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Tensile properties of concrete at very early ages
Construction and Building Materials 134 (2017) 563–573
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Tensile properties of concrete at very early ages Duy H. Nguyen a, Vinh T.N. Dao a,⇑, Pietro Lura b,c a
School of Civil Engineering, The University of Queensland, Australia Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland c Institute for Building Materials, ETH Zurich, Switzerland b
h i g h l i g h t s Testing for reliable tensile properties of very early-age concrete is reviewed. The current dearth of such reliable data is emphasised. Key features of an improved direct tensile testing system are presented. Newly-collected reliable data and their analysis are reported. Fundamental tensile properties of concrete at very early ages are presented.
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 11 November 2016 Accepted 28 December 2016
Keywords: Early-age concrete Direct tensile test Tensile properties Fracture properties Cyclic loading Tensile relaxation Digital image correlation
a b s t r a c t Proper knowledge of tensile properties of concrete from very early ages is essential for effective control of not only early-age cracking but also of residual stress due to restrained early-age deformation as well as for demolding and handling of young concrete members in the precast industry. Despite significant past research, such knowledge is currently inadequate and based on experimental data with questionable reliability, due mainly to the considerable challenges in testing early-age concrete. This paper first highlights the challenge and importance of collecting reliable test data on full tensile stress-deformation curves for very early-age concrete. Through identifying and effectively addressing critical drawbacks in previous test setups, an improved direct tensile testing system that can reliably capture simultaneously stress and deformation of concrete from the age of several hours after mixing has been successfully developed. Key features of the improved setup, including the air-bearing box for friction minimisation and digital image correlation for non-contact full-field deformation capturing, are then reported in the paper. Based on the newly collected data, fundamental tensile properties of concrete at very early ages are re-assessed and presented. Such properties include tensile strength, Young’s modulus, strain at peak stress, fracture energy, performance under cyclic loading and tensile relaxation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Early-age cracking may occur in concrete structures from as early as several hours after casting. Very often, these early-age cracks would further propagate and render these structures unserviceable at later stages due to subsequent shrinkage and loading. As a result, the performance characteristics and service life of affected concrete structures can be severely compromised. The effective control of such cracking is thus of particular concern for highway pavements, bridges, tunnels, liquid reservoirs [1–3], as
⇑ Corresponding author. E-mail address: [email protected] (V.T.N. Dao). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.169 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
well as critical structures such as concrete containments of nuclear power plants and storage facilities for hazardous substances [3,4]. The basic underlying mechanism for such cracking is the tensile stress (or strain) due to restrained early deformation in the concrete reaching its tensile capacity. Importantly, even when earlyage cracking does not occur, the residual stress due to restrained early-age deformation can substantially reduce the remaining tensile-carrying capacity of concrete. Such effect, if not properly accounted for, may critically compromise the performance of concrete structures in service loads and environment [5]. Accordingly, a proper understanding and control of the abovementioned early-age cracking risk and residual stress from restrained early-age deformation in concrete structures requires, among other things, an adequate knowledge of tensile and fracture
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properties of concrete from very early ages. Such knowledge of early-age tensile properties is also highly beneficial in precast industry to optimize the timing for prestressing, demoulding and handling of precast members [6,7]. Having recognised the significance of such knowledge, different research groups have attempted to measure the relevant tensile properties of concrete since the early 1970s, with useful but still rather limited reported outcomes [8–14]. In [13], using a tensile wedge splitting setup, the load-displacement up to peak stress of young concrete specimens could be captured – However, the post-peak behaviour could not be obtained, rendering the determination of fracture properties challenging. In fact, the full stressstrain curves have been reported by only a few groups [9,10]. This is believed to be due mainly to the considerable practical challenges to be overcome when testing early-age concrete to obtain reliable stress-deformation curves, which requires reliable capturing of both stress and deformation at the same time. Specifically:
This paper reports key initial outcomes of a research program ongoing at The University of Queensland aimed at addressing the above-mentioned knowledge gaps, including:
Due to the low tensile strength to be measured, friction between the test specimen and its supporting surface must be minimised if reliable loading is to be recorded. Various measures have been attempted [8,9], including the use of a mercury bath [15], roller bearing [8] or materials with a low coefficient of friction such as Teflon [16]. The best available method is possibly the inclusion of an air-bearing box which essentially ‘‘floats” the test specimen on a thin layer of air, thus effectively eliminating all friction – The system was first proposed by Hannant’s group [9] and subsequently further improved by Dao et al. [10]. Due to its fragility, disturbance to test specimens must be minimised throughout the moulding, handling and testing processes. In particular, such fragility of early-age concrete has important implications on reliable deformation measurement – Methods for deformation measurement in previous research can be categorised into three groups:
The setup for direct tensile testing of concrete test specimens used in this study, as schematically shown in Fig. 1, is based on its earlier version [10] with significant improvement to allow reliable simultaneous capturing of stress and deformation. During testing, the test apparatus is placed on the horizontal platform of a displacement-controlled Instron 5985 testing frame. The test apparatus itself comprises an air-bearing box ⑦ and a lever arm ④ attached to a small steel frame ①. The lever arm is pin-connected to the frame and is self-balanced in the test position, enabling the direction but not the magnitude of the force applied through the Instron loading machine ⑤ to be altered. The test specimen ② is placed on top of the air-bearing box ⑦ with one end pin-connected to the load cell ⑥ and the other to the lever arm. ④ The test specimen and its dimensions are given in Fig. 2: The minimum dimension of 70 mm is greater than the average width of the fracture process zone, which is about three times the maximum aggregate size used [23]. The curved transitions aim to promote failure in the middle section while eliminating significant stress concentrations that could initiate undesirable cracking within the transitions [10]. The two notable features of this unique test setup include:
Strain gauges are attached to the mould [10,17], giving total deformation of the whole specimen – Deformation over the region of interest has to be estimated from the measured total deformation, typically by finite element modelling. This ‘‘estimation” procedure inevitably has introduced additional uncertainties that have not been appropriately taken into account, raising doubts over the reliability of obtained results. Strain gauges are attached to the posts cast into concrete [18,19] – The likely movement of the posts in young concrete and the resulting disturbance may have significantly compromised the measurement accuracy; but again, the effect of such movement and disturbance has not been adequately quantified. Digital Image Correlation (DIC): DIC uses high definition professional cameras for reliable, non-contact capturing of the required displacement fields. DIC has been used successfully to track the free surface of flowing concrete [20] and capture surface deformation of mortar specimens [21]. However, there seem limited reported studies in which the reliable deformation of concrete surface captured by DIC is synchronised with reliable loading to provide the required loaddisplacement curves. Roziere et al. [16] reported one such study on concrete specimens from the age of 7 h after casting – Accordingly, this technique may not be appropriate for concrete of earlier ages due to the weaker, wetter and softer nature of the concrete surface at this stage. Nguyen and Dao [22] appear to be the first to have reported a successful use of DIC for direct tensile testing of concrete from 3 h after mixing.
An improved direct tensile test that enables the reliable and simultaneous capturing of deformation and stress of concrete specimens from very early ages. On the basis of newly-collected tensile stress-deformation curves, improved knowledge of such important tensile properties of early-age concrete, such as: tensile strength, Young’s modulus, fracture energy, as well as concrete performance under cyclic loading and tensile relaxation.
2. Experimental study 2.1. Direct tensile testing setup
Air-bearing box ⑦ with the top plate having 32 holes of 1 mm in diameter symmetrically distributed under the test specimen: During testing, an air pressure of 140 kPa is supplied to float the test specimen on a thin layer of air, effectively eliminating the friction between the test specimen and the supporting base.
Fig. 1. Schematic illustration of the direct tensile test setup.
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Fig. 2. Test mould and concrete specimen with dimensions and patterned AOI.
Digital Image Correlation (DIC) with two high resolution professional cameras (⑨,⑩ in Fig. 1): This enables the noncontact capturing of the desired deformation over the whole area of interest (AOI, Fig. 2) without any disturbance to the test specimen. Both cameras are synchronized to work together with the loading process by a software developed in-house at The University of Queensland. The angle formed by each camera axis and the vertical line perpendicular to the top surface is within the recommended optimal range of between 10° and 20° for best analysis quality [24]. Four reference pattern areas (Fig. 2) are also applied to the steel mould at four corners of the AOI to facilitate subsequent system verifications. 2.2. Pattern material and application The principle of Digital Image Correlation (DIC) technique is to capture images of the AOI with a surface pattern of speckles (Fig. 2) at a predetermined time interval. The deformation over the AOI is then obtained through analysing the change in position of these speckles by comparing the captured images. As a result, to enable reliable capturing of required deformation in this study, the applied surface pattern must: stick to the concrete surface to ensure no relative displacement between the speckles and the concrete; have appropriate quality and contrast to ensure suitably good images for subsequent image analysis; and, dry as quickly as possible to facilitate the testing process. Following a number of trials of different potential pattern materials, RustOleum industrial fast drying spray paint was found to satisfy all three desirable performance characteristics as mentioned above. During the application of paint, due care was taken to ensure the paint was very thin and not continuous. The quality of applied patterns was then evaluated both visually and analytically: Based on recommended criteria for the mean intensity gradient [25] and subset entropy [26], the optimal subset size was found to range between 40 and 60 pixels – which was consistently achieved for all test specimens. In addition, due to the fast drying nature of the chosen paint [27] and testing started within 10 min after spraying, the influence of paint’s chemical substances on measured properties of concrete was deemed negligible. The application of chosen surface pattern as in Fig. 2 enabled the deformation capturing by DIC of: the AOI region of concrete surface, and the four corners of the two steel mould halves.
2.3. Reliability of deformation captured by DIC To assess the reliability of the deformation captured by DIC, the measured deformation over the AOI region of concrete surface and of the steel mould halves was compared with the input displacement rate of 8.35 104 mm/s in Fig. 3. Prior to the formation of major cracking, the average relative displacement rate between two mould halves (Fig. 3b) and between two AOI’s edges (Fig. 3a) were much smaller than the input displacement rate. This was also observed in previous studies [16] and was possibly due to the combined effect of various factors, including system slacks, distributed microcracking and slippage between concrete and steel mould. After breakage following major cracking, the relative displacement between the two specimen’s halves should equal the applied displacement. Indeed, the relevant displacement of the steel mould halves (Fig. 3b) and of the AOI region of concrete surface (Fig. 3a), obtained from DIC analysis, were 8.35 104 mm/s, which was consistent with the input displacement rate. This clearly confirms the reliability of deformation captured by DIC in this study for very early-age concrete. Accordingly, the improved test setup (Fig. 1) successfully enables the reliable and simultaneous capturing of deformation and loading of concrete specimens from very early ages. Basing on the data collected using this setup, improved knowledge of important early-age tensile properties of concrete can be obtained, which are reported in the subsequent sections. 2.4. Concrete mix design and test procedure A typical concrete mix design with a slump of 80 mm and a 28day characteristic compressive strength of 32 MPa was used throughout the test series. Details of the mix design are given in Table 1. The aggregates used were locally available Hornfels aggregates with the particle size distributions given in Table 2. The material batch sheet for cement, which was a commercially available general purpose Portland cement, is as shown in Table 3. Significant effort was made to ensure the consistency of all ingredients, mix design, curing and test conditions for all specimens throughout the test duration: (i) All ingredients were stored in sufficient quantity for the whole study; (ii) Moisture condition of aggregates were regularly checked to allow adjustment of mixing water accordingly; and (iii) The mixing, placement and curing of concrete were kept the same as far as practically possible to ensure similar mix quality. All mix ingredients were stored in the laboratory condition for at least 7 days prior to usage. The concrete was mixed in a 70litre rotating mixer and placed directly into the steel moulds, which was then compacted via a vibrating table. Test specimens
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(a) Displace ment rate of AOI’s edges.
(b) Displacement rate of two mould halves.
Fig. 3. Verification of the reliability of deformation captured by DIC.
Table 1 Concrete mix design. Ingredients
Portland cement (kg)
20 mm aggregate (kg)
10 mm aggregate (kg)
Coarse sand (kg)
Fine sand (kg)
Water reducer (l)
Water (l)
Quantity (/m3)
310
790
280
495
366
1.09
195
Table 2 Particle size distributions of aggregates. Sieve size (mm)
Cumulative passing (%)
40 19.5 14 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075 0
20 mm aggregate
10 mm aggregate
Coarse Sand
Fine Sand
100 98.5 59 9.6 1.5 1.2 1.1 1 0.9 0.8 0.5 0
100 100 100 86.8 6.8 2 1.7 1.5 1.3 1.1 0.4 0
100 100 100 100 99.5 75.5 46.3 26.9 14.1 7.3 3.3 0
100 100 100 100 97.4 83.1 68.7 44.7 16.5 4.5 1 0
Table 3 Material batch sheet for Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
19.7%
5.4%
2.8%
63.7%
1.2%
2.8%
3.2%
C3S
C2S
C3A
C4AF
Fineness
Na2O equivalent
61.8%
9.9%
9.6%
8.5%
380 m2/kg
0.54%
were then covered by damp cloth to prevent moisture loss and subsequently transferred to the test room with temperature and humidity controlled at 20 ± 2 °C and 60 ± 5%, respectively. Concrete age was measured from the time all mixing water was added. For each specimen, approximately 15 min before testing, the damp cloth was removed, exposing the AOI for pattern application. Flat RustOleum industrial fast drying spray paint was chosen for the pattern material due to its fast drying capacity as well as negligible gloss: White paint was applied first, followed by black paint after some waiting time for the white paint to dry out. The optimal waiting time (approximately 5–10 min) was determined based on the adhesive capacity of the paint to the concrete surface. The specimen was then carefully placed and connected into test position, followed by the installation of the DIC camera system.
Loss on Ignition
The camera capturing frequency was set at 2 s. During testing, the two sides of the test specimen were moved apart at a constant rate of 8.35 104 mm/s. This rate is the same as that in [10], and is an order of magnitude smaller than that of between 0.3 and 1.2 mm/min in [8] and 0.75 mm/min in [9]. The testing rate was chosen to ensure the capturing of the post-peak tensile stressstrain curves in most cases. A slower rate, though giving a better resolution in the post-peak region, would lead to undesirably long test duration for each specimen. For the applied displacement rate of 0.05 mm/min, test duration for a specimen was typically between 10 and 40 min for monotonic loading and 40 to 60 min for cyclic loading. The nominal age used in subsequent sections refers to the age of first loading during a test. Undoubtedly, due to ongoing hydration, certain changes in concrete properties
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during a test duration would occur and should be kept in mind when interpreting test data. It should also be noted that any shrinkage due to surface drying or hydration was included in the deformation captured by DIC in the test setup.
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were found to vary over only a small range of within ±6% of the mean value and have a standard deviation of 0.6 MPa, giving confidence to the consistency of concrete quality across all batches. 3.2. Direct tensile strength
3. Experimental results and discussion Based on the data recorded by DIC and load cells, the strain can be calculated over a gauge length of 70 mm of the AOI while the stress can be determined based on the recorded tensile load and the gross cross-sectional area of 70 mm by 100 mm of the middle region (Fig. 2). Typical stress-strain curves obtained for concrete at different ages from mixing are shown in Fig. 4: For concretes of later ages, the reduced number of data points after peak stress was mainly due to the faster crack propagation. A higher image capturing frequency would provide improved measurement of the post-peak behaviour. From these stress-strain curves, a number of important concrete properties at very early ages can be derived. Such properties include tensile strength, Young’s modulus, tensile strain capacity and fracture energy. Importantly, the performance under cyclic loading and tensile relaxation of early-age concrete in direct tension are also investigated. 3.1. Compressive strength at 28 days The 28-day compressive strength was determined for concrete of each batch in accordance with the relevant Australian Standards [28,29] and plotted in Fig. 5. The 28-day compressive strengths
The development of the direct tensile strength with age from mixing is plotted in Fig. 6a. It can be observed that the tensile strength of the concrete under investigation was negligibly small during the first 3 h or so but accelerated quickly in the following few hours. Since wet curing was provided for every specimen up until the start of their tensile testing, the contribution of matric suction in such increase in tensile strength should be negligible, if not zero. The increase in measured tensile strengths was thus due almost entirely to the ongoing hydration of cementitious material. The rapid early-age strength growth also suggests that every additional hour of effective curing of concrete that can be afforded on the construction site is highly beneficial in ensuring sufficiently high tensile capacity, thereby reducing the cracking risk. The tensile strengths obtained in this study were plotted together with results from other studies [8,10,17] in Fig. 6b. A similar trend of tensile strength development with time is clearly evidenced, indicating a rather high consistency in the order of magnitude of direct tensile strength of early-age concrete measured in different studies. The slight differences in reported tensile strengths among studies are possibly due to the combined effects of the following: Mix designs and curing procedures used [8,17,30]; Effectiveness of measures to minimise friction between test specimens and supporting surfaces, as briefed in Section 1; Accuracy of load recording devices; and, Assumed times of zero for determining the age of concrete: While this would not affect the comparison of data within a given study, such comparison among studies is slightly affected. Fig. 6a also presents a comparison of the obtained direct tensile strengths with corresponding values as given by relevant models in currently available standards (Table 4). The significant deviation between the models predictions and measured values (Figs. 6 and 7) clearly highlights the rather limited predictive capability of these models. Such poor capability is due mainly to the current dearth of reliable data on tensile properties of very early-age concrete, prompting the need to generate more reliable data as highlighted in Section 1 ‘‘Introduction”.
(a) Monotonic loading.
(b) Cyclic loading. Fig. 4. Typical tensile stress-strain relationships.
3.3. Young’s modulus In this study, the Young’s modulus was determined by a linear fit to the initial ascending part of stress-strain curves between 5% and 40% of the tensile strength, following relevant Australian standards [33]. The obtained results are plotted in Fig. 7. Similar to the direct tensile strength, the Young’s modulus was found to be negligibly small during the first 5 h or so but then started to increase quickly thereafter. There also exists significant deviation between the measured Young’s moduli and corresponding values predicted by different available models. Furthermore, the Young’s modulus obtained in this study is considerably less scattered than that in past research [10]. The above results are then plotted against comparable Young’s modulus from a previous study [10] in Fig. 7b. In [10], similar concretes were also tested for full stress-strain behaviour using the same test frame. However, only the relative displacement between the two steel mould halves (DLMould_Avg) was measured using two
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Fig. 5. Compressive strength at 28 days.
(a) This study
(b) This study versus previous ones
Fig. 6. Tensile strength development with age from mixing.
Table 4 Models for prediction of tensile strength and Young’s modulus. BS EN 1992-1-1:2004 [31] Tensile strength (MPa): fctm(t) = bcc(t) fctm Young’s modulus (MPa): Ecm(t) = [fcm(t)/fcm]0.3 Ecm qffiffiffiffiio n h ‘‘Age” coefficient: bcc ðtÞ ¼ exp s 1 28 t
JSCE: 2007 [32] (Eq. (1)) (Eq. (2)) (Eq. (3))
s = 0.25 for class N cement (this study); s = 0.20 for class R cement; s = 0.38 for class S cement. where t – concrete age in days; fcm, fctm and Ecm – 28-day compressive strength, tensile strength and Young’s modulus; fcm(t) = bcc(t) fcm – concrete’s compressive strength at age t.
For ordinary Portland cement at up to 3 days of age: p p Tensile strength (MPa): ftk(t) = c f0 c(t) = 0.44 f0 c(t) p p Young’s modulus (MPa): Ee(t) = U(t) 4.7 103 f’c(t) = 3431 f0 c(t)
(Eq. (4)) (Eq. (5))
where f’c(t) – concrete’s compressive strength at age t; t – concrete age in days; c – a coefficient varied with the degree of drying of concrete; U(t) – early creep factor, taken as 0.73 at up to 3 days of age.
LVDTs attached to the steel mould halves. The required deformation over the AOI region (DLAOI) was then estimated from the measured deformation (DLMould_Avg). On the basis of finite element modelling [10], the ratio DLAOI/DLMould_Avg was assumed as: 0.50 before peak stress was reached, and 0.90 after peak stress. To assess whether such estimation is acceptable, the ratio DLAOI/DLMould_Avg was determined using reliable deformation captured by DIC in this study (Section 2.3) and plotted in Fig. 8: DLAOI: is the relative displacement between the two extreme edges of AOI region of concrete surface (line ①). DLMould_Avg: is the average relative displacement between the two steel mould halves (line ④, being the average of lines ② and ③).
It can be observed that: In the post-peak period following major cracking, the ratio DLAOI/DLMould_Avg was fairly constant at around 0.95, which is in rather good agreement with the assumption in [10]; Prior to the formation of major cracking, the ratio varied over a large range initially before stabilising at around 0.2. This was significantly different from the assumed value of 0.5 in [10]. This provides at least a partial explanation for the significantly more scattered distribution of Young’s modulus in [10] compared to that in this study. 3.4. Relationship between direct tensile strength and Young’s modulus The direct tensile strength versus Young’s modulus from this study and available standards are plotted in Fig. 9. It can be observed that:
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(a) This study
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(b) This study versus previous ones
Fig. 7. Young’s modulus development with age from mixing.
(a) Relative displacements, ΔL
(b) ΔLConcrete
/ΔLSteel mould
Fig. 8. Assessing the reliability of deformation reported in a typical previous study [10].
Fig. 9. Direct tensile strength versus Young’s modulus.
There was a very strong linear relationship between the direct tensile strength and Young’s modulus for studied concretes during the first 10 h after mixing. The ratio of Young’s modulus (MPa) to direct tensile strength (kPa) appeared to be independent of concrete age and was estimated by linear best fitting to be a constant of 7.88. A similar observation was also reported in [10]. Such linear relationship between direct tensile strength and Young’s modulus of early-age concrete can be conveniently used to determine either property if the other property is
known. Further work is needed to confirm such relationship for other types of cement, mix designs and curing regimes. The prediction by JSCE:2007 [32] based on the square root of compressive strength at the respective age is in good agreement with results of this study; while that given by BS EN 1992-1-1 [31] is not. Whereas this correct prediction of the linear relationship between the two properties by JSCE:2007 is acknowledged, the need to improve its currently poor capability in predicting individual early-age concrete properties (as evidenced in Figs. 6 and 7) remains.
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3.5. Strain at peak stress The evolution of strain at peak stress over time recorded in this research and major past studies [3,8–10,16] is presented in Fig. 10, from which the following observations can be made: The strain at peak stress appeared to decrease over time within the test age range, and possibly reaching a minimum value at around 8–9 h of age. This corresponds well with the general trend reported in the literature [8,34–38], which suggests an initial sharp decrease of strain at peak stress from thousands of microstrain before reaching a stable value of between 100 le and 200 le. Compared to previous studies, the strain at peak stress in this study appears to be consistent with a clear evolution over time, which agrees well with Kasai et al. [8]. Test results from Dao et al. [10], on the other hand, suffer large variation (Fig. 10b) due to the difficulty in displacement recording as discussed in Section 3.3. With limited test results, Hannant et al. [18] and Roziere et al. [16] reported lower strain at peak stress, possibly due to different mix designs and/or cement types employed. The tensile strain capacity value of 70 le suggested in CIRIA C660 [3] for crack control of early-age concrete seems appropriate (Fig. 10b).
Fig. 11. Fracture energy development with age from mixing.
3.6. Fracture characteristics Using the obtained test data, fracture properties can be calculated based on a stress-crack opening curve following the fictitious crack model proposed by Hillerborg [39]. The obtained development over time of fracture energy GF is plotted in Fig. 11: The fracture energy GF was observed to increase with time at an increasing rate over the observed period of 10 h after mixing. There appears a good agreement between the time evolution of GF in this study and in [10], despite the difference in methods to determine deformation (Section 3.3). This agreement was possibly due to the governing influence of the tensile strength and post-peak deformation, which were similar in both studies. The effect of the variation in pre-peak deformation (Section 3.3) was negligible. The fracture energy GF is plotted against the corresponding tensile strength ft and Young’s modulus E in Fig. 12, together with their best-fit linear trendlines. It can be seen that the fracture energy increases with increasing values of tensile strength and Young’s modulus, similar to observations reported in [10]. How-
Fig. 12. Fracture energy versus tensile strength and Young’s modulus.
ever, their relationships are only weakly linear as evidenced by the relatively small R-square values of 0.82 and 0.9, compared to stronger linear relationships reported in [10].
3.7. Early-age concrete under cyclic loading The behaviour of concrete, both in its hardening and hardened states, is dependent upon its load history. While the concrete
Fig. 10. Strain at peak stress development with age from this and major past studies.
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performance under cyclic loading in its mature state is relatively well-researched [40], our understanding of such performance at very early ages is very limited. An improved knowledge of behaviour of hardening concrete under cyclic loading is therefore required to provide a better basis for proper assessment of earlyage cracking risk. Accordingly, using the improved direct tensile setup, four specimens were subjected to cyclic loading to assess its effect on tensile Young’s modulus of early-age concrete. For each load cycle, the test specimen was subjected to a constant displacement rate of 0.05 mm/min up to about 40% of the tensile strength at that age, estimated from tensile strength development over time of parallel series of specimens (Fig. 7). A typical stress-strain curve in cyclic loading is shown in Fig. 4b and a summary of the results is presented in Fig. 13. As evidenced in Fig. 13, when a single test specimen was subject to repeated tensile load cycles in this study, the Young’s modulus in subsequent load cycles were found to be higher than those in preceding cycles, which is in stark contrast to the typical decreasing trend reported for mature concrete [41,42]. Such difference is possibly due to the combined effects of the following:
Fig. 14. Testing scheme for concrete relaxation under constant deformation.
Table 5 Age, tensile strength and initial tensile stress of relaxation specimens. Age (h:min)
ft (kPa)
r0 (kPa)
r0/ft
5:05 6:40
39.8 122.2
17.2 52.4
0.43 0.43
Note: r0 – applied stress at the beginning of relaxation test. ft – direct tensile strength obtained from parallel specimens.
On the one hand, due to the ongoing hydration during this early stage, there were considerable increases in Young’s modulus over the time intervals between load cycles, as evidenced in Fig. 7. In contrast, for mature concrete, such ongoing hydration is deemed negligible. On the other hand, due to the microcracking and damage accumulated over the load cycles, Young’s modulus was compromised. The accumulated microcracking affects the properties of both early-age and mature concretes. 3.8. Relaxation behaviour of early-age concrete Adequate knowledge of creep and relaxation behaviour of earlyage concrete is required for the proper assessment of performance of early-age concrete, including its cracking risk [12]. To trial the capability of the current direct tensile setup for relaxation testing and subsequently to complement currently available data [12,43], in this part of the study, the variations over time of stresses in two test specimens subject to constant deformation were investigated (Fig. 14). The imposed deformations were chosen to ensure similar initial tensile stresses r0 in all specimens, which were approximately 43% of the corresponding tensile strengths at start of the relaxation tests (Table 5). (The ratios of stress to tensile strength would become increasing smaller during a relaxation test due to the increasing tensile strength over time as a result of ongoing hydration but remain within the typical range for this type of
Fig. 15. Tensile relaxation of very early-age concrete.
test [44,45].) Test duration for each specimen was approximately 60 min from the time t0 that r0 was reached. The obtained relaxation behaviour is plotted in Fig. 15. For both specimens, the stress in the concrete was found to initially decrease over time at increasingly diminishing rates, which is consistent with typical relaxation behaviour of concrete [44,45]. This suggests the appropriateness of this test setup for the study of relaxation behaviour of early-age concrete. The stress in specimen 2, however, increased slightly after about 10 min, due possibly mainly to the effect of ongoing hydration. Further work is ongoing to generate more data to form the basis for improved understanding of such relaxation behaviour of young concrete, taking due account of the effects of loading and other mechanisms. 4. Summary and conclusions
Fig. 13. Young’s modulus of early-age concrete under cyclic loading.
This paper first highlights the current dearth of reliable data on tensile properties of very early-age concrete, despite their significance in assessing cracking risk and residual stress as well as in precast industry. This is argued as a result of the considerable practical problems to be overcome when testing very early-age
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concrete due to its weak nature. Through identifying and effectively addressing critical drawbacks in previous test setups, an improved direct tensile testing system that can reliably capture simultaneously stress and deformation of concrete from the first several hours after mixing has been successfully developed and reported. Such capturing has been achieved for concrete from the age of 3 h in this study, but this starting age would be earlier or later depending on the mix design and curing. Two key features of this setup are: Air-bearing box to effectively eliminate the friction between the test specimen and supporting base. Digital Image Correlation (DIC) to capture the required deformation over the whole desired area of interest. The reliability of DIC capturing of very early-age concrete is clearly demonstrated through comparing with input displacement rate. The improved test setup is then used to generate data on tensile stress-strain curves to form the basis for improved knowledge of such important tensile properties of early-age concrete as tensile strength, Young’s modulus, fracture energy: The overall development trends over time are found to be in general agreement with limited available data from previous studies. All tensile strength, Young’s modulus and fracture energy are negligibly small during the first 4 h or so after mixing but increase quickly afterwards. The highly linear relationship between direct tensile strength and Young’s modulus is particularly noted. Importantly, the Young’s modulus obtained in this study is considerably less scattered than that in past research. This is due mainly to the more reliable capturing of deformation using DIC in this study. It is also found that the improved direct-tensile test setup can be conveniently used to study the behaviour of early-age concrete under direct-tensile cyclic loading and relaxation. Initial results suggest that: Different from its general decrease due to cyclic loading observed in mature concrete, Young’s modulus of early-age concrete specimen in subsequent loading cycles is found to increase but at a rate substantially lower than for the case of no accumulated microcracking. This is possibly due to the combined and counteracting effects of ongoing hydration of cementitious materials and accumulation of micro-damage as a result of cyclic loading. The tensile relaxation of early-age concrete is generally consistent with typical relaxation behaviour reported in literature. However, further data is needed to better understand such relaxation behaviour under the influence of loading, hydration and other mechanisms. With the reliability and capability of the improved direct tensile testing setup clearly established, further work is ongoing to provide a better basis for rational design of concrete structures taking due account of the effect of early-age performance characteristics. Acknowledgements Thanks are due to Renee Shi, Denton Liu, Qiule Qu, Guanwei Zhang, Yang Xu, and Technical staff within the School of Civil Engineering at the University of Queensland for their help with the experimental work.
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Nantung, Saw-Cutting Guidelines for Concrete Pavements: Examining the Requirements for tIme and Depth of SawCutting, 2009, p. 813. [14] L. Buffo-Lacarrière, S. Baron, F. Barré, D. Chauvel, A. Darquennes, J.P. Dubois, J. Gayete, F. Grondin, B. Kolani, H. Lançon, A. Loukili, G. Moreau, C. Rospars, A. Sellier, J.M. Torrenti, Restrained shrinkage of massive reinforced concrete structures: results of the project CEOS.fr, Eur. J. Environ. Civil Eng. (2015) 1–24. [15] D.M.F. Orr, G.F. Haigh, An apparatus for measuring the shrinkage characteristics of plastic mortars, Mag. Concr. Res. 23 (74) (1971) 43–48. [16] E. Roziere, R. Cortas, A. Loukili, Tensile behaviour of early age concrete: new methods of investigation, Cement Concr. Compos. 55 (2015) 153–161. [17] J. Abel, K. Hover, Effect of water/cement ratio on the early age tensile strength of concrete, Transp. Res. Rec. 1610 (1998) 33–38. [18] D.J. Hannant, J. Branch, M. Mulheron, Equipment for tensile testing of fresh concrete, Mag. Concr. Res. (1999) 263–267. [19] J. Branch, D.J. Hannant, M. Mulheron, Factors affecting the plastic shrinkage cracking of high-strength concrete, Mag. Concr. Res. (2002) 347–354. [20] Z. Shan, Z. Yu, J. Shi, Experimental investigation of flow of fresh selfcompacting concrete in improved L-box, Constr. Build. Mater. 84 (2015) 30– 38. [21] A. Messan, P. Ienny, D. Nectoux, Free and restrained early-age shrinkage of mortar: influence of glass fiber, cellulose ether and EVA (ethylene-vinyl acetate), Cement Concr. Compos. 33 (3) (2011) 402–410. [22] D. Nguyen, V. Dao, A novel method for tensile testing of very early-age concrete, in: S.T. Smith (Ed.), The 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), Byron Bay, Australia, 2014, pp. 47–52. [23] Z.P. Bazant, B.H. Oh, Crack band theory for fracture of concrete, Mater. Struct. 16 (93) (1983) 155–177. [24] B.L. Karihaloo, Fracture Mechanics and Structural Concrete, Longman, Harlow, England, 1995. [25] B. Pan, Z. 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Standard, Eurocode 2: Design of Concrete Structures, Part 1–1: General Rules and Rules for Buildings, British Standard, UK, 2004, p. 230. [32] JSCE Concrete Committee, English Version of Standard Specification for Concrete Structures, Design, Japan Society of Civil Engineers, Japan, 2007, p. 503. [33] Standards Australia, Methods of testing concrete – AS1012.17, Method 17: Determination of the static chord modulus of elasticity and Poisson’s ratio of concrete specimens, Australia, 1997.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Tensile properties of concrete at very early ages Duy H. Nguyen a, Vinh T.N. Dao a,⇑, Pietro Lura b,c a
School of Civil Engineering, The University of Queensland, Australia Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland c Institute for Building Materials, ETH Zurich, Switzerland b
h i g h l i g h t s Testing for reliable tensile properties of very early-age concrete is reviewed. The current dearth of such reliable data is emphasised. Key features of an improved direct tensile testing system are presented. Newly-collected reliable data and their analysis are reported. Fundamental tensile properties of concrete at very early ages are presented.
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 11 November 2016 Accepted 28 December 2016
Keywords: Early-age concrete Direct tensile test Tensile properties Fracture properties Cyclic loading Tensile relaxation Digital image correlation
a b s t r a c t Proper knowledge of tensile properties of concrete from very early ages is essential for effective control of not only early-age cracking but also of residual stress due to restrained early-age deformation as well as for demolding and handling of young concrete members in the precast industry. Despite significant past research, such knowledge is currently inadequate and based on experimental data with questionable reliability, due mainly to the considerable challenges in testing early-age concrete. This paper first highlights the challenge and importance of collecting reliable test data on full tensile stress-deformation curves for very early-age concrete. Through identifying and effectively addressing critical drawbacks in previous test setups, an improved direct tensile testing system that can reliably capture simultaneously stress and deformation of concrete from the age of several hours after mixing has been successfully developed. Key features of the improved setup, including the air-bearing box for friction minimisation and digital image correlation for non-contact full-field deformation capturing, are then reported in the paper. Based on the newly collected data, fundamental tensile properties of concrete at very early ages are re-assessed and presented. Such properties include tensile strength, Young’s modulus, strain at peak stress, fracture energy, performance under cyclic loading and tensile relaxation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Early-age cracking may occur in concrete structures from as early as several hours after casting. Very often, these early-age cracks would further propagate and render these structures unserviceable at later stages due to subsequent shrinkage and loading. As a result, the performance characteristics and service life of affected concrete structures can be severely compromised. The effective control of such cracking is thus of particular concern for highway pavements, bridges, tunnels, liquid reservoirs [1–3], as
⇑ Corresponding author. E-mail address: [email protected] (V.T.N. Dao). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.169 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
well as critical structures such as concrete containments of nuclear power plants and storage facilities for hazardous substances [3,4]. The basic underlying mechanism for such cracking is the tensile stress (or strain) due to restrained early deformation in the concrete reaching its tensile capacity. Importantly, even when earlyage cracking does not occur, the residual stress due to restrained early-age deformation can substantially reduce the remaining tensile-carrying capacity of concrete. Such effect, if not properly accounted for, may critically compromise the performance of concrete structures in service loads and environment [5]. Accordingly, a proper understanding and control of the abovementioned early-age cracking risk and residual stress from restrained early-age deformation in concrete structures requires, among other things, an adequate knowledge of tensile and fracture
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properties of concrete from very early ages. Such knowledge of early-age tensile properties is also highly beneficial in precast industry to optimize the timing for prestressing, demoulding and handling of precast members [6,7]. Having recognised the significance of such knowledge, different research groups have attempted to measure the relevant tensile properties of concrete since the early 1970s, with useful but still rather limited reported outcomes [8–14]. In [13], using a tensile wedge splitting setup, the load-displacement up to peak stress of young concrete specimens could be captured – However, the post-peak behaviour could not be obtained, rendering the determination of fracture properties challenging. In fact, the full stressstrain curves have been reported by only a few groups [9,10]. This is believed to be due mainly to the considerable practical challenges to be overcome when testing early-age concrete to obtain reliable stress-deformation curves, which requires reliable capturing of both stress and deformation at the same time. Specifically:
This paper reports key initial outcomes of a research program ongoing at The University of Queensland aimed at addressing the above-mentioned knowledge gaps, including:
Due to the low tensile strength to be measured, friction between the test specimen and its supporting surface must be minimised if reliable loading is to be recorded. Various measures have been attempted [8,9], including the use of a mercury bath [15], roller bearing [8] or materials with a low coefficient of friction such as Teflon [16]. The best available method is possibly the inclusion of an air-bearing box which essentially ‘‘floats” the test specimen on a thin layer of air, thus effectively eliminating all friction – The system was first proposed by Hannant’s group [9] and subsequently further improved by Dao et al. [10]. Due to its fragility, disturbance to test specimens must be minimised throughout the moulding, handling and testing processes. In particular, such fragility of early-age concrete has important implications on reliable deformation measurement – Methods for deformation measurement in previous research can be categorised into three groups:
The setup for direct tensile testing of concrete test specimens used in this study, as schematically shown in Fig. 1, is based on its earlier version [10] with significant improvement to allow reliable simultaneous capturing of stress and deformation. During testing, the test apparatus is placed on the horizontal platform of a displacement-controlled Instron 5985 testing frame. The test apparatus itself comprises an air-bearing box ⑦ and a lever arm ④ attached to a small steel frame ①. The lever arm is pin-connected to the frame and is self-balanced in the test position, enabling the direction but not the magnitude of the force applied through the Instron loading machine ⑤ to be altered. The test specimen ② is placed on top of the air-bearing box ⑦ with one end pin-connected to the load cell ⑥ and the other to the lever arm. ④ The test specimen and its dimensions are given in Fig. 2: The minimum dimension of 70 mm is greater than the average width of the fracture process zone, which is about three times the maximum aggregate size used [23]. The curved transitions aim to promote failure in the middle section while eliminating significant stress concentrations that could initiate undesirable cracking within the transitions [10]. The two notable features of this unique test setup include:
Strain gauges are attached to the mould [10,17], giving total deformation of the whole specimen – Deformation over the region of interest has to be estimated from the measured total deformation, typically by finite element modelling. This ‘‘estimation” procedure inevitably has introduced additional uncertainties that have not been appropriately taken into account, raising doubts over the reliability of obtained results. Strain gauges are attached to the posts cast into concrete [18,19] – The likely movement of the posts in young concrete and the resulting disturbance may have significantly compromised the measurement accuracy; but again, the effect of such movement and disturbance has not been adequately quantified. Digital Image Correlation (DIC): DIC uses high definition professional cameras for reliable, non-contact capturing of the required displacement fields. DIC has been used successfully to track the free surface of flowing concrete [20] and capture surface deformation of mortar specimens [21]. However, there seem limited reported studies in which the reliable deformation of concrete surface captured by DIC is synchronised with reliable loading to provide the required loaddisplacement curves. Roziere et al. [16] reported one such study on concrete specimens from the age of 7 h after casting – Accordingly, this technique may not be appropriate for concrete of earlier ages due to the weaker, wetter and softer nature of the concrete surface at this stage. Nguyen and Dao [22] appear to be the first to have reported a successful use of DIC for direct tensile testing of concrete from 3 h after mixing.
An improved direct tensile test that enables the reliable and simultaneous capturing of deformation and stress of concrete specimens from very early ages. On the basis of newly-collected tensile stress-deformation curves, improved knowledge of such important tensile properties of early-age concrete, such as: tensile strength, Young’s modulus, fracture energy, as well as concrete performance under cyclic loading and tensile relaxation.
2. Experimental study 2.1. Direct tensile testing setup
Air-bearing box ⑦ with the top plate having 32 holes of 1 mm in diameter symmetrically distributed under the test specimen: During testing, an air pressure of 140 kPa is supplied to float the test specimen on a thin layer of air, effectively eliminating the friction between the test specimen and the supporting base.
Fig. 1. Schematic illustration of the direct tensile test setup.
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Fig. 2. Test mould and concrete specimen with dimensions and patterned AOI.
Digital Image Correlation (DIC) with two high resolution professional cameras (⑨,⑩ in Fig. 1): This enables the noncontact capturing of the desired deformation over the whole area of interest (AOI, Fig. 2) without any disturbance to the test specimen. Both cameras are synchronized to work together with the loading process by a software developed in-house at The University of Queensland. The angle formed by each camera axis and the vertical line perpendicular to the top surface is within the recommended optimal range of between 10° and 20° for best analysis quality [24]. Four reference pattern areas (Fig. 2) are also applied to the steel mould at four corners of the AOI to facilitate subsequent system verifications. 2.2. Pattern material and application The principle of Digital Image Correlation (DIC) technique is to capture images of the AOI with a surface pattern of speckles (Fig. 2) at a predetermined time interval. The deformation over the AOI is then obtained through analysing the change in position of these speckles by comparing the captured images. As a result, to enable reliable capturing of required deformation in this study, the applied surface pattern must: stick to the concrete surface to ensure no relative displacement between the speckles and the concrete; have appropriate quality and contrast to ensure suitably good images for subsequent image analysis; and, dry as quickly as possible to facilitate the testing process. Following a number of trials of different potential pattern materials, RustOleum industrial fast drying spray paint was found to satisfy all three desirable performance characteristics as mentioned above. During the application of paint, due care was taken to ensure the paint was very thin and not continuous. The quality of applied patterns was then evaluated both visually and analytically: Based on recommended criteria for the mean intensity gradient [25] and subset entropy [26], the optimal subset size was found to range between 40 and 60 pixels – which was consistently achieved for all test specimens. In addition, due to the fast drying nature of the chosen paint [27] and testing started within 10 min after spraying, the influence of paint’s chemical substances on measured properties of concrete was deemed negligible. The application of chosen surface pattern as in Fig. 2 enabled the deformation capturing by DIC of: the AOI region of concrete surface, and the four corners of the two steel mould halves.
2.3. Reliability of deformation captured by DIC To assess the reliability of the deformation captured by DIC, the measured deformation over the AOI region of concrete surface and of the steel mould halves was compared with the input displacement rate of 8.35 104 mm/s in Fig. 3. Prior to the formation of major cracking, the average relative displacement rate between two mould halves (Fig. 3b) and between two AOI’s edges (Fig. 3a) were much smaller than the input displacement rate. This was also observed in previous studies [16] and was possibly due to the combined effect of various factors, including system slacks, distributed microcracking and slippage between concrete and steel mould. After breakage following major cracking, the relative displacement between the two specimen’s halves should equal the applied displacement. Indeed, the relevant displacement of the steel mould halves (Fig. 3b) and of the AOI region of concrete surface (Fig. 3a), obtained from DIC analysis, were 8.35 104 mm/s, which was consistent with the input displacement rate. This clearly confirms the reliability of deformation captured by DIC in this study for very early-age concrete. Accordingly, the improved test setup (Fig. 1) successfully enables the reliable and simultaneous capturing of deformation and loading of concrete specimens from very early ages. Basing on the data collected using this setup, improved knowledge of important early-age tensile properties of concrete can be obtained, which are reported in the subsequent sections. 2.4. Concrete mix design and test procedure A typical concrete mix design with a slump of 80 mm and a 28day characteristic compressive strength of 32 MPa was used throughout the test series. Details of the mix design are given in Table 1. The aggregates used were locally available Hornfels aggregates with the particle size distributions given in Table 2. The material batch sheet for cement, which was a commercially available general purpose Portland cement, is as shown in Table 3. Significant effort was made to ensure the consistency of all ingredients, mix design, curing and test conditions for all specimens throughout the test duration: (i) All ingredients were stored in sufficient quantity for the whole study; (ii) Moisture condition of aggregates were regularly checked to allow adjustment of mixing water accordingly; and (iii) The mixing, placement and curing of concrete were kept the same as far as practically possible to ensure similar mix quality. All mix ingredients were stored in the laboratory condition for at least 7 days prior to usage. The concrete was mixed in a 70litre rotating mixer and placed directly into the steel moulds, which was then compacted via a vibrating table. Test specimens
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(a) Displace ment rate of AOI’s edges.
(b) Displacement rate of two mould halves.
Fig. 3. Verification of the reliability of deformation captured by DIC.
Table 1 Concrete mix design. Ingredients
Portland cement (kg)
20 mm aggregate (kg)
10 mm aggregate (kg)
Coarse sand (kg)
Fine sand (kg)
Water reducer (l)
Water (l)
Quantity (/m3)
310
790
280
495
366
1.09
195
Table 2 Particle size distributions of aggregates. Sieve size (mm)
Cumulative passing (%)
40 19.5 14 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075 0
20 mm aggregate
10 mm aggregate
Coarse Sand
Fine Sand
100 98.5 59 9.6 1.5 1.2 1.1 1 0.9 0.8 0.5 0
100 100 100 86.8 6.8 2 1.7 1.5 1.3 1.1 0.4 0
100 100 100 100 99.5 75.5 46.3 26.9 14.1 7.3 3.3 0
100 100 100 100 97.4 83.1 68.7 44.7 16.5 4.5 1 0
Table 3 Material batch sheet for Portland cement. SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
19.7%
5.4%
2.8%
63.7%
1.2%
2.8%
3.2%
C3S
C2S
C3A
C4AF
Fineness
Na2O equivalent
61.8%
9.9%
9.6%
8.5%
380 m2/kg
0.54%
were then covered by damp cloth to prevent moisture loss and subsequently transferred to the test room with temperature and humidity controlled at 20 ± 2 °C and 60 ± 5%, respectively. Concrete age was measured from the time all mixing water was added. For each specimen, approximately 15 min before testing, the damp cloth was removed, exposing the AOI for pattern application. Flat RustOleum industrial fast drying spray paint was chosen for the pattern material due to its fast drying capacity as well as negligible gloss: White paint was applied first, followed by black paint after some waiting time for the white paint to dry out. The optimal waiting time (approximately 5–10 min) was determined based on the adhesive capacity of the paint to the concrete surface. The specimen was then carefully placed and connected into test position, followed by the installation of the DIC camera system.
Loss on Ignition
The camera capturing frequency was set at 2 s. During testing, the two sides of the test specimen were moved apart at a constant rate of 8.35 104 mm/s. This rate is the same as that in [10], and is an order of magnitude smaller than that of between 0.3 and 1.2 mm/min in [8] and 0.75 mm/min in [9]. The testing rate was chosen to ensure the capturing of the post-peak tensile stressstrain curves in most cases. A slower rate, though giving a better resolution in the post-peak region, would lead to undesirably long test duration for each specimen. For the applied displacement rate of 0.05 mm/min, test duration for a specimen was typically between 10 and 40 min for monotonic loading and 40 to 60 min for cyclic loading. The nominal age used in subsequent sections refers to the age of first loading during a test. Undoubtedly, due to ongoing hydration, certain changes in concrete properties
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during a test duration would occur and should be kept in mind when interpreting test data. It should also be noted that any shrinkage due to surface drying or hydration was included in the deformation captured by DIC in the test setup.
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were found to vary over only a small range of within ±6% of the mean value and have a standard deviation of 0.6 MPa, giving confidence to the consistency of concrete quality across all batches. 3.2. Direct tensile strength
3. Experimental results and discussion Based on the data recorded by DIC and load cells, the strain can be calculated over a gauge length of 70 mm of the AOI while the stress can be determined based on the recorded tensile load and the gross cross-sectional area of 70 mm by 100 mm of the middle region (Fig. 2). Typical stress-strain curves obtained for concrete at different ages from mixing are shown in Fig. 4: For concretes of later ages, the reduced number of data points after peak stress was mainly due to the faster crack propagation. A higher image capturing frequency would provide improved measurement of the post-peak behaviour. From these stress-strain curves, a number of important concrete properties at very early ages can be derived. Such properties include tensile strength, Young’s modulus, tensile strain capacity and fracture energy. Importantly, the performance under cyclic loading and tensile relaxation of early-age concrete in direct tension are also investigated. 3.1. Compressive strength at 28 days The 28-day compressive strength was determined for concrete of each batch in accordance with the relevant Australian Standards [28,29] and plotted in Fig. 5. The 28-day compressive strengths
The development of the direct tensile strength with age from mixing is plotted in Fig. 6a. It can be observed that the tensile strength of the concrete under investigation was negligibly small during the first 3 h or so but accelerated quickly in the following few hours. Since wet curing was provided for every specimen up until the start of their tensile testing, the contribution of matric suction in such increase in tensile strength should be negligible, if not zero. The increase in measured tensile strengths was thus due almost entirely to the ongoing hydration of cementitious material. The rapid early-age strength growth also suggests that every additional hour of effective curing of concrete that can be afforded on the construction site is highly beneficial in ensuring sufficiently high tensile capacity, thereby reducing the cracking risk. The tensile strengths obtained in this study were plotted together with results from other studies [8,10,17] in Fig. 6b. A similar trend of tensile strength development with time is clearly evidenced, indicating a rather high consistency in the order of magnitude of direct tensile strength of early-age concrete measured in different studies. The slight differences in reported tensile strengths among studies are possibly due to the combined effects of the following: Mix designs and curing procedures used [8,17,30]; Effectiveness of measures to minimise friction between test specimens and supporting surfaces, as briefed in Section 1; Accuracy of load recording devices; and, Assumed times of zero for determining the age of concrete: While this would not affect the comparison of data within a given study, such comparison among studies is slightly affected. Fig. 6a also presents a comparison of the obtained direct tensile strengths with corresponding values as given by relevant models in currently available standards (Table 4). The significant deviation between the models predictions and measured values (Figs. 6 and 7) clearly highlights the rather limited predictive capability of these models. Such poor capability is due mainly to the current dearth of reliable data on tensile properties of very early-age concrete, prompting the need to generate more reliable data as highlighted in Section 1 ‘‘Introduction”.
(a) Monotonic loading.
(b) Cyclic loading. Fig. 4. Typical tensile stress-strain relationships.
3.3. Young’s modulus In this study, the Young’s modulus was determined by a linear fit to the initial ascending part of stress-strain curves between 5% and 40% of the tensile strength, following relevant Australian standards [33]. The obtained results are plotted in Fig. 7. Similar to the direct tensile strength, the Young’s modulus was found to be negligibly small during the first 5 h or so but then started to increase quickly thereafter. There also exists significant deviation between the measured Young’s moduli and corresponding values predicted by different available models. Furthermore, the Young’s modulus obtained in this study is considerably less scattered than that in past research [10]. The above results are then plotted against comparable Young’s modulus from a previous study [10] in Fig. 7b. In [10], similar concretes were also tested for full stress-strain behaviour using the same test frame. However, only the relative displacement between the two steel mould halves (DLMould_Avg) was measured using two
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Fig. 5. Compressive strength at 28 days.
(a) This study
(b) This study versus previous ones
Fig. 6. Tensile strength development with age from mixing.
Table 4 Models for prediction of tensile strength and Young’s modulus. BS EN 1992-1-1:2004 [31] Tensile strength (MPa): fctm(t) = bcc(t) fctm Young’s modulus (MPa): Ecm(t) = [fcm(t)/fcm]0.3 Ecm qffiffiffiffiio n h ‘‘Age” coefficient: bcc ðtÞ ¼ exp s 1 28 t
JSCE: 2007 [32] (Eq. (1)) (Eq. (2)) (Eq. (3))
s = 0.25 for class N cement (this study); s = 0.20 for class R cement; s = 0.38 for class S cement. where t – concrete age in days; fcm, fctm and Ecm – 28-day compressive strength, tensile strength and Young’s modulus; fcm(t) = bcc(t) fcm – concrete’s compressive strength at age t.
For ordinary Portland cement at up to 3 days of age: p p Tensile strength (MPa): ftk(t) = c f0 c(t) = 0.44 f0 c(t) p p Young’s modulus (MPa): Ee(t) = U(t) 4.7 103 f’c(t) = 3431 f0 c(t)
(Eq. (4)) (Eq. (5))
where f’c(t) – concrete’s compressive strength at age t; t – concrete age in days; c – a coefficient varied with the degree of drying of concrete; U(t) – early creep factor, taken as 0.73 at up to 3 days of age.
LVDTs attached to the steel mould halves. The required deformation over the AOI region (DLAOI) was then estimated from the measured deformation (DLMould_Avg). On the basis of finite element modelling [10], the ratio DLAOI/DLMould_Avg was assumed as: 0.50 before peak stress was reached, and 0.90 after peak stress. To assess whether such estimation is acceptable, the ratio DLAOI/DLMould_Avg was determined using reliable deformation captured by DIC in this study (Section 2.3) and plotted in Fig. 8: DLAOI: is the relative displacement between the two extreme edges of AOI region of concrete surface (line ①). DLMould_Avg: is the average relative displacement between the two steel mould halves (line ④, being the average of lines ② and ③).
It can be observed that: In the post-peak period following major cracking, the ratio DLAOI/DLMould_Avg was fairly constant at around 0.95, which is in rather good agreement with the assumption in [10]; Prior to the formation of major cracking, the ratio varied over a large range initially before stabilising at around 0.2. This was significantly different from the assumed value of 0.5 in [10]. This provides at least a partial explanation for the significantly more scattered distribution of Young’s modulus in [10] compared to that in this study. 3.4. Relationship between direct tensile strength and Young’s modulus The direct tensile strength versus Young’s modulus from this study and available standards are plotted in Fig. 9. It can be observed that:
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(a) This study
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(b) This study versus previous ones
Fig. 7. Young’s modulus development with age from mixing.
(a) Relative displacements, ΔL
(b) ΔLConcrete
/ΔLSteel mould
Fig. 8. Assessing the reliability of deformation reported in a typical previous study [10].
Fig. 9. Direct tensile strength versus Young’s modulus.
There was a very strong linear relationship between the direct tensile strength and Young’s modulus for studied concretes during the first 10 h after mixing. The ratio of Young’s modulus (MPa) to direct tensile strength (kPa) appeared to be independent of concrete age and was estimated by linear best fitting to be a constant of 7.88. A similar observation was also reported in [10]. Such linear relationship between direct tensile strength and Young’s modulus of early-age concrete can be conveniently used to determine either property if the other property is
known. Further work is needed to confirm such relationship for other types of cement, mix designs and curing regimes. The prediction by JSCE:2007 [32] based on the square root of compressive strength at the respective age is in good agreement with results of this study; while that given by BS EN 1992-1-1 [31] is not. Whereas this correct prediction of the linear relationship between the two properties by JSCE:2007 is acknowledged, the need to improve its currently poor capability in predicting individual early-age concrete properties (as evidenced in Figs. 6 and 7) remains.
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3.5. Strain at peak stress The evolution of strain at peak stress over time recorded in this research and major past studies [3,8–10,16] is presented in Fig. 10, from which the following observations can be made: The strain at peak stress appeared to decrease over time within the test age range, and possibly reaching a minimum value at around 8–9 h of age. This corresponds well with the general trend reported in the literature [8,34–38], which suggests an initial sharp decrease of strain at peak stress from thousands of microstrain before reaching a stable value of between 100 le and 200 le. Compared to previous studies, the strain at peak stress in this study appears to be consistent with a clear evolution over time, which agrees well with Kasai et al. [8]. Test results from Dao et al. [10], on the other hand, suffer large variation (Fig. 10b) due to the difficulty in displacement recording as discussed in Section 3.3. With limited test results, Hannant et al. [18] and Roziere et al. [16] reported lower strain at peak stress, possibly due to different mix designs and/or cement types employed. The tensile strain capacity value of 70 le suggested in CIRIA C660 [3] for crack control of early-age concrete seems appropriate (Fig. 10b).
Fig. 11. Fracture energy development with age from mixing.
3.6. Fracture characteristics Using the obtained test data, fracture properties can be calculated based on a stress-crack opening curve following the fictitious crack model proposed by Hillerborg [39]. The obtained development over time of fracture energy GF is plotted in Fig. 11: The fracture energy GF was observed to increase with time at an increasing rate over the observed period of 10 h after mixing. There appears a good agreement between the time evolution of GF in this study and in [10], despite the difference in methods to determine deformation (Section 3.3). This agreement was possibly due to the governing influence of the tensile strength and post-peak deformation, which were similar in both studies. The effect of the variation in pre-peak deformation (Section 3.3) was negligible. The fracture energy GF is plotted against the corresponding tensile strength ft and Young’s modulus E in Fig. 12, together with their best-fit linear trendlines. It can be seen that the fracture energy increases with increasing values of tensile strength and Young’s modulus, similar to observations reported in [10]. How-
Fig. 12. Fracture energy versus tensile strength and Young’s modulus.
ever, their relationships are only weakly linear as evidenced by the relatively small R-square values of 0.82 and 0.9, compared to stronger linear relationships reported in [10].
3.7. Early-age concrete under cyclic loading The behaviour of concrete, both in its hardening and hardened states, is dependent upon its load history. While the concrete
Fig. 10. Strain at peak stress development with age from this and major past studies.
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performance under cyclic loading in its mature state is relatively well-researched [40], our understanding of such performance at very early ages is very limited. An improved knowledge of behaviour of hardening concrete under cyclic loading is therefore required to provide a better basis for proper assessment of earlyage cracking risk. Accordingly, using the improved direct tensile setup, four specimens were subjected to cyclic loading to assess its effect on tensile Young’s modulus of early-age concrete. For each load cycle, the test specimen was subjected to a constant displacement rate of 0.05 mm/min up to about 40% of the tensile strength at that age, estimated from tensile strength development over time of parallel series of specimens (Fig. 7). A typical stress-strain curve in cyclic loading is shown in Fig. 4b and a summary of the results is presented in Fig. 13. As evidenced in Fig. 13, when a single test specimen was subject to repeated tensile load cycles in this study, the Young’s modulus in subsequent load cycles were found to be higher than those in preceding cycles, which is in stark contrast to the typical decreasing trend reported for mature concrete [41,42]. Such difference is possibly due to the combined effects of the following:
Fig. 14. Testing scheme for concrete relaxation under constant deformation.
Table 5 Age, tensile strength and initial tensile stress of relaxation specimens. Age (h:min)
ft (kPa)
r0 (kPa)
r0/ft
5:05 6:40
39.8 122.2
17.2 52.4
0.43 0.43
Note: r0 – applied stress at the beginning of relaxation test. ft – direct tensile strength obtained from parallel specimens.
On the one hand, due to the ongoing hydration during this early stage, there were considerable increases in Young’s modulus over the time intervals between load cycles, as evidenced in Fig. 7. In contrast, for mature concrete, such ongoing hydration is deemed negligible. On the other hand, due to the microcracking and damage accumulated over the load cycles, Young’s modulus was compromised. The accumulated microcracking affects the properties of both early-age and mature concretes. 3.8. Relaxation behaviour of early-age concrete Adequate knowledge of creep and relaxation behaviour of earlyage concrete is required for the proper assessment of performance of early-age concrete, including its cracking risk [12]. To trial the capability of the current direct tensile setup for relaxation testing and subsequently to complement currently available data [12,43], in this part of the study, the variations over time of stresses in two test specimens subject to constant deformation were investigated (Fig. 14). The imposed deformations were chosen to ensure similar initial tensile stresses r0 in all specimens, which were approximately 43% of the corresponding tensile strengths at start of the relaxation tests (Table 5). (The ratios of stress to tensile strength would become increasing smaller during a relaxation test due to the increasing tensile strength over time as a result of ongoing hydration but remain within the typical range for this type of
Fig. 15. Tensile relaxation of very early-age concrete.
test [44,45].) Test duration for each specimen was approximately 60 min from the time t0 that r0 was reached. The obtained relaxation behaviour is plotted in Fig. 15. For both specimens, the stress in the concrete was found to initially decrease over time at increasingly diminishing rates, which is consistent with typical relaxation behaviour of concrete [44,45]. This suggests the appropriateness of this test setup for the study of relaxation behaviour of early-age concrete. The stress in specimen 2, however, increased slightly after about 10 min, due possibly mainly to the effect of ongoing hydration. Further work is ongoing to generate more data to form the basis for improved understanding of such relaxation behaviour of young concrete, taking due account of the effects of loading and other mechanisms. 4. Summary and conclusions
Fig. 13. Young’s modulus of early-age concrete under cyclic loading.
This paper first highlights the current dearth of reliable data on tensile properties of very early-age concrete, despite their significance in assessing cracking risk and residual stress as well as in precast industry. This is argued as a result of the considerable practical problems to be overcome when testing very early-age
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concrete due to its weak nature. Through identifying and effectively addressing critical drawbacks in previous test setups, an improved direct tensile testing system that can reliably capture simultaneously stress and deformation of concrete from the first several hours after mixing has been successfully developed and reported. Such capturing has been achieved for concrete from the age of 3 h in this study, but this starting age would be earlier or later depending on the mix design and curing. Two key features of this setup are: Air-bearing box to effectively eliminate the friction between the test specimen and supporting base. Digital Image Correlation (DIC) to capture the required deformation over the whole desired area of interest. The reliability of DIC capturing of very early-age concrete is clearly demonstrated through comparing with input displacement rate. The improved test setup is then used to generate data on tensile stress-strain curves to form the basis for improved knowledge of such important tensile properties of early-age concrete as tensile strength, Young’s modulus, fracture energy: The overall development trends over time are found to be in general agreement with limited available data from previous studies. All tensile strength, Young’s modulus and fracture energy are negligibly small during the first 4 h or so after mixing but increase quickly afterwards. The highly linear relationship between direct tensile strength and Young’s modulus is particularly noted. Importantly, the Young’s modulus obtained in this study is considerably less scattered than that in past research. This is due mainly to the more reliable capturing of deformation using DIC in this study. It is also found that the improved direct-tensile test setup can be conveniently used to study the behaviour of early-age concrete under direct-tensile cyclic loading and relaxation. Initial results suggest that: Different from its general decrease due to cyclic loading observed in mature concrete, Young’s modulus of early-age concrete specimen in subsequent loading cycles is found to increase but at a rate substantially lower than for the case of no accumulated microcracking. This is possibly due to the combined and counteracting effects of ongoing hydration of cementitious materials and accumulation of micro-damage as a result of cyclic loading. The tensile relaxation of early-age concrete is generally consistent with typical relaxation behaviour reported in literature. However, further data is needed to better understand such relaxation behaviour under the influence of loading, hydration and other mechanisms. With the reliability and capability of the improved direct tensile testing setup clearly established, further work is ongoing to provide a better basis for rational design of concrete structures taking due account of the effect of early-age performance characteristics. Acknowledgements Thanks are due to Renee Shi, Denton Liu, Qiule Qu, Guanwei Zhang, Yang Xu, and Technical staff within the School of Civil Engineering at the University of Queensland for their help with the experimental work.
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