CHAPTER 5: BEHAVIOR OF SILTY SOIL REINFORCED BYCEMENTUNDER CBR, UU, CONSOLIDATION, ANDSHEARTEST
5.6. GRAIN SIZE DISTRIBUTION OF SOILCEMENTMIXTURE
Todeterminethegrainsizeofcement-treatedsoil,theSEM(scanningelectronic microscopy) method can be used to evaluate the properties of soils improved by cementthroughelectronicmicroscopescanningandX-raydiffractiontesting[124].
Voidratio (e) Secent modulus (MPa)
Untreated 3%
5%
7%
10%
However, this method has some disadvantages, including a small sample that cannot berepresentedforallsoils.Additionally,thequantityofgrainsizedistributionforall soils could not be determined. In this study, sieve analysis and hydrometer analysis were performed to determine grain size distribution. Although this method cannot give an image of structural soil or illustrate its properties, a large amount of soil can be used to investigate.
So, the quantity of grain size and accuracy can be obtained. Table 5.5 shows the difference between SEM and sieve-hydrometeranalysis.
Table 5.5:The difference between SEM and sieve-hydrometer analysismethods SEM method Sieve -hydrometer analysis method - A small sample that cannot be
represented for all soils
- A sample with large quantities can be represented for all soils.
- Investigating the soil structure - Can not investigate the soil structure.
- Can not determine the grain size
distribution - Determine the grain size distribution.
Using particle size analysis, the effects of cement treatment on the structure of the modified soil were examined. After 28 days of curing, sieve analysis and hydrometer analysis were performed on cement-treated soil samples using ASTM C136 [125] and ASTM D4440 [118], respectively. In general, the particle size ofthe treated soil was larger than that of the untreated soil, as illustrated in Figure5.21.
100 80 60 40 20 -
10 1
0.1 Grain size
(mm)
0.01 0.001
Figure 5.21:The grain size distribution of the untreated soil and the cement-treated soil after 28 days of curing.
The increase in cement content resulted in a greater fraction of sand-
sizeparticles and a larger median particle size,D50. Similar results were found for the
Percent of finer (%)
particle size distribution of cement-treated clay as measured by the Carlo Erba mercuryintrusionporosimeter[62].Itrevealedatransitionfrompredominantlyclay- sized particles to silt-sized particles. Due to hydration and pozzolanic processes in cement, the creation of fabric and bonding in cement-treated soil induces anincrease inparticlesize.Inthisinvestigation,itappearsthatthelatereffectpredominatedand caused the particle size to increase. While it appears that the fabric and bondings did not entirely form due to the low cement content (i.e., less than 10%) and the soaking procedure of the treated specimens. The size improvement in fine particles was also observed in the cement-treated soft Singapore marine clay by Chew et al. [62], who concluded that there was a shift from predominantly clay-size particles to silt-size particles. The increase in sand size fraction of cement-treated soil was quantified further by analyzing the proportion of sand-sized particles and fine contents shown in Table5.6.
Table 5.6:Percent of sand and fines with median particle size of untreated and treated soil specimens after 28 days of curing
Cement content,cm
(%) % sand
(%)
% fines (%)
Median particle size,
D50(mm) Coefficient
0% (untreated) 12.3 87.7 0.006 0
3% 13.9 86.1 0.010 0.018
5% 16.4 83.6 0.011 0.048
7% 19.0 81.0 0.014 0.077
10% 24.1 75.9 0.016 0.135
Consider the dry mass of sand size particles and fine particles
areMsandMf,respectively, the percent of sand particles in the untreated soil should be:
%𝑆𝑢𝑛𝑡𝑟𝑒𝑎𝑡𝑒𝑑 =𝑀𝑠 ×100% (5.1)
𝑀𝑠+𝑀𝑓
When mixing soil with cement, the total dry weight of the cement-treated soil,Mtreatedincludedthedrymassofthesoil,themassofcementandthemassofhydration,and
cementitious products, which were evaluated asfollows:
𝑀𝑡𝑟𝑒𝑎𝑡𝑒𝑑=(𝑀𝑠+𝑀𝑓)×[1 +(1+𝛼)𝑐𝑚] (5.2)
inwhichwasthedrymassratiobetweenhydrationandcementitiousproductsandcemen t.Thevalueofwasreporteddifferentlydependingonthecompositionof the cement and types of soils. At 28 days of curing, Zhu et al.[126]
reportedthatthev a l u e o fw a sa b o u t 0 . 1 6 f o r t h e m i x t u r e o f c e m e n t w i t h l a k e a n d m a r i n e sediments (high plasticity clay) and 0.21 for that with river sediment(highplasticitysilt). For the hydration of Portland cement only, Chu et al.
[127] found thatthemassof water related to complete hydration was about 25.2%
(i.e.=
0.252),whichwasclosetothevalue=0.23reportedbyConcreteSociety[128]atcompletehydr ation.Theh y d r a t i o n a n d c e m e n t i t i o u s p r o d u c t s i n c r e a s e d p a r t i c l e s i z e i n c e m e n t - treated soil specimens. By assuming a uniform condition in the mixture, the massof sand-sized particles in the treated sample was evaluated as follows:
𝑀𝑠_𝑡𝑟𝑒𝑎𝑡𝑒𝑑=𝑀𝑠×[1 +(1 +𝛼)𝑐𝑚]+𝛽𝑀𝑓×[1 +(1+𝛼)𝑐𝑚] (5.3) in whichis the coefficient that accounts for the effects of cement on integrating the fine particles with the sand-sized particles. Meanwhile, the first term is the new dry mass of sand-size particles mixed with cement with hydration and cementitious products. The percentage of sand-sized particles in the treated soil should be:
%𝑆𝑡𝑟𝑒𝑎𝑡𝑒𝑑= %𝑆𝑢𝑛𝑡𝑟𝑒𝑎𝑡𝑒𝑑+𝛽%𝐹𝑢𝑛𝑡𝑟𝑒𝑎𝑡𝑒𝑑 (5.4) The percent of sand-size particles in the untreated soil as the first term in the above equation, illustrates that the cement and its hydration and cementitiousproductsdonotcontributetotheincrementinthevalueof
%Streated,However,itmightincrease the particle size and form bonds between them. The
increment in particle
sizeduetocementtreatmentwasalsoreportedingranularsoilmixedwith2%cement content [129]. It also concluded that the cement bonds were difficult to destroy by hand but might be destroyed under confining pressure and monotonicshearing.
The values offor the cement-treated soil at 28 days were given in Table 5.6, in which it increased from 0.018 to 0.135 when increasing the cement content from
3%to10%.Inotherwords,upto13.5%ofthefinecontentinthesoilwastransferred to sand-size particles when treated with 10% of the cement contents. The increasein
Shear stress,(kN/mkPa)
(b) =100 kPa 10%
7%
ted clay untrea 5%
3%
0% ( (a) = 50 kPa
10%
7%
5%
3%
0% (untreated clay)