# The Time Variation Law of Concrete Compressive Strength: A Review
## 1 Introduction  
Concrete is one of the most widely used construction materials because of its good strength and durability relative to its cost [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. However, it is not an everlasting construction material. Due to the interaction of various factors in the surrounding environment, concrete materials will gradually deteriorate over time, thus reducing their mechanical properties [13; 14; 15]. Among them, compressive strength is an essential mechanical index of concrete [16; 17; 18; 19; 20; 21].  
Many factors cause variations in the compressive strength of concrete, such as carbonation, temperature effect, chloride ion erosion, freeze-thaw cycle, sulfate attack, etc. [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Currently, the research methods for studying the time variation law of compressive strength generally include taking statistical analyses on laboratory experimental data or field test data. The statistical analysis of laboratory experimental data is the main method for studying single factors affecting concrete, such as carbonation, freeze-thaw cycle, temperature effect, etc. [37; 38; 39; 40; 41; 42; 43]. In many cases, however, the variation in concrete strength is the result of the comprehensive effects of various factors. The superposition of carbonation and freeze-thaw cycles, dry and wet cycles and sulfate attack, loading and freeze-thaw cycles, carbonation and chloride ion attack and other influencing factors on the compressive strength of concrete were studied based on laboratory experimental data [30; 44; 45; 46; 47; 48; 49]. In addition, the long-term time variation law of concrete specimens under the coupling effect of multiple factors was studied [50; 51; 52; 53]. However, concrete compressive strength is influenced by a number of factors, and concrete structures mostly experience long-term loads in practical engineering. Thisphenomenon is generally difficult to emulate in a laboratory environment. Therefore, the statistical analysis of field test data is becoming the main method for studying the time variation law of compressive strength. Many researchers analyzed the field test data of existing concrete buildings and obtained a long-term time variation law [54; 55; 56]. In addition, the compressive strength of concrete is also influenced by internal factors such as cement type, curing conditions, and water-cement ratio [57; 58; 59; 60; 61; 62; 63].  
The study of the time variation law of concrete compressive strength, which is the basis of structural resistance calculations, is very necessary for assessing time-dependent reliability and predicting the subsequent service life of structures or components. There are many studies on the time variation law of the compressive strength of concrete. However, there is a lack of summary and comparison of its variation laws. In this paper, the time variation law of compressive strength was reviewed from both external and internal factors. External factors include carbonization, freeze-thaw cycle, temperature effect, sulfate attacks, etc. The internal factors include cement type, curing conditions, and water-cement ratio, etc. Firstly, the mathematical models of single external factors on concrete compressive strength with time were summarized. Secondly, the time variation laws of concrete under the coupling action of two or more factors were analyzed, and the time variation models of compressive strength in existing buildings were compared. Finally, the influences of internal factors on the strength of concrete were briefly outlined.