The performance, under drop-weight impact load, of hybrid cement composite (HCC) elements, consisting of a top layer of plain concrete (PC) and a bottom layer of fibre reinforced concrete (FRC), in comparison with full-depth FRC and PC was studied. Apart from improving the tensile capacity of PC and saving fibre steel reinforcements of FRC, the results showed that HCC can effectively control the deformations and enhance the impact performance of the structural members as its outcomes were similar to that of a full-depth FRC. The analytical studies using Hughes empirical formulae (HEF) and yield line theory (YLT) adopted to investigate the practical use of HCC showed that they are applicable for design such HCC elements against impacts.
Under impact situations, the material is required to absorb a large amount of energy within a short duration (
It is also generally assumed that the beneficial effect of fibres in concrete matrix is much more significant in tension than that in compression (
An alternative approach, based on the concept of functionally graded concrete (
Nevertheless, casting in layers at fresh state did not always guarantee the good bond between layers even when HCC element is subjected to static load (
In spite of the significance of the concept for being applied in structures subjected to impact (
The objective of this paper is to study performance of structural HCC element under drop-weight impact load compared to the monolithic counterparts of PC and FRC. Maximum impact load, accumulated energy absorption capacity and mode of failure against multiple impacts are aspects for assessing the impact performance among them. The study also focuses on the dynamic responses (local and global) and interface of HCC elements in order to comprehend whether there is a delamination between layers.
The structural elements were made by using a Portland cement CEMI 52.5 (OPC), densified silica fume (DSF) and limestone filler (LF) as finer components, siliceous sand and siliceous aggregate with maximum size 10 mm (CA). A high range water reducer admixture (HRWRA) was used to keep w/c under control considering that the self-compacting mixes were designed to have a flow spread of about 650 mm and a 28-day compressive strength of 80–85 MPa (
Mix proportions and main characterizations
Unit | PC | FRC | |||
---|---|---|---|---|---|
Mix proportion | OPC | kg/m3 | 500 | 500 | |
DSF | 45 | 45 | |||
LF | 50 | 50 | |||
Sand | 821 | 821 | |||
CA | 670 | 670 | |||
SF | – | 80 | |||
HRWRA | 14 | 16 | |||
W/C | 0.35 | 0.35 | |||
Rheological characterization | Flow spread | df | mm | 660 | 620 |
Flow time | T50 | s | 6 | 6.5 | |
Mechanical characterization at 28 days | Compressive strength |
fc | MPa | 84.6 | 82.1 |
Elastic modulus |
Ec | GPa | 38.3 | 38.1 | |
Splitting tensile strength |
fct1 | MPa | 5.8 | 8.9 | |
Flexural tensile strength |
fct2 | MPa | 7.8 | 11.2 | |
Residual flexural strength |
fr1 | MPa | – | 9.7 | |
fr2 | MPa | – | 6.4 | ||
fr4 | MPa | – | 2.2 |
Average of three sample results.
Individual test result.
The types of structural elements were all slabs of size 310 mm×310 mm×60 mm. This size was adopted to fit the steel frame span used with the drop-weight tower. Regarding to the casting of HCC slabs, they were prepared in two steps by using the wet-on-wet method proposed for designing a layered functionally graded concrete (
Slabs used for experimental test
Material | PC | HCC | FRC |
---|---|---|---|
Number of slab cast | 4 | 4 | 4 |
Schematic view (units in mm) |
The instrumented drop-weight impact tower used was a Dynatup 8250 included in
Drop-weight tower used in this study.
The cross-head has an indication flag, which is used together with the laser sensor to trigger the Instron data acquisition system, used for recording during test the load vs time curve once the impactor was in contact with the slab. The machine had also an electro-mechanical system and a computer connected to the acquisition system. The electro-mechanical system was operated manually, to move the weight-load cell-impactor system freely up and down along the two stainless steel circular guide rods. Next to guide rods, there were two shock absorbers, which play the role of stopping the cross-head to protect the impactor from collision with other components. Grease was applied to guide rods in order to reduce friction along the rods and to ensure a controlled and smooth fall. The weight together with load cell and the impactor was dropped freely below the earth's gravitational acceleration of 9.81 m/s. An overall view of the apparatus and also a detail view of the testing area are shown in
Lastly, the machine is complemented with a mobile rigid steel frame, as shown in
Slab support condition.
All of slabs were tested on a 354 mm diagonal span length with the impactor striking them at the mid-span of their top in as-cast direction, which was marked by the diagonal cross-lines, as it can be seen in
Finish of impact load test on a slab.
It has been reported by some authors (
where,
E = kinetic energy lost by the weight (J)
M = drop-weight impact (kg)
νi=impact velocity at the corresponding time (m/s)
As expressed in equation [
It must be noticed that prior to beginning each drop sequence the impactor was manually move down in order to check whether the laser sensor system was performed properly and its position was in coincidence with the centre of slab (cross-lines) or subsequently with that of previous impact. Afterward, the cross-head together with the weight, load cell and impactor was moved upwards for beginning test process and corresponding measurements.
As given above in
When the structure receives an impact, other important consideration is to examine the local (such as indentation, spalling phenomenon, cone cracking, etc.) and global (crack formation, propagation, with, etc.) response or damage of structure (
Crack width determined by mean of magnifying glass.
Impact faces of PC slabs after impact load test a) S1; b) S2 and c) S3.
Velocity of impactor, maximum impact loads, penetration depth and energy absorption capacity obtained from each experimental test
Material | ID | Impact up to failure | Velocity of impactor (m/s) | Maximum load after each impact (kN) | Maximum penetration depth after each impact (mm) | Energy absorption capacity after each impact (J) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
No. | Weight (kg) | Impact I | Impact II | Impact III | Impact I | Impact II | Impact III | Impact I | Impact II | Impact III | Impact I | Impact II | Impact III | ||
PC | S1 | 1 | 33.14 | 4.24 | – | – | 34.37 | – | – | 2.45 | – | – | 249.72 | – | – |
S2 | 1 | 33.14 | 4.19 | – | – | 62.42 | – | – | 2.98 | – | – | 163.06 | – | – | |
S3 | 1 | 33.14 | 4.18 | – | – | 47.64 | – | – | 2.03 | – | – | 257.68 | – | – | |
HCC | S4 | 2 | 33.14 | 4.23 | 4.22 | – | 52.38 | 58.50 | – | 12.31 | 35.99 | – | 301.04 | 282.60 | – |
S5 | 2 | 33.14 | 4.20 | 4.25 | – | 59.73 | 25.86 | – | 11.09 | 39.43 | – | 295.56 | 250.85 | – | |
S6 | 2 | 33.14 | 4.23 | 4.25 | – | 50.44 | 52.65 | – | 11.31 | 32.14 | – | 299.53 | 292.41 | ||
FRC | S7 | 3 | 33.14 | 4.24 | 4.24 | 4.23 | 63.66 | 72.56 | 58.11 | 7.43 | 5.90 | 16.49 | 300.40 | 300.26 | 230.34 |
S8 | 2 | 44.02 | 4.21 | 4.23 | – | 60.52 | 64.56 | – | 12.56 | 19.59 | – | 395.71 | 397.67 | – | |
S9 | 2 | 44.02 | 4.21 | 4.21 | – | 70.23 | 40.410 | – | 8.06 | 24.53 | – | 394.61 | 401.45 | – |
As it was pointed out above, all of slabs were subjected to impact load in as-cast direction, hence, in case of HCC slabs, the impact or top face was that without fibre inclusion or PC layer and, consequently, the distal or bottom face was that with fibre inclusion or FRC layer. The weight level chosen to test first was 33.14 kg. However, when testing of the first two FRC slabs with that weight, they truly showed better behaviour under multiple impacts compared to PC and HCC tested previously. For instance, the FRC slab, which was used for preliminary test, failed only after four impacts. Therefore, it was attempted to study the limitation of this material, and the weight was raised up to 44.02 kg for testing the last two FRC slabs (S8, S9).
The PC slabs were broken immediately after a single impact of the weight 33.14 kg, as it can be seen in
In case of the other FRC slabs S8 and S9, since the impact weight was increased up to 44.02 kg, the impactor penetrated fully into the slabs after double impacts. The penetration depth of these slabs was moderately higher than that of slab S7 after the first impact, as shown in
The failure of the PC slabs under impact was catastrophic, as it can be seen in
On the contrary, under the first impact of weight 33.14 kg, the crack was not formed on the impact face of the HCC slabs, except the spalling of PC at the area of impact point; on the distal face, several cracks originating from centre to the edges of slab were observed and the maximum width was of 0.1 mm, as crack map is shown in
Crack formation in the impact and distal faces of HCC slab S6 after a) impact I (Units of crack width in mm); b) impact II.
Under the same drop-weight impact of 33.14 kg the impact face of FRC slab S7was slightly indented by spherical-ended body of an impactor and suffered less damage than HCC and PC slabs did after the first impact. Several cracks were propagated on the distal face with the maximum width of 0.08 mm, as crack map is seen in
Crack formation on the impact and distal faces of FRC slab S7 after a) impact I; b) impact II; c) impact III (Units of crack width in mm).
The spalling phenomenon is explained in the literature that due to the reflection of elastic wave from distal face to impact face causing the tensile strain higher than failure tensile strain of material (PC or FRC) and, consequently, led to chipping or splintering of material in the vicinity of impact point (
Load versus time history curve of PC slab S3, HCC slab S5 and FRC slab S7 after the single impact of weight 33.14 kg.
According to the load versus time history curves, it was seen that the slab S3 did not show the secondary peak. After reaching the maximum load, the curve fell down steeply and the impact experienced only during about 2 ms. This indicated why the penetration depth was only 2÷3 mm, as given in
Since all of tested slabs have damaged after a single impact such as failure of PC slabs or crack propagation in HCC and FRC slabs, the data in terms of maximum impact load from impact I of
Average maximum impact load after a single impact and accumulated energy absorption for each type of slab
Materials | Maximum impact load from Impact I (kN) | Ratio with HCC | Accumulated energy absorption (J) | Ratio with HCC | ||
---|---|---|---|---|---|---|
Average | Deviation | Average | Deviation | |||
PC | 48.14 | 11.50 | 0.88 | 223.49 | 42.85 | 0.39 |
HCC | 54.18 | 4.00 | 1 | 574.00 | 17.14 | 1 |
FRC | 63.85 | 4.50 | 1.17 | 806.81 | 19.80 | 1.41 |
As given in
While PC and FRC slabs were built monolithically, the HCC slabs were built in two layers of FRC and PC mixes, the maximum impact load of HCC slabs was approximately 13–74% greater than that of PC slabs and about 18–39% smaller than that of FRC slabs. Yet, the HCC slabs absorbed 2.6 times more energy than PC slabs did and only 41% less than FRC slabs did.
Although there was a scatter of obtained data, the test results emphasized an enhanced impact performance of HCC slabs under impact load compared to that of PC slabs. Undoubtedly, FRC slabs showed better behaviour than HCC slabs did, in terms of ductility and toughness under impact load, but it had to take into account that the volume fraction of fibre used in HCC slabs was only a half of that in FRC slabs.
In general, the global structural response and failure of the PC, HCC and FRC slabs were quite similar and characterised by the formation of flexural cracks emanating from the centre towards the corners and edges of the slab, as same as reported in other studies on concrete slabs subjected to drop-weight impact test (
Distal face of HCC slab S6 at post-test: a) bell-shaped failure; b) typical crack formed across the layers of HCC slabs on lateral side after impact load.
During the contact period with impactor, the PC slabs lost their integrity swiftly, hence, the local response of the slabs was not observed. Looking at
The global damage of HCC slabs was observed that the radial cracks have also developed across the layers on lateral sides, as seen in
Owing to complexities in evaluating structural damage such as penetration, perforation, spalling and scabbing due to impact load, design criteria so far developed have been mainly dependent on experimental tests and empirical formulae (
where,
xem=empirical penetration depth (mm)
Nh=impactor nose-shape factor (Nh=1.26 for spherical-ended bodies in this study)
Ih=non-dimensional impact factor and
S = strain-rate factor and
M = drop-weight impact (kg)
Vo=impactor velocity (m/s)
d = impactor diameter (mm)
ft=tensile strength (MPa)
where,
xp=required thickness of target to prevent perforation (mm)
xsc=required thickness of target to prevent scabbing (mm)
β = safety factor in the range of 1.2–1.3 and equals to 1.3 adopted for this study
Using the tensile strength of PC, FRC and HCC, which have been reported previously elsewhere in (
Empirical results of material PC, HCC and FRC by Hughes formula
Material | ID | Real thickness of tested slabs | Experimental penetration depth xex | Empirical penetration depth xem | Empirical required thickness of target to prevent perforation xp | Empirical required thickness of target to prevent scabbing xsc |
---|---|---|---|---|---|---|
Impact I | ||||||
mm | ||||||
PC | S1 | 60.49 | 2.45 | 11.08 | 45.87 | 63.03 |
S2 | 60.55 | 2.98 | 10.98 | 45.67 | 62.81 | |
S3 | 60.32 | 2.03 | 10.96 | 45.63 | 62.77 | |
HCC | S4 | 60.92 | 12.31 | 10.03 | 43.71 | 60.36 |
S5 | 60.75 | 11.09 | 9.98 | 43.61 | 60.25 | |
S6 | 60.97 | 11.31 | 10.03 | 43.71 | 60.36 | |
FRC | S7 | 60.84 | 7.43 | 9.74 | 43.11 | 60.00 |
S8 | 60.56 | 12.56 | 10.68 | 45.05 | 62.13 | |
S9 | 60.94 | 8.06 | 10.68 | 45.05 | 62.13 |
In order to protect concrete structure under impact load properly, the real thickness of slabs needs to be greater than required thickness to prevent both perforation (xp) and scabbing (xsc). Looking into the empirical results in
Not like the case of material PC, under the same drop-weight impact of 33.14 kg the required thickness of slab to prevent perforation (xp) and scabbing (xsc) are smaller than real thicknesses of HCC slabs and FRC slab S7. It is mainly due to tensile strength of material FRC and HCC are higher than that of material PC. It depicts that HCC slabs and the FRC slab S7 can sustain the impact load of drop-weight 33.14 kg, indeed, after a single impact, those slabs have merely damaged very slightly, the crack width in distal face was equal or smaller than 0.1 mm, as it can be seen in
In case of FRC slabs S8 and S9 the required thickness of slab to prevent and scabbing (xsc) was higher than the real thickness of slabs and the xem and xex of those slabs are also quite similar; consequently, those slabs are not able theoretically to withstand under impact load of drop-weight 44.02 kg. However, it needs to bear in mind that the empirical formulae only employ tensile strength of FRC and does not take into account the post-cracking behaviour or toughness of FRC, which is an essential property of FRC as well. This is a deficiency of empirical formulae which needs to be enhanced.
It is noted that the xem and xex of PC slabs have a big discrepancy about 11 mm and 2.5 mm respectively, as shown in
In order to assess the static ultimate load of slabs, analysis of slab in failure regime can be done based on an extended yield line theory (
Where:
Ps=static load (kN);
Mp = elastic moment (kN.m) and
ft=tensile strength (MPa)
y = depth of slab under tensile stress (m)
I = second inertia moment per unity (m4)
The ultimate static load of each slab has been calculated and included in
Correlation between dynamic and static loads
Material | ID | Ultimate dynamic load (kN) | Ultimate static load (kN) | ||||
---|---|---|---|---|---|---|---|
Each value | Mean value | Deviation | Each value | Mean value | Deviation | ||
PC | S1 | 34.37 | 48.14 | 11.5 | 37.37 | 37.71 | 0.29 |
S2 | 62.42 | 38.07 | |||||
S3 | 47.64 | 37.68 | |||||
HCC | S4 | 58.50 | 56.96 | 3.09 | 48.42 | 48.53 | 0.27 |
S5 | 59.73 | 48.90 | |||||
S6 | 52.65 | 48.26 | |||||
FRC | S7 | 72.56 | 69.12 | 3.36 | 54.85 | 54.37 | 0.45 |
S8 | 64.56 | 54.49 | |||||
S9 | 70.23 | 53.77 |
Despite of the limited number of data, three slabs for each material, the significant outcomes of HCC have been revealed in this study. The maximum impact load and accumulated energy absorption capacity of HCC slabs were not too much lower than that of FRC slabs and considerably higher than that of PC slabs. HCC slabs entailed only a half of fibre volume fraction used for FRC slabs. That implied an advantage in terms of material saving or cost reduction. The adhesive failure or delamination did not occur; an interface of HCC slabs could be relied on being intact under impact load. Those are important highlights proving the viability of HCC. In view of further research, it would be desirable to conduct more slabs including full-scale HCC slabs with different thickness of PC and FRC layers under impact load test (this would of course require much larger funding).
The experimental and analytical results from drop-weight impact load tests on PC, FRC and HCC slabs showed the following conclusions:
The maximum impact load of HCC slabs was about 13–74% higher than that of PC and 18–39% lower than that of FRC slabs. However, compared to the PC slabs failed swiftly after a single impact, the HCC slabs behaved quasi-ductile and only failed after double impacts of the same drop weight used for testing PC slabs. This occurred thank to the fibre inclusion in HCC slabs, which bridged the cracks and maintained the integrity of slabs avoiding a sudden failure.
The accumulated energy absorption of HCC slabs was 2.6 times higher than that of PC slabs and only 41% lower than that of FRC slabs, taking into account that the total fibre used in HCC slabs was only a half of that of the FRC ones.
The global dynamic response of PC, HCC and FRC slabs was quite similar and characterised by the formation of flexural cracks originating from the centre towards the corners and edges of the slab. However, the frustum cone-shaped plug was pushed off from HCC slabs at failure stage due to the punching shear local response. The fibre type and content used in FRC layer of HCC slabs was quite relevant which resulted in transient behaviour from PC layer to FRC layer without any sign of delamination between them.
The empirical Hughes formulae, which employ tensile strength of material for design structure against impact, were used to evaluate the slabs PC, HCC and FRC under impact as same as experimental condition. Empirical results were somewhat in agreement with experimental results in this study. However, since they are nothing but mathematical formulae, they could not have included so far the full role of fibres such as toughness enhancement and crack control in their formulae.
Based on an extended yield line theory, analytical analysis of slabs under static load in failure regime and the obtained dynamic test results have showed that their correlation of dynamic and static load of PC, FRC and HCC material was in the similar order of magnitude as prescribed in design code for concrete structures against impact.
The work presented here was partially funded by the EU-6FP-011817-2 TUNCONSTRUCT project and the JAE program of CSIC, which are gratefully acknowledged. The authors also would like to express their gratitude for valuable comments of reviewers.