Determination of the deformation characteristics of the foam concrete as a sub-base

2.1. Experimental field

Measurements in the experimental field can be realized at controlled boundary conditions to achieve reliable results. Scheme of the experimental field is plotted in Fig. 1. The Fig. 1 shows the positions of FWD testing sites. PLT testing was performed in the middle row in the same position as a FWD testing. In total, 12 FWD and 4 PLT tests were performed. Steel frame above the field served as a counterweight for PLT testing (Fig. 1, section A-A').

Fig. 1Scheme of the experimental field. Circles represent the PLT and LWD testing sites, dimensions in mm

Subgrade consists of anthropogenous clay of intermediate plasticity with rigid consistency. No groundwater was observed in the boreholes. Surface of the subgrade was leveled without compaction. We assume the use of the FC 400 foam concrete in the foundation slabs of family houses where mechanical and thermal properties are important. Modification of basic formula allows to decrease the bulk density and increasing of the thermal resistance at the same time with remaining almost the same mechanical characteristics compared to FC 500. Subgrade under the foundation structures of family houses is not always compacted so we prepared the subgrade in this manner. The values given in the Table 1 were obtained by the laboratory testing of a clay samples taken from experimental field. Characteristics of the clay in the subgrade are in Table 1.

Non-woven polypropylene geotextile with the unit weight of 200 g·m^-2 was laid on the subgrade as a permanent part of the FC design. Two foam concrete layers were created. First layer with the thickness of 12 cm was placed on the geotextile and second one with the thickness of 10 cm was placed on the first layer after measurements. Total thickness of the foam concrete layers was 22 cm.

PLT and LWD tests were carried out on the clayey subgrade and on the both foam concrete layers as plotted in Fig. 1. Construction of the field is shown in Fig. 2.

Fig. 2Construction stages of the experimental field: a) thermometer on the subgrade, b) geotextile on the subgrade, c) leveling of the fresh FC mixture, d) final FC layer prepared for realization of the second FC layer with the thickness markers

Temperature at the top of the FC layer, at the bottom of the layer and in the very upper layer of the subgrade was measured during the testing to clarify the temperature regime of the compound, especially during the winter season (Fig. 3). Vertical lines indicate the date of the PLT and LWD testing on particular layer.

Fig. 3Temperature in the experimental field during testing: a) top of the FC layer, b) bottom of the FC layer, c) clayey subgrade

Measurements on the second FC layer were carried out before (2nd FC layer – test 1) and after winter season (2nd FC layer – test 2). Surface of the FC was covered with the foil between the test days. Excitation of temperature is clearly visible at the top and less at the bottom of FC. No freezing was observed in the top layer of the subgrade (Fig. 3(c)) but the lowest temperature reached – 4.6 °C at the bottom of FC and – 16.4 °C was recorded at the top of FC (end of 1/19).

2.2. Plate load test

PLT testing was carried out with the rigid steel loading plate with the diameter of 357 mm. Load was added in stages at which steady settlement of the loading plate was recorded. Maximum contact stress was 0.15 MPa for the clayey subgrade, 0.3 MPa for the first FC layer and 0.5 MPa for the second FC layer. Theory for calculation of $E_{v2}$ modulus is based on the principles of elastic half-space. The equations for the calculation are given in the Slovak technical standard [18] and in certain variations they can also be found in the foreign standards. Strain modulus was calculated for the middle third of the overall pressure interval following the Eq. (1):

where: $E_{v2}$ – second load cycle strain modulus (MPa), $\mu$ – Poisson’s ratio (0.40 for the subgrade and 0.25 for the foam concrete), $r$ – radius of the loading plate (0.1785 m), $\mathrm{\Delta }p$ – selected stress interval (MPa), $\mathrm{\Delta }y$ – loading plate settlement difference for given stress interval (mm).

Clayey subgrade was tested using conventional measurement beam supplied as a regular part of PLT equipment (Fig. 4). The Fig. 4 shows the PLT testing on the clayey subgrade. Rigid steel plate was loaded up to maximum vertical contact stress in several loading stages, when attenuation of plate settlement was achieved in every stage. Depression around the loading plate doesn’t affect the beam legs because of small stiffness of the soil. Calculated strain modulus $E_{v2}$ from the second load cycle for the clayey subgrade varied in interval from 11.5 to 13.4 MPa.

Fig. 4PLT testing on the subgrade with conventional measurement beam

Because of high stiffness of the FC layer, we assumed that deflection radius around the loading plate could affect the legs of conventional measurement beam shipped with the loading plate for PLT testing. Therefore, larger beam with the legs situated outside the FC slab was created (Fig. 5). Polymer tube was reinforced by the strip of steel at which vertical holders for deformeters were attached. These were placed to identify the deflection of the FC slab surface. We tried to reduce the use of steel elements because of thermal expansivity of steel but some tempering was necessary to stabilize the deflection of the beam. Additionally, we screened the test site to avoid the direct radiance by sunrays and wind.

The plate was loaded by the piston connected with the hinge so only vertical force without moment effect was applied. Vertical contact stress and corresponding steady vertical settlement were recorded. Typical load/settlement curves for PLT tests for both load cycles are plotted in Fig. 6.

Fig. 5Large beam with the deformeters during the PLT test on the FC layer

Fig. 6Typical load/settlement curves for PLT test: a) on the subgrade, b) on the 1st FC layer, c) on the 2nd FC layer

Propagation of the deflection curve for the first and second FC layer is plotted in Fig. 7.

Fig. 7Deflection curve during PLT testing: a) measurement on the 1st FC layer, b) measurement on the 2nd FC layer – test 1, c) measurement on the 2nd FC layer – test 2

Deflection radius for the 12 cm FC layer is smaller than half width of the slab so the entire depression fits the slab (Fig. 7(a)). For the 22 cm slab, the edges are lifted, so deflection radius is larger than half width of the slab (Fig. 7(b) and 7(c)). As we can see, standard measurement beams cannot be used for PLT testing on the FC slabs because of the influence of the depression around the loading plate. On the other hand, the testing is still possible with conventional loading plate and pressure cylinder.

2.3. Light weight deflectometer

Light Weigh Deflectometer (LWD) equipment is based on impact theory. The 10 kg weight falls from the height of 0.755 m on the damping pad on the steel circular loading plate. The impact causes a contact stress of 0.1 MPa (vertical force of 7.07 kN). The impact interval is 17.9 ms what is related to the vehicle tire pass at the velocity of 60 km·h^-1. This stress causes the deflection under the loading plate $y$. The dynamic deformation or impact modulus $E_{vd}$ is calculated following the Eq. (2):

where: $E_{vd}$ – dynamic deformation modulus (MPa), $F$ – impact force (7.07 kN), $d$ – loading plate diameter (0.3 m), $y$ – deflection under the loading plate, $\mu$ – Poisson’s ratio (0.40 for the subgrade and 0.25 for the foam concrete).

Scheme of the LWD test equipment with typical test output is plotted in Fig. 8.

Fig. 8Scheme of the LWD testing apparatus and the graphic output of the test

Value of the modulus $E_{vd}$ and deflection $y$ is displayed on the display of the control unit after 3rd impact. A 3 LWD test sites were situated close to the corresponding PLT test sites (Fig. 1).

The testing repeated on the same place immediately after the first test causes the increase of $E_{vd}$ values. The test sites for measurements were therefore moved after the winter period in order to avoid the influence of the previous testing.