Discussion
The entrained air void system in concrete is intended to act as a release mechanism for the hydraulic pressure caused by the 9% expansion of water volume when moist concrete freezes. It is made up of a quantity of relatively small air bubbles in very close proximity (<0.008 in.) to the tiny capillary pores. Properly spaced and sized, the air voids significantly reduce the distress caused by freeze-thaw cycles. As the water turned into a solid, the pressure builds in the void system, and when the pressure exceeds the tensile strength of the matrix, distress occurs and the concrete deteriorates. In a functioning void system, the water from thawing ice returns to the capillary pores form the air bubbles and the matrix remains intact. The petrographic examination shows a range of air contents form 2.5% (too low) to 7.9%. The test examples on both ends of the air content spectrum showed reasonable sizes (small bubbles resulting in specific area totaling greater than 600 in.² per cubic inch of volume) and reasonable separation, however, the total volumes of air contents, in several samples, were too low. The concrete failed and showed very persistent characteristics of D-cracking coarse aggregate.

"D-cracked concrete resembles frost-damaged concrete caused by paste deterioration. D-cracks are closely spaced crack formations parallel to transverse and longitudinal joints that later multiply outward from the joints toward the center of the pavement panel. D-cracking is a function of the pore properties of certain types of aggregate particles and the environment in which the pavement is placed. Due to the natural accumulation of water under pavements in the base and sub-base layers, the aggregate may eventually become saturated. Then with freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate." (PCA Design & Control of Concrete Mixtures, 14th edition, pg. 89) The petrographic examination of cores taken near the joints revealed distress that consisted of sets of several cracks concentrated through limestone coarse aggregate particles. This indicates, that the aggregates were the main initiating source of distress (D-cracking).

Aggregates containing reactive silica minerals can react with alkali hydroxides in concrete. This reaction called alkali-silica reaction (ASR) is potentially harmful only when it produces significant expansion. this reaction is fueled by absorbed salts and water. The air voids provide some relief from the expanding gel. A surprising quantity of ASR gel indicating that ASR likely was not the primary cause of cracking. In petrographic thin sections, several coarse aggregate limestone particles exhibited inclusions of silica rich minerals identified as chalcedony, known to be ASR reactive. ASR of portland cement concrete generally takes considerable time to develop significant expansion without other influences. This concrete exhibited a significant quantity of gel for its relatively short life span.

Conclusion
The technical evaluation discussed above has shown the need for improved durability considerations for the concrete placed at Cedar Creek. The fact is that many residential and commercial streets and sidewalks are subject to severe exposure (wetting-drying and deicing salt absorption), and therefore, significant changes in concrete mix design/components are needed to meet these exposure and durability requirements.

Industry wide, it has been demonstrated that life cycle costing makes concrete the long term product of choice.

Clearly, the soundness of the rock and the competency of the air void system are the primary factors at work in these difficult problem. The formation of ASR gel was greatly accelerated due to the initial freeze- thaw cracks. These concrete short comings were greatly accelerated by the extreme environmental exposures. The combination of hard non-absorptive rock, and ASR mitigating cementitious systems address these two concerns. Recent specification changes to require more durable aggregates, adequate air contents, and mineral admixtures (to address the potential of ASR) tend to address these issues that caused the deterioration observed in Cedar Creek. However, it is Ash Grove's position that any aggregate (including limestone) can be used if it is shown to demonstrate the durability requirements for its application. In all exterior concrete exposed to freezing and thawing, the concrete must have the appropriate quantity (6.0% minimum after placement) and quality of air-void system, and the placement, joining and curing must meet the minimum standard good practice requirements for concrete exposed to severe conditions and deicer salts.

Ash Grove supports the intent of the newly recognized KCMMB specifications and Recommends Testing Procedures for the Evaluation of Aggregates and Concrete for Durability (assuming measures incorporation slag or pozzolans are taken for ASR mitigation) as follows: