- Technical Paper - INTERNAL RELATIVE HUMIDITY MEASUREMENT ON MOISTURE DISTRIBUTION OF MORTAR CONSIDERING SELF-DESSICATION AT EARLY AGES Januarti Jaya EKAPUTRI *1 , Amiltra BONGOCHGETSAKUL *2 , Tetsuya ISHIDA *3 and Koichi MAEKAWA *4 ABSTRACT Using Blast Furnace Slag (BFS) with low water-binder ratio in concrete mixture to improve the performance of concrete can cause early-age cracks due to autogeneous shrinkage. In this study, the internal relative humidity (IRH) of mortar under ambient condition was measured by using new proposed method to study the effect of water-binder ratio and BFS in mixture on moisture loss. Decrement of IRH in mortar specimens at low water-binder ratio depends on moisture diffusion and self desiccation. Using BFS in mortar mixture leads lower IRH compare to pure Portland cement. Keywords: autogeneous shrinkage, internal relative humidity, water-binder ratio, blast furnace slag 1. INTRODUCTION Physical and chemical properties of concrete always change with time depending on moisture, curing, environment and etc. These material properties such as concrete strength, elastic modulus, creep, and shrinkage are significantly influenced by heat of hydration and moisture content in concrete at early ages. Consequently, for that reason it is necessary to know the behavior of moisture distribution in concrete structures under arbitrary ambient condition. Internal relative humidity (IRH) is an effective parameter to assess the moisture condition in concrete. It also relates to moisture content to shrinkage strain When there is no moisture movement allowed with the environment, shrinkage in concrete at early ages will occur due to cement hydration. Water consumption of un-hydrated cement results relative humidity in the concrete pores (IRH) reduced. This phenomenon is known as self-desiccation. It causes volume change called autogeneous shrinkage [1]. This deformation may cause micro cracking due to restrained auotogeneous shrinkage. It may form macro-cracks after connecting into continuous crack pattern [2]. This is a serious matter regarding durability, aesthetics and strength as well. Problems such as autogeneous shrinkage and change of IRH have been a great concern on using high performance concrete with low water-binder ratio. In practice, measuring IRH at early ages is a tough work because of its accuracy. From saturated condition to dry condition, measurable reduction in IRH will only occur after pores with radii of 50 to 100 nm have been emptied [3]. IRH measurement also is necessary to indicate the performance of curing method when weight loss is impossible to be measured in practical. Due to some disadvantages and toughness from existing method in measuring IRH, a new method of measurement is proposed. In addition, experimenting durability of concrete structure in long span (10-100 years) is not practical. Consequently, simulation program will be useful to evaluate durability performances over the whole service life since cement comes into contact with water. A thermodynamics oriented model [8] so-called DuCOM has been developed in the author’s research group According to this computational system, microphysical properties of cementitious materials can be precisely predicted. In this research, DuCOM can compare experiment results to analytical results. 2. IRH MEASUREMENT There are many types of hygrometers for relative humidity measurement. Chilled mirror, resistive and capacitive sensors seem to be most popular in concrete research. In this research capacitive sensors were adopted. The capacitive sensor used for IRH needs proposed by Z.C. Grasley [3]. It has accuracy ± 1.8% for RH and ± 0.3° C for temperature. It is not simple to measure IRH, because to get a reliable reading value, it is necessary to wait until the probe and surrounding concrete pore vapor are stable. The sensor includes a capacitive polymer-sensing element for relative humidity and a band-gap temperature sensor is used in this research (Figure 1). This In trial experiment based on existing method proposed by Grasley [4], the sensor and connector were wrapped using waterproof/breathable fabric made from polytetrafluoroethylene (PTFE) and sealed with *1 PhD Student, Dept. of Civil Engineering, University of Tokyo, JCI Member *2 Master‘s Degree, Dept of Civil Engineering, University of Tokyo *3 Associate Prof., Dept. of Civil Engineering, University of Tokyo, JCI Member *4 Prof., Dept. of Civil Engineering, University of Tokyo, JCI Member