3.2　Fracture Energy

The fracture energy is an important parameter used to describe the fracture characteristic of concrete material.Threepoint bending tests on a notched concrete beam is an indirect method recommended by RILEM TC50-FMC to determine the fracture energy of concrete and mortar.Figure 6(a)shows clearly that the fracture energy of PC concrete and AAS gepolymer concrete increases with the compressive strength.The average fracture energy of PC concrete increases from 127.1 N/m to 177.2 N/m(i.e.a 39.4% increase)when the compressive strength grade increases from C30 to C70.Although the fracture energy improvement of AAS geopolymer concrete is not as significant as that of PC concrete,the value still increases from 177.2 N/m to 207.9 N/m(i.e.a 17.3% increase).Overall,the fracture energies of AAS geopolymer concrete are all higher than those of PC concrete at all three compressive strength grades.The difference of fracture energy between PC concrete and AAS geopolymer concrete appears to reduce as the compressive strength increases,since sodium silicate can reduce the flocculation of slag grains and the wall effect.As a result,the initial porous ITZ around aggregate in AAS geopolymer concrete is smaller than that in PC concrete[10].

Figure 6　Fracture energy of PCC/AASC and AASM/PCM with the same compressive grade

The calculated average fracture energy of PCM and AASM is plotted against the compressive strength grade in Figure 6(b).The fracture energy of PCM is found to increase with the compressive strength,but the AASM exhibits an opposite trend.The fracture energy reduction in the case of AASM may be due to the use of a higher alkali concentration activator,which can generate more serious autogenous and drying shrinkage and shrinkage cracks[13].At M30,the average fracture energy of PCM is 100.9 N/m,which is 24.7%lower than that of AASM.Nevertheless,when the compressive strength increases to M70,the average fracture energy of PCM is 119.1 N/m,which is 23% higher than that of AASM.When the strength is relatively low,it was found that the matrix of AASM is more homogenous than that of PCM and the ITZs in AASM are denser and stronger than in PCM,leading to more energy consumption for crack propagation.When the compressive strength becomes high,it could be found more micro-cracks occurred in AASM than in PCM.Probably due to these existing flaws,AASM exhibits more brittle behavior and the energy consumed for crack propagation during the whole testing procedure is reduced[10].

The comparison of the fracture energy of PC mortar and AAS geopolymer mortar and concrete specimens is shown in Figure 7.Regardless of the matrix type(i.e.PC or AAS),due to the addition of coarse aggregates,the fracture energy of concrete is evidently higher than that of mortar.For both the PC and AAS series,the dif ference of the fracture energies between the mortar and concrete samples grows with the increase in the compressive strength.

Figure 7　Comparison of fracture energy between mortar and concrete

The fracture energy GF calculated from the P-δ curves of AASFC with different parameters are shown in Figure 8.The ultimate loads Pu of the TPB tests are also presented in the Figure 8 for reference.As expected,the ultimate loads Pu and the fracture energy GF of AASFC beams increased with the alkali concentration,the modulus and the slag/FA mass ratio while decreased with the increase of water/binder ratio,in other words,increased with the compressive strength.It can be seen from Figure 8(a)that the improvement of the average ultimate load Pu of AASFC beams(i.e.from 2 443 N to 3 214 N)with the increase of alkali concentration from 3%to 5% was 31.5%,and the fracture energy increased from 118.3 N/m to 137.6 N/m with a 16.3% increase.Although the imp rovement of the ultimate loads Pu of AASFC beams with modulus increase(from 1.0 to 2.0)was not significant(i.e.from 3 202 N to 3 344 N and only a 4.4%increase),the average fracture energy still increased from 128.8 N/m to 157.7 N/m(i.e.a 22.4%increase)Figure 8(b).Similarly,the improvements of the ultimate loads Pu and the fracture energy GF were 19.2% and 14.1%,respectively(i.e.from 2 993 N to 3 567 N and from 118.7 N/m to 135.4 N/m)with the increase of slag/FA mass ratio from 50/50 to 100/0(Figure 8(c)).In addition,the increases of the ultimate loads Pu and the fracture energy GF with the decrease of water/binder ratio from 0.50 to 0.40 were 30.0% and 26.4%,respectively(Figure 8(d)).