1 Introduction
2 Experimental Program
2.1 Materials and Mixture Proportions
Oxide composition % by mass | Cement | Silica fume |
---|---|---|
CaO | 62.25 | – |
SiO2
| 21.22 | 93.16 |
Al2O3
| 4.68 | 1.13 |
Fe2O3
| 3.68 | 0.72 |
MgO | 3.63 | 1.6 |
Na2O | 0.25 | – |
K2O | 0.75 | – |
SO3
| 1.74 | 0.05 |
L.O.I. | 1.37 | 1.58 |
code | Cement (kg/m3) | Silica fume (kg/m3) | w/cm | Water (kg/m3) | Coarse aggregate (kg/m3) | Fine aggregate (kg/m3) | Plasticizer (%) |
---|---|---|---|---|---|---|---|
NPC | 400 | 0 | 0.5 | 200 | 956 | 778 | 1.2 |
SFC | 370 | 30 | 0.5 | 200 | 959 | 784 | 1.2 |
2.2 Casting and Curing of Concrete Specimens
Code | Density (kg/m3) | Air content (%) | Slump (mm) | Curing | Compressive strength (MPa) | |
---|---|---|---|---|---|---|
7 days | 28 days | |||||
NPC | 2370 | 2.7 | 80 | 0-D | 26.0 | 31.3 |
1-D | 30.0 | 36.6 | ||||
3-D | 30.8 | 37.9 | ||||
6-D | 33.5 | 39.0 | ||||
27-D | 33.5 | 39.8 | ||||
SFC | 2355 | 1.6 | 65 | 0-D | 26.0 | 37.1 |
1-D | 29.9 | 45.3 | ||||
3-D | 32.9 | 51.4 | ||||
6-D | 34.4 | 53.6 | ||||
27-D | 34.4 | 55.2 |
2.3 Exposure Condition
Ion type | K+
| Ca++
| Mg++
| SO4
−−
| Na+
| Cl−
| Total salt |
---|---|---|---|---|---|---|---|
Concentration (ppm) | 470 | 480 | 1600 | 3300 | 12600 | 23,400 | 41,850 |
2.4 Sampling and Testing
2.5 Chloride Diffusion Coefficient (D c ) and Surface Chloride Content (C s ) Calculation
3 Results and Discussion
3.1 Chloride Profiles at Varying Exposure Time
3.2 Chloride Diffusion Coefficient (D c )
Curing | NPC | SFC | ||||
---|---|---|---|---|---|---|
a
|
b
|
R
2
|
a
|
b
|
R
2
| |
27-D | 2.2991 | −0.760 | 0.95 | 0.0008 | −0.357 | 0.83 |
6-D | 0.0165 | −0.432 | 0.99 | 0.0005 | −0.296 | 0.78 |
3-D | 0.1863 | −0.568 | 0.94 | 0.0004 | −0.286 | 0.89 |
1-D | 3.5321 | −0.728 | 0.93 | 0.0026 | −0.378 | 0.87 |
0-D | 13.688 | −0.797 | 0.90 | 0.3849 | −0.659 | 0.91 |
3.3 Relationship Between Diffusion Coefficient and Curing Time
Curing condition |
k
c,cl
* (NPC) |
k
c,cl
(SFC) | |
---|---|---|---|
1 day wet curing | 1.77 | 1.54 | 2.08 |
3 days wet curing | 1.26 | 1.10 | 1.50 |
7 days wet curing**
| 1.00 | 1.00 | 1.00 |
28 days wet curing**
| 0.64 | 0.68 | 0.79 |
3.4 The Influence of the Curing Time on Surface Chloride Content (C s )
Sample | Curing |
k
|
R
2
|
---|---|---|---|
NPC | 27-D | 7.00E−09 | 0.95 |
6-D | 4.00E−09 | 0.79 | |
3-D | 4.00E−09 | 0.83 | |
1-D | 4.00E−09 | 0.95 | |
0-D | 4.00E−09 | 0.99 | |
SFC | 27-D | 4.00E−09 | 0.77 |
6-D | 1.00E−09 | 0.95 | |
3-D | 2.00E−09 | 0.91 | |
1-D | 2.00E−09 | 0.76 | |
0-D | 6.00E−09 | 0.98 |
3.5 The Influence of the Wet Curing Time on Time-to-Corrosion Initiation of Concrete Structures
4 Conclusion
-
A wet curing extension decreases difference between initial and long-term diffusion coefficients due to improvement of concrete cover quality and blocking the ingress of aggressive substance in initial ages. This reduction in early age diffusion preserves concrete against high rate of chloride penetration at early ages.
-
As the length of exposure period to marine environment increased the effects of initial wet curing became less pronounced. This might be due to the curing effects of the seawater which compensate for the differences observed in early age diffusion coefficient due to the duration of initial wet curing. In long-term ages, a 27 days wet curing is the only curing regime which preserves its efficiency in reducing diffusion coefficient in both of NPC and SFC mixtures.
-
A power functional relationship is derived between curing factor (k curing = D t /D 0, where D 0 is the diffusion coefficient of no-cured concrete, D t is the diffusion coefficient of wet cured specimen) and time of wet curing (t curing ) at early ages.
-
The general trend of surface chloride shows its increment as the time goes on. The SFC specimens have more surface chloride content in comparison with NPC specimens in early ages presumably due to the higher level of chloride binding and sorptivity. But as the time goes, the rate of surface chloride content increment is lower in SFC specimens in contrast to NPC specimens.
-
Both plain and silica fume specimens show that 27 days wet curing causes tangible increase in time-to-corrosion initiation and service life of concrete structures. It seems that no difference is observable between the time-to-corrosion initiation values of the curing times less than 6 days in both silica fume and plain specimens based on time dependent results from field. It might happen because of the availability of continuous capillary pores in concrete specimens (w/c of 0.50) with less amount of wet curing.