International Concrete Abstracts Portal

This paper reports results of an investigation of the use of pullout testing and the maturity method to predict the early age strength of concrete. Concrete specimens, composed of 12 variations of 2 cement types, 2 aggregate types, and 3 water-cement cured at different temperatures: 37OF (2.8°C), and the outdoor environ-Cylinder compression and pullout tests were performed on specimens at ages ranging from 12 to 168 hours (7 days). Regression equations for cylinder strength and maturity, and pullout force and maturity are developed where the maturity is modified by changing the datum temperature from -10° C to 0° C to improve the predictive capabilities. A model for the prediction of the cylinder strength in terms of maturity is developed, as well as a oncrete strength by the pullout ombining cylinder strength and The reliability of the pullout model was affected by the comparative rates of strength gain of the cylindrical specimens and the slab specimens.

Pulse Velocity/Strength relationships can be estab-lished for concrete but they are influenced by many factors. Of particular significance are the proportions and composition of the components, age s curing conditions and moisture content of the concrete. Cement type, air-entrainment and curing temperatures -influence to a lesser degree. Pulse velocity correlates well with strength at early ages but is insensitive to even major increases in strength at later ages. A relationship established at early ages therefore is not applicable as the concrete matures. A rela-tionship determined on sound concrete during its development stage cannot be used to predict the strength of concrete that is deteriorating. Such a relationship should be established on cores from the concrete in question. A relationship established using laboratory-cured specimens, cannot be used with assurance to follow strength development in a structure. A Pulse Velocity Strength relationship can be confused by cracks, voids or other discontinuities in the concrete.

Embedded reinforcement may have a significant effect on ultrasonic pulse velocity measurements taken through structural concrete members. Reliable corrections are essential if test locations cannot avoid the influence of the steel. Extensive laboratory experimental work demonstrates major shortcomings in all currently accepted allowance procedures and confirms that bar diameter is an essential variable to be incorporated. The effect of bars passing across the pulse path is shown to be less than for bars of similar size running along the path. A correction procedure is proposed which can meet many practical combinations of bar size, bar orientation and concrete properties with significantly greater accuracy than possible by established methods.

The paste efficiency concept based on the pulse velocity difference between a control specimen and the concrete in the structure can be used to estimate the air-dried cube strength and hence in-situ concrete strength. This paper presents extensive test evidence to substantiate the validity of the paste efficiency principle. It is shown that the use of air-dried cubes (represent-ing in-situ concrete) produces consistent results; this has the further advantage that the method can be used to estimate the probable strength distribution in a structure prior to construction. The results show that in reality the k values relating control specimen strength to structure strength depend on concrete mix proportions, cement content, size and type of dense coarse aggregate and the type of concrete ie normal or lightweight. Based on this project, a set of k values is recommended, which should enable in-situ strength to be estimated to within + 10%; and these values should apply to site conditions with reasonably good quality control. The paper shows that pulse velocity measurements based on the paste efficiency concept can offer a reliable and consistent method of estimating air-dried cube strength which is shown to have some correlation to core strength.

A discussion is presented advocating the use of in-situ testing of concrete and a brief description is given of the "Break-off" method. Some data are presented from an experimental investigation of "Break-off" testing applied to slip-formed concrete. Six case histories of the use of the test under field conditions are discussed and some consideration is given to its future applications in concrete testing. In particular the role of the test in concrete construction in developing countries and as a means to improve testing efficiency in the pre-cast and pre-stressed concrete industries are considered. Relative cost and time requirements are compared for break-off testing versus conventional laboratory testing. The authors consider that in-situ testing and the break-off method in particular will be used increasingly as more demands are put upon the performance of concrete in a variety of applications.