### 3.1 The Effect of the Adding Aggregate Volume on the Concrete Strength

The conventional concretes with strength grade varying from C30 to C80 are prepared with various kinds of aggregates, cement, mineral admixture and mixing proportions, the aggregate interlocking concretes are prepared with scattering-filling aggregate process by adding 20 % volume ratio of coarse aggregates to substitute the conventional concretes mixtures, the 7 and 28 days strengths of those concretes with different content of adding coarse aggregate contents are illustrated in Fig. 2.

It can be found that the strengths of all those concretes with different mix proportions increase obviously with the 20 % (the volume ratio of scattering concrete in the finished concrete) adding aggregate ratio only different in the extent. The increasing ratio can be as high as 25 %. It is easy to find that concretes with 50–100 MPa have been prepared with only 362–440 kg/m^{3} of cement or binder when 20 % coarse aggregate is scattered to substitute the original concrete mixtures through some simple calculation, this can be never achieved with the conventional concrete placing process. The scattering-filling coarse aggregate can prepare concrete with very high coarse aggregate, although the “pre-placed aggregate” concrete process (Neville 1981) can prepare concrete with as low cement dosage as this process, but it can only prepare low or middle strength concrete. The scattering-filling aggregate method can produce consolidated concrete with less cement but higher strength and performances at the same time, the decrease on cement dosage not only give the concrete good cost competitive but also good environmental harmonization. The optimal volume percentage of the scattering aggregate in the concrete depends on the void ratio of coarse aggregate in the original concrete and the flowability of the concrete, the looser the aggregated packs and the more flowable, the more the scattering aggregate can be added, around 5–12 % of aggregate can be scattered in the pavement concrete. The scattering-filling aggregate process can be potentially apply in nearly all kind of conventional plastic and flowing concrete, is especially suitable to be used in pumped concrete for its high mortar content.

### 3.2 The Shrinkage of the Coarse Aggregate Interlocking Concrete

The shrinkage crack of concrete sometime results in damage of concrete structures, increasing of the aggregate can reduce the crack of concrete. The relation between the concrete shrinkage and the content of the aggregate in the concrete can be described as (Mindess et al. 2003):

{S}_{\text{c}}={S}_{\text{p}}{(1-\mathit{\alpha})}^{n}

(1)

Where *S*_{c} and *S*_{p} is the shrinkages of the concrete and the paste respectively, α is the volume fraction of the aggregate in the concrete and *n* is a constant which varies between 1.2 and 1.7 depending on the elasticity of the concrete. The shrinkage curves of the concretes with different scattering-filling aggregate replacement volume fractions are illustrated in Fig. 3.

Figure 3 indicates that with the increase of the replacement volume fraction of the scattering-filling aggregate, the shrinkage of the concrete decreases significantly. With increasing of the volume fraction of the extra coarse aggregate is added to the concrete, the shrinkage of the concrete decreases markedly. The aggregate scattering-filling process is a very effective approach to reduce the shrinkage of the high strength concrete, which is highly desired by engineers (Mindess et al. 2003).

### 3.3 The Micro-Hardness Distribution of the Aggregate Paste ITZ

Figure 4 illustrated the micro-hardness distribution of the ITZ in ordinary concrete and scattering-filling concrete, the rigid origin of X axis is the edge of the aggregate can be found in the microscopy. It is easy to see that the micro-hardness of the scatting-filling aggregate is much higher than that of the ordinary concrete. Generally, the ITZ has the lower hardness than aggregate and cement paste (Ping and Beaudoin 1992). If the edge of ITZ is where the micro-hardness begin to increase, the width of the ITZ in the ordinary concrete is around 110 μm but the ITZ in the scattering-filling aggregate concrete is 90 μm. This test result also can be explained with the water absorption of the air dry coarse adding aggregate which enhance the microstructure of the ITZ.

### 3.4 The Mechanism of the Coarse Aggregate Interlocking Concrete

There are many kinds of process to produce concrete with high coarse aggregate volume fraction, e.g. pre-placing aggregate concrete, Roller-compacted concrete, and some concrete with low cement content (Neville 1981; Ping and Beaudoin 1992; Nadeau 2003). Unfortunately, those processes typically can only prepare low or middle strength concrete (Neville 1981), the result of Fig. 2 indicated that concrete with a strength degrade ranging from C30 to C80 MPa can be prepared with this scattering-filling coarse aggregate process although this kind of aggregate interlocking concrete has very low cement dosage. The mechanism of the concrete process can be explained with following three aspects.

First of all, the coarse aggregate volume fraction increases dramatically in this type of concrete when more than 20 % of coarse aggregate is scattered in the mixture, the coarse aggregates can interlock with each other, the friction forces among the aggregate contribute to the strength enhance in a certain extent. The second, the coarse aggregate distributes more evenly in this kind of concrete than the concrete prepared with conventional process. With the vibration, the segregation of the coarse aggregate toward the bottom of the form and the of the cement paste toward the top may result in the conventional concrete (Mindess et al. 2003), the surface of the concrete will be very weak, but when scatting-filling process is applied, the coarse aggregate will fill evenly at the top of the form. The third, the addition of extra amount of air dry coarse aggregates acts as somewhat water reducer, not only reduces the W/C of the paste close to the aggregate and make the paste stronger so the ITZ of the concrete is enhanced. With the volume fraction of the aggregate increasing, as a composite material, the concrete will become stronger and stiffer.