concrete and strength have been proposed presented and presented with numerous experimental data and the correlation relationship. Some figures by Whitehurst [12] suggested some figures for concrete approximately of 2400 kg/m 3 for concrete density are given as very poor , poor, doubtful , good and excellent, for 2000 m/s and below ,3000-3500 , 3500-4500,and 4500 m/s and above, UPV values respectively. Based on experimental results. Tharmaratnam and Tan [13] gave the relationship between the ultrasonic pulse velocity in a concrete (Vc) and concrete compressive strength (fc) as: fc = a e bv Where:a, b = parameters dependent upon the properties of material. Keating et al., (1989) [14] investigated the relationship between the cube strength for cement paste and direct ultrasonic longitudinal pulse velocity (DUPV) in the first 24 hours. For concrete cured at room temperature, it is realized that the relative change in the pulse velocity is higher than the observed rate of strength in the first few hours. Anyway, a mutual correlation between these two parameters can be reduced. [15] The relation between the ultrasonic pulse velocity and the compressive strength was developed by several studies, most important equations were found by following equations:

Raouf, Z and Ali Z.M. Equation:
Raouf and Ali (1983) developed Equation from 650 test results collected from the results of students, the experimental mixes and the other taken from the tested cube, which send to National Centre for Construction Labs.
With confidence, limit equal to (10%) as shown in Equation (1

2.1.3-Jones R. Equation:
In 1962, Jones has presented a non-linear equation to relate ultrasonic velocity and compressive strength as shown in equation (

Popovics et al. Equation:
Popovics have used Klieger experimental results in 1957, for a mathematical comparison for use of DUPV and surface ultrasonic waves (SUPV), respectively, for strength estimation. The best-fit formula for the relationship between concrete strength and DUPV for the seventh day experimental results by Klieger is presented by equation (4)

Elvery and lbrahim Equation:
Elvery and lbrahim carried out test to examine the relationship between concrete cube strength and ultrasonic pulse velocity for ages of 3 h over a curing temperature range from 1 to 60C. The authors developed equation below for 28 day age with correlation equal to (0.74) (Elvery and lbrahim, 1976).

Materials
More than 150 cubes specimens with dimensions of 150x150x150 mm were performed for the goal of the study, which is consisted of the measurement, and comparison of UPV obtained using direct and indirect transducer arrangements on the same specimens at an age of 7, 28, 56 days. Table -2-present the composition of the mix.

Cement
Ordinary Portland cement (Type 1) satisfy the requirement of Iraqi Specifications No. 5, (1984), [16] was used throughout this work, which has high (C3S) cement compound (49.67%). Tables (3) and (4) show the physical and the chemical test results of the used cement.

Table-4-Main compounds and chemical composition of the
Ordinary Portland cement.

Aggregate
Natural sand was brought from AL-Habaniya site and separated by sieving. Its grading satisfies the requirement of B.S specification No.882/1992 [17] for fine grading.
Tables (5) and (6) show the physical properties and grading of fine aggregate (natural sand) respectively compared with the requirement of B.S.882/1992 .Natural coarse aggregate was brought from AL-Nibaae site and separated by sieving. Its grading satisfies the requirement of B.S specification No.882/1992. Tables (7) and (8) show the grading of coarse aggregate and the content of sulphate compared with the requirement of B.S.882/1992.

Water
In this work, tap water is used for mixing. Table (9) shows the chemical composition of Ramadi City water.

Superplasticizer
In this study, the super-plasticizer used is Gelunium 51 (superplasticizer, high range water and reducing agent) which complies with the requirements of ASTM C494-05 [18] type F; its properties are shown in table-10.

4-Results and discussion
A relationships were established in this study between the results of non-destructive testing (DUPV , SUPV) and those from of mechanical testing of specimens (Table 11). These values are plot in graphs, Matlab is used to extract the curves (regression line), and then coefficients of determination R 2 are obtained for each regression line. Figures (2), (3), (4), (5) and (6) show the DUPV, SUPV strength development and density, with concrete age for the same mix batches. The DUPV and SUPV strengths of concrete and density appear that an increasing with the age advancement. These five curves of the relationship between (DUPV and SUPV), compressive strength, and density were drown for concrete with the age 7, 28, and 56.
Table (12), shows the relationship between regression curves and the data points for concrete with the ages 7, 28, 56 days. The equations for the curves (Fig 5 and 6) for these three ages as shown in table (13). Where:-σD = compressive strength (MPa) for (DUPV) σS = compressive strength (MPa) for (SUPV) v = the ultrasonic pulse velocity (m/s) The curves in (Figs 5 and 6) at age (7,28, and 56) show a good relationship between concrete strength and (DUPV, SUPV), with a high coefficient of determination for this particular mixture (R 2 ) of 0.8113 , 0.7091 , 0.9478 for DUPV and 0.8506, 0.9023, 0.7964 for SUPV respectively as shown in table (12), this indicate the relationship between regression curves and the data points.
Then the compressive strength of the specimen obtained from the destructive tests was compared with the predicted strength. (Table 14). Almost the deviation of the results was about ±10% from the true value between the compressive strength of the specimen and the predicted strength. This verifies the suitability of the proposed relationship curves for prediction of hardened concrete strength with a measured UPV value.

5-Conclusions
Most of the reliable and a valuable method of examining the interior of a body of concrete is ultrasonic pulse velocity measurement, which is, describe as non-destructive manner. This method was used to assess the mechanical compressive strength of concrete. The main objective of this paper is to investigate the relationship between compressive strength of concrete and the ultrasonic pulse velocity (UPV) to understand the influence of the age, density of concrete on the relationship between compressive strength and UPV.
Based on a compressive experimental investigation involving more than 150 cubes (15x15x15) cm that came from the same mix butches. Specific conclusions are as follows:-1-The difference between the values of resistance obtained by destructive and non-destructive tests decreased considerably at the age of 28 days.
2-In this research it is verified that, as shown in Figures (5) and (6), tests on concrete specimens represent that the value of the concrete strength increases as UPV increases.
3-To determine the concrete strengths of the concrete mix proportions at the range of density, the equations obtained from the simulation curves can be used, as shown in table 12.
4-This study indicates that the ultra-pulse velocity tests are very sensitive and Very important data can be provide by used this test about making the decision for concrete conditions.