Abstract
The rheological properties of fresh mortar are directly related to concrete workability.The flowability or filling ability of concrete can be evaluated by measuring rheological properties of mortar with considering size and fraction of coarse aggregates.The previous researches proposed correlation between rheological parameters and flow test results such as mini-slump or channel flow tests.This research aims to establish yield stress equation of freshly mixed mortar based on flow table test results,which is flow test for mortar having relatively poor workability.In this study,a number of flow table tests,numerical simulations and rheological measurements using concentric vane rheometer were conducted,and Sisko&Bingham model can generalize the flow behavior of fresh mortars.
The workability of cement-based materials determines its construction performance in field applications.The workability or flowability can be generalized by rheological properties such as yield stress,viscosities.The relatively high flowable mortar's yield stress can be estimated by simple flow tests using conventional correlation models.Previous researches proposed correlation models to estimate the Bingham parameters through the mini-slump flow test.Tregger et al.[1]established yield stress equations based on theoretical and experimental approaches,and Roussel et al.[2]updated the theoretical yield stress equation with considering surface tension.Kim et al.[3]also proposed correlation equations to obtain yield stress and plastic viscosity of high flowable mortar using channel flow test,which is one of the gravity-induced flow tests for freshly mixed mortar.
This study proposed a model to correlate results of flow table tests with rheological parameters for poor workable cement mortar.The flow table tests and rheological measurements using vane rheometer of 4 test mixes were conducted,and Sisko and Bingham model quantified viscosity curves and flow curves which obtained by rheological measurements.A numerical simulation using volume of fluid technique also gived correlation of rheological parameters and flow table test results.Finally,the yield stress equations with respect to spread diameter after 5 and 25 drops,the two fitting coefficients are also proposed with high determination coefficient(R 2).
The flow table test can evaluate the workability of mortar using an impactinduced disturbance on a sample.The ASTM standards[4,5]specify the requirements for the flow table and accessory apparatus used in making flow test for consistency of mortar in tests of hydraulic cement,which were shown in Figure 1.
Figure 1 Flow table and rheometer
The mix proportions of a test mortar are regularized by 0.35∶1∶1.5 for considering the rheophysical state of SCC,and 0.5∶1∶3 for considering standard method of determination of cement strength(ISO679 Methods of testing cements.Determination of strength).The dosage of polycarboxylate ether(PCE)superplasticizer was set as maximum 0.1%with respect to mass of cement.The type of PCE is called methoxy polyethylene glycol(MPEG).The mix proportions are listed on Table 1.
Table 1 Mix proportions of test mortars
The procedure for flow table tests is(1)mixing by mechanical mixer for 3 minutes,(2)pouring a sample into a mold and tamping,(3)lifting the mold and measuring spread diameters after 5 drops and 25 drops,and repeating the procedure with interval in an hour.The maximum measurement time is 3 hours.Finally,total 16 flow table tests(4 mixes×4 intervals)were conducted.
Rheological measurements using commercialized viscometer(Discovery HR-1,TA Instruments)were also conducted.The concentric vane rotor was used,and its dimensions areφ55 mm×85 mm of container andφ15 mm×38 mm of 4-blades vane.The measurement protocol was flow sweep test which measured viscosity curves
relations)or flow curves
relations).
The data measured by rheometer can be generalized by various rheological models[6].The cement-based material's rheological behavior was commonly generalized by Sisko model for viscosity curves and Bingham model or Herschel-Bulkley model for flow curves.
The correlation between measured yield stress and flow table test database were proposed.Figure 2 showed the experimental and simulated correlation between yield stress(τ0)and spread diameter(D5 or D25).The common trend is that increasing rheological parameter is directly related to decreasing spread length as expected.The experimental correlation equation can be functionalized as
where k1 and k2 are fitting coefficients,and k1 has the unit of“mm/Pa”.The fitting coefficients were determined as k1=4 583 mm/Pa,k2=-1.216 for D5 with high coefficients of determination(R 2＞0.85 for D5).
The flow table tests of 4 mortar mixes were conducted,and their rheological behavior were also measured by vane rheometer.A numerical simulation using VoF technique can give us the spreadability of mortar accordance with its rheological properties.The measured rheological behavior which is represented by viscosity curves or flow curves could be generalized by fluid models,which are Sisko & Bingham fluid model.The correlations between yield stress and spread diameter obtained by flow table test were presented,and the yield stress equations and fitting coefficients were finally proposed by regression analyses with high coefficients of determination.
Figure 2 Yield stress-Spread diameter relations