Research On Early-age Shrinkage Performance And Cracking Risk Of Concrete

Shrinkage and cracking of concrete are principal factors in concrete durability. Inrecent years, Chinese scholars have done a number of studies on concrete shrinkage andcracking. However, the method and device of the experiment are still in exploration orself-development stage. Slab test of the early-age anti-crack experiment which is used tosimulate the impact of rebar on concrete in projects had been added to the nationalstandard in2009. Based on these discoveries, we make a further study on the volumedeformation and cracking of early-age concrete by slab cracking device and providebasis for the concrete application in projects.The main contents of this paper are as follows:1. Test the compressive strength, splitting tensile strength and elastic modulus ofthe early-age concrete and perform regression on elastic modulus in the super-earlystage.2. Study the changes of early-age concrete volume and mass during free shrinkageand one-sided drying.3. Test the concrete cracking resistance by slab cracking device，and stress-straincharacteristics by FBG sensors in constraint conditions. Find out the correspondencebetween them.The conclusion shows that: the compressive strength, splitting tensile strength andelastic modulus of concrete will decrease when the water-cement ratio increase.Compared with the reference concrete, fly ash reduces the compressive strength ofconcrete.Both for free shrinkage and one-sided drying shrinkage, the early-age concrete willbe decreased by adding fly ash and reducing water-cement ratio. For the concrete whichadding no fly ash, it can increase the one-sided drying shrinkage value by increasingwind speed since wind accelerates the loss of water on concrete surface. However, forthe concrete which adding too much fly ash, it has no significant effect on the one-sideddrying shrinkage since wind speed is under5.5meters per second. For free shrinkage ofconcrete, the loss of water can always be increased by accelerating wind speed.However, for drying shrinkage of concrete, the wind speed and the radio must meetsome certain requirements. The use of fly ash can increase the loss of water.The workability of fresh concrete has a great influence on slab cracking test.Adding fly ash will increase the risk of cracking if the water-cement ratio is under0.40.But it can control cracking if the water-cement ratio is under0.36. The total crackingarea of concrete per unit area increases with adding fly ash and accelerating wind speed.It is easily to detect the temperature which restrains the concrete inside and the strainchanges using FBG sensors. Buried FBG sensors in concrete is a good method for monitoring and warning in an early stage since the cracking time can be early detectedby sensors rather than naked eyes.