Roller compacted concrete-tensile strength of horizontal joints
A. C. Bettencourt Ribeirol, J. Diez-Cascon and A. F. Gonfalves
ABSTRACT
Roller Compacted Concrete is a very dry material consolidated by the use of a powerful external vibratory compaction. This material is used mainly in gravity or arch/gravity dams, and pavements. RCC dam construction methodology, by concrete layers, involves a high number and an extensive area of horizontal joints. These joints are the weakest parts of RCC in terms of strength and permeability, being one hindrance to more widespread use of RCC. This study analyses the influence of different parameters on the direct tensile strength of the joints, namely the relative humidity in the concrete surface, the setting and hardening state of concrete, and the cold joint treatment. The results obtained indicated that the decrease in the joint quality with the time of exposure is continuous being important the compaction of the upper layer before the initial setting time of the lower layer. Cold joint treatments should expose aggregate particles.
INTRODUCTION
The horizontal joints are the weakest parts of RCC in terms of strength and permeability. In order to control the quality of the joints it is usual to specify a maturity index (M.I.) to evaluate when a cold joint treatment is needed. This M.I. is calculated as: M.I. = AST x TE, where, AST = Average Surface Temperature, TE = Time of Exposure. Another simplified method to control the joints is based only on the time of exposure. The use of these methods is in general insufficient to avoid lower strength and higher permeability in the joint. In fact, the values specified for M.I or TE are usually very high and do not take into account the actual state of the setting/hardening of the concrete. The quality of the joint highly depends on the capacity to avoid a weak transifon zone between layers. If the lower layer is too stiffwhen the upper layer is compacted, a weak transition zone will occur. The mix between layers, which leads to a certain concrete homogeneity at the joint level, only occurs if the concrete of the lower layer maintains thixotropy and enough workability. On the other hand, according to the Arrhenius law, the maturity of concrete is not expressed by a linear function of the temperature.
Since the quality of the joint depends on the properties of the surface of the lower layer, research has been
done concerning the study of practical methods to identiffy the different setting and hardening phases which could be correlated with the characteristics of the joint between layers. The objective is to know which parameters should be defined to obtain a block of mass concrete, avoiding major changes in the homogeneity of concrete structure.