organic content

1. Pavement Research Group 18 SEAGC - Singapore - 2013
Effect of organic content on carbonation rate of
cement stabilised soils

2. Carbonation:
2
Carbonation of cement mixture is the reaction of the calcium hydroxide
Ca(OH)2 and calcium silicate hydrate CSH with carbon dioxide CO2
Ca(OH)2+CO2 CaCO3+H2O (1)
(3CaO.2SiO2.3H2O)+3CO2 (3CaCO3.2SiO2.3H2O) (2)
•
Pavement typical section
CaO
Ca(OH)23CaCo3
H2O
CO2
Energy
C02 H20
Chemical reaction loop

3. Signs of Carbonation:
3
• Surface weakening and scabbing ,bleeding of surfacing .
• Loss of cementation of layer from bottom upward then rutting and
shearing
• Return of soil plasticity
• Expansion cracking and decreasing density
• Fine powdery layer between wearing course and base layer
ScabbingSurface weakening and cracking

4. Effect of Carbonation:
4
• Loss of cement products such as CSH , Ca(OH)2
• Decrease of PH from about 12.4 to 8.3
• Decrease of UCS or CBR
• Reduction of compaction degree
• Shrinkage of CSH
• Micro cracking
• Electrical conductivity reduction of paste

5. Carbonation Control
5
 Avoiding delays between completion of cemented soil layers
Immediate compaction of cemented soil layers
Using cement about 0.5-1% more than Initial Cement Consumption
(PH>12.4)
 Curing of cemented layer by placing one moist layer of soil
Using more non plastic fillers

6. Carbonation Case histories
6
 44 Known cases in Southern Africa
State Highway 36 and 59 in Houston (USA)
Reid Hwy ,... (Western Australia)
Hot and arid climate location

7. Tests of Carbonation:
7
Phenolphthalein (0.5%)
PH<8.4 (colourless)....(pink)....PH>11(Red)
Phenol Red (0.4%)
PH<6.8 (yellow).......PH>8.4(Red)
Dilute HCL (5N)
CaCO3 effervescence
Phenolphthalein solution spraying
BS EN 14630:2006

8. Tests of Carbonation:
8
Hand Brushing Tests:
ASTM D 559 – 96 or AASHTO T 135-97
Standard Test Methods for Wetting and Drying Compacted Soil-Cement Mixtures:
12 cycle of
-Submerge in potable water for 5h
-Drying in oven 71o c for 42 h
-Stroking sample2 and weighting
18 to 20 vertical brush stroke
on sides
2 To 4 vertical brush stroke
on ends
Hand brush method

9. Tests of Carbonation:
9
Residual UCS test
S. A. Shihata and Z. A. Baghdadi(2001)
Like Standard Test Methods for Wetting and Drying Compacted Soil-Cement Mixtures
12 cycle of
-Submerge in potable water for 5h
-Drying in oven 71o c for 42 h
- UCS test

10. Main findings in soil cement
10
Netterberg et al (1984)
Carbonation rate about 3-50 mm/year.
Curing method :bitumen primer, spray grade emulsion, plastic sheets or
moist sand as and early surfacing at most two or three weeks after
compaction.
Bagonza et al (1987)
Clayey sand (a poor quality calcrete) from Botswana
Carbonation rate of water spraying method about 6 mm just after curing ,
25 mm after 25 days exposure in air.
Paige-Green et al (1990)
Studied about 44 case of carbonation through 119 cases in South Africa
and provided complete picture of identifying, testing and curing methods.
carbonation rate can be in the range of 0.5 - 2 mm/day in exposed
conditions and 2 - 50 mm/year in covered conditions

11. Carbonation in concrete
11
When the depth of concrete’s carbonation reaches the reinforcement,
passivity is lost and metal’s corrosion can proceed easily in the presence of
moisture and oxygen
𝑥𝑥 =
𝑤𝑤0 − 0.3
0.3(1 + 3𝑤𝑤0)
𝑡𝑡 Uchida et al (1928)

12. Carbonation rate
12
Where [CO2] = molar concentration of carbon dioxide (mol/m3)
[Ca (OH)2] = molar concentration of calcium hydroxide (mol/m3)
[CSH] = molar concentration of calcium silicate hydrate (mol/m3)
De.CO2 = effective diffusivity of carbon dioxide (m2/s)
Papadakis et al. (1990)

13. Diffusivity of CO2
13
𝐷𝐷𝐷𝐷. 𝐶𝐶𝐶𝐶𝐶 = (1.64 ∗ 10−6)ԑ𝑝𝑝
1.8[1 − 𝑅𝑅𝑅𝑅/100] 2.2
[Ca(OH)2]+3[CSH]=
33000
1+
𝑤𝑤
𝑐𝑐
𝜌𝜌𝑐𝑐
𝜌𝜌𝑤𝑤
+
𝑎𝑎
𝑐𝑐
𝜌𝜌𝑐𝑐
𝜌𝜌𝑎𝑎
CO2 = 42 . y CO2
where yCO2 is ambient carbon dioxide content by volume
(0.03% - 0.05%).
Key assumptions for the equations
•Hygrothermal equilibrium (determination of water distribution in pores from ambient relative humidity and
temperature)
•complete cement hydration and any pozzolanic activity of concrete in experimental Values.
Richardson (2002)
Papadakis et al. (1990)

14. Result of model in CTB material
14
Effect of cement content on carbonation rate for
RH=50%

15. Result of model in CTB material
15
Effect of cement content on carbonation rate for
RH=50%

16. Second ambiguity
16
Paige et al (1990) found that concentration of carbon dioxide can reach up to
450 times more than normal ambient condition. This subject confirmed by
carbonation upward direction from bottom of layer.
They also noted that the CO2 content in pavement is same in either case of
topsoil removal or keeping.

17. Earth Work
17
Specifications often
indicate organic content
<2-5%
Franklin et al. ( 1973)
the maximum dry density of soil
decreases rapidly when the organic
content exceeds 8% - 10%.

18. Organic Content of Soil Material
18
Carbon content is commonly used to characterise the amount of
organic matter in the soil.
Organic content = 1.724 * per cent organic carbon (3)
In other words, given the presence of 5% organic content in sub-grade
soil, there is the potential for producing up to 2.9% of carbon dioxide.
ASTM International, “Special Procedures for Testing Soil
and Rock for Engineering Purposes”, STP 479, 1970

19. Conclusion
19
 Organic contents can cause rapid deterioration of the cemented
layers in pavement. As such, it is advisable to decrease the organic
content of the sub-grade layer to as low a level as possible.
 Reviewing the organic content requirement pavement material
and sub grade layer while soil cement is involved in designs

20. Thank you