 Article
 Open Access
Performance of High Strength SelfCompacting Concrete Beams under Different Modes of Failure
 Raya Hassan Harkouss^{1}Email author and
 Bilal Salim Hamad^{1}
https://doi.org/10.1007/s400690140088x
© The Author(s) 2014
 Received: 7 March 2014
 Accepted: 18 August 2014
 Published: 1 October 2014
Abstract
Selfconsolidating concrete (SCC) is a stable and cohesive high consistency concrete mix with enhanced filling ability properties that reduce the need for mechanical compaction. Limited standards and specifications have been reported in the literature on the structural behavior of reinforced selfcompacting concrete elements. The significance of the research presented in this paper stems from the need to investigate the effect of enhanced fluidity of SCC on the structural behavior of high strength selfconsolidating reinforced concrete beams. To meet the objectives of this research, twelve reinforced concrete beams were prepared with two different generations of superplasticizers and designed to exhibit flexure, shear, or bond splitting failure. The compared beams were identical except for the type of superplasticizer being used (second generation sulphonatedbased superplasticizer or third generation polycarboxylatebased superplasticizer). The outcomes of the experimental work revealed comparable resistance of beam specimens made with selfcompacting (SCC) and conventional vibrated concrete (VC). The dissimilarities in the experimental values between the SCC and the control VC beams were not major, leading to the conclusion that the high flowability of SCC has little effect on the flexural, shear and bond strengths of concrete members.
Keywords
 construction materials
 concrete admixtures
 selfconsolidating concrete
 highstrength concrete
 reinforced concrete beams
 structural behavior
1 Introduction
Selfconsolidating concrete (SCC) is distinguished by its high fluidity, passing ability and cohesiveness characteristics that eliminate or reduce to a minimum the need for mechanical compaction. Reducing the intervention of the human factor in the concreting stage improves the quality of the project under construction.
The advantages associated with SCC have led to the adoption of this relatively advanced technology in many contemporary projects, even before the release of specifications, testing techniques and standards that reflect the behavior of structural elements cast using high consistency concrete.
The research reported in this paper is concerned with the effect of enhanced fluidity of SCC on the structural behavior of reinforced concrete beam elements designed to exhibit different critical modes of failure. The hypothesis to be tested is whether the high consistency of SCC will negatively affect the shear strength of reinforced SCC members and the bond strength of spliced bars in such members.
Accordingly, a threephase research program was conducted to study the effect of two types of superplasticizers on the mechanical performance of plain and reinforced concrete elements. Sulphonated naphtalene formaldehydebased (SNF) admixture was chosen to represent the conventional type of second generation superplasticizer commonly used by the concrete industry in the production of high strength workable concrete. On the other hand, polycarboxylate ethersbased superplasticizer (PCE), a high range water reducing admixture, was the third generation superplasticizer incorporated in the development of the SCC mixes in this research.
The difference in the dispersion mechanisms of the second and third generation superplasticizers is expected to reflect on the mechanical properties of concrete, a point that was elaborated in Phases 1 and 2 of the AUB research program. To limit the number of variables merely to the type of admixture used in the concrete mix, the experiments of the first two phases of the research aimed at establishing an optimum mix design with a common dosage of second or third generation superplasticizer that would ensure the minimum workability characteristics for vibrated concrete (VC) and the high consistency properties for SCC. In the first phase, comparative studies of high strength mortar mixes prepared with second generation (SNF) or third generation (PCE) superplasticizer were conducted. In the second phase, the comparative studies were carried on concrete mixes rather than mortar mixes. The two studies unveiled that a dosage of 1.6 % of second generation or third generation superplasticizer is satisfactory.
The research reported in this paper constitutes the third phase of the experimental program. It investigates and compares the structural behavior of reinforced concrete beams cast using the optimal high strength SCC and VC mixes established in the first two phases of the research with a second generation (SNF) or third generation (PCE) superplasticizer content of 1.6 % of the total weight of cement.
2 Literature Review
Few studies were found dealing with high strength SCC beams produced using PCE based admixtures. A common procedure was followed in the majority of these studies where beams prepared with SCC, frequently comprising fly ash or silica fume powders, were compared with control beams cast using VC mixes made with different constituents and mix proportions. The overlapping effect of the numerous variables engaged in those studies often resulted in losing the track on the effect of each variable on the behavior of the reinforced concrete specimens.
The majority of the research reported in the literature review agreed on the equivalence of the bond strength between normal concrete and SCC (Domone 2006). Desnerck et al. (2010). studied the bond characteristics of different bar diameters in beam specimens cast using selfcompacting concrete SCC and conventional vibrated concrete VC having an ${f}_{c}^{{}^{\prime}}$ of approximately 60 MPa. The concrete mixes were designed differently where SCC mixes involved PCE superplasticizers and limestone fillers, two additional constituents that were excluded from the conventional VC mix design. The aggregate distribution of VC and SCC mixes was also different. The outcomes of the research study concluded on the similarity of the bond strength between VC and SCC beams for large bar diameters whereas the bond strengths for SCC appeared to be superior in beams with small bar diameters.
Turk et al. (2008) also inspected the bond strength of tension lap splices in SCC beams. Beam specimens with 16 and 20 mm bars were used to compare the behavior of SCC and VC elements having a compressive strengths ranging from 41.5 to 44 MPa. The stability of SCC mixes was maintained using silica fume. The selfcompactness of concrete was attained using PCE superplasticizer whereas sulphonated melaminebased superplasticizer was used for the normal concrete mix. Different concrete mix proportions were adopted. The study led to a conclusion that the enhanced filling ability of SCC results in higher bond strengths.
ForoughiAsl et al. (2008) reported on pullout tests designed to study the effect of SCC on bond strengths. Different bar diameters were tested. The mix designs of the SCC and the companion normal concrete NC specimens were the same except for the addition of the silica fume and PCE superplasticizers in the SCC specimens. The experimental data gathered revealed slightly higher bond strengths for the SCC specimens.
This similarity in the behavior of SCC and normal concrete specimens was not reflected in the research papers studying the shear resistance of reinforced concrete beam elements. The shear capacity of normal VC appeared to overcome that of SCC. Veerle Boel (2010) tested the shear capacity of beam specimen made with SCC and VC. The SCC mix proportions were marked by the high limestone filler content and the low river gravel volumes. The SCC specimen contained 43 % lower aggregate content. Boel associated the lower shear capacity of the SCC beams to the lower aggregate interlock caused by the fewer coarse aggregates.
Hassan et al. (2008) also conducted an experimental investigation on the shear strength of SCC beams. The concrete mixes were designed differently where SCC contained 25 % coarse aggregate content lower than NC. The difference in volume dedicated for coarse aggregate was compensated by an addition in the sand content of the SCC mixes. The experimental results indicated a similarity in the overall failure mode in terms of the cracking pattern, crack width and height in SCC and NC beams. The ultimate shear capacity of SCC beams appeared to be lower than their NC counterparts. According to the researchers, the lower shear strength could be attributed to the decrease in coarse aggregate content that used to provide additional resistance to shear through aggregate interlock mechanisms.
Sharifi (2012) studied the flexural behavior of SCC beams having an average concrete compressive strength of 30 MPa. SCC mixes included micro silica and limestone powder to control the mix stability. According to the researcher, the theoretical calculations regularly followed to find the moment capacity of reinforced concrete beams are conservative and reliable in the estimation of SCC beam capacities.
The involvement of different types of fillers in the concrete mix design, as mentioned in the previous reported research, has an impact on the hydration of cement and consequently on the concrete microstructure and the hardened concrete properties. In addition, the use of different mix proportions and the variation in the type of coarse aggregates (river gravel or crushed limestone) will also affect the properties of concrete and its behavior in handling the tensile stresses at the microstructural level. Accordingly, conclusions related to the effect of enhanced consistency of SCC mixes drawn from the comparison studies between SCC and conventional vibrated concrete would be more reliable if identical mix constituents are used to avoid any factors that might affect the structural behavior.
3 Research Objectives
The main objective of the research program reported in this paper was to study the structural behavior of high strength SCC beams cast using third generation PCE and designed to fail in flexure, shear, or bond splitting. Accordingly, the behavior of SCC beams and control VC beams was compared. The two types of beams had identical geometrical, structural and concrete mix designs but were made with different types of superplasticizers. This methodology distinguishes the current research from previously conducted research studies found in the literature review, and makes it significant. The objective of the study stems from the need to test the hypothesis that the high fluidity of SCC could adversely affect the shear strength of SCC members and the bond strength of bars anchored in fullscale structural members. The hypothesis is partially supported by the reported shear studies in the literature and the fact that the previous studies, bond and shear, included different constituents between the SCC and the normal concrete mixes.
4 Materials and Methods
4.1 Variables and Specimen Design
Variables of the test program.
Beam type  Beam notation  Concrete mix  Mode of failure 

Flexural beams  SCCFB1  SCC  Flexure 
SCCFB2  SCC  Flexure  
VCFB1  VC  Flexure  
VCFB2  VC  Flexure  
Shear beams  SCCSHB1  SCC  Shear 
SCCSHB2  SCC  Shear  
VCSHB1  VC  Shear  
VCSHB2  VC  Shear  
Bond beams  SCCBB1  SCC  Bond splitting 
SCCBB2  SCC  Bond splitting  
VCBB1  VC  Bond splitting  
VCBB2  VC  Bond splitting 
Accordingly, the beams are identified by a three part notation system. The first term indicates the type of concrete mix used in the casting of the beam (SCC or VC). The second term specifies the preset mode of failure (F for flexure, SH for shear, and B for bond splitting). The third term designates the listing number of the two replicates (B1 or B2).
The beam specimen was 2,000 mm long with a distance of 1,800 mm between supports. The width of the beam was 200 mm and the depth was 300 mm.
4.2 Constituent Materials
4.2.1 Concrete
The twelve beams were cast at a readymix plant. With the exception of the type of superplasticizer, the mix proportions were identical and designed to produce a nominal concrete compressive strength of 60 MPa.
The consistency of the VC mix was adjusted using a second generation (SNF) superplasticizer while selfconsolidation characteristics of SCC were provided by a third generation (PCE) superplasticizer.
Concrete mix proportions.
Constituent materials  Mix proportioning 

Cement (kg/m^{3})  575 
Natural sand 0–1.18 mm (kg/m^{3})  453 
Crushed sand 0–4 mm (kg/m^{3})  371 
Coarse aggregates 4–10 mm (kg/m^{3})  807 
Water (kg/m^{3})  194 
Bulk dosage of SNF or PCE superplasticizer by weight of cement (%)  1.6 % 
SNF or PCE based superplasticizer (kg/m^{3})  9.2 
Fresh concrete properties.
Concrete mix type  % Bulk SP  Slump (mm)  Spread flow test (mm) 

VC  1.60  210  – 
SCC  1.60  –  790 
Standard 150 × 300 mm cylinders taken from the SCC and VC concrete batches produced at the readymix plant, were cast and tested to determine the concrete compressive strength ${f}_{c}^{{}^{\prime}}$, the tensile strength f_{ t }, and the modulus of elasticity E_{ c }.
Average hardened concrete properties.
Strength (MPa)  SCC  Theoretical  VC  Theoretical 

${f}_{c}^{{}^{\prime}}$  62.4  62.4  57.9  57.9 
E _{c}  35,133  35,624  33,103  34,746 
f _{t}  4.3  4.7  3.8  4.5 
f _{r}  6.0  4.9–7.8  5.1  4.7–7.5 
f_{t}/f_{r}  72 %  –  75 %  – 
4.2.2 Steel Reinforcement
Yield and ultimate strengths of longitudinal and transverse reinforcing bars.
Rebar size  f_{ y } (MPa)  f_{ u } (MPa)  E_{ s } (MPa)  

Bottom reinforcement  2 Φ 20  632.0  743.0  200,000.0 
Top reinforcement  2 Φ 12  557.0  667.0  290,000.0 
Shear reinforcement  2 Φ 8  569.0  661.0  220,000.0 
The shear reinforcement consisting of closed hoop stirrups were also dimensioned taking into account a design concrete cover of 30 mm on the 4 sides. With reference to ACI 31811 (2011), a minimum inside bend diameter of 4 d_{b} was adopted for the stirrups with a minimum extension length of 50 mm at the free end of the bar, equivalent to 6 d_{b}.
In order to monitor the strain in the steel bars during testing, each beam had one strain gage sealed to each of the two bottom reinforcing bars. In the flexural and shear beams, the strain gages were located at the middle of the bars, whereas in the bond beams the strain gages were placed at the end of the splice length.
4.2.3 Admixtures
4.2.3.1 Sulphonated Naphtalene FormaldehydeBased Superplasticizer (Dransfield 2003)
4.2.3.2 Polycarboxylate Ethers (PCE) Based Superplasticizer: Third Generation SP (Dransfield 2003)
4.3 Testing Procedure
5 Analysis of Test Results
Ultimate loads and maximum deflections at failure.
Beam type  Beam notation  P_{ max } (kN)  Δ_{ max } (mm) 

Flexural beams  SCCFB1  158.4  15.5 
SCCFB2  149.6  19.4  
VCFB1  154.8  13.5  
VCFB2  157.2  30.2  
Shear beams  SCCSHB1  132.3  14.9 
SCCSHB2  107.4  8.2  
VCSHB1  111.5  7.8  
VCSHB2  127.9  11.2  
Bond beams  SCCBB1  93.1  6.6 
SCCBB2  87.2  5.3  
VCBB1  87.7  9.9  
VCBB2  90.5  8.0 
5.1 Hardened Concrete Properties
The following sections present the equations for the modulus of elasticity, the splitting tensile strength and the modulus of rupture that were found to best represent the experimental outcomes.
5.1.1 Modulus of Elasticity
5.1.2 Splitting Tensile Strength
The findings of Dewar (1964), stating that the splitting tensile strength can reach 70 % of the flexural strength at 28 days, were reflected in this research where the reported tensile strengths attained 70–75 % of the flexural strength respectively in SCC and VC mixes.
5.1.3 Modulus of Rupture
5.2 Bond Strength
The proper performance of reinforced concrete members in flexure or direct tension comes as a result of adequate force transfer between reinforcing bars and concrete. In their studies on the behavior of fullsize reinforced concrete elements in bond splitting modes of failure, researchers opted to use splice specimens that appeared to be effective in providing realistic experimental data. Accordingly, four beam specimens were cast to test and compare the bond strength of steel reinforcement in SCC beams.
Test results of bond beams.
Specimen notation  SCCBB1  SCCBB2  VCBB1  VCBB2 

P at bond splitting failure (kN)  93  87  88  91 
Number of cracks  14  14  15  15 
Max. flexural crack width at splice end (mm)  0.20  0.20  0.20  0.20 
The test results of the bond beams indicate that the high fluidity characteristics of SCC mixes have no impact on the bond characteristics of reinforcing bars, a statement that refutes the hypothesis presented earlier in this paper.
5.3 Shear Strength
The shear strength of SCC is designated by the ultimate load that triggered the appearance of the first diagonal crack and was evaluated through the close monitoring of the development of this crack. Since the concrete shear strength in beams is dependent on the tensile characteristics of concrete and is independent of the area of transverse reinforcement allocated for a concrete section, the concrete shear strength results of the flexure and shear beams were considered. In the analysis of the test results, the experimental values were compared to the theoretical estimation of the concrete strength in shear. The maximum shear capacity carried by concrete was computed using Eqs. (115) of Sect. 11.2.2.1 of the ACI Building Code ACI 31811 (2011). The ACI equation for the shear strength considers the effect of the longitudinal reinforcement and the applied moment on the shear resistance of reinforced concrete beams.
Test results of shear beams.
Specimen notation  SCCSHB1  SCC SHB2  VCSHB1  VC SHB2 

P at first diagonal crack (kN)  50  50  40  45 
P at ultimate shear failure (kN)  132  107  112  128 
Theoretical ultimate shear (kN)  93  93  91  91 
Number of cracks  10  12  10  13 
Max. shear crack width (mm)  1.50  1.50  1.50  1.50 
The experimental results revealed an average concrete shear capacity of 50 kN in SCC beams and 42.5 kN in VC beams compared to a theoretical concrete shear load of 55.5 kN for SCC beams and 53.8 kN for VC beams. A reduction factor ϕ of 0.75 was used in the computation of all the theoretical concrete shear capacities.
Considering identical mix designs for SCC and VC, SCC demonstrated better shear resistance than its VC counterpart. Consequently, the high consistency of selfconsolidating concrete has little effect on the concrete capacity in shear.
On the other hand, both SCC and VC beams have shown a concrete capacity that is lower than the nominal capacity computed using Eq. (115) of ACI 31811 (2011). The ACI report on high strength concrete (ACI 363R 2010) states that with increasing concrete compressive strength, the actual concrete contribution in the shear resistance reveals lower values than the ones predicted through the more complex ACI equation for V_{ c } due to the reduction in the aggregate interlock for HSC.
In the case where the concrete shear resistance was computed using the simplified Eq. (113) of the ACI 31811 (2011), the theoretical concrete shear capacity of SCC becomes 50 kN compared to 48.2 kN for VC beams. The simplified equation has proven to be the best fit equation for the estimation of the SCC shear capacity and can be considered as reliable in the simulation of the behavior of SCC beams in shear. The adoption of Eq. (113) appears to conservatively cover the effect of the reduction in the aggregate interlock in SCC beams through providing theoretical estimations that are equal to the experimental values. In contrast, this same equation is not representative of the behavior of VC beams.
The results of the shear failure of SCC and VC beams revealed an equal average ultimate shear capacity of approximately 120 kN in both beam types. The theoretical ultimate shear capacity was found to be equivalent to 93 kN in SCC beams and 91 kN in VC beams.
Crack patterns were very similar for replicate identical shear beams and were very similar for the VC and SCC beams. The diagonal crack widths constant for identical shear and flexural beams indicate the consistency of the test results.
These outcomes refute the hypothesis statement declaring that the high consistency of SCC will negatively affect the shear strength of SCC members. Also, the effect of the aggregate interlock on the concrete shear capacity was found to be more pronounced in VC beams than its is in SCC beams. This difference can be associated to the enhanced hydration of cement and the improved cohesiveness of the concrete mix.
5.4 Flexural Strength
Test results of flexure beams.
Specimen notation  SCCFB1  SCCFB2  VCFB1  VCFB2 

P at first diagonal crack (kN)  50  50  45  40 
P at yielding (kN)  92  107  84  99 
Δ_{y} (mm)  5.41  6.22  4.54  4.94 
P at failure (kN)  158  150  155  157 
Theoretical ultimate load (kN)  120  120  118  118 
Δ_{ u } (mm)  15.5  19.4  13.5  19.8 
1/μ (%)  34.9  32.1  33.6  24.9 
Number of cracks  14  15  15  14 
Max. flexural crack width (mm)  0.6  0.6  0.6  0.6 
Max. flexural crack height (cm)  23.3  23.1  22.4  20.6 
Theoretical flexural crack height (cm)  21.8  21.7  21.6  21.6 
Max. shear crack width (mm)  1.25  1.25  1.25  1.25 
The number of cracks and the crack width measurements disclosed similar values for the VC and SCC beams.
The loads at yielding and at ultimate for the four flexural beams are listed in Table 9.
The average yielding load for the two replicate beams was similar for SCC (P_{ y } = 99.5 kN) and VC (P_{ y } = 91.5 kN).
It was noticeable that the flexural crack height was greater in the SCC and VC beams than it was predicted using cracked section analysis. According to the ACI report on high strength concrete (ACI Committee 363R 2010), this behavior can be foreseen in HSC beams where shallower compression zones are required to maintain equilibrium in flexure.
In reference to Eqs. (7)–(12) the ductility index was taken as the ratio of the deflection at failure to the deflection at the load producing reinforcement yielding.
Beyond the yield load, the flexural beams exhibited a shear mode of failure with an average maximum load in SCC beams of 154 and 156 kN in VC beams. These results confirm the findings of the shear beam experimentation analysis.
6 Conclusions
Twelve beam specimens were cast using either SCC or VC mixes. The beams were tested in flexure to investigate their structural behavior in three modes of failure: flexure, shear or bond splitting.
The concrete mixes were performed at a readymix plant with a bulk dosage of 1.6 % of second generation (SNF) superplasticizer and third generation (PCE) superplasticizer used respectively for the VC and SCC beams.
Using the MTS machine, the reinforced concrete beams were subjected to two concentrated loads located at onethird and twothird of the beam span length creating a constant moment region in the middle. The beam deflection, cracking and the tension reinforcement straining were closely monitored.

Maximum crack widths were reported for the vertical cracks at midspan, at splice ends and under the two concentrated applied loads. As for the diagonal cracks, the maximum crack widths were measured at the supports in the flexure, shear and bond beams. The experimentations on flexure, shear, and bond beams cast using SCC and VC revealed similar cracking patterns and demonstrated consistent beam responses to load increments.

The average splitting load failure of the bond beams appeared to be the same in SCC and VC beams which indicates that the bond between steel and concrete is not affected by the high flowability of SCC mixes.

Although several studies confirm that the shear capacity of SCC is lower than that of VC, the results of this study have demonstrated that the high consistency of SCC has no adverse effect on the shear strength of concrete. A comparison between the experimental and theoretical concrete capacities has shown an insignificant difference between the predicted and the actual shear resistances of SCC.

The SCC and VC flexure beams exhibited similar behavior under identical loading conditions, leading to the conclusion that the high fluidity of concrete has little impact on the flexural strength of reinforced concrete beams.
Declarations
Acknowledgments
The authors gratefully acknowledge the University Research Board at the American University of Beirut for supporting this program. Also, the assistance of Mr. Helmi ElKhatib, Supervisor of the testing laboratories at AUB, is appreciated.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
Authors’ Affiliations
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