## 3 Results and Discussion
### Storage Modulus  
The effect of sand volume fraction on the magnetorheological response determined by measuring the storage modulus of the mixtures with and without magnetic field application is shown in Figures 8 and 9. In the absence of the external magnetic field, the mixtures all have a storage modulus in a similar range after 300 s (G\({}^{\prime}\)300s \(<\) 150 kPa). When a magnetic field of 0.65 T is applied, the initial rate of increase in storage modulus (and hence the structural build-up) and final storage modulus after 300s of all the mixtures increases significantly to varying degrees. This is consistent with previous studies in which the storage modulus of cementitious pastes containing MNPs was measured with/out a magnetic field. The increase in storage modulus is due to the increased structuration caused by the formation of magnetic clusters within the paste. The magnitude of increase in the storage modulus under a magnetic field provides an indication of the degree of magneto responsiveness and ability to form clusters. Generally, a larger relative increase in storage modulus indicates a higher magneto response [5; 7; 8; 29].  
Figure 8: Storage modulus evolution with/out magnetic field application.  
Figure 7: Effect of aggregate content (after [17]).  
Increasing the volume percentage of sand particles from 0 to 20% had the effect of increasing the magnetorheological response of the mixture, as shown in the storage modulus evolution in Figure 8 and final storage modulus after 300 s in Figure 9. It is hypothesised that in up to 20% volume of sand, the presence of sand grains in the mixture does not have a negative impact on the ability of the magnetic particles to form chains or clusters. Due to the low packing density, the sand grains are loosely packed within the cementitious paste, giving the magnetic particles more freedom to move and form clusters. As the percentage volume of sand is increased from 20 to 40%, it is theorised that the presence of the sand grains begins to inhibit the movement of the magnetic particles, thereby lowering the magnetorheological response. At 40% sand volume, the magnetorheological response is the lowest. The result thus suggests that there is a threshold volume percentage of fine aggregate that can be added to the paste after which the magnetorheological effects begin to decline due to the presence of aggregates. In practice, it is more desirable to formulate the mixture composition for maximum packing density to improve the strength and durability indexes of the concrete [30]. It can thus be hypothesised that for more practical mixture compositions containing fine aggregates of even larger sizes than used in this study, the magnetorheological response would not be as high as its paste counterparts. Nevertheless, despite the M4 having the lowest magneto response, the magnitude of increase in storage modulus is still noteworthy. The storage modulus increased by a factor of 4.2. Moreover, in comparison to the time sweep tests that are performed in nearly static conditions, formwork leakage flows occur in dynamic conditions, of which the particle concentration also plays a key role in the blocking mechanisms. The higher sand volume could have a positive effect on the blocking mechanisms despite the lower magneto response. This is discussed in the following section.