Influence Of Lightweight Aggregate Types On Physical And Mechanical Properties Of Concrete

Aggregate is an indispensable part of concrete,which accounts for more than 70% of the total volume of concrete.In recent years,there have been more and more high-rise buildings and long-span bridge structures.Natural aggregate has a large apparent density and consumes a lot of energy in the production process,which neither conforms to the concept of green development nor meets the special density requirements of modern engineering for concrete.Lightweight aggregate concrete uses industrial waste materials and artificial lightweight aggregates to replace sand and stone.It can reduce the dependence on non-renewable resources and promote the reuse of solid waste.There are many kinds of lightweight aggregates.Different lightweight aggregates have respective characteristic,which have various effects on concrete.The development of light aggregate concrete with different aggregate types can meet the application needs of diverse projects.According to the above reasons,we selected 4 kinds of lightweight aggregates: spherical fly ash ceramsite,crushed stone fly ash ceramsite,expanded polystyrene particles(EPS),Super Absorbent Polymer particles(SAP).Two kinds of cement stone matrix are designed,which have different strength grades.Then we combine each aggregate systems with different strength grades of cement matrix in order to research the effect of lightweight aggregate characteristics on the physical and mechanical properties of concrete.In this paper,firstly,different light aggregate density classes,cylinder compressive strength,water absorption and release properties,grading,internal curing effect,and reinforcement modification were researched.Then the concrete was prepared by combining aggregate system with cementite matrix of different strengths.The laws regarding the workability,physical and mechanical properties,dry shrinkage properties of round spherical haydite concrete,crushed stone shaped haydite concrete,EPS concrete and SAP concrete were obtained,and analyzed the reasons.The main research contents and results are as follows:We designed 2 strength classes of cementite matrix in our experiments: high strength cementite matrix and medium strength cementite matrix.First,the test uses silicate cement as the main cementitious material and silica fume as well as fly ash as the mineral admixtures.The experimental design was carried out using the D-optimal design method in the response surface method.Based on the closest packing theory,a prediction model on the workability and 28 d compressive strength of concrete was established with cement,silica fume,fly ash,water reducing agent and water-binder ratio as independent variables.The validation results showed that the flow rate and 28 d compressive strength of high-strength cementite matrix were 153 mm and 133.5 MPa,respectively.the model predicted values with high accuracy.Then,the optimal ratio of medium strength cement stone matrix was determined by single-factor test.Under this ratio,the flow degree of cement stone matrix can reach 276 mm,the wet capacity is 1810kg/~3,the 3d compressive strength is 23.8MPa,and the 28 d compressive strength is 58.8MPa.The differences in basic mechanical properties and microstructure of round spherical fly ash ceramic granules,crushed stone shaped fly ash ceramic granules,EPS granules,and SAP granules were researched.Aggregate gradations in the particle size range of 3 to 5 mm and 5 to 8 mm were optimized based on aggregate bulk density and void ratio as indicators,and shell modification studies were further carried out on round spherical fly ash ceramic granules and EPS granules.The results show that there are differences in density,cylinder compression strength and water absorption of different types of light aggregates.The apparent density of round spherical ceramic granules is the largest and the cylinder compression strength is the highest,which are 1753kg/~3 and 9.7MPa respectively.The apparent density of EPS granules is the smallest,only 13kg/~3,which almost does not have the bearing capacity.The water absorption multiplier of SAP granules is the largest,which is up to 93 times of its own mass for 1h,and EPS granules basically do not absorb water.As for the microstructure of aggregates,the surface of round spherical ceramic granules and crushed stone shaped ceramic granules are rough and have holes.The former has a well-developed and interconnected internal pore structure,while the other has a dense pore wall structure.The surface of expanded polystyrene particles resembles honeycomb structure and the internal pore wall is folded,and the surface shape of super absorbent polymer granules is smooth with slight internal grooves.After the optimization of aggregate gradation,the maximum bulk density of each aggregate system and the lowest corresponding void ratio were found at V(5~8mm):V(3~5mm)= 35:65 for spherical ceramic particles,V(5~8mm):V(3~5mm)= 35:65 for gravel-shaped ceramic particles,V(5~8mm):V(3~5mm)= 25:75 for expanded polystyrene particles,V(5~8mm):V(3~5mm)= 40:60 for super absorbent polymer granules.The pre-absorption of ceramic granules facilitates the preparation of fresh concrete with good workability.By controlling the water absorption time of super absorbent polymer granules can prepare aggregates with different particle sizes.The stress analysis of the core-shell light aggregate shows that the aggregate stress is mainly borne by the outer shell,and the maximum stress value is mainly distributed at the interface between the shell and the core aggregate.In addition,the shell-building enhancement is obvious.Based on the previous work,concrete with different light aggregate systems/different cementite matrix was prepared.The results of the paste-aggregate ratio research show that the fresh concrete has good workability at a paste-aggregate ratio of 45:55.The workability of concrete composed with different strength matrices is influenced by the type of aggregate in a consistent pattern: SAP concrete > EPS concrete > spherical haydite concrete > crushed stone shaped haydite concrete.The degree of pre-wetting treatment of aggregates and the particle morphology characteristics are the main factors affecting the workability of concrete.The differences in density,water absorption,and cylinder compressive strength of the aggregates themselves are the main reasons for the variations in dry apparent density,water absorption,and mechanical properties of the concrete.Analysis of the stress-strain curves revealed that the form of damage of haydite concrete is brittle failure,while EPS concrete and SAP concrete showed ductile failure.The pre-absorption of light aggregate has an inhibitory effect on dry shrinkage.Compared with EPS concrete,the dry shrinkage of crushed stone shaped haydite concrete and round spherical haydite concrete decreased by 58.9% and 37.26%,respectively.X-ray diffraction analysis showed that the aggregate with internal water storage could influence the hydration process and hydration product distribution of cementitious materials inside the concrete.The core-shell type light aggregate can slightly increase the dry density of concrete,significantly reduce the water absorption,significantly improve the frost resistance and significantly increase the compressive strength.