How are Strength and Permeability of Concrete Interrelated?

Now-a-days durability of concrete is major concern in mix design and proportioning and permeability of concrete is a way to penetrate chemical agents and as well as physical damage. So significance of permeability has to realize clearly and proper mitigation measure is required.

Strength of concrete is the most important property which is interrelated to all other properties. Here we will find relation between strength and permeability. We can easily realize that the smaller porosity will produce stronger concrete.

Again amount of void in concrete defines it permeability property. So both term is interrelated through capillary porosity.

So it is considered that the measures influencing strength also influence concrete permeability. Reduction in capillary voids of paste matrix obviously reduce permeability and also increase strength of concrete.

Say with low water-cement ratio we can increase strength of concrete and also the permeability of concrete also reduce. The figure shown below provides exponential relation between strength and permeability with porosity.

Do you know usual porosity of cement paste and natural aggregates? 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 exceed 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.

Does Inclusion of Aggregate in Concrete Increase Permeability?

At first we will learn about theoretical effect of aggregate inclusion. Aggregate may be of highly permeable or of low permeable; let’s consider low permeable aggregate for concrete. Cement paste in compare to aggregate is highly permeable especially in concrete of high water/cement ratio (at high capillary porosity at early age of hydration).

The expected performance is- final concrete will be of low permeability. The theory behind this is- aggregate particles in the system provide an obstruction of flow channel within the matrix of cement paste.

So it is expected that neat cement paste with respect to concrete or mortar have high permeability. Thus aggregate will be used with advantage. Is the practical situation are the some?

(a)

(b)

The answer is definitely no. In practical field the inclusion of aggregate in cement paste or mortar results increasing permeability and also in significant degree.

The larger in size of aggregate, the greater will be co-efficient of permeability. The typical permeability co-efficient of different concrete are given below: 

Low strength concrete defined above usually used for dam construction. Influence of maximum aggregate size and w/c ratio on permeability of concrete shown below.

What is Soluble Soil? How does Foundation Settlement Occur in Soluble Soil?

Soluble soils have particles that are soluble in water. These types of soils are found in many region of the world but they commonly found in arid region. The soluble soil particles may be

-halite

-magnesium carbonate

-calcium carbonate

-gypsum

Halite particles are salt; the deposit of which may form in

-salt playas

-sabkhas

-salinas

The mineral, calcium carbonate, we are discussing here is called caliche and gypsum is known as gypsiferous soil. Sabkhas is marshes of coastal salt.

Dear reader we are discussing here very dangerous type of soil that may result severe foundation settlement under infiltration of water. In arid region the water infiltration is very low and before development no problem is arisen.

When a project is developed or surroundings are developed, water is available from many sources like leak in water or sewer pipes, irrigation process or watering in garden/lawn. This water when infiltrate into ground results settlement of finished or under construction structures.

What are the Features for Engineering Classification of Soil?

Requirements for effective and ideal system for soil classification are as follows:

a. A scientific approach is developed for this system

b. The system should be simple

c. Subjective element should be removed as far as possible in rating soil

d. Based on only few identical properties, a limited No. of different groups should be formed

e. In making such group, the selected properties, upon which grouping is done, should be useful in practical engineering applications.

f. A fair accuracy is obtained, in indicating probable performance under particular field condition of soil.

g. The classification terminology should be based on well known and accepted soil terminology. This helps one to understand and use the terms easily.

h. The tests required to classify soil, to belong to certain category, should be of manual test and also they should be simple visual test. It is expected to limit tests as few as possible.

i. The group definition in soil classification should have such boundary that significant variation in properties of soil is identified clearly. j. The classification system should be such that all engineers accept it.

Now do you think all requirements can be fulfilled? if expect this, it will be very ambitious about these requirements for soil classification, as soil is at a time complex and heterogeneous material and it is not possible to group it in simple classification. So system of soil classification is likely satisfactory for only the specific type of project considering which system is developed.

What are the Common Aggregates for Concrete?

The commonly used concrete aggregate are sand, crushed stone, gravel, crushed blast furnace slag and rarely or experimentally waste from demolition and construction. Here we will discuss about fine and coarse aggregate. We have already learnt that 4.75mm size is demarcation line between coarse and fine aggregate.

Gravel is formed by disintegration of rock by weathering; it is used as coarse aggregate for concrete. There have another importance of gravel in embankment and dam construction; it is very much important element of granular filter.

Sand is the most common fine aggregate in concrete industry. It is formed from the natural weathering of rock or crushing stone. We have discussed about recycled sand like crushed glass sand. This fine aggregate (glass sand) have adverse impact on concrete hydration generating alkali-aggregate reaction which may be controlled by pozzolana or other supplementary cement aggregate.

Crushed stone is generally produced by industrial crushing of

-rocks

-boulders

-large cobblestones

It is very popular coarse aggregate in concrete industry.

Some by-products of industry are also now using in concrete advantageously. Blast-furnace slag is a by-product in iron industry which is crushed to form aggregate. The blast furnace slag is solidified under slow cooling in normal atmospheric conditions before crushing.

Aggregate derive from demolition waste and also from construction includes-recycled concrete, stone rubbles or bricks.

African Soil Stabilization for Mud Wall Rendering

In various part of world including Africa still mud wall is serving as building component. We, here, will discuss about African technologies to stabilize soil for protecting mud wall.

We will discuss about traditional materials more accurately natural materials as stabilizing agent. The materials have range from plant extract to cow dung. They are mixed with soil to render mud wall and purpose of treatment is to provide water resistance.

We will discuss about the country like Ghana, Upper Volta, Nigeria, Sudan and Ethiopia etc.

Ghana

As stabilizer, an extract from boiled banana stems is mixed with soil. This technique is observed in the northern Ghana where lateritic soil is usually treated in this way.

Upper Volta

A varnish is provided to mud wall which is extracted from a plant local name is ‘am’. The present name of Republic of Volta is Burkina Faso. The country banks at upper part of Volta River which was colony of French community.

Nigeria 

Laso

In northern part of Nigeria, extract collected form vine (Vitis pallida) used as stabilizer for water proof mud. The local name of the plant is dafara.

Makuba

This is made of fruit pod from tree of lowest bean and serve as water proofing agent.

Sudan

In Sudan mud houses known as ‘Jaloos’ are treated by water proofing materials produced locally from horse or cow dung. The local name of this is ‘Zibla’.

West Asia

Since biblical period, straw is using in this purposes with mud.

Ethiopia

Straw is used to mix with soil paste having local name chika. They prefer straw of maize known as chid.

Why is Permeability of Mortar/Concrete Greater than that of Cement Paste?

Permeability of system depends on following parameter of capillary voids:

-size of voids

-volume of voids

-continuity of voids

In case of heterogeneous material like concrete or mortar, there have also another consideration of zone that remains between cementing medium and aggregate.

This zone is interfacial transition zone which lies between cement paste and aggregate not between fine aggregate and cement paste. ACI code specifies maximum size of coarse aggregate which are not discussing here.

Maximum size of aggregate influence interfacial transition zone and also microcracks remains in this zone. In our previous post we have discussed how is permeability associated to cracking in this zone?

In cement paste the interfacial transition zone is absent thus reducing microcracks. The differential strain is also less as homogeneous consistency remains in this system. The absence of cracks more accurately less cracks in cement paste makes them more resistant to permeability.

In case of concrete or mortar, the microcracks are generated due to shrinkage cracks within interfacial transition zone; while interfacial transition zone is weak in early period of hydration which ultimately interconnected to form a continuity to make water to flow thus increasing permeability.

The permeability of aggregate also have influence on permeability of concrete, the co-efficient of permeability of different aggregate and their influence on concrete permeability will be discussed in our next post.

Relative Comparison of Stabilizer for Soil Block

We know cement, bitumen, lime and industrial by-products are used as stabilizers. Of these cement is most popular and used extensively. Soil that is stabilized with lime show less water resistant and less strength relative to stabilization with cement.

But both types of block can be produced; here more perfect word is compacted by Cinva Ram. Now what is Cinva Ram? It is a block making device, obviously block of stabilized soil.

Now we will learn about some block making devices. We have already introduced Cinva Ram other popular devices are

-Balram

-Astram

-Ellson block master.

Of these Astram can produce about 500 blocks on average. Production rate of Balram is about 1000 blocks. Again production means pressing stabilizer soil to desire shape, size and expected pressure.

The block size can be pressed by Astram is 23X19X8 cm, that of Balram is 23X11X7.5 cm.

The purposes of stabilizing are

-To increase bearing strength of soil block

-To improve their resistance against moisture or water

All stabilizers improve these properties more or less, one can improve strength but may have poor performance against water repellent property, some perform fair in both properties of block. Cement bricks are produced by treating soil generally with 5% cement and bitumen-treated bricks are stabilized by 1% bitumen.

The strength and moisture repellent property will be our concern here. Cement stabilized bricks have better strength but have poor performance in water/moisture resistance in relation to bitumen stabilized blocks. In our last part we learnt that cement blocks are both stronger and more water/moisture resistant than lime stabilized block.

The bearing strength of cement block and bitumen block are respectively 4 N/mm² and 3N/mm².

So where water resistance is the primary criteria we should go for bitumen as stabilizer and where strength is main criteria we should go for cement as stabilizer.

The promotion of soil block is patronized by ASTRA (Association of Science and Technology for Rural Area) at Indian Institute of Science (IISc) and NBO RHDC both of Bangalore. They played most important role in introducing soil block as building construction materials in Karnataka.

CBRI, Roorkee have conducted extensive experiment over bitumen stabilized block to decrease quantity of bitumen by replacing industrial by-products like bagasee and flyash. They found that with the application of by-product as additive bitumen quantity of bitumen can be reduced up to 0.5% from 1.0% as usual as stated above. Rice husk has been tried also for stabilizing soil and found promising results. Dear reader we have discussed about rice husk as pozzolana for concrete in previous post.

Influence of Microcracking on Permeability of Concrete

Dear reader we have heard in previous posts about microstructure of concrete, here we will learn about a zone, an essential element of microstructure that control almost all properties of concrete which lies between aggregate and cement paste.

This zone is interfacial transition zone. In this discussion it plays important role, there lies microcracks in this zone. The extent and quantity of microcracks in this zone will be discussed in next post.

We know size of aggregate and its grading as well have influence on bleeding characteristics within concrete mixtures, thus influencing surrounding interfacial transition zone of aggregate.

During early age of concrete when hydration commenced for a while, the interfacial transition zone remain in week condition and have every possibility to be cracked. We know due to thermal shrinkage and drying shrinkage, differential strains are produced between aggregate particles and cement paste. The external superimpose load may also produce such strains.

The strains from these sources lead to crack in weak interfacial transition zone. This cracks generated in interfacial transition zone can not be seen with our naked eye. But don’t feel safe as they are sufficiently bigger than most of capillary cavities present in matrix of cement paste.

This system of microcracks is inter-connected by propagation process and increase permeability of system significantly. This is the microstructural cause of development of permeability.

What are the Additives for Soil Block Stabilization?

Extensive researches were and are conducting for stabilizing of soil for different purposes, may be for bearing foundation or making soil block. We have defined stabilized soil block in our previous post. Around 130 stabilizing agent are identified and tested for stabilization purposes including lime, bitumen and cement.

It is necessary to test thoroughly to identify perfect stabilizer to be used in soil. In our following discussion we will provide information about suitability of different additives off course for soil stabilization.

At first we will start with cement; the soil that is most suitable for cement stabilization should have particles size distribution of having fine clay.

We should keep in mind that we are discussing mainly about stabilizing soil block. Some aspects may match with soil stabilization for foundation bearing or other purposes of soil treatment.

We have to keep in mind that organic soil should be handled carefully and not suggested to provide stabilization. Dear reader we have published many posts about problematic soil like organic, black cotton soil, collapsible soil in our previous posts.

Highly plastic soil has also treated same to organic soil. This type of soils is very clayey and generally as stabilizing additive lime is chosen. In case of sandy soil bitumen is suitable additive. For the stabilizer discussed above and to be discussed, the quantity depends on type of soil. A rough guideline is provided below for normal situation; it should keep in mind that all essential tests have to perform to achieve desired soil block from a selected type of soil.

Sandy soil: 5% cement as stabilizer

Clayey soil: (5-6) % hydrated lime +2% cement

Ordinary red soil having clay content up to 25%: 2.5% hydrated lime.

The percentage, provided, of additives are observed to be most suitable to reach

-Desire strength of stabilized blocks/bricks

-Weather resistance of desire degree

-Economy of production

Industrial by-products like flyash can also be used as stabilizer and many study is conducted over the application of these by-products in different field of civil engineering like concrete technology, soil stabilization etc.

The cementing medium produced by combining lime with flyash performed almost comparable to Portland cement for both sandy soil and for clay soil they are used successfully.

Advantage of Stabilized Soil Block over Burnt Brick

Stabilized soil block are block formed by stabilizing soil with cement, hydrated lime, clay, bitumen and industrial by products like bagasse and well known flyash. They can be used as building block.

We all know that burnt brick is the most popular building block all over the world. The main problem with this is emission of green house gases and subsequent pollution to environment. The energy consumption is also high. Normally coal and sometimes gas are used to burn raw brick specimen.

Stabilized soil block in contrast to burnt brick show significant saving (about 70%) of energy. Regarding cost involvement it is also cheaper than burnt bricks by (20-40) %.

A study was undertaken by Karnataka housing board on performance of stabilized soil blocks that are used in house construction found that such block may have compressive strength of the order of (3-7)N/mm².

When we are concern about conservation of energy and environment as well, we need building materials that require less energy of production and soil block provides us a way to achieve this goal. In engineering requirement like strength it also provides sufficient strength to construct two storied building.

A compressive strength of 3.5 N/mm² can be achieved by stabilizing soil with

-5% cement

-or 2.5% cement +2.5%hydrated lime

In Indian context, soil block is replacing burnt brick gradually especially in Karnataka. Many private buildings in urban and rural area are now constructing with this block in southern states and becoming popular as building construction materials.