Research Of Degradation Mechanism And Improvement Of Long-term Strength Of Autoclaved Concrete

Accelerated curing methods are usually adopted for precast concrete members to obtain higher concrete strength in a short time,which is useful to ensure the following construction.Autoclaved curing,as one of the effective accelerated thermal curing methods,has been widely utilized all over the world today.However,at the same time,there are still some problems in autoclave curing,which are mainly focused on the degradation of the compressive strength of concrete at its later stage,which causes certain safety risks to a project.With the development of society and the extension and application of prefabricated concrete members,higher requirements are required for the long-term performance,quality and later age strength of prefabricated concrete members.The adverse effects of autoclave curing on long-term compressive strength need to be solved urgently.In this paper,autoclaved concrete is taken as a research background,and the changes of compressive strength of autoclaved concrete were studied by adding different nano-admixtures and changing the subsequent curing methods.Meanwhile,combined with the moisture contents of concrete,the changes of internal hydration products and micro-morphology were analyzed by mi-cro-experiments.The change mechanism of concrete compressive strength was ana-lyzed from micro-scale points,and the compressive strength development was also studied through fitting regression analysis.The main conclusions are listed as follows.1.Autoclave curing can increase the porosity of concrete,deteriorate the internal micro-pore structures,reduce the proportion of harmless pores and less harmful pores,increase harmful pores and more harmful pores,and transform the tobermorite formed into other forms of hydration products at the later stage.In addition,it makes the inte-rior of concrete dry,and reduces the later age hydration degree relative to concrete under standard curing,which is not beneficial to the long-term compressive strength development of concrete.2.The addition of nano-SiO₂ and nano-CaCO₃ can improve the compressive strength of concrete under autoclave curing to a certain degree,but the later age strength of concrete will still decrease.Adding nano-materials can increase the con-tent of tobermorite in concrete,but tobermorite will be converted into other forms of hydration products at the latter age,which is not beneficial on the long-term strength development.The addition of nano-materials can improve the pore structures.Specially,the incorporation of nano-CaCO₃ can reduce the porosity of concrete,however nano-SiO₂ can increase it,which is mainly ascribed to that the high pozolanic activity of nano-SiO₂ will aggravate the internal thermal damages on con-crete.The adding of nano-materials and slag in concrete under autoclave curing can increase concrete early age porosity,reduce the later age porosity,improve the pore structure distribution,partly compensate the thermal damages,and enhance the later age compressive strength.The main reasons are that slag has pozzolanic activity and secondary hydration with calcium hydroxide,produces more hydration products with high compactness,and the compressive strength of nano-CaCO₃ is higher than that of nano-SiO₂ in the later age.This is because the thermal damages on concrete with nano-SiO₂ is more serious at the early stage,which is not beneficial on the the later strength development.3.After 7 days of subsequent water-curing,the compressive strength,hydration degree,pore structure and compactness of autoclaved concrete can be evidently im-proved,and the thermal damages caused by autoclaving can be partly compen-sated.Seven days of secondary water curing can replenish water into autoclaved con-crete with nano-SiO₂,which is beneficial on the further hydration of unhydrated ce-mentious particles.It makes the internal pore structures more compact,and the harm-ful pores transforme into less harmful pores,and improve the later age strength.The compressive strength of autoclaved concrete with slag and nano-SiO2 can be effec-tively improved by subsequent 7 day water-curing.After autoclave curing,the interior of concrete is dry,and there are more slag unhydrated.The 7 day secondary water curing can promote the further hydration of the unhydrated cementitious particles.The calcium hydroxide generated can also take part in the secondary pozzolanic reac-tions with the added slag,which results in more hydration products,improves the in-ternal pore structures,and effectively alleviates the thermal damages caused by early autoclave curing.As a result,it can increase the later age compressive strength.The effects of long-term water-curing on strength development of autoclaved concrete was the best.Saturated water state can promote further hydration of unhydrated cementi-tious materials,improve the internal pore size,enhance the hydration degrees,and compensate the thermal damages caused by autoclave curing at the greatest extent.With the increase of concrete age,the compressive strength of autoclaved concrete increases,and tends to be stable after 90 days of age.4.Exponential fitting can be used to obtain a more appropriate strength devel-opment curve.The fitting function is y=?₀-a×b^x,in which ?₀ is the long-term compressive strength,and the compressive strength development of concrete fits well.For blank concrete,the negative effects of autoclave curing was partly alleviated after7 days of secondary water curing.The stable value of compressive strength in the later period could reach that under the standard curing,and the compressive strength in the later period of long-term water curing was higher than that under the standard curing.The long-term compressive strength of autoclaved slag concrete with nano-materials can be effectively improved.The long-term compressive strength of autoclaved specimens with nano-SiO₂ and nano-SiO₂ slag can be effectively enhanced by subse-quent water curing for 7 days.This shows that the subsequent water curing coupled with different admixtures can promote the subsequent hydration reaction,effectively improve the later age strength.The subsequent water curing with nano-SiO₂ and slag can effectively improve the later age strength,and obtains the best effect,and the spe-cific enhancement of the latter age strength was 21.2%relative to that of the blank autoclaved concrete.