Strength durability index (SDI) for improved concrete strength and durability assessment
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The use of concrete maturity as a non-destructive method to determine in situ strength has grown in recent years. Numerous published reports and papers have demonstrated this non-destructive technique to be a good indicator of compressive strength; provided, the placed concrete receives adequate curing and strict quality control is enforced at the batching plant. Although, great care may be taken to ensure adequate curing at the job site, there exists a real possibility for the degree of hydration to be less than required. It is this uncertainty (the amount of available or provided moisture) during curing that exposes a weakness in determining concrete maturity and strength based on current methods. Current maturity functions consider only internal concrete curing temperature and concrete age to determine a maturity index (expressed either as a time-temperature factor or as an equivalent age). They do not directly consider the availability of moisture during hydration, but rather assume sufficient moisture is present throughout the curing process. It has been shown through previous research conducted by Texas Tech University for TxDOT that these two inputs (i.e., time and temperature) are not sufficient to accurately predict cast-in-place bridge deck concrete strength in the field for various curing conditions. Therefore this study explores moisture availability from a variety of curing conditions and is a first step in the development of a Strength Durability Index (SDI) which considers temperature, concrete age, and internal concrete pore humidity. This investigation included exposing a concrete mixture to three different curing regimes and a number of curing durations. Both concrete temperature and pore humidity were monitored to help distinguish the effects on concrete strength and permeability. This investigation, together with the current Arrhenius equivalent age function and findings by other researchers in the area of water diffusion in concrete, help develop the framework for a better strength and durability prediction tool for field-cast concrete under a variety of curing conditions.