| With rapid urbanization of a growing population,residential buildings and infrastructures are building year by year,leading to sharply reduction in permeability of urban surface ground.As a result,numerous environmental disasters,such as flash flooding,urban heat island,and ground subsidence,happen more and more frequently in many Chinese cities.To increase the permeability of urban ground,the concept of "sponge city”was proposed by national government,meanwhile rainwater can be used rationally or permeates into soil.Since road area accounts for 10%to 20%of the total urban area,permeable pavement is suggested as an effective measure to counterpart the urban flooding and to compensative the groundwater.Pervious concrete,a key material for permeable pavement,usually presents high mechanical properties or high drainage properties,it’s difficult to balance the strength and permeability simultaneously for a given pervious concrete,which limits the application of pervious concrete.In available literature,heavy trials experiments are needed to determine the mixture proportion of pervious concrete,and the performance of pervious concrete is roughly estimated based on experiences.Owing to the lack of theoretical deduction and calculation,it is very diff-icult to determine the mixture proportion(especially for the volume of cement paste)of pervious concrete to avoid drainage of cement paste and void clogging during casting.In addition,relationship between the skeleton parameters and the properties of pervious concrete has not yet be established,thus the mixture proportion of pervious concrete can not be designed appropriately based on target performance,or the properties of pervious concrete can not be predicted accurately.In this thesis,the relationship between the rheological properties of the cement paste and the maximum paste coating thickness on the surface of the aggregate was established,and then drainage of cement paste can be evaluated by comparing the designed paste coating thickness and the predicted maximum paste coating thickness.The skeleton structure of pervious concrete was characterized in terms of matrix strength fc,the number of contact point N,the width of contact zone W,and the paste thickness between aggregates T,and then linked to the strength and permeability of pervious concrete.Finally,mixture proportion,including cement paste composition,aggregate size,and paste/aggregate ratio,can be calculated or optimized based on the skeleton parameters,which are closely related to the mechanical properties and permeability of pervious concrete.The research significances are summarized in detail as follows:To simulate interactions between aggregate and cement paste during mixing,casting,and moulding,a paste drainage test on flow table was developed to measure the paste coating thickness on the surface of the aggregate(PCT).The influences of test variables,in terms of the number of flow table drops,the sieve mesh size,and the aggregate size,on PCT were investigated,and the clogging of voids at the bottom of the pervious concrete was observed.The optimized variables of paste drainage test were flow table drops of 2,sieve mesh size of 5.6 mm(No.3.5),and aggregate size of 8.0-11.2 mm.Under this condition,maximum paste coating thickness(MPCT)without clogging can be measured,with high precision and reliability,as the standard deviations of all results lied in range of ±3%.Four series of cement pastes with different rheological properties were designed by changing water/binder ratio,superplasticizer dosage,the type and mixture proportion of cementitious materials.Based on the rheological properties and MPCT of all cement pastes,relationship between yield stress(τ)and MPCT can be expressed as:MPCT = aln(b · τ),that between viscosity(η)and MPCT can be described as:MPCT = ced ·η,and MPCT = f,sm can be used to correlate mini-slump spread(s)and MPCT.The deviation of MPCT predicted based on yield stress or apparent viscosity was within ±5%,while that estimated based on mini-slump spread was lower ±8%,indicating that the relationships proposed can be regarded as a reliable method to predict the MPCT and voids clogging of pervious concrete.Image analysis(IA)was employed to characterize the number of contact point(N),the width of contact zone(W),the paste thickness between aggregates(7),and pore structure in the cross-sections of pervious concrete,then relationship between the skeleton structure and properties of pervious concrete was established.The planar porosity of pervious concrete Pp increased linearly with the increase of total paste area of contact zone(CTPA =N×W×T):Pp =PO-αCTPA.Equivalent void size(Dp)can be calculated by the number of contact point and CTPA using:DP = 2((P0-αCTPA)/πγNβ)1/2.The relationship between skeleton parameters and the compressive strength of pervious concrete(Fc)can be described as:Fc =(φ’dfc-φ’d)ln(CTPA-Co),and k15=4ω/π((P0-αCTPA)2)/γNβ can be used to correlate skeleton parameters and the permeability coefficient(k15)of pervious concrete.Mix proportion design method of pervious concrete was calculated according to the mechanical properties and permeability.The experimental results indicated that the relationship between the average size of aggregates(da)and N can be expressed as:N=NO(da)θ,that between paste/aggregate ratio(Vp/VA)and target paste thickness(TPT)can be described as:VP/VA=6TPT/da,and paste thickness between aggregate t can be calculated by:t=1.82TPT.During mixture proportion design,the matrix strength fc,the total paste area of contact zone CTPA and the number of contact point N were chosen according to properties requirement.Then,the width of the contact zone W and the paste thickness between aggregates T were selected keeping CTPA constant,and the MPCT should be larger than TPT(TPT≤MPCT).According to the mix proportion design method proposed,pervious concrete with 28d compressive strength of 37.2MPa and permeability of 14.1mm/s was prepared using diabase aggregate with good sphericity.Furthermore,the compressive strengths and permeability coefficient of all pervious concretes designed were higher than their target value,and the maximum deviation between the measured and predicted compressive strengths was only 4.2%,indicating that target mechanical properties and permeability can be achieve by the mixture proportion design method proposed.In this study,the relationships between the MPCT and the rheological properties of cement paste was clarified,and empirical equations of the skeleton structure and the mechanical properties,permeability of pervious concrete were established,then mixture proportion of pervious concrete was designed according to target properties.The methods proposed can be further used for structural characterization,performance and void clogging prediction,the experimental data laid theoretical foundation and technical support for high performance pervious concrete and its industrial application,all of which will helpful to the development of"sponge cities",sustainability and environmental protection of urban area. |
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