A Simple Mix Proportion Design Method Based on Frost Durability for Recycled High Performance Concrete Using Fully Coarse Recycled Aggregate

Durability design of recycled high performance concrete（RHPC） is fundamental for improving the use rate and level of concrete waste as coarse recycled aggregate（CRA）. We discussed a frostdurability-based mix proportion design method for RHPC using 100 % CRA and natural sand. Five groups of RHPC mixes with five strength grades（40, 50, 60, 70 and 80 MPa） were produced using CRA with four quality classes, and their workability, 28 d compressive strengths and frost resistances（measured by the compressive strength loss ratio and the relative dynamic modulus of elasticity） were tested. Relationships between the 28 d compressive strength, the frost resistance and the CRA quality characteristic parameter, water absorption, were then developed. The criterion of a CRA maximum water absorption limit value for RHPC was suggested, independent of its source and quality class. The results show that all RHPC mixes achieve the expected target workability, strength, and frost durability. The research results demonstrate that the application of the proposed method does not require trial testing prior to use.

A MODEL FOR CONCRETE DURABILITY DEGRADATION IN FREEZE-THAWING CYCLES

Up to now,how to control the durability of concrete structures is still a pending research topic in civil engineering.Owing to the fact that concrete durability can be affected by many non-mechanical factors,it is very difficult to research durability only based on mechanics. However,the phenomenon of failure caused by non-mechanical factors is similar to that caused by loads.As frost durability reflects the comprehensive durability of concrete in a certain sense, the specific failure process is analyzed and a damage model is introduced,which can describe the durability degradation process of concrete in the whole freeze-thawing cycle.

A Damage Model of Concrete under Freeze-Thaw Cycles

The frost durability of concrete is considered from structural engineering points of view.Specific failure process is analyzed and a damage model is established,which can describe the deterioration of concrete during the whole freeze thawing process.The model is verified by test data.The parameters of model can explain the effect of pore structures or water to binder ratio on frost durability of concrete.

Evaluation of the Relationship between Blocking Patterns and Duration of Spring Frost Waves:The Case of Iran

In order to evaluate the effects of blocking patterns on the duration of frost waves across Iran,the minimum daily temperatures of 60 weather stations were collected from 20 March 1968 to 21 June 2014.The findings,which were obtained based on the distribution tables of reported temperatures and their frequency variation graphs,resulted in identifying 114 frost waves.We obtained the daily weather maps at 500 hPa from NCEP/NCAR during the peak days of durable frosts,and it was revealed that the formation of low pressures over 20°-70°E and the dominant southwest-northeast troughs at 500 hPa facilitated the influx of cold air from both eastern Europe and central Siberia toward Iran,leading to the average frost duration of 12.4 days.Furthermore,low-pressure centers appeared in a dipole pattern in northern Iran over Caspian Sea and created durable frosts with an average duration of 15.4 days.The effect of omega shaped pattern of the geopotential height on durable frosts demonstrated a sharp difference with two recent patterns leading to the frost occurrence with a 13.3-day duration averagely.Additionally,the slow speed of cold western currents in the rex type of blocking from 30°to 50°N caused durable frosts of 14.3 days.Also,we found a specific reduction in the frequency of blocking patterns,which has occurred in the past four decades.Mostly,the blocking and non-blocking patterns cause long-and short-duration frosts over Iran,respectively.