Self-compacting concrete (SCC) is flowing in nature and hence viscosity modified agent (VMA) or higher amount of binder is used for stability of mix. The mix proportion of SCC is also different from normal vibrated concrete (NC). This modifies the properties of concrete at fresh as well as at hardened state. Three mixes, one VMA type SCC, one powder type SCC and one NC, are considered with identical water to binder ratio. Properties at fresh state such as workability and segregation resistance and at hardened stage such as compressive strength, elastic modulus, shrinkage, creep and elastic shortening are determined and compared. Powder type SCC shows superior properties at fresh stage. Also compressive strength of powder type SCC is found better than that of VMA SCC and NC. Observations show more shrinkage, elastic shortening and creep for powder type SCC as compared with VMA SCC and NC.
Self-compacting concrete (SCC) can be defined as a concrete which can completely consolidate under its own weight and does not require any external efforts like vibration, floating, poking etc. The mandatory requirements for this concrete are ability of passing, ability of filling and ability of being stable. For getting passing and filling ability of concrete, high range water reducer like superplasticizer is used which goes ahead toward hampering stability of mix. One way of achieving stability of mix is providing higher amount of binder material which is classified as powder type and other way is using chemical additives like viscosity modifying agent (VMA) which is classified as VMA type self-compacting concrete. To increase the quantity of binder material, it is added with filler material like fly ash, blast furnace slag, lime stone powder and silica fume [
The studies had been carried which give an idea about similarities and changes between various properties of self-compacting concrete and normally vibrated concrete. Bertil Persson et al. [
The investigation is undertaken to study properties of two types of self-com- pacting concrete: VMA type (VMA SCC) and powder type (SCC 550) and normally vibrated concrete (NC) at fresh and hardened state. The mixes with equal binding material content of 400 kg/m3 for NC and VMA SCC were prepared. One mix of powder type self-compacting concrete with 550 kg/m3 of binder content has been prepared. Water to binder ratio was kept constant for all mixes. The properties at fresh stage which includes workability, segregation resistance and at hardened stage which includes compressive strength, elastic modulus, shrinkage, creep and elastic shortening were found out and compared. Prediction of shrinkage, creep and elastic modulus is done with ACI 209 R-92 and BS EN 1992-1-1-2004 and compared.
At fresh state, the fundamental properties of self-compacting concrete are its filling ability, passing ability and segregation resistance. For workability measurement; slump flow test, V-Funnel test and L-box test were decided to carry out which signify filling ability and passing ability of mix. V-funnel five minutes test and visual inspection of slump flow have been used for confirmation of segregation resistance of SCC.
Mix proportion for both powder type and VMA type SCC is opted by using EFNARC guidelines [
Types | Cement | Fly Ash | Fine aggregates | Coarse aggregates | |
---|---|---|---|---|---|
10 mm | 20 mm | ||||
Specific Surface area | 325 m2/kg | 374 m2/kg | - | - | - |
Specific gravity | 3.15 | 2.3 | 2.79 | 2.9 | 2.9 |
Fineness Modulus | - | - | 2.93 | 6.09 | 8.11 |
Appearance | Reddish Coloured liquid |
---|---|
pH | 6 - 8 |
Specific Gravity @ 30˚C | 1.12 |
Chloride content | Nil |
Alkali content | Less than 1.5 g Na2O equivalent/litter |
VMA Type SCC | Normal Concrete | Powder Type SCC | ||
---|---|---|---|---|
Cement (kg) | 400 | 400 | 330 | |
Fly Ash (kg) | 0 | 0 | 220 | |
Natural Sand (kg) | 1114 | 688 | 1130 | |
Coarse Aggregates (kg) | 10 mm | 401 | 412 | 321 |
20 mm | 172 | 619 | 137 | |
Water/binder Ratio | 0.33 | 0.33 | 0.33 | |
Superplasticizer % | 1.60 | 0. 5 | 1.30 | |
VMA % | 0.20 | - | - |
At hardened state, compressive strength, modulus of elasticity, shrinkage, creep and elastic shortening of concrete were determined. To find compressive strength of concrete, cubes of size 150 mm has been prepared. The compressive strength has been taken at the age of 7, 28 and 56 days. To measure modulus of elasticity of concrete, cylinders of 150 mm diameter were used. For shrinkage measurement, prismatic specimens of cross section 100 × 100 mm were prepared. It is proposed to measure total shrinkage of concrete. All samples prepared for shrinkage measurement are stack vertically and change in length due to shrinkage of sample is measure with the dial gauge of accuracy 0.001 mm (
Prismatic samples of size 100 × 100 × 1200 mm were casted for pretention prestressing system. High tensile strand cable of 12.5 mm diameter having tensile strength of 1750 N/mm2 is used for prestressing work. The cable is placed at the centre of the sample intended for zero eccentricity. This is to ensure continues compressive stress on concrete section after release of prestressing cable. Strain gauge points are affix by adhesive along with lateral sides of the specimen at the position of prestressing reinforcement (
The results of workability for both SCC are given in
(a) Compressive strength
Compressive strength of all concrete mixes is shown in
Test | VMA Type SCC | Powder Type SCC | Permissible Limits |
---|---|---|---|
Slump Flow diameter (mm) | 690 | 680 | 650 - 800 |
V-Funnel Time T0 (Sec) | 11.2 | 8.2 | 6 - 12 |
V-Funnel at T5 minute (Sec) | 13.8 | 10.9 | ≤T0 + 3 |
L-Box T20 cm (Sec) | 1.2 | 0.89 | 1 ± 0.5 |
L-Box T40 cm (Sec) | 2.3 | 2.34 | 2 ± 0.5 |
L-Box Blocking Ratio (H2/H1) | 0.8 | 0.86 | 0.8 - 1.0 |
while it is considerable for SCC 550. This is due to presence of fly ash in SCC 550 mix which has hydration properties but reacts slowly and shows its effect late [
(b) Elastic modulus of concrete
(c) Shrinkage, creep and elastic shortening
Experimental values obtain for shrinkage strain (εs) of all mixes along with time are shown in
Type of Concrete | Value of Multiplying factor for |
---|---|
VMA SCC | 4604 |
NC | 4844 |
SSC550 | 4503 |
matching with the predicted values by ACI 209 R-92 whereas the predicted value of shrinkage by BS EN 1992-1-1-2004 is observed less. The predicted values by ACI 209 R-92 are 5% more than measured values while predicted values by BS EN 1992-1-1-2004 are 10% less than measured values for NC. For VMASCC, predicted values by ACI 209 R-92 are closely matching with measured values while BS EN 1992-1-1-2004 underestimates the values by 14%. In powder type SCC, the estimated values for shrinkage are less than measured values. While ACI 209 R-92 predicted values are 17% less than measured values, for BS EN 1992-1-1-2004, difference is 54% for SCC550 concrete (
When prestressing force is transfer to concrete, it undergoes decrease in length because of its elastic properties. Before releasing tendon, again jack was connected to prestressing cable behind anchors. The load was then gradually released on concrete. Strain reading was taken just before and after releasing the tendon. The difference of these reading is noted as elastic shortening (εe). The results are shown in
After releasing tendon, total strain (εt) is measured with interval of a seven days for first four week and then every twenty eight days till 112 days. The creep strain (εc) is calculated as per Equation (1).
εc = εt ? εs ? εe (1)
By observation of
Mix | Ratio of binding material to concrete | Ratio of coarse aggregates to concrete |
---|---|---|
VMA SCC | 0.18 | 0.254 |
NC | 018 | 0.452 |
SSC550 | 0.233 | 0.234 |
and less coarse aggregates in SCC 550 leads to more shrinkage, creep and elastic shortening.
Three mixes, one normal vibrated concrete, one VMA type Self compacting concrete and one powder type self-compacting concrete, were prepared with identical water/binder ratio. Properties at fresh state such as workability and at hardened stage such as compressive strength, modulus of elasticity, shrinkage, creep and elastic shortening were determined and compared.
・ At fresh state, the SCC 550 is found superior as it is more cohesive and stable.
・ Compressive strength of SCC 550 is more than that of VMA SCC and NC at 28 days. The rise in strength at 56 days is also more for SCC 550.
・ Predictions for elastic modulus of concrete by code ACI 318-89 are comparable with experimental values obtained. Values of constant for multiplication with
・ Shrinkage of NC and VMA SCC is observed identical and the shrinkage of SCC 550 is seen more. Observed values for NC and VMASCC are matching with predictions by ACI 209 R-92 whereas shrinkage for powder type SCC observed is more than prediction.
・ Elastic shortening is observed more in SCC 550 in comparison to VMA SCC and NC.
・ Creep of NC and VMA SCC is identical with a little higher value for VMA SCC while creep for SCC 550 is observed more. Observed creep values are closely matching with ACI 209 R-92 predictions for SCC 550.
・ Powder type self-compacting concrete is superior at fresh state and also has more compressive strength but has more shrinkage, elastic shortening and creep as compared with normal vibrated concrete and VMA type self-com- pacting concrete.
Bhirud, Y.L. and Sangle, K.K. (2017) Comparison of Shrinkage, Creep and Elastic Shortening of VMA and Powder Type Self-Compacting Concrete and Normal Vibrated Concrete. Open Jour- nal of Civil Engineering, 7, 130-140. https://doi.org/10.4236/ojce.2017.71008