Investigation of Pulse Velocity-Strength Relationship of Hardened Concrete

The purpose of this paper is to investigate the relationship between the ultrasonic pulse velocity (UPV) and the compressive strength of concrete. The specimens used in the studies were made of concrete with a paste content of 36% and the constituents of the specimens varied in different water-cement ratios (w/c) and coarse aggregate contents by weight. The UPV measurement and compressive strength tests were carried out at the concrete age of 1, 3, 7, 14, and 28 days. The experimental results show that the relationship between UPV and the compressive strength of concrete is significantly influenced by age and coarse aggregate content. The UPV and the compressive strength of concrete grow with age, but the growth rate varies with mixture proportion. To simplify the analysis task, this study chose hardened concrete (at an age of 28 days) as the subject for analysis. It is found that with the same content of coarse aggregate, a clear relationship curve can be drawn to describe the UPV and compressive strength of hardened concrete. This paper proposes the UPV and strength relationship curves for concrete having different contents of coarse aggregate. These curves were verified to be suitable for prediction of hardened concrete strength with a measured UPV value.

Keywords: concrete strength; mixture proportions; nondestructive evaluation; ultrasonic pulse velocity.

INTRODUCTION

Some work in previous literature made use of the ultrasonic pulse velocity (UPV) of concrete to predict compressive strength and it is fundamental in such research work to study the relationship between UPV and compressive strength.1-6 At early age of concrete, the pulse velocity increases rapidly relative to strength.7,8 Previous research results indicated that the density of concrete ties positively to UPV and compressive strength. Pulse velocity is influenced by many variables, however, including mixture proportions, aggregate type, age of concrete, moisture content, and others.1 The factors significantly affecting the concrete strength might have little influence on UPV. As a result, a strength estimate made with the pulse velocity method is not reliable if a preestablished calibration curve is not available.2

Several previous studies1-5 concluded that, for concrete with a particular mixture proportion, there is a good correlation between UPV and the compressive strength; yet a relationship with a wide variation will be acquired if the data of UPV and the compressive strength of concrete having different mixture proportions are put together and analyzed. No clear rules have been presented to describe how the relationship between UPV and the compressive strength of concrete changes with its mixture proportion. Therefore, there exists a high uncertainty when one tries to make use of UPV to predict the strength of concrete in different mixture proportions.

A previous study9 showed that UPV of hardened concrete is predictable based on its mixture proportion. In addition, it has been known that the compressive strength of concrete corresponds with the mixture proportion; thus, this study tries to adopt the mixture proportion of concrete as a medium to investigate the relationship between UPV and the compressive strength of hardened concrete. This paper uses a mixture proportion of concrete with a paste content of 36% to make cylindrical concrete specimens and the specimen constituents vary widely in water/cement ratio (w/c) and coarse aggregate content by weight. The UPV measurement and compressive strength tests were carried out at the age of 1, 3, 7, 14, and 28 days. The influence of three factors-age, w/c, and coarse aggregate (CA) content-on UPV and the compressive strength of concrete is studied and used to determine how these factors affect the relationship between UPV and the compressive strength of concrete.

RESEARCH SIGNIFICANCE

Currently, the influence of mixture proportion on the relationship between UPV and the compressive strength of concrete is unclear. This study uses concrete mixture proportion as a medium to link the interrelated relationship between UPV and the compressive strength of concrete and to clarify its influence. Furthermore, this paper proposes a new direction to establish a clear relationship curve between UPV and the compressive strength of concrete to improve the application of the UPV method on nondestructive evaluation of concrete strength.

EXPERIMENTAL DETAILS

Materials

Materials used for making specimens include cement, fine aggregate (FA), CA, and high-range water-reducing admixture. The cement used was portland Type I. River sand with a saturated-surface dry (SSD) density of 2.62 and crushed stone with an SSD density of 2.60 were used as fine and coarse aggregates, respectively. Both sand and crushed stone were from the same source. The grading curves for fine and coarse aggregates are shown in Fig. 1. The pulse velocity of sand was measured by using the twophase model as discussed in a previous paper9 to be approximately 4960 m/s (16,273 ft/s). The pulse velocity of the coarse aggregate was measured to be approximately 5100 m/s (16,732 ft/s).