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CPTP Status Report - Task 65 Engineering ETG Review Copy
Chapter 2 - CPTP Focus Areas
CPTP Focus Area 2: Improved Materials
Projects in this area are looking at ways to improve or advance concrete material selection process and concrete mix design procedure to result in durable concrete paving mixes that can be placed and finished effectively for slipform operations. Highlighted projects within this focus area are described below.
Test Standards to Identify Compatible Combinations of Individually Acceptable Concrete Materials
Properly designed and constructed concrete pavements can provide 20 to 25 years of initial service life without significant maintenance. In recent years, however, cases of early-age problems and premature deterioration have resulted from use of incompatible materials. Highway professionals have noted instances of early loss of workability (early stiffening), delayed set (retardation), early-age cracking due to excessive shrinkage, and lack of proper air void system. These problems affect long-term performance and even construction productivity. Important expected outcomes under CPTP Task 4 are the advancements and improvements in testing and characterizing of PCC mixtures to mitigate premature failures in paving concrete. The objectives of this project are to develop practical test protocols and criteria to assess the effects of combinations of materials for concrete pavements on the following three areas:
The materials under evaluation include portland cement and blended cements; fly ash, slag, silica fume, and other pozzolans; chemical admixtures; and aggregates. The researchers will also consider the effects of production and placement methods, factors influencing aggregate segregation, bleeding and cohesiveness relative to finishability, and the influence of environmental conditions such as temperature. Deficiencies in existing test methods for assessing the suitability of materials for making concrete will be identified.
The test protocols developed will be incorporated into guidelines for evaluating and qualifying combinations of materials to be used in pavements. An implementation plan with recommendations and materials for dissemination of the information developed during this project will also be provided.
Concrete Mixture Optimization
Over the last several years a number of studies have been conducted that dealt with various aspects of the effect of concrete components on the performance of the resulting concrete pavements. The wealth of information now available is too great to be practically assimilated and combined from existing guidelines, reports, tables and predictive models in order for a pavement or materials engineer to derive the optimal mix for a given paving project. Under CPTP Task 64, a coordinated effort is being made to take the results of previous work (including work performed in CPTP Task 56) and integrate them into a computer-based system that will guide the concrete materials engineer in selecting the optimal mix for a particular project. Factors that are being considered include pavement structural design (loading effects), early-age and long-term environmental effects, the construction process, desired service life, available local materials and cost.
Concrete Durability Testing
Under CPTP Task 56, research projects are being conducted to develop additional information and test data to support improvements in characterizing the alkali-silica reactivity (ASR) and freeze-thaw (F-T) durability of paving type concrete mixtures. In a series of laboratory tests, researchers are evaluating the durability of high performance concrete (HPC) with lower amounts of entrained air. These tests will help define more effective air content requirements for HPC. In another study under Task 56, laboratory testing is being conducted to develop a fast, reliable test for assessing ASR potential of concrete mixtures. There is currently no accepted rapid test method to evaluate the ASR susceptibility of concrete mixtures. The commonly used ASTM C1260 test procedure specifically states that it is to be used to assess aggregates and not combinations of aggregates and cementitious materials. The results of this study will result in guidelines for proper use of ASTM C1260 for evaluating combinations of cementitious materials and aggregates.