1. INTRODUCTION
⌅Concrete
is the most commonly used construction material worldwide due to the
availability of its ingredients, economic benefits, and outstanding
mechanical and durability-related properties. However, in 2018, the International Energy Agency (IEA) linked concrete production to approximately 7% of the global carbon
dioxide emissions, mainly due to Portland cement production and
aggregates processing (11. International Energy Agency (2018) Cement technology roadmap plots path to cutting CO2 emissions 24% by 2050 - News - IEA. Iea [April], 1-3.
).
In light of the current urgency to adopt new measures to reduce the
environmental impact of the production of concrete, recycling and
reusing concrete can help reduce the consumption of non-renewable
aggregates by transforming them into recycled concrete aggregates (RCA)
while utilizing the adhered residual cement paste to reduce the new
cement demand. However, there are still doubts on the material’s
variability and quality, especially concerning alkali-silica reaction
(ASR), one of the most damaging distress mechanisms impacting concrete,
which significantly reduces the service life of affected structures,
leading to early demolition thus creating large amounts of waste.
Recent
studies have evaluated the potential for “secondary” induced expansion
of ASR-affected recycled concrete made of reactive coarse and fine RCA (2-72.
Abbas, A.; Fathifazl, G.; Isgor, O.B.; Razaqpur, A.G.; Fournier, B.;
Foo, S. (2009) Durability of recycled aggregate concrete designed with
equivalent mortar volume method. Cem. Concr. Comp. 31 [8], 555-563. https://doi.org/10.1016/j.cemconcomp.2009.02.012.
3.
Fathifazl, G.; Abbas, A.; Razaqpur, A.G.; Isgor, O.B.; Fournier, B.;
Foo, S. (2009) New mixture proportioning method for concrete made with
coarse recycled concrete aggregate. J. Mater. Civ. Eng. 21 [10], 601-611. https://doi.org/10.1061/(asce)0899-1561(2009)21:10(601).
4.
Hayles, M.; Sanchez, L.F.M.; Noël, M. (2018) Eco-efficient low cement
recycled concrete aggregate mixtures for structural applications. Constr. Build. Mater. 169, 724-732. https://doi.org/10.1016/j.conbuildmat.2018.02.127.
5.
Ahimoghadam, F.; Sanchez, L.F.M.; de Souza, D.J.; Andrade, G.P.; Noël,
M.; Demers, A. (2020) Influence of the recycled concrete aggregate
features on the behavior of eco-efficient mixtures. J. Mater. Civ. Eng. 32 [9], 04020252. https://doi.org/10.1061/(asce)mt.1943-5533.0003323.
6.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
7.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
);
these studies found that the source of the reaction (i.e., reactive
coarse or fine aggregates) significantly affects the deterioration
mechanism through its crack generation and propagation, kinetics, and
potential of secondary expansion (66.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
, 77.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
). Nevertheless, very few works appraised the impact of ASR on the mechanical properties of RCA concrete (88.
Zhu, Y.; Zahedi, A.; Sanchez, L.F.M.; Fournier, B.; Beauchemin, S.
(2021) Overall assessment of alkali-silica reaction affected recycled
concrete aggregate mixtures derived from construction and demolition
waste. Cem. Concr. Res. 142, 106350. https://doi.org/10.1016/j.cemconres.2020.106350.
).
This work aims to evaluate the influence of ASR on the direct shear
strength of RCA-affected concrete to better understand the role of the
aggregate in such concrete mixtures. Moreover, correlations with
microscopic analysis are also conducted to enhance understanding of the
findings (66.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
, 77.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
).
It is worth noting that to improve the text’s readability, a
description of all acronyms (in order of appearance) used in this work
is presented in Table 1.
Acronyms | Description |
---|---|
ASR | Alkali-silica reaction |
RCA | Recycled concrete aggregate |
DRI | Damage rating index |
CC | Conventional concrete |
OVA | Original virgin aggregate |
RM | Residual mortar |
EV | Equivalent volume mix design method |
CCA | Closed crack in the aggregate |
OCA | Open crack in the aggregate |
OCAG | Open crack in the aggregate with gel |
DAP | Disaggregated particles |
CCP | Crack in the cement paste |
CCPG | Crack in the cement paste with gel |
CAD | Debonded aggregate |
CPT | Concrete prism test |
RH | Relative humidity |
ITZ | Interfacial transition zone |
NCP | New cement paste |
RCP | Residual cement paste |
2. BACKGROUND
⌅2.1. Alkali-silica reaction (ASR)
⌅Alkali-silica reaction (ASR) is a reaction between the alkalis from the cement paste (i.e., Na+, K+,OH-)
and uncrystallized silica within the aggregates. ASR is one of the
leading causes of early concrete deterioration in Canada and worldwide (99. Sims, I.; Poole, A.B. (2017) Alkali-aggregate reaction in concrete: A world review. Taylor and Francis, London (2017). https://doi.org/10.1201/9781315708959.
). Concrete expansion levels are generally used to reflect ASR-induced damage. Figure 1 shows a qualitative crack propagation model in ASR-affected
conventional concrete (CC), presenting A) sharp cracks and B) onion skin
type cracks (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
).
Sharp cracks are generally initiated within the aggregate particles at a
slightly damaged degree (i.e., 0.05% expansion level). As the reaction
continues to a moderate expansion level (i.e., 0.12%), the previously
generated sharp cracks propagate towards the aggregate’s edges. At high
expansion levels (i.e., 0.20%), the sharp cracks begin to extend into
the cement paste. At very high expansion levels (i.e., 0.30%), the sharp
cracks formed in the aggregates and the cement paste form an extensive
cracking network and decrease the physical integrity of the material.
Meanwhile, onion skin type cracks follow the aggregate’s edge,
elongating with increasing expansion until entering the cement paste.
Mechanical properties (i.e., compressive strength tensile strength, and
modulus of elasticity) of concrete affected by ASR are therefore reduced
as a function of the expansion level, regardless of the aggregate type
(i.e., coarse vs. fine aggregates) and nature (i.e., mineralogy) (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
, 1111.
Sanchez, L.F.M.; Drimalas, T.; Fournier, B.; Mitchell, D.; Bastien, J.
(2018) Comprehensive damage assessment in concrete affected by different
internal swelling reaction (ISR) mechanisms. Cem. Concr. Res. 107, 284-303. https://doi.org/10.1016/j.cemconres.2018.02.017.
). De Souza et al. (1212.
Barreto Santos, M.; de Brito, J.; Santos Silva, A.; Hawreen, A. (2020)
Effect of the source concrete with ASR degradation on the mechanical and
physical properties of coarse recycled aggregate. Cem. Concr. Comp. 111, 103621. https://doi.org/10.1016/j.cemconcomp.2020.103621.
)
observed that the direct shear strength of ASR-affected specimens
reduces as a function of ASR expansion, observed even at low expansion
levels, which was attributed to the adverse effect of ASR on the
aggregate interlock of the concrete specimens.
).
2.2. Recycled concrete aggregate (RCA)
⌅Recycled
concrete aggregate (RCA) is a multi-phase material composed of original
virgin aggregate (OVA) and residual mortar (RM). RCA is often
considered a lower quality material (i.e., presence of residual mortar -
RM, impurities, deterioration, and high variability, among many
others), resulting in lower concrete performance. Several mix-design
procedures have been developed to increase the overall performance of
recycled concrete. Among these mix-design techniques, the Equivalent
Volume (EV) method (55.
Ahimoghadam, F.; Sanchez, L.F.M.; de Souza, D.J.; Andrade, G.P.; Noël,
M.; Demers, A. (2020) Influence of the recycled concrete aggregate
features on the behavior of eco-efficient mixtures. J. Mater. Civ. Eng. 32 [9], 04020252. https://doi.org/10.1061/(asce)mt.1943-5533.0003323.
)
improves the hardened and fresh state properties and results in a more
sustainable concrete mixture. The EV method is based on the mix-design
technique developed by Fathifazl et al. (33.
Fathifazl, G.; Abbas, A.; Razaqpur, A.G.; Isgor, O.B.; Fournier, B.;
Foo, S. (2009) New mixture proportioning method for concrete made with
coarse recycled concrete aggregate. J. Mater. Civ. Eng. 21 [10], 601-611. https://doi.org/10.1061/(asce)0899-1561(2009)21:10(601).
) and its modified version (44.
Hayles, M.; Sanchez, L.F.M.; Noël, M. (2018) Eco-efficient low cement
recycled concrete aggregate mixtures for structural applications. Constr. Build. Mater. 169, 724-732. https://doi.org/10.1016/j.conbuildmat.2018.02.127.
).
The EV mix-design technique aims to proportion recycled concrete
mixtures with the same volume of aggregates (coarse and fine) and cement
paste as a companion CC. Indeed, there are concerns regarding the use
of RCA due to the presence of past deterioration such as ASR (1212.
Barreto Santos, M.; de Brito, J.; Santos Silva, A.; Hawreen, A. (2020)
Effect of the source concrete with ASR degradation on the mechanical and
physical properties of coarse recycled aggregate. Cem. Concr. Comp. 111, 103621. https://doi.org/10.1016/j.cemconcomp.2020.103621.
, 1313.
Peng, Z.; Shi, C.; Shi, Z.; Lu, B.; Wan, S.; Zhang, Z.; Chang, J.;
Zhang, T. (2020) Alkali-aggregate reaction in recycled aggregate
concrete. J. Clean. Prod. 255, 120238. https://doi.org/10.1016/j.jclepro.2020.120238.
),
which can be influenced by the RCA production (i.e., aggregate
crushing, washing, and storage) and the remaining reactivity potential (14-1814.
Shehata, M.H.; Christidis, C.; Mikhaiel, W.; Rogers, C.; Lachemi, M.
(2010) Reactivity of reclaimed concrete aggregate produced from concrete
affected by alkali-silica reaction. Cem. Concr. Res. 40 [4], 575-582. https://doi.org/10.1016/j.cemconres.2009.08.008.
15.
Ideker, J.H.; Adams, M.P.; Tanner, J.; Jones, A. (2013) Durability
assessment of recycled concrete aggregates for use in new concrete:
Phase I-Revised. TREC Final Reports. https://doi.org/10.15760/trec.15.
16.
Johnson, R.; Shehata, M.H. (2016) The efficacy of accelerated test
methods to evaluate alkali silica reactivity of recycled concrete
aggregates. Constr. Build. Mater. 112, 518-528. https://doi.org/10.1016/j.conbuildmat.2016.02.155.
17.
Mukhopadhyay, A.K.; Geiger, B.J.; Button, J. (2010) Use of
alkali-silica reaction-affected recycled concrete aggregate in hot-mix
asphalt. Transp. Res. Rec. 2179, 1-9. https://doi.org/10.3141/2179-01.
18. Li, X.; Gress, D.L. (2006) Mitigating alkali-silica reaction in concrete containing recycled concrete aggregate. Transp. Res. Rec. 1979, 30-35. https://doi.org/10.3141/1979-06.
). Trottier et al. (66.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
, 77.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
)
found that the kinetics and deterioration process (i.e., crack width,
crack length, and propagation path) are affected by the severity of
initial damage and its location (i.e., OVA or RM). However, the
influence of this type of aggregate on the aggregate interlock,
determined through the direct shear test, remains unknown. Techniques
used to assess ASR damage in concrete.
2.2.1. Damage Rating Index (DRI)
⌅The damage rating index (DRI) is a microscopic tool developed (1919. Grattan-Bellew, P.E. (1995) Laboratory evaluation of alkali-silica reaction in concrete from Saunders generating station. ACI Mater. J. 92 [2], 126-134. https://doi.org/10.14359/9763.
) and further modified (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
, 2020.
Villeneuve, V.; Fournier, B.; Duchesne, J. (2012) Determination of the
damage in concrete affected by ASR- the damage rating index (DRI).
Proceedings of the 14th ICAAR, Austin, TX.
) to
appraise damage in ASR-affected concrete. After polishing the surface of
a concrete section (i.e., cut longitudinally), distress features are
counted with the help of a stereomicroscope (15 to 16x magnification) in
a 1 cm2 grid drawn on the reflective surface. The distress
feature counts are then weighted according to their location and
importance (i.e., 0.25 for closed cracks in the aggregate - CCA; 2 for
open cracks in the aggregate without and with gel and disaggregated
particles - OCA, OCAG, and DAP, respectively; and 3 for cracks in the
cement paste without and with gel and debonded aggregate - CCP, CCPG,
and CAD, respectively). The overall assessment is then normalized to 100
cm2 resulting in the DRI number. This microscopic tool is an
effective technique for assessing the degree of damage of ASR-affected
concrete regardless of the aggregate type and concrete strength (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
, 2020.
Villeneuve, V.; Fournier, B.; Duchesne, J. (2012) Determination of the
damage in concrete affected by ASR- the damage rating index (DRI).
Proceedings of the 14th ICAAR, Austin, TX.
, 2121.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Bedoya, M.A.B.; Bastien, J.;
Duchesne, J. (2016) Use of Damage Rating Index to quantify alkali-silica
reaction damage in concrete: Fine versus coarse aggregate. ACI Mater. J. 113 [4], 395-407. https://doi.org/10.14359/51688983.
), as well as for recycled mixtures (6-86.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
7.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
8.
Zhu, Y.; Zahedi, A.; Sanchez, L.F.M.; Fournier, B.; Beauchemin, S.
(2021) Overall assessment of alkali-silica reaction affected recycled
concrete aggregate mixtures derived from construction and demolition
waste. Cem. Concr. Res. 142, 106350. https://doi.org/10.1016/j.cemconres.2020.106350.
). Furthermore, the extended version of DRI, presenting characteristics in absolute (counts/100 cm2)
and relative (%) values without the weighting factors, gives a more
thorough evaluation and comprehension of the ASR-induced damage
development. (88.
Zhu, Y.; Zahedi, A.; Sanchez, L.F.M.; Fournier, B.; Beauchemin, S.
(2021) Overall assessment of alkali-silica reaction affected recycled
concrete aggregate mixtures derived from construction and demolition
waste. Cem. Concr. Res. 142, 106350. https://doi.org/10.1016/j.cemconres.2020.106350.
, 1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
).
2.2.2. Direct shear setup test
⌅The
direct shear strength of concrete measures the material’s aggregate
interlock.; compression and tension are the governing forces in
measuring the latter. The transfer of shear forces across inner cracks
occurs through two mechanisms: a) dowel effect and b) shear friction (2222.
Haskett, M.; Oehlers, D.J.; Mohamed Ali, M.S.; Sharma, S.K. (2011)
Evaluating the shear-friction resistance across sliding planes in
concrete. Eng. Struct. 33 [4], 1357-1364. https://doi.org/10.1016/j.engstruct.2011.01.013.
).
While the dowel effect is related to reinforcement used in concrete,
shear friction is the frictional resistance of cracks to sliding (2222.
Haskett, M.; Oehlers, D.J.; Mohamed Ali, M.S.; Sharma, S.K. (2011)
Evaluating the shear-friction resistance across sliding planes in
concrete. Eng. Struct. 33 [4], 1357-1364. https://doi.org/10.1016/j.engstruct.2011.01.013.
) used in reinforced concrete design known as aggregate interlock (2323. Concrete design handbook (1985). Canadian Portland Cement Association, Canada (1985).
). Barr and Hasso (2424. Barr, B.I.G.; Hasso, E.B.D.; Liu, K. (1985) Shear strength of FRC materials. Comp. 16 [4], 326-334. https://doi.org/10.1016/0010-4361(85)90285-X.
)
proposed a setup using a modified cylindrical concrete specimen, where a
semi-circular notch was applied on each side, expected to ensure a
shear failure at this location. Gao et al. (2525.
Gao, H.; Zhiqiang, W.; Chengshou, Y.; Aihua, Z. (1979) An investigation
on the brittle fracture of KI-KII composite mode crack. Acta Metall. Sin. 15 [3], 380-391.
) introduced a new setup to evaluate the brittle fracture of reinforced concrete composites. The setup proposed by Gao et al. (2525.
Gao, H.; Zhiqiang, W.; Chengshou, Y.; Aihua, Z. (1979) An investigation
on the brittle fracture of KI-KII composite mode crack. Acta Metall. Sin. 15 [3], 380-391.
) was adopted by Barr and Hasso (2626. Barr, B.; Hasso, E.B.D. (1986) Development of a compact cylindrical shear test specimen. J. Mater. Sci. Lett. 5, 1305-1308. https://doi.org/10.1007/BF01729401.
) for further analysis in 100 by 200 mm cylindrical specimens, using a circumferential notch of 20-25 mm in depth (Figure 2). Ultimately, De Souza et al. (2727.
de Souza, D.J.; Sanchez, L.F.M.; de Grazia, M.T. (2019) Evaluation of a
direct shear test setup to quantify AAR-induced expansion and damage in
concrete. Constr. Build. Mater. 229, 116806. https://doi.org/10.1016/j.conbuildmat.2019.116806.
) utilized the last version of the setup proposed by Barr and Hasso (2626. Barr, B.; Hasso, E.B.D. (1986) Development of a compact cylindrical shear test specimen. J. Mater. Sci. Lett. 5, 1305-1308. https://doi.org/10.1007/BF01729401.
) to evaluate the effects of ASR on the direct shear strength of affected concrete specimens.
).
3. SCOPE OF WORK
⌅As aforementioned, only a few works have evaluated the impact of RCA’s initial damage on the mechanical properties and deterioration of concrete made of ASR-affected RCA, while the influence of ASR on the mechanical properties of CC is somewhat well understood. This work aims to assess the influence of previously damaged RCA and its severity on the aggregate interlock captured by the direct shear strength test. As such, CC specimens incorporating a reactive coarse aggregate (i.e., Springhill - Greywacke) were manufactured in the laboratory and stored under conditions enabling ASR-induced expansion and deterioration. The specimens were split into two groups: slightly deteriorated (i.e., 0.05%) and severely deteriorated (i.e., 0.30%) concrete; upon reaching the above expansion levels, the specimens were crushed, and RCA material was obtained. RCA concrete specimens made of slightly and severely ASR-deteriorated coarse aggregates were fabricated and stored at conditions enabling secondary ASR-induced development. Secondary damage (i.e., expansion levels) was monitored over time to compare the ASR kinetics of recycled concrete with a companion CC affected by ASR. The direct shear test was then conducted on specimens reaching expansion levels of 0.05%, 0.12% and 0.20%, followed by complementary microscopic analysis (i.e., damage rating index) on a separate set of specimens at each expansion level to better understand the role of the RCA in concrete affected by ASR.
4. MATERIALS AND METHODS
⌅4.1. Concrete manufacturing
⌅To
fabricate the control CC specimens (i.e., 100 mm by 200 mm, 35 MPa
concrete cylinders) and concrete specimens to be used to produce the
RCA, a highly reactive coarse aggregate (i.e., Springhill - Greywacke)
was combined with natural non-reactive sand sourced locally in Ottawa.
All concrete specimens were mix-proportioned following the concrete
prism test (CPT) as per ASTM C1293 (2828. ASTM C1293 - 20a standard test method for determination of length change of concrete due to alkali-silica reaction. https://www.astm.org/Standards/C1293.
);
CSA general use (ASTM type 1) Portland cement was used in the mixture.
To accelerate ASR development, the total alkali content of the mixture
was raised to 1.25% Na2Oeq. by cement mass after
adding reagent grade NaOH. The specimens were moisture-cured in 100%
relative humidity (RH) and 20°C, and then de-moulded after 24 hours.
Holes of 5 mm in diameter by 15 mm in depth were then drilled in both
ends of the specimens, and stainless-steel gauge studs were glued using a
fast-setting cement paste slurry on both ends for the length change
measurements. The specimens were left to moist-cure for an additional 24
hours before the initial reading. The specimens were then stored over a
film of water in 22-litre plastic containers lined with an absorbent
cloth and then placed in an environmental chamber (i.e., 100% RH and 38oC).
The abovementioned absorbent cloth was installed on the lid and inside
the buckets to prevent the formation of droplets and minimize the effect
of leaching. Prior to the periodic monitoring of the length change
measurements, the containers were removed from the environmental chamber
and cooled down to 20°C for 16 ± 4 hrs. The specimens were taken out
one by one during the measurement period, and buckets were returned to
the environmental chamber immediately after each measurement. Two levels
of expansion representing distinct damage degrees were selected to
produce the RCA: 0.05% and 0.30% (i.e., slight and severe,
respectively). The specimens were then jaw crushed to produce reactive
coarse RCA ranging from 4.75 mm to 19 mm in size. After crushing, the
RCAs were stored in sealed buckets in conditions preventing further ASR
development (12oC). The RM content of slightly damaged RCA
and severely damaged RCA were 46% and 51.5%, respectively. The RM
content was determined following the procedure proposed by Abbas et al. (2929.
Abbas, A.; Fathifazl, G.; Burkan Isgor, O.; Razaqpur, A.G.; Fournier,
B.; Foo, S. (2008) Proposed method for determining the residual mortar
content of recycled concrete aggregates. J. ASTM Int. 5 [1], 1-12. https://doi.org/10.1520/JAI101087.
), which involves five cycles of freezing and thawing. All recycled mixtures were proportioned using the EV method (55.
Ahimoghadam, F.; Sanchez, L.F.M.; de Souza, D.J.; Andrade, G.P.; Noël,
M.; Demers, A. (2020) Influence of the recycled concrete aggregate
features on the behavior of eco-efficient mixtures. J. Mater. Civ. Eng. 32 [9], 04020252. https://doi.org/10.1061/(asce)mt.1943-5533.0003323.
) while keeping the total cement content of the system equal to 420 kg/m3 (i.e., using the CPT mixture proportions as the companion mixture) as presented in Table 2.
To raise the alkali content, the recycled mixtures also incorporated
reagent grade NaOH and were cast, cured, prepared, stored, and monitored
over time, following the same procedures as aforementioned for the CC.
It is worth noting that to assess the effect of initial damage on the
future performance of the RCA concrete, only the alkali contribution
from the new cement paste was considered. The total alkalis of the
system was kept constant, and the stage of the initial reaction was not
considered in the total alkali content calculation.
Raw materials | Springhill CC | Slightly damaged RCA-concrete (0.05% -SPR-RCA) | Severely damaged RCA-concrete (0.30% -SPR-RCA) |
---|---|---|---|
Coarse aggregate-highly reactive (kg/m3) | 934 | 1040 | 1048 |
Sand-non reactive (kg/m3) | 823 | 774 | 791 |
Cement (kg/m3) | 420 | 340 | 331 |
Water (kg/m3) | 189 | 153 | 149 |
4.2. Experimental procedures
⌅4.2.1. Direct shear test
⌅The
direct shear test was used to evaluate the direct shear strength (i.e.,
aggregate interlock) of the recycled mixtures. Three specimens per
concrete mixture (i.e., CC, slightly and severely damaged recycled
mixtures) and expansion level selected for this study (i.e., 0%, 0.05%,
0.12% and 0.20%) were prepared for the direct shear test. A
circumferential 22 mm deep (2626. Barr, B.; Hasso, E.B.D. (1986) Development of a compact cylindrical shear test specimen. J. Mater. Sci. Lett. 5, 1305-1308. https://doi.org/10.1007/BF01729401.
)
and 5 mm wide (i.e., the width of the diamond blade on the masonry saw)
notch at the center of the specimens was cut to ensure a shear failure
at the notch. The specimens were then tested in accordance with the
setup and procedure proposed by Barr and Hasso (2626. Barr, B.; Hasso, E.B.D. (1986) Development of a compact cylindrical shear test specimen. J. Mater. Sci. Lett. 5, 1305-1308. https://doi.org/10.1007/BF01729401.
), as shown in Figure 2. A loading rate of 100 N/s was selected for this study. The following Equation [1] was then used to calculate the direct shear strength:
Where denotes the failure load (N), denotes the cylinder diameter (mm), and denotes the depth of the notch (mm).
4.2.2. Damage Rating Index (DRI)
⌅One
specimen per mixture (i.e., CC, slightly and severely damaged recycled
mixture) and expansion level (i.e., 0%, 0.05%, 0.12% and 0.20%) was cut
in half longitudinally and polished with diamond-impregnated rubber
disks of successive grits (i.e., 30, 60, 140, 280, 600, 1200, 3000)
through the use of a mechanical polishing machine prior to conducting
the microscopic assessment. A grid of 1 cm by 1 cm was drawn on the
reflective surface, and each square was observed through a
stereomicroscope at 16x magnification. Distress features were then
counted in each square while applying a weighting factor to each feature
as per (2020.
Villeneuve, V.; Fournier, B.; Duchesne, J. (2012) Determination of the
damage in concrete affected by ASR- the damage rating index (DRI).
Proceedings of the 14th ICAAR, Austin, TX.
). The sum of the weighted counts was normalized to 100 cm2 to obtain a DRI number. Moreover, the extended version of the DRI
(i.e., without the application of the weighting factors) was also
performed as per Sanchez et al. (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
).
5. RESULTS AND DISCUSSION
⌅5.1. ASR development and kinetics
⌅Figure 3 illustrates ASR-induced average expansion levels as a function of time. A standard deviation of 0.01%-0.04%, 0.02%-0.04% and 0.02%-0.03% was obtained for the CC and recycled mixtures made of slightly damaged and severely damaged RCA, respectively. Expansion measurements were carried out for 150 days on the CC at which an expansion level of 0.31% was reached, whereas after 188 days, levels of expansion of 0.24% and 0.28% were obtained for the slightly and severely damaged recycled mixutres, respectively. A similar trend is observed at the beginning from 0-0.03% of expansion (i.e., 29 days), after which both recycled mixtures increase in expansion up to 0.09% and 0.06% at 51 days for the severely and slightly damaged RCA, respectively, while the CC attains only 0.05% at 52 days. After approximately 63 days, the CC swelled at a faster rate when compared to both recycled mixtures. The severely and slightly damaged recycled mixtures, as well as the CC, reached an expansion level of 0.05% at 36, 44 and 49 days, and an expansion level of 0.12% at 79, 94 and 75 days, respectively. Meanwhile, an expansion of 0.20% was observed after 94, 128, 157 days for the CC, severely and slightly damaged recycled mixtures, respectively.
).
5.2. Direct shear test
⌅The direct shear strength and reductions at distinct expansion levels of the CC and recycled concrete mixtures are presented in Figures 4a and 4b, respectively. Results indicate that the direct shear strength of all mixtures lessens as a function of induced expansion and ASR development. The CC’s direct shear strength begins to decrease only after 0.05% of expansion (i.e., initially at 8.81 MPa) which can be attributed to the cracks being present within the aggregates as presented in Figure 1; thus, cracks propagating through the interfacial transition zone (ITZ) during shear failure. Hereafter, the decrease in direct shear strength is linear for the CC, reaching 5.82 MPa at 0.20% of the expansion. The recycled mixtures on the other hand, present a lower initial shear strength when compared to the CC. The slightly damaged RCA displayed an initial direct shear strength of 7.2 MPa. As expansion advanced, at low and moderate expansion levels (i.e., 0.05% and 0.12%, respectively), the direct shear strength reduced to 5.81 MPa and 5.44 MPa (i.e., losses of 20% and 24%, respectively). At an expansion level of 0.20%, a significant loss is observed (i.e., loss of 44%), reducing the direct shear strength to 4.02 MPa. Likewise, the severely damaged RCA with an initial direct shear strength of 6.8 MPa presented a similar trend through the low and moderate expansion levels (i.e., losses of 20% and 29%, respectively). Interestingly, no significant loss was observed at a high expansion level (0.20%), unlike the slightly damaged RCA (i.e., from 29% to 34%).
5.3. Microscopic assessment
⌅The result from another study by Trottier et al. (66.
Trottier, C.; Zahedi, A.; Ziapour, R.; Sanchez, L.; Locati, F. (2021)
Microscopic assessment of recycled concrete aggregate (RCA) mixtures
affected by alkali-silica reaction (ASR). Constr. Build. Mater. 269, 121250. https://doi.org/10.1016/j.conbuildmat.2020.121250.
)
with the same RCA particles and mixture proportions, was used to
complement the results obtained through the direct shear test and
understand the role of the RCA particles. Due to the multi-phase nature
of the aggregates, the cracks observed in the cement paste were
classified into two categories: a) cracks in new cement paste (NCP) and
b) cracks in the residual cement paste (RCP). It should be noted that
the crushing and weathering of the aggregates could result in closed
cracks inside the aggregates, and these cracks are not necessarily
attributed to ASR (3030.
McCarthy, M.J.; Csetenyi, L.J.; Halliday, J.E.; Dhir, R.K. (2015)
Evaluating the effect of recycled aggregate on damaging AAR in concrete. Mag. Concr. Res. 67 [11], 598-610. https://doi.org/10.1680/MACR.14.00260.
).
However, a decrease in such cracks is a result of closed cracks in the
aggregates opening due to the expansion of silica gel within these
sites. As such, the CC mixture shows an increase in the proportions of open cracks in the aggregates (from 12% to 40%), as well as cracks in the cement paste (from less than 1% to 10%) up to moderate expansion level (i.e., 0.12%). Afterwards, their proportions remain constant (Figure 5a) although the number of cracks keeps increasing, as shown in Figure 5b. The severely damaged mixture (i.e., 0.30%-SPR-RCA) displays a significantly higher portion of open cracks in the aggregate with and without gel when compared to the slightly damaged mixture (i.e., 0.05% -SPR-RCA)
before being subjected to secondary damage (28% and 10%, respectively).
The proportion of open cracks in the aggregate without and with gel increases up to 45% for slightly damaged RCA and 36% for the severely damaged RCA concrete at 0.20% of expansion (Figure 5a).
Meanwhile, the cracks in the cement paste present a significantly
smaller proportion overall yet, similar proportions in both recycled
mixtures are observed, increasing with expansion (i.e., from 5% to 20%).
).
The DRI numbers generally increase with increasing expansion, as presented in Figure 5b. The CC mixture displays the highest DRI values, followed by the severely then slightly damaged RCA mixtures. Before being subjected to ASR, only a negligible level of damage was observed for all concrete mixtures (i.e., DRI values of 138, 73, and 171, for CC, slightly and severely damaged RCA, respectively). At 0.05% and 0.12% of expansion levels, the CC mixture presents the highest DRI numbers (i.e., 307 and 465, respectively) between all mixtures, while the slightly and severely damaged RCA mixtures achieve 161 and 261 for low (i.e., 0.05%) expansion and 268 and 390 for moderate (i.e., 0.12%) expansion, respectively. A significant increase in the DRI number is found for the slightly damaged RCA concrete at the expansion of 0.20%. (i.e., 470) whereas the severely damaged RCA and CC mixtures increased to 514 and 621.
5.4. Discussion and overall assessment
⌅The
shear strength of recycled mixtures presented interesting results when
compared to the CC mixture. At low expansion levels (i.e., 0.05%), CC
mixtures presented cracks mainly within the aggregates (1010.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Duchesne, J. (2015) Reliable
quantification of AAR damage through assessment of the Damage Rating
Index (DRI). Cem. Concr. Res. 67, 74-92. https://doi.org/10.1016/j.cemconres.2014.08.002.
, 1111.
Sanchez, L.F.M.; Drimalas, T.; Fournier, B.; Mitchell, D.; Bastien, J.
(2018) Comprehensive damage assessment in concrete affected by different
internal swelling reaction (ISR) mechanisms. Cem. Concr. Res. 107, 284-303. https://doi.org/10.1016/j.cemconres.2018.02.017.
, 2121.
Sanchez, L.F.M.; Fournier, B.; Jolin, M.; Bedoya, M.A.B.; Bastien, J.;
Duchesne, J. (2016) Use of Damage Rating Index to quantify alkali-silica
reaction damage in concrete: Fine versus coarse aggregate. ACI Mater. J. 113 [4], 395-407. https://doi.org/10.14359/51688983.
). This behaviour was well captured by the direct shear test (i.e., no reduction of the shear strength as per Figure 4)
since no “aggregate interlock loss” is expected when cracks remain
within the aggregate particles. Conversely, at moderate expansion levels
(i.e., 0.12%), some of the cracks developed within the aggregates
reached the cement paste (Figure 1),
decreasing the aggregate interlock due to the splitting of the
particles, which significantly reduces the shear strength of the
affected material. In addition, the location of cracks (i.e., aggregate
vs cement paste) and their severity (i.e., number, crack length and
crack width) significantly affected the shear strength of CC (3131.
Ziapour, R.; Trottier, C.; Sanchez, L. (2020) Assessment of AAR-induced
expansion and damage through the direct shear test. Proceedings of the
16th ICAAR. LX, Portugal.
). On the other hand, RCA
mixtures showed a lower initial shear strength when compared to CC,
likely due to: a) the distinct microstructure (i.e., multi-phase nature
and increased number of ITZ) of RCA particles and, b) the
crushing/processing inducing micro-cracks, which might have compromised
the aggregate interlock capacity of RCA concrete as previously observed
in the literature (2222.
Haskett, M.; Oehlers, D.J.; Mohamed Ali, M.S.; Sharma, S.K. (2011)
Evaluating the shear-friction resistance across sliding planes in
concrete. Eng. Struct. 33 [4], 1357-1364. https://doi.org/10.1016/j.engstruct.2011.01.013.
, 3232.
Rahal, K.N.; Al-Khaleefi, A.L. (2015) Shear-friction behavior of
recycled and natural aggregate concrete-An experimental investigation. ACI Struc. J. 112 [6], 725-734. https://doi.org/10.14359/51687748.
, 3333.
Hayles, M. (2018) Investigation into the mechanical properties and
structural behaviour of recycled concrete members. [MSc Thesis] Civil
Engineering Department, University of Ottawa, Ottawa (2018). https://doi.org/10.20381/ruor-22063.
).
Conversely to the CC mixtures where the shear strength remained
constant from 0% to 0.05% of expansion, the recycled mixtures showed a
19% and 20% shear strength loss. This difference in the shear
strength reduction in recycled mixtures compared to CC is likely due to
the previous ASR damage, resulting in the secondary damage having a
short initiation period leading towards cracks extending to the cement
paste in recycled mixtures before such cracks are observed in CC (Figure 5b).
Moreover, the influence of the previous ASR damaged was captured by the
direct shear test where recycled mixtures made of slightly deteriorated
RCA concrete displayed a greater initial shear strength when compared
to RCA made of severely deteriorated particles. The DRI numbers obtained
for the slightly and severely damaged recycled mixtures prior to being
subjected to secondary ASR damage (i.e., 73 and 171, respectively)
support the trend observed through the shear strength. Yet, a DRI number
of 138 was reported for the CC, while a higher shear strength was
observed at 0% of expansion. Therefore, the number of cracks does not
necessarily govern the shear strength. Moreover, the open cracks in the
aggregates are significantly lower in number for the slightly damaged
RCA yet, similar numbers are observed for the severely damaged mixture
and the CC. At 0.05% of expansion, although the shear strength in CC
remains constant, the number of open cracks in the aggregate increases
significantly while a smaller increase is observed in both recycled
mixtures, highlighting the influence of the nature of the RCA presenting
previous damage. Furthermore, the width of cracks may have played an
essential role in the direct shear strength of recycled mixtures.
Recycled mixtures at low expansion levels presented significantly wider
cracks than CC (i.e., 0.15 mm and 0.10 mm, respectively) (77.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
).
An
interesting behaviour is observed at 0.20% of expansion for the
slightly damaged recycled mixture observed through both the shear
strength and the DRI number. The shear strength thus presents a higher
loss for the slightly damaged mixture while the DRI number increases
significantly with the number of open cracks in the aggregate being
similar to that of the severely damaged mixture, further highlighting
the difference between the RCA particles subjected to different levels
of previous damage. In addition, at high expansions (i.e., 0.20%),
cracks were widest in the slightly damaged RCA concrete reaching 0.25
mm, compared to 0.20 mm for severely damaged RCA concrete and CC as
previously reported (77.
Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021)
Microscopic characterization of alkali-silica reaction (ASR) affected
recycled concrete mixtures induced by reactive coarse and fine
aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
).
The sudden loss of direct shear strength observed in the recycled
concrete made of slightly damaged RCA at an expansion level of 0.20% may
be linked to wider observed cracks. Nevertheless, the reactive
potential of the RCA particles was captured by both tools used in this
study. It is worth noting that it is generally agreed that the shear
strength of concrete not only depends on the location and width of
cracks, but the crack directionality could also play an important role
in the shear strength of concrete specimens (3434.
Tanaka, Y.; Kishi, T.; Maekawa, K. (2005) Experimental research on the
structural mechanism of RC members containing artificial crack in shear. Doboku Gakkai Ronbunshu. [802], 109-122. https://doi.org/10.2208/jscej.2005.802_109.
, 3535. Pimanmas, A.; Maekawa, K. (2001) Shear failure of RC members subjected to pre-cracks and combined axial tension and shear. J. Materials. Conc. Struct. Pavem. 53 [690], 159-174. https://doi.org/10.2208/jscej.2001.690_159.
), thus the impact of crack directionality on the shear strength of RCA concrete is recommended topics for future work.
6. CONCLUSIONS
⌅This study aimed to appraise the direct shear strength reduction of concrete containing ASR reactive RCA while understanding its behaviour through a microscopic analysis. The results were compared with a companion CC, and the key findings of this study are presented below:
-
The results gathered in this research indicate that the severity of past deterioration in RCA affects the shear strength of recycled concrete; it is worth noting that the location and type of these distress features are of great significance towards the shear strength of recycled concrete.
-
Although severely damaged RCA displays a higher level of damage through the DRI number for all three secondary expansion levels selected in this study, slightly damaged RCA presented a significantly higher loss of direct shear strength and a significant increase in the DRI number at 0.20% of the expansion.
-
The direct shear strength setup used in this study was able to capture the increase in ASR damage in CC and the recycled mixtures. The influence of the primary damage in the RCA particles was also highlighted through both the shear strength and DRI numbers which emphasizes the necessity to distinguish the type of RCA used in new concrete mixtures.
-
Overall, the direct shear strength of RCA concrete depends on the original concrete properties and the secondary induced deterioration taking place in the RCA concrete. This was observed by Trottier et al. (77. Trottier, C.; Ziapour, R.; Zahedi, A.; Sanchez, L.; Locati, F. (2021) Microscopic characterization of alkali-silica reaction (ASR) affected recycled concrete mixtures induced by reactive coarse and fine aggregates. Cem. Concr. Res. 144, 106426. https://doi.org/10.1016/j.cemconres.2021.106426.
) when developing a qualitative model of crack generation and propagation of RCA. Further investigation of the mechanical properties is required to understand the effects of initial and secondary damage on RCA concrete. -
The overall crack width of the system plays an essential role in assessing the direct shear strength of ASR-affected specimens. This was clearly observed in this work, especially at high expansion levels.