# Evaluation of Early Age Concrete Pavement Strength by Combined Nondestructive Tests
## 1 Introduction  
Pavement engineers need accurate information on in situ concrete properties to make decisions on early age pavement construction procedures. When to perform procedures, such as joint sawing and opening to traffic, relies on the pavement's ability to withstand stresses applied during construction. Early age concrete properties change rapidly and there is a risk of damage if concrete is subjected to loading prematurely. Onsite concrete testing must be fast and reliable to make proper decisions on procedure timing and scheduling. Current practices involve pouring separate specimens using the same concrete mixture in the field to be tested for compressive or flexural strength in a laboratory. This practice, called destructive testing, allows for a direct strength measurement for specimens, but is laborious, time-consuming, and may not be representative of field strength [1]. Using separate specimens ignores the effects of load, climate, edge support conditions, time of construction, design features, or other factors that may affect pavement performance but are irreplicable in a laboratory setting [2]. Variability within the pavement due to small environmental changes, concrete mixture changes, or placement differences may also be overlooked [3]. The pavement industry has recognized these limitations and has begun implementing nondestructive methods which directly inspect, monitor, or test systems without removing serviceability. Environmental, boundary, or curing conditions of in situ pavements are more likely to be accounted for in nondestructive tests [3, 4]. Concrete material properties, such as strength, density, or moisture content, are estimated by empirically correlating the property to variables measured by nondestructive tests [5]. Various nondestructive tests have been performed for years to evaluate concrete pavements, often for estimating in situ concrete strength or quality control. This paper proposes the combined use of twonondestructive methods in determining early age concrete strength development: maturity method and ultrasonic testing. The maturity method is a popular nondestructive test that relies on the relationship between early age concrete temperature and the rate of cement hydration to estimate concrete strength. Maturity is defined as the product of concrete age and temperature or the equivalent age at a specified temperature [3; 4]. When temperature is measured in the field using an imbedded sensor, the equivalent strength can be obtained through the maturity-strength relationship. An important aspect of maturity is the predictive ability of this relationship. Temperature data are predictable based on historical data on ambient temperature and heat of hydration. Since maturity is a function of temperature, temperature models can be used to predict the change in maturity and therefore the future strength development. This is a major benefit in scheduling early age construction practices and a key advantage to maturity. The maturity method relies on a calibration between maturity index and strength. This preconstruction laboratory work must be done for each concrete mix and if any deviations occur in the field (changes in the water-to-cement ratio, aggregate, cement type, curing conditions, etc. ), the relationship is no longer valid. Field use depends on temperature sensors and it is assumed the sensor location is representative of the entire pavement, which can ignore potential issues in consolidation or curing [3; 4; 6]. Despite limitations, maturity is an accurate and low labor method for estimating in situ concrete strength. The maturity method has been used for many years in research [3; 4; 7; 8; 9; 10; 11; 12; 13], has an ASTM standard for consistent use [14], and has been implemented in over half of state specifications as an acceptable strength measurement. Ultrasonic testing allows for analyzing internal characteristics of structures using penetrating, mechanical waves.