Application of Shotcrete as Replacement of Concrete

We know shotcrete is projected pneumatically on backup surface. Then they built up required section gradually i.e. one side of formwork can be handled.

Thus economy of this process relative to concrete is achieved only when obstructions are less. Say a section having no or less ties.

We have discussed in previous posts that more skilled operator relative to concrete is required; equipment and form of placing are also more expansive than our conventional concrete. Besides cement consumption in this process is also high.

So application of shotcrete is carefully chosen to reach economy. The primary selection is based on less obstruction in projection like

-Thin section

-Light reinforcement in section

The examples of these types of sections are roofs/ slabs specially

-Folded plate

-Shells

-Tunnel lining

-Prestressed tanks

In the repair work and stabilizing purposes and fire proofing also shotcrete plays important role. Some examples are

-Repair of concrete which is deteriorated due to different physical and chemical agents

-In stabilization of rock slopes

-Fireproofing steel members by encasing them with shotcrete; such thin overlay may be provided also over masonry and concrete.

For erosion resistance shotcrete can be applied to concrete exposed to surface runoff of rain /stormwater or any source of running water; but accelerator like washing soda should be used to provide flash set.

The accelerator reduces strength of shotcrete significantly, but to make possible repair work, we have to sacrifice strength.

We have forgotten important information that a thickness of up to 4 in. (10 mm) is generally provided.

What is Gunite or Sprayed Concrete (EU)

In gunite or sprayed concrete or mortar are conveyed through hose to project pneumatically to a surface at a high velocity which is required to construct or repair work.

Sprayed concrete terminology is a term in EU and significantly general. But both gunite and sprayed concrete are nothing but synonym of shotcrete.

The properties of ginite are identical that of traditionally placed concrete or mortar having same proportions. In this method significant skill of nozzleman is important. We will discuss about different method of applying shotcrete in our next post.

In any method strength and finishing of surface depend especially on control on accurate placing of materials with the help of nozzle i.e. performance depend on skill of operators.

ACI certifies the nozzleman for shotcrete operation and we should hire only certified nozzleman for our shotcrete operation.

We have discussed where to go to be shotcrete nozzleman and who patronize this certification in our previous posts. You can visit this for more information.

Dear reader we will discuss about application of shotcrete/gunite/sprayed concrete in our next post. Please stay with us.

What is No Fines Concrete and Difference between It & Normal Concrete?

We know concrete consists of cementing agent, fine aggregate and coarse aggregate which gain its strength while hydration in presence of water. If we remove coarse aggregate from concrete entirely, it will perform as mortar. But what will happen when fine aggregate is eliminated?

The result is also a concrete-no fine concrete. This is a specially type of lightweight concrete. Thus coarse aggregate mass in no fine concrete is coated by cement paste; whereas in normal concrete the coarse aggregates are surrounded by paste consist of cement and sand (fine aggregate).

The coating may be of up to 0.05 in (1.3 mm) thick. In normal concrete voids between coarse aggregates are filled by fine particles and dense mass is produced. But in this concrete, large voids are formed with in concrete resulting low density.

Low density of concrete will result lightweight concrete. Like other traditional lightweight concrete, it is also low strength. We know low density (large void) always produce a weak concrete.

But some advantages are also achieved. The large voids prevent water movement through capillary action. Dear reader, we here provided just definition and tried to differentiate it with normal concrete. In our upcoming post we will discuss about density, strength, placing, thermal and shrinkage properties and as well durability aspects of these concrete.

What is the Difference between Shotcrete and Concrete?

Dear reader we have discussed about shotcrete in our previous posts but not mentioned the difference between it and concrete. This nothing but a mass of concrete or mortar but the difference is placement technique.

In this technique concrete or mortar is conveyed by using a hose, transfer through it and projected pneumatically at a high velocity to/onto a required backup surface.

In normal concreting operation, we have to use temping or vibration techniques to expel out any short of voids. In shotcrete the compaction is done by the exerted force of jet that impact the backup surface.

As impact compacts shotcrete materials onto the surface to support material itself avoiding sloughing or sagging, no additional temping or vibration is required in shotcrete operation.

This technique of shotcreting is even applicable for vertical face and overhead too. So formwork is not required in shotcrete operation whereas in normal concrete work significant time and cost involved in this respect.

The workmanship required for shotcrete operation must be of considerably higher than normal concreting work. Dear reader in our previous post we have discussed about requirements of ACI shotcrete nozzleman.

What is Subsidence in Foundation Soil?

Subsidence is nothing but sinking of large (considerable) area of ground surface in downward direction. Subsidence may be a consequence of extraction of groundwater and soil as well.

These extractions may lead to compression of underlying soil (porous texture) or sometimes rock structure. The subsidence is considered as secondary influence on foundation settlement as no direction influences are observed (as of consequence of extraction of groundwater or oil).

So the settlement of structure above it due to this is considered secondary settlement. Other than extraction, there have special case of downward displacement where wet clay deposit remain in soil strata.

Foundation suffers downward displacement when wet clay deposits are dried out and very often downward displacement of foundation due to desiccation of such layer are termed as settlement.

These displacements are not always downward; the reverse displacement is also observed. In rainy season when moisture is introduced to clay deposit the expansion of volume of deposit is happened in some active clay soil. We have discussed about activity of clay in our previous post.

In such case, initially settled foundation may be even heaved. So it is not a permanent settlement (in case expansive clay). The seasonal influence in such deposit may lead to cyclic shrinkage and heave on foundation soil and should be carefully studied. We have published many posts about expansive soil in our previous posts.

Achieving Excellent Finishing of Concrete with Permeable Formwork

Dear reader in our last post we have discussed about durability of concrete poured in permeable formwork. A concrete member which has visibility in vertical face can be designed to have excellent finished surface with such formwork.

Most concrete member has visible vertical face and bottom horizontal face (say beam). In case of column and reinforced concrete wall vertical face is also available. In case of slab either top surface is finished with neat cement finish or provided with tiles/mosaic.

The bottom surface of slab sometimes has ribbed appearance according to architectural requirements. Most case plaster is provided to exposed face of concrete member.

When a fresh concrete is poured, we know bleeding is common phenomenon. With permeable formwork, the surface achieved is free from streak of bleeding and also free from pack-marks due to entrapped air.

Removing these two, permeable formwork produces an enhanced aesthetic appearance to exposed surface. So with the advantage of greater durability, excellent architectural appearance is also available.

The less bleeding phenomenon provides additional advantage of early finishing. There have also disadvantage of such formwork which will be discussed in next post. Till then goodbye.

How is Durability of Concrete Increased with Permeable Formwork?

Permeable formwork has special advantage of least susceptible to unfriendly exposure around concrete surface. It is noticed that with the application of such special formwork water-cement ratio of concrete near surface is considerably lowered.

Now question is-what is the extent of reduction in water-cement ratio?

-The depth up to which water-cement is changed is 20 mm from face of member near formwork.

-The degree of reduction is about is about 0.15. The reduction in water-cement ratio is not uniform. This is 0.15 at zone next to formwork and gradually decreases to reach no reduction at 20 mm from the surface of formwork.

Now why is water-cement ratio important? Water-cement ratio is the most important parameter that has influence in almost all properties of concrete. This great reduction in water-cement ratio results reduction in permeability of water and surface absorption as well near outer surface of concrete where they are in contact with hostile environment.

But is 20 mm enough for clear cover of concrete from reinforcing steel? When a concrete mass is cast in direct contact with soil, the expected clear cover is 75 mm (maximum value is provided here). But outer 20 mm of less permeable concrete greatly increase durability.

The another important change is surface hardness. This property is also increased enforcing resistance against erosion and cavitations.

What is Permeable Formwork for Concrete?

This is a special development of concrete formwork where filter, draining and structural supports are provided with this system. This type of formwork is specially designed for vertical and inclined surface of concrete which furnished with propylene fabric supported by plywood backing having drain holes.

The fabric is strong and supplied in project in rolls having variable length. The fabric must be chemically inert.

The purposes of this system are:

a. Filter:

To facilitates air and water to pass out from fresh concrete by allowing a passage but essentially retains cement/fine solids.

b. Drain:

The drain holes discussed above allows water and air to transfer from formwork to environment.

c. Structural support:

The system keep fabric and backing element in position until the formwork is removed after sufficient maturity of concrete member is gained. The concrete pressure is also resisted by this system.

If the above purposes are served successfully, we will produce a concrete of excellent visual appearance and greater durability as surface hardness of concrete produced in this process is greatly increased.

The water permeability and absorption in surface of concrete is also greatly reduced.

The only limitation we know is-high drying rate due to less bleeding which is expressed in the form of plastic shrinkage cracking.

What are the Parameters for Defining Design Foundation Settlement

Foundation of a building transfers load of the structure to the safe stratum. But always hard stratum is not available to support or economy of structure doesn’t allow designer to use deep foundation . In these cases engineers need to depend on shallow foundation or foundation not supported on firm stratum.

Even in case of pile foundation, there need to check for settlement of pile group. In case of clay deposit long time settlement is very important and in case of construction where project duration is limited the time required to achieve desired degree of consolidation is very important.

Therefore a geotechnical engineer has primary obligations to determine settlement characteristics of proposed structure. The parameters provided below are frequently required in defining settlement behavior of structure:

a. Total settlement

b. Maximum value of differential settlement

c. Settlement rate

d. Maximum magnitude of angular distortion

Here we are elaborating above parameters as follows:

a. Total settlement

Total settlement is also termed as maximum settlement, ρmax. This is the largest magnitude of settlement that any part of foundation of structure suffered during its service life. As shown in fig. below:

b. Maximum differential settlement:

This is a relative settlement defined as largest difference between settlements of two different foundation of same structure as shown in fig above. There should not have any doubt that maximum differential settlement (Δ), not necessarily to experience near foundation suffering maximum settlement. Any two foundations having maximum difference in settlement are considered here.

c. Settlement rate

During construction work gradually loads are applied to foundation soil say 2-3 years or may be several years where skyscrapers are founded. Some structures need to go in operation within shortest expected duration say highway, dam etc. when socio-economical or political aspects are considered.

So we can leave some settlement during its service and essentially safe fraction of consolidation before structure is left to service. In this regard we have to know rate of settlement as most construction work have time constraint.

Again settlement consideration for building is not same for highway construction. Different methods are introduced by geotechnical engineers to change rate of settlement, some have discussed here discussed in this blog, and some are coming soon.

d. Maximum angular distortion:

This term is expressed as δ/L. For the expression we can easily realize that it is nothing but differential settlement (not maximum differential settlement) between two locations in foundation soil divided by distance between two locations but should be less than tilt.

Now what is tilt? MacDonald and Skempton (1986) defined this term as rotation of entire structure. The maximum angular distortion is the highest value of δ/L as shown in fig above, again the location of maximum angular distortion not necessarily to locate at maximum settlement or total settlement.

Now how these parameters are used in foundation design?

At first total/maximum or maximum differential settlement of structure proposed for in-situ soil conditions are determined. Thus allowable settlement rates are determined and finally they are compared to maximum settlement parameters.

If these parameters exceed allowable settlement values, the foundation type required or required mitigation measures to limit settlement are designed.

Comparison between Permeability of Aggregate and Cement Paste of Concrete

Do you know, what are the usual porosity of cement paste and natural aggregate? The usual capillary porosity of hardened cement paste in concrete lies between (30-40) percent.

In natural aggregate this value is usually 3 percent and in rare cases exceeds 10 percent. In concrete production our usual interest is to make it less permeable and we will try to keep permeability of aggregate as low as possible.

Thus cement paste have large porosity. Does it mean cement paste have more permeability? We are providing a table below; try to understand the comparison between permeability of cement paste and rocks. Here rocks represent aggregate that we are discussing.

Consider, granite have co-efficient of permeability 5.36X10-9 cm/s which have similar permeability of cement paste having water cement ratio 0.70 (the paste is sufficient matured).

Now consider an aggregate particles having 3% porosity but have permeability of a poor cement paste of mature concrete. Form this table it is clear aggregate of above range have permeability that varies with that of cement paste having water/cement ratio of (0.38 to 0.71).

Here a term is used ‘matured’ cement paste, when do they become mature? Of different water-cement ratio maturity comes after different periods as below:

Water-cement ratio	Maturity period (days)
0.4	3
0.5	14
0.6	180
0.7	365

 

The curing condition is moist curing. Dear reader in our next post we will discuss about permeability contribution of aggregate in relation to their porosity.

Factors Leading Slope of Soil Mass to Fail

Various modern methods for testing soils are developed in recent years and as well stability analysis techniques improved. Various dynamic testing facilities invented that help us to take decision a soil slope either it is safe or unsafe under both static and dynamic loading. Before learning analysis techniques we have to realize the factors that lead to failure of a soil mass.

Failure of soil mass may occur along curved or plane surface. When a soil mass (considerably large) slides relative to remaining mass, failure is commenced. Usually an outward and essentially downward movement of soil mass is observed.

A slope is failed when forces acting on soil mass that result failure are greater than shearing resistance developed along critical failure surface. The factors that lead failure to slope of soil mass are of two categories:

a. Factors increasing shear stress

b. Factors reducing shear strength

a. Factors increasing shear stress:

The factors that increase shear stress exceeding shear strength of soil mass results slope failure. The stress may be increased due to following cases

1. Stresses exerted by water under saturation of soil mass

2. Surcharge loads

3. Seepage pressure

4. Steepening of earth slopes due to natural erosion or excavation

b. Factors reducing shear strength:

If the stresses applied are same but soil mass lose its shear strength, the slope is also failed. The shear strength can be lost due to

1. Increase in moisture content and pore water pressure

2. Cyclic loading or shock

3. Weathering

The factors discussed above (two types) actually are of same sources i.e. when water content increases, stress is increased with simultaneous decrease in strength of soil mass and due to this natural slopes are commonly failed in rainy season.

What is the Slope Stability of Soil Mass?

A slope means soil surface that is inclined and unsupported. In many engineering purposes we need to form earth slopes having adequate safety against stability. As earth consists of soil (or rock,) we can use the term stability of soil mass.

Now why do we need to form earth slope? The purposes are to form

-Railway formation

-Highway embankment

-Canal banks

-Earth dams

-And in other many locations

We can notice many natural slopes that remain in stable condition i.e. not failed (shear failure) which is stabilized naturally but may fail under vibration or under saturation. Many landslides were reported during earthquake and slope failure due to excessive rainfall. Our concern here is artificial or man-made slopes.

When economy is concern we need to establish steepest slopes as they cost minimum. When safety issue is considered, such steep slopes are usually not stable. As both economy and safety are important in a project, it needs to compromise between them.

So we have to provide a slope that is neither too flat nor too steep. So our aim will be to cut or provide possible steepest slope which is safe and stable. Failure of slope is often take life and property and geotechnical engineers have to evaluate stability of slopes proposed for the project.

Different methods are established to check stability and now-a-days more economical slopes can be provided with these tools. Dear reader in our next post we will learn about factors that lead to failure of slopes of soil mass.

What is Halite? How is Foundation Settlement Associated with This?

Dear reader in our last post we have discussed about soluble soil where halite, a rock salt, was discussed in relation to soluble particles. This salt is soluble and when remains in soil in sufficient quantity, the soil structure fails as it find moisture; the halite particles are dissolved.

Halite has isometric crystals of NaCl and they usually are colorless or can be said white. But every mineral have some impurities which render a change in color that may be

-light/dark blue

-purple

-pink or red

-orange or yellow

-gray

In the Canada and United States there have extensive underground salt bends (halite). This doesn’t always produce problem to us; we are using this in both residential and municipal purposes as deicing agent, but our concern is here foundation settlement, so we are not discussing this point here.

The soils containing such soluble salt particles are hard and dense enough to consider safe against foundation load or overburden pressure. But when water infiltrate through them the settlement is observed.

Two types of foundation settlement are observed as follows:

a. The particles of soil can be bonded by salt particles which become weak in contact with water and soil structure is collapsed.

b. The soluble material is dissolved out in water and as solid portion lost in significant fraction the settlement of foundation is occurred.