What is the Basis of Analysis of Slope Failure of Soil Mass?

We know that shear strength is the key issue that determines stability of slope. Slope of soil mass should be such that soil mass is safe against failure of slope along any conceivable (plane or circular) surface. Now elastic or plastic theories are using increasingly to establish method of analysis. But common methods of analysis are developed base on the limiting equilibrium.

Now what is limiting equilibrium? Here it is assumed that soil mass is on the brink of failure. The method (limiting equilibrium) is statically indeterminate. Due to indeterminacy, we cannot predict stress-strain relationships working along assumed surface and as usual we need some assumptions. With the help of assumption we will try to make the system statically determinate. This facilitated to produce sufficient equilibrium equation to solve the problem conveniently.

The forces that are active along failure surface and trying to result failure and their resultant are determined in analysis. The shear strength available in system is also determined. 

In all indeterminate system, certain assumptions are required to solve the system and these assumptions depend on the analysis method. In analysis of soil mass the assumptions made are as follows:

a. A two-dimensional stress system is assumed in analysis, the stress components in third direction are considered as zero. Third direction means the stress along normal to section of soil mass?

b. Strength parameter cohesion intercept (C) and angle of internal friction (φ) are known.

c. Coulomb’s equation regarding shear strength is assumed to be applicable to this system.

d. The next assumption is water levels and seepage conditions of the soil mass are known.

e. Pore water pressure can be estimated based on (d).

f. It is assumed that plastic failure condition will be achieved along critical surface. Plastic failure condition means shear stress in any points of critical surface will be sufficiently large to become all obtainable shear strength mobilized.

g. In addition to the assumptions discussed above some assumptions are made depending upon method of stability analysis regarding forces distribution and their magnitude along anticipated planes.

Using these assumptions, the resultant forces and shear strength are determined. The factor of safety of slope is calculated using resulting forces available in the system and actuating forces.

Rendering of No-Fines Concrete

Dear reader we were discussing about no-fines concrete in previous posts; many aspects we have discussed but do we know feasibility of this concrete? This concrete has advantage of less or negligible capillary suction.

But this concrete has high absorption. This may lead to vulnerable of use this concrete as exterior wall. The external walls of building should be provided with rendering on both faces of walls.

The rendering not only act against water absorption but also reduce permeability of air. The advantages of rendering are always associated with disadvantage of poor performance in sound acoustic.

Rendering and or painting reduce the special property of better sound-absorption of no-fines concrete. This is due to closing the pores by painting or and types of rendering.

Therefore where acoustic properties of wall are of primary objective, the rendering should not be provided on that side of walls that are of an element of sound acoustic system. The other side may be rendered where absorption of environmental agents is an important aspect of design.

In aesthetical point of view, open-textural wall of this concrete provides addition advantage of providing better rendering. Dear reader every product has some advantages and disadvantages and we have overcome these by our technical knowledge and proper application. Dear reader in our next post we will discuss about beneficiary effects of no fines concrete having large pores.

Thermal Conductivity of No-Fines Concrete

In simple word thermal conductivity is a measure of ability of a material (here concrete) to conduct flux of heat through concrete. The units used are j/m²s⁰C/m or Btu/ft²h⁰F/ft.

In normal concrete, this property depends largely on composition of concrete mix. The typical value of thermal conductivity is about 1.4 to 3.6  j/m²s⁰C/m when concrete is saturated.

Normally density of concrete doesn’t influence (or influence negligibly) the conductivity of normal concrete. But when lightweight aggregate is concerned, this value varies with density of such concrete.

In lightweight concrete there have more air voids and we know air have low conductivity; so this concrete conducts less heat through them.

Our concern is here no fines concrete, which is also of lightweight and should have less thermal conductivity relative to ordinary concrete. The typical value for this concrete varies between (0.69 to 0.94)  j/m²s⁰C/m when the aggregate is itself normalweight.

Again when lightweight aggregate is used to produce this lightweight concrete the thermal conductivity drop down to 0.22 j/m²s⁰C/m.

But thermal conductivity is increased considerably when moisture content in concrete is very high. This property is important in determining thermal strain, cracking, warping (at very early age of concrete) and designing thermal insulation. We have discussed about thermal conductivity of normal concrete and some posts about behavior concrete under exposure to fire and explosion. You are invited to read these posts.

Comparison of Thermal Expansion of No-fines Concrete with Normal Concrete

Thermal properties of any type of concrete like thermal conductivity, thermal diffusivity and thermal expansion are considered in concrete design and construction due to following reasons:

a. Thermal strains

b. Warping

c. Cracking

d. To have idea about temperature gradient

e. Providing joints in concrete say expansion joints and contraction joints.

Here our concern is about thermal expansion and we were discussing about no-fines concrete. Here this property will be compared with normal concrete. So we should have idea about thermal expansion of normal concrete.

The co-efficient of thermal expansion depends on

• Mix composition


• Hygral state during temperature change

In following table we are providing thermal expansion after 2 years in relation to aggregate content in concrete.

The linear coefficient for thermal expansion of normal concrete is 6.67X10^-6 per0F and volumetric coefficient for thermal expansion of normal concrete is 20X10^-6 per ⁰F. (Both are at 20⁰C).

Now come to the point no-fines concrete; the coefficient of thermal expansions for this concrete are about (0.6-0.8) of thermal expansions of normal concrete both in linear and volumetric consideration.

But the true value of this coefficient depends on aggregate types that are chosen for concrete batching. This coefficient is very important in designing joints where concrete may suffer large movement under thermal action and significant structural damage may be incorporated with this expansion. Normally high rise buildings, bridge construction and in structurally indeterminate members they are very important.

Shrinkage of No-Fines Concrete

Dear reader volume change in concrete is an important parameter and carefully studied in concrete engineering. Volume change may be swelling and shrinkage. We will discuss about shrinkage of no-fines concrete in this post.

Dear reader, at first, we will learn about shrinkage of ordinary concrete. The usual value of shrinkage in ordinary concrete is (520 X10⁶-780 X10⁶). In no-fines concrete we will find significant lower shrinkage than that of ordinary concrete.

The typical value of shrinkage in no fines concrete lies between 120X10⁶ to 200X10⁶. Defining shrinkage is always referenced by relative humidity; here considered relative humidity is very low.

Dear reader, we have already learnt that fine aggregate is absent in this special type of concrete and has a thin layer of cement paste that coat the aggregate. This thin coating of paste is also suffer drying shrinkage but is greatly restrained by aggregate (coarse aggregate).

But rate of volume change under shrinkage is very rapid. There have available large area of coated surface which is exposed to environment (air) and having great contact surface, the shrinkage is increased at greater rate.

The rate can be described with an example like (typical value):

-Total volume change is completed around one month (usually more than month but not greater than 5 weeks)

-Within 10 days half of total shrinkage is completed.

Strength of No-Fines Concrete

Dear reader, for any type of concrete, strength is a very important parameter which is related to other parameter too. Say a strong concrete have more density i.e. have less void and low void produce a durable concrete in all respect.

In our previous post we have discussed about density and water-cement ratio of no fines concrete and we know this have low density having large size voids. Obviously this will produce weak concrete and we have already defines them as light weight concrete.

The compressive strength can be achieved with this concrete usually varies from 200 to 2000psi (1.5 to 14 Mpa). The range is large and depends mainly on density of concrete mass. The density depends on cement content.

 
Unlike normal concrete, this concrete is less dependent on water/cement ratio in respect of strength development. Dear reader we have discussed about optimum water cement ration in last post.

The rate of strength development of no-fines concrete is usually identical as that of normal concrete. Regarding flexural strength, we can include, this is 30% of its compressive strength. This value is comparatively higher than that of normal concrete.

The modulus of elasticity has direct relation with strength say for concrete of 700 psi this value is 1.3X106 psi (10 Gpa). 

Water-Cement Ratio for No-Fines Concrete

Dear reader we know water-cement ratio is the controlling factor for strength of concrete. Here, in no-fines concrete, water-cement ratio is not main controlling factor.

There have a narrow range of water-cement ration, which is optimum for a given batch of aggregate. We have already discussed about recommended aggregate for no-fines concrete. Aggregate grading and shape have influence on water-cement ratio.

An aggregate having more surface area will need more water than an aggregate of less surface area. Again a rough aggregate can held coating of cement paste more than smoother one.

We were discussing about optimum water/cement ratio, when provided water cement ratio is higher than optimum, the cement paste have tendency of draining out away from surface of aggregate particles.

Whereas, when provided water-cement ratio is lower than optimum, the paste becomes less adhesive which leads to deviation from desired composition of this concrete.

Again aggregate absorption plays an important role in defining water-cement ratio and in prediction of optimum water-cement ratio must included this factor.

However, in general, the water content for concrete mix can be applied as 180 kg/m3 of concrete mass. Thus cement content required to coat aggregate sufficiently is considered in defining water-cement ratio.

We can provide a typical value of W/C ratio 0.38-0.52. But the strength achieved with this W/C ration should examined by test. Dear reader in our next post we will discuss about strength of no-fine concrete in our next post.

What is the Usual Density of No-Fines Concrete?

Dear reader we have already learnt that the density of no-fines concrete varies with grading of aggregate. The grading requirement, size and shape of aggregate already discussed there. The density of such concrete is measured using density of ingredients comprising concrete.

The simple calculation method is summing bulk density of

-Aggregate, in actual from of compaction, (kg/m3)

-Cement concrete (kg/m3)

-Water content (kg/m3)

In FPS unit we will use lb/ft3 instead of Kg/m3. Now why do we calculate density in such way?

This due to applying very little or no compaction effect; if provided compaction is very negligible.

We can use both normal weight aggregate and lightweight aggregate in nofines concrete. When normal weight aggregate is used, the density of this concrete lies between (1600-2000) kg/m3.

When lightweight aggregate is used; the density of no-fines concrete may reach as low as 640 kg/m3.

For different aggregate/cement ratio (measured by volume), for different water/cement ratio (measured by mass) the densities of no-fines concrete are provided in following table (where (10-19) mm aggregates are used). 

Formwork Requirement for No Fine Concrete

Dear reader we have discussed in previous post about compaction of no-fines concrete. Now we know much compaction effort is not required for no-fines concrete and preferably no. Thus vibration is not applied to concrete and ultimately to formwork.

Again no-fines concrete, produce low pressure on formwork. We can select a large size and lightweight forms for such concrete. Segregation is also reported negligible, which allows us to place concrete from greater dropping height.

So long and narrow vertical member like column and concrete shear wall etc. can be cast at one lift; multiple lift casting is avoided, which allow us to select large form length.

Here coarse aggregate is coated with cement paste; no slurry of cement sand is available in the system to seep or come out from leaks in the forms. For sealing leak or leak proof formwork, we have to invest both time and money.

Therefore we can use expanded metal open mesh of steel supported on light timber frames. But it is essential to ensure that forms and its bracing must have sufficient strength to support concrete without suffering deformation.

But considering economy of construction, the forms should be designed such that transportation, handling and ease of fixing are ensured having sufficient ductility and strength to use repeatedly for future project or future extension of existing project.

Compaction of No Fines Concrete

Dear reader here we will learn about compaction of no-fines concrete. We have discussed about workability requirements of no-fines concrete in previous post. Workability is important here as it defines ease of concrete compaction without producing any sort of segregation.

But in no-fines concrete, no compaction is recommended. We know about the Arching; this may occur near corners of four or around obstacles. To avoid arching in this particular location compaction can be used.

We know compaction can be applied by manual and mechanical method. The manual methods are temping or rodding or spading. The mechanical methods include power temping, vibration and shake tables etc.

Vibration may be applied in this concrete but for very small duration; otherwise cement paste can run off from coarse aggregate particles. Thus no-fines concrete has little or no tendency to segregate.

In concrete placement, dropping height of concrete has great importance. In long member, where concrete have to travel long distance, we generally cast member in different phase to avoid segregation. In many situations considerable waste of time is also reported due to this.

No fine concrete can be placed from greater dropping height, as it has inherent non-segregating property. In many case it can be placed from height as much as three storeys.

Dear reader in our next post we will learn about requirements for formwork for no-fines concrete. Please stay with us.

How does Shrinkage Do Harm to Concrete?

We have all concerned about shrinkage crack in concrete; why do shrinkage cracks occur and how do they do harm to concrete?-We have less idea. Here we like to provide an overall idea about shrinkage; in our upcoming post we will discuss this in microstructural point of view.

You will be surprised that before applying any superimposed load or its self weight (while formwork is not removed) Strain arrived in concrete. This is due to changes in temperature and humidity in environment.

The fresh concrete is cohesive and moist; when exposed under ambient humidity, the moisture is lost resulting drying shrinkage. This not the only causes of shrinkage strain but also there have another agent.

This is heat, produced due to hydration of cement and when hot concrete element is cooled under ambient temperature, shrinkage also attributed.

When a massive element (mass concrete) hydrated, a significant temperature rise is observed. The reason of rising temperature is poor dissipation process involved in this large member. Thus significant shrinkage occurs under cooling which is called thermal shrinkage.

The above strains from any sources and reasons are considered detrimental when concrete member is restrained against movement, tensile stress is generated.

The tensile stress in most case associated with cracks upon exceeding tensile strength of concrete. We know concrete have mere percentage of tensile strength as compared to compressive strength.

These cracks are fought by embedded steel in suitable location and suitable proportion. We have provided ACI code provision for temperature and shrinkage reinforcement for concrete in our previous post.

Workability Requirement for No Fines Concrete

Dear reader we were discussing about no fines concrete throughout last two posts and including this post, several posts will be published very soon. Workability of concrete is very important property that defines ease of work with concrete. We know different tests are conducted to examine workability of normal concrete.

But these have no workability test for such concrete; we know no fines concrete is special type of concrete where fine aggregate is eliminated; therefore workability requirement is also changed. Here only visual inspection of concrete is enough where we will examine whether or not coarse particles is coated with cement paste sufficiently.

We know a thin cement layer is expected over coarse aggregate, therefore no-fines concrete should be placed (conveyed and compacted) very rapidly to avoid drying out of thin cement paste coating. An improperly placed (here delayed) concrete will obviously produce weak concrete (low strength).

So, though simple test is enough to check workability but great care in placement is important to produce desired strength of such concrete. Dear reader in our next post we will learn about compaction of no fine concrete. Till then good bye.

Aggregate for No Fines Concrete

Dear reader in our previous posts we have learnt that no fines concrete has large voids and low density. Here we will learn about aggregate for this concrete. In normal concrete, the density of concrete depends to some extent on aggregate grading.

In no fine aggregate also grading is important parameter in analyzing density of such concrete. Density varies primarily with grading of aggregate. Dear reader we hope that you know about well graded, gap graded and uniform graded aggregate.


Well graded mass of aggregate can be packed to greater density than uniform graded aggregate or gap graded aggregate. If aggregate mass have same size of aggregate (uniform graded) low density concrete will be obtained.

Regarding aggregate size we have included:

Usual aggregate size ranges from 3/8” to 3/4".

Tolerance is

• Oversize 5 percent


• Undersize 10 percent

The limit of under size aggregate should be greater than 3/16 in.

Elongated and flaky particles as usual have to be avoided. Sharp edge and low strength aggregate should be avoided; this due to local crushing under load, the edge may break down.

To ensure uniform coating around aggregate, damp aggregate should be used in mixing.