There are many different additives that can be used in cement mixes which are used to improve certain properties of the mixture depending on the application. This project focuses on foam (cellular), fly ash, blast furnace slag, and silica fume as additives and partial replacement of cement grouts.
Grout is used to improve many inadequate or failed soils by injecting stabilizing materials. Cement grout is typically injected into granular soils to improve bearing capacity, reduce settlements and permeability and mitigate liquefaction (Akbulut, 2003). Grout is in general injected until the soil cannot accept any more grout however even small amounts of grout injected into soils can significantly improve soil characteristics (Ali, 1992).
Each additive has unique properties that contribute to improvement of certain properties of the grout. The specific changes in properties of a grout mixture can often be either an advantage, disadvantage or both depending on the application. The effects and applications of the cement additives or alternatives are given below for each of the additives.
Figure 1: Foam used for for grout mixture (Cellular Concrete, 2014)
Foam or cellular grout is a cement grout mixture that contains foaming agents (surfactants) (Bruce, 2005). The foaming agents create many small air voids in the mix that reduce the unit weight and improve flow of the mixture. Foam grout density ranges from about 30-80 pcf (480-1300 kg/m3) which result in 28-day compressive strengths of 50-1200 psi (350-8300 kPa). The density and compressive strengths of the mix are tradeoffs: the higher the density, the higher the compressive strength. To achieve a specific compressive strength, different mix designs should be tested to find a minimum density to achieve the desired strength (Henn, 2003). Figure 2 shows the tradeoff between density and compressive strength for cellular grouts with varying degrees of foaming. With increased quantities of foam the density (unit weight) decreases resulting in a decrease in compressive strength.
Figure 2: Variation of compressive strength with unit weight and cure time for foam grouts (Vipulanandan, 2000)
Foam grout is typically used as a low cost option when strength is not a requirement (McGillivray, 2012). The air voids in foam grout allow the material to be somewhat compressible and therefore a good material for increased energy absorption. This property makes foam grout a good option for seismic areas, highways, and airport runways (Henn, 2003). It can also be used as backfill for tunnels and pipelines and fill material. It is also used to fill in the ring between the outside of a pipe and it’s surroundings (backpack grouting) (Vipulanandan, 2000).
Sliplining has been used with increasing popularity to replace existing concrete sewer pipes. In this process a new pipe is introduced within the existing pipe and the annular space between the pipes is filled with grout to support the new pipe and control infiltration. Poor grouting mixes and practices have resulted in many problems when it comes to sliplining including “unwanted buoyant uplift, excess deflection or collapse of the new liner pipe.” To avoid these problems it is very important to use a lightweight grout with good flow properties which is why foam grouts are typically used for these applications (Vipulanandan, 2000).
Although foam grout is generally used as a lightweight material to fill voids, it can also be used for stabilization purposes such as protecting slopes against earthquakes or preventing liquefaction. Typically, to stabilize a soil with grout the void space is completely filled with a particulate or chemical grout but often the same soil could be adequately stabilized by grouting the particle contacts without filling all the void space. Ali and Woods show how particle contact grouting can be accomplished with the use of a foam grout. By introducing bubbles through the foaming process, sand specimens were able to be grouted to various degrees of cementation. A micrograph showing cemented particles surrounded by void space can be seen below in Figure 3. For large scale remediation projects, the amount saved by not completely filling the void space can be significant (Ali, 2009).
Figure 3: Micrograph showing open pores and pendular elements formed between Ottawa 20-30 sand particles (Ali, 2009)
In 2003 a sand/cement/foam grout was used by McGillivray et al. to successfully treat sinkholes in Hillsborough Florida. The foam used for this project was a synthetic material which came in a concentrate form. Using a foam generator, shown in Figure 4, a foaming agent with small stable bubbles was created and dispensed directly into mixing trucks, shown in Figure 5. The mixing trucks, which originally contained a partial load of sand/cement/fly ash grout mixes the foaming agent with the grout mixture to create a pre-formed foam grout, shown in Figure 6. For this application, the grout was only required to be slightly stronger than the surrounding soil so a target strength of 3 MPa was used. Laboratory compressive strength tests of the foam grout used for the project had an average strength of 3 MPa with a standard deviation of 0.6 Mpa. Through experimentation it was shown that a 60% to 40% grout/foam mixture would result in a 20 to 25% savings on cost for a typical sinkhole remediation project compared to using traditional grouts.
Figure 4: Foam Generator Setup (McGillivray, 2012)
Figure 5: Foam being added to mixing truck (McGillivray, 2012)
Figure 6: Foam grout after pumping (McGillivray, 2012)
Figure 7: Fly ash (Portland Cement Association)
Figure 8: Fly ash particles (University of Kentucky, 2014)
Fly ash (ASTM C618) is a by-product of the combustion of coal in power plants. In cement mixes, a portion of the cement can be substituted with fly ash due to its pozzolanic properties. Fly ash is a electrically precipitated powder produced from crushed coal. The fine (10 micrometer) particles are made up of silicate glass spheres containing silica, alumina, iron and calcium. The particle gradation is slightly more coarse than portland cement. Since fly ash is a waste product, it’s properties can vary by source (Weaver, 2007).
Fly ash comes in two different types: Class C and Class F. Class F is rather inexpensive and has pozzolanic characteristics but cannot set without a source of calcium (lime or cement). Class F fly ash is produced from anthracite or bituminous coal and cures slowly. Class C has both cementitious and pozzolanic characteristics, so it can set by itself without cement. It is produced from subbituminous or lignite coal and can be pulverized to improve hydraulic properties. If more than 15% by weight of cement is Class C fly ash, the grout can deteriorate due to expansive tendencies of the Class C fly ash. There should be no more than 10% carbon used in grouts containing fly ash since more water will be required (Weaver, 2007).
Typically for a fly ash/cement grout, 15-20% of the cement is replaced by grout but due to economic and environmental pressure, high volume fly ash/cement grouts (grouts containing > 55% fly ash) are being used with greater regularity. A few important property changes that occur from the use of high volume fly ash use include a decrease in flow time for low water/cement ratios, significantly increased stability for high water/cement ratios, a decrease in setting time due to the slow reaction of fly ash (shown below in Figure 9) and a reduction in modulus of elasticity (Mirza, 1999).
Figure 9: Effect of 60% fly ash on the initial setting time of portland cement and portland cement/fly ash mixtures (Mirza, 1999)
Because of its typically inexpensive nature, fly ash is often used as a partial cement replacement for high volume applications such as soil, rock or oil well grouting (Mirza, 1999).
In the process of sliplining, briefly explained above in the foam grout section, significant amounts of fly ash are often used as a partial replacement of cement for foam grouts. Fly ash is primarily used to reduce cost, decrease shrinkage, increase flowability and manipulate other mechanical and chemical properties of a specific grout (Vipulanandan, 2000). An example of the shrinkage reduction of foam grouts from the use of class C fly ash can be seen below in Figure 10:
Figure 10: Reduction in shrinkage of a foam grout from the use of 50% fly ash replacement of cement (Vipulanandan, 2000)
Since fly ash is used as a cement substitute in many different concrete and grout mixes, it has positive environmental impacts. Of the 71 million tons of fly ash produced in 2004, about 40% of it was recycled by way of cement substitution and has the potential to save 10 million tons of carbon dioxide emissions annually (Portland Cement Association).
When tunnels were constructed through the Lower Chalk Marl in the UK, there were many constraints on the grout used around the 20 mm gap between the concrete tunnels and the soil. The grout mix had to be pumped a long distance, but upon arrival at the construction location, must set quickly. Additionally, there was a lot of water in the soil which required the grout to be anti-washout and not bleed significantly. The grout also needed to gain a minimum 28 day strength of 8 MPa. After many tests, the ideal grout mix contained 50% Portland cement and 50% fly ash. It also contained superplasticizer and stabilizer to maintain pumpability and flowability. The large amount of fly ash was most likely used due to the excessively wet environment of placement. The anti-washout and anti-bleed properties were most likely met by the use of fly ash.
Figure 11: Blast furnace slag (Portland Cement Association)
Ground-granulated blast furnace (GGBF) slag (ASTM C989) is a by-product of the production of iron. It requires an alkaline medium to initiate hydration. Slag does not require a high pH to activate, instead addition of portland cement will create formation of calcium silicate hydrate. Slag consists of silicates and aluminosilicates of calcium and other bases. Slag is classified into three grades. The properties can vary between sources, but are fairly consistent for a single source (Weaver, 2009).
Since use of slag in cement decreases the need to landfill large amounts of slag, up to 3 million tons of carbon dioxide emissions can be eliminated annually (Portland Cement Association).
Ground granulated blastfurnace slag is being used in mining applications. Instead of disposing the large amounts of tailings (soils/waste removed from mines), this material can be made into a slurry and mixed with a small amount of cementitious material. This mixture can be used to fill parts of mines that are no longer in use. Slag is frequently used (mixed with Portland cement) as the cementitious binder in this mixture due to its ability to produce high strength, low permeability material as compared to Portland cement alone. The addition of slag can also increase homogeneity and increase strength gain of the mixture (Jefferis, 2012). Blast Furnace Slag has also been widely used in Poland on dam foundation treatment (Weaver, 2009).
In 1918 a moveable bridge carrying carrying the Amtrak railroad over the Thames river was constructed. In 2006 work to expand the bridge caused significant movement of one of the piers when the new piles, located 20’ from the existing 40’ wide by 99’ long caisson, were driven below the depth of the caisson. To stabilize the caisson, compaction grouting was first implemented but inclinometer data, shown in Figure 12, revealed that movement of the sand layer beneath the caisson was still occurring. Instead of compacting the sporadically dense sand, the grout was escaping into the overlying layer of organic silt. After compaction grouting failed, permeation grouting was used to stabilize the underlying sands. Because of the variability of the sand and the great depth, a typical cement grout would require too many boreholes due to inadequate permeation. Therefore, a commercially available blast furnace slag based cement with a maximum particle size of 10 microns and a Blaine Fineness greater than 900 cm2/gr was used. Because blast furnace slag delays the set time, which is a disadvantage for this application, a diutan gum blend was utilized to stabilize the grout during the 24 hour set time. It required approximately 270,000 gal of grout to initially stabilize the pier followed by an additional 350,000 gal to further stabilize the influence zone.
Figure 12: Pier movement vs. grouting progress (Kaeck, 2009)
A water intake tunnel below the Niagara River was contaminated with organic wastes from a landfill. Because of this, regulatory authorities requested that the tunnel be grouted closed. This project was complicated due to the depth of the tunnel, the amount of grout that needed to be applied and the fact that water would not be removed from the tunnel before grouting.
Since the grout needed to travel 25 meters below the ground and up to 1600 meters along the tunnel, the mix needed to have a set time of more than 24 hours to allow significant amounts of grout to be in place before it begins to set. The required grout also needed to be able to easily displace water and set in a saturated environment. It was also required that the grout have low viscosity, a 28 day compressive strength of 100-200 kPa or greater and a permeability of 10-6 cm/s or less.
Many different grout mixes were tested to find the best option for this project. Variations of bentonite clay, fly ash, foam and slag were tested with Portland cement. In the end, a mix of 75% slag with 25% Portland cement, among other additives (including bentonite) was used. This grout easily displaced water and set under water in tests. It also had the desired strength (200-800 kPa) and permeability (10-6-10-7 cm/s). Finally, with the addition of slag, the mix had low viscosity, low bleed, and low shrinkage.
Figure 13: Silica fume (Portland Cement Association)
Silica fume (ASTM C1240) is a by-product of the extraction of silicon or ferrosilicon manufacturing. The particles are composed of glassy spheres almost entirely made up of SiO2. The diameter of the particles ranges from about 0.1 to 0.15 micrometers and is generally condensed and precipitated to prevent the particles from going airborne. Silica fume can either be used as a replacement for cement within a grout mixture or as an additional component. As a substitution for cement typically only 4-10% by weight of cement is substituted (Weaver, 2007). Only small amounts of silica fume are substituted because it has been shown that 4-10% is the optimal amount to improve grout strength and elastic modulus. However, pumpability and strength are tradeoffs to grouts and especially important for silica fume since it does not greatly increase the strength of grout as a replacement for cement (Akbulut, 2003). Using silica fume as a partial replacement for cement can also be a good way to create a lighter weight material, for a given compressive strength, silica fume can replace approximately 3 to 4 times its weight of cement (Aitcin, 1984).
Using silica fume as an additive as opposed to a replacing agent, can significantly increase the strength and decrease the permeability of a cement grout. This is the product of the silica fume reacting with the lime released by the hydration of the cement and creating a “compact secondary CSH”. In addition to increasing the strength and decreasing the permeability of a grout, this reaction increases the stability and decreases the threat of chemical attack (Aitcin, 1984). The use of varying amounts of condensed silica fume as an additive for a cement grout to increase strength can be seen below in Figure 14:
Figure 14: Comparison of compressive strength values for varying amounts of condensed silica fume (Aitcin, 1984)
Due to its many advantages, listed above, silica fume as either an additive or a partial replacement for cement grouts, is applicable to many different construction applications. Because silica fume drastically increases the cohesive properties of a grout, it is an ideal additive for underwater grouts. The decreased permeability resulting from the use of a silica fume/cement grout makes it a useful additive for post-tensioning applications. Silica fume can also be an extremely useful additive to reduce or eliminate unwanted reactions in chemically harsh environments. The use of silica fume/cement grouts for oil well drilling applications slows or even stops gas leakage from the grouted well. Shotcrete, which is often used in underground applications such as tunnels and mining, also often uses a large quantity of silica fume to increase adhesion (Norchem, 2013).
In the 1990s in Vancouver, British Columbia, compression screw piles were grouted in soft, organic and unconsolidated soils. The use of grout around these piles improved the load capacity in the soft soils. Silica fume was used in the grout mix to minimize shrinkage, improve strength, improve corrosion protection of the piles, and bond well to the piles. The addition of silica fume also increased the density and flowability of the grout.
In one test site, an ungrouted pile driven to 15.8 m (52 ft) had a capacity of 169 kN while a grouted pile in the same location had a capacity of 320 kN driven to a depth of 13.7 m (45 ft). At the other test site, both grouted and ungrouted piles were driven to the same depth. However, the grouted pile had a capacity of 302 kN while the ungrouted pile had a capacity of 200 kN and the grouted pile deflected less at failure. In general, grouting improved the capacity of the piles at this site by about 60%.
There are many options for cement mix designs in grouting applications. This report only touches on a few key additives. Any number of these additives can be used to improve specific properties in grout mixes, but may impact other properties. When deciding to use a new method of cement grouting, many tests should be run to determine whether or not the grout meets the needs of the project. A summary of the primary advantages and applications of the cement additives discussed in this report are presented below in Table 1. As noted earlier, whether a specific property change is an advantage or disadvantage may often depend on the specific application.
Table 1: Summary of primary advantageous property changes and applications for cement grout additives including foam, fly ash, blast furnace slag and silica fume