Influence of Aggregate Shape on Water-Cement Ratio of Concrete

Water requirement for concrete to be compacted fully for given effort or practical effort is a measure of workability; also placing without segregation and efficient finishing are also important in workability of concrete. A workable concrete is desirable in concrete engineering to facilitate placing, compaction and other subsequent operations.

Water required for particular mix design is measured by the weight of cement as a ratio of it. Now we will learn about the influence of aggregate shape over workability of concrete. We concrete engineers are familiar with aggregate shapes of 

-round

-Angular

-elongated

-flaky etc.

Now we have to consider which shape has more surface area and posses more voids which are very important in measuring water requirement. Actually, angular, flaky and elongated shaped aggregate produce a concrete of harsh property while round or cubical shaped aggregate results a concrete of smooth, less frictional resistant i.e. can be compacted easily with a less compaction effort.

Later three types of aggregate have more surface area and more voids as well which leads to more placement and compaction difficulties. These types of aggregate require huge water to have only optimum compaction which leads to selection of large water-cement ratio. We have discussed about relation between strength and W/C ratio of concrete.

In case of round shape aggregate, comparatively less volume of water produce concrete of more workability. Dear reader you perhaps read our posts regarding high performance concrete, where one of the main aims is to reduce W/C ratio to minimum value. In many cases only 0.25 W/C is expected. Though this degree of W/C ratio cannot be achieved only by selecting right aggregate shape, but aggregate shape plays a important role over there.

The advantage of round aggregate is observed in case of river sand or gravel. This aggregate provides greater workability in comparison to aggregates that derived from crushing.

Belt Conveyors in Concrete Transportation

Dear reader we have discussed about different way of transporting/handling of concrete in delivery or placing end. Still some methods haven’t published. In this post we will discuss about belt conveyors, sometimes called concrete belt conveyors.

So far in concrete transportation system, belt conveyors are used with limitation extent considering some problems associated with this type of concrete transporting. The main objection against this system is segregation tendency of concrete which is not allowed in concrete engineering. We have discussed different aspects of segregation in our previous post; you can read these for more information.

So we were on segregation. The main sources and locations of segregation tendency are:

a. Transportation on steep inclination

b. Where directional changes are provided

c. In transfer points

d. Just above the roller while belt passes over it.

e. Vibration of rubber belt is also responsible for concrete segregation

Another shortcoming of this process, not included above, are drying and consequent stiffening of concrete while it passes over long distance exposed to hostile ambient environment. When this exposure is hot, windy or dry weather, the losing of moisture takes place severely. In most cases it is required to remix concrete at delivery end before final placing and subsequent compaction work.

With above bad words, you may be confused about applicability of belt conveyors. But technology is advancing rapidly to facilitate transporting concrete process and it brings some modification in conveyors system. Modern conveyors have adjustable reach, variable speed towards back and forth, travelling diverter. This system extremely useful when enormous amount of concrete has to be transferred quickly through a relatively less accessible area.

We have discussed about losing moisture; in this regard it is wise to cover the belt partially or completely depending on weather condition. Portable conveyor system is available for transporting short lift or distance. The discharge end must be designed such that the arrangement minimize segregation of concrete and ensure discharging of entire mortar form belt.

Cement Weighing and Error in Concrete Mixing

Though the title of this post is cement weighing, we will discuss about consequence of not weighing cement in concrete batching. In small concreting works, it is usual to add cement as bag considering each bag having weight 50 kg. Cement bags are considered 1.25 cubic feet and weight of 50 kg.

In actual condition this 50 kg bag, due to inherent properties of products and as well the quality and efficiency of machines, may not have exact 50 kg of cement. So an error limit has to be set for packaging of cement. In this regard, Standard of Weights and Measures (packages) Rules, 1977 provides margin of variation by 1% (this may be short or excess in quantity) for each bag.

Cement is packed in factory as 50 kg per bag; but due to error in packaging, stated above, and also during transportation and unloading and reloading operations in different places, some cement may be lost. This is prominent when jute bags are used in packing.

Now we are receiving cement bags of less weight and sometimes this may lack 5 kg or more of cement i.e. lacking more than 10% of weight and designing mix proportion according to volume batching, considering 1.25 cft obviously lead to error. This error is also observed in weigh batching when 50 kg bag is considered.

Now-a-days we are using plant-mixed concrete, with either hauling equipment or sometimes some mixing is left for agitating hauling equipments. They are well known as ready mixed concrete (RMC). We have provided numerous information about ready mixed concrete from our experience and from ASTM and ACI specifications and requirements in this blog. In RMC, weigh batching is done successfully and economically. In most cases, in Indian subcontinent, volumetric batching are used and we experienced in many cases excess consumption of valuable components of concrete. So for important and large jobs of concreting, cement should be measured accurately to have exact cement content in concrete for expected mix proportioning.

Bearing Capacity of Plastic Silt and Clay Soil

We have discussed about difficulties in piling in plastic clay, contact pressure of saturated clay, heaved piles, and numerous posts about expansive clay soil specially on black cotton soil/regur. Our posts on expansive soil become very popular. Dear reader here we will discuss about this clay soils and determination approach of bearing capacity of clay. Here we are grouped plastic silt and clay in same class as plastic silt in saturated condition act like medium clay and sometimes soft clay. So in foundation design approach they are treated as same class.

We know that primary bearing capacity of soils depends on shearing resistance of them. Being relatively fine grained the pore water pressure cannot be released easily in these types of soils and they can be assumed impermeable to some extent especially just after load is applied to the soil. Thus undrained condition prevails.

So when structural loads are transferred to these soils in saturated conditions through foundation, excess pressure due to pore water is generated in fairly impermeable medium of soil mass and there have no way to dissipate these pressure quickly.

So for a short period just after loading, undrained condition leads to Φ=0 analysis. In deriving strength, undrained shearing strength is used. We know that this strength in one half of unconfined compressive strength i.e.

S=C=1/2 X q_u

Considering the consolidation of such soil, the results derived on the basis of Φ=0 analysis are on safe side. Dear reader we will discuss this term elaborately in the next post. The thin walled tube samplers are successfully used to take specimens to set under laboratory testing but more accurate methods can be used depending on economy. In our sister blog “structural concrete foundation engineering” we have published a post discussing elaborate method of exploration of this soil in relation to economy.

Behavior of Concrete Microcracking Under Loading

Cracking of any form can lead to failure of concrete members. So in concrete engineering, this term is studied and handled with care. We have provided a basic idea about microcracking in the last post; here in this post we will discuss about the cracking pattern, elongation of cracking and appearing new cracks under loading. We have already learnt that this types of cracking remains in the concrete even in preloading state. Let’s learn about what happen when concrete becomes stressed to its ultimate loading.

Microcracking in concrete under
lightmicroscope (Green llines)

Microcracking is determined by optical microscope as it is often not visible under human eyes. But a upper limit of cracking (0.1 mm) that can be detected by eyes without any visual instrument, is found in some findings. For engineering applications a lower limit of cracking can be set.

We will discuss about behavior of microcracking under cyclic loading in the next post. In this post we will discuss about gradually application of loads. The microcracks show stability under increasing load up to around 30 percent of ultimate load. Beyond this stress the cracks expanded with their number, length and width. As we discuss in the last post the cracking weaken concrete under its tensile stress.

The white lines are mortar cracks and black lines stand for combined cracking in concrete under uniaxial tension

Now the stress level depicted above is not fixed, it’s have sensitivity of W/C (water to cement) ratio in the paste. But in this stage cracks propagates slowly.

So far we are talking about cracking between interface of cement paste and coarse aggregates. When stress reached near ultimate capacity of concrete, cracks formed in the mortar i.e. bond between fine aggregate and cement paste are destroyed with the gradual increment. 

When load is increased to (70 to 90) percent of ultimate strength of concrete, cracks make their way through the mortar. A new pattern of cracking in concrete in continuous form is developed which release the bond between fine aggregate and cement paste.

The cracking state discussed above is fast propagation stage. As discussed in last post, reaching stable stage depends on water to cement ratio. Here, in this fast propagation cracking stage, the starting point depends on strength i.e. initiation of this stage is higher in higher strength concrete and lower in normal strength concrete.

Higher strength concrete shows better performance than normal strength concrete in both stages i.e. the number, width and length of cracks, in all respects, high strength concrete performs well. Cumulative length of cracking of such types is measured using the neutron radiography. The high strength concrete possesses lower values in cumulative length of microcracks as usual.

The commencement of fast propagation stage of cracking corresponds to discontinuity in volumetric strain. We will discuss about volumetric plotting in determining poisson’s ratio elaborately in our upcoming post. Here we are providing a basic idea about this. We all know about stress-strain graph; when volumetric strain is counted as strain and steady but rapid increasing of loading is applied, above particular stress level, poisson’s ratio shows rapid increment, due to formation of extensive vertical cracks. When stress level is increased further the volumetric strain variations changes sign; we are talking about this point.

What would be under sustained loading? Under sustain load failure is matter time. Both high strength and normal strength concrete fails under this sustained loading. 

Structural Sandwich Without Earthquake Agitation, Savar Bangladesh

People are asking their engineers about earthquake safety and vulnerability of their dwelling or owned structures. But we have seen structural collapse of huge commercial building without any influence of earthquake (24th April, Rana Plaza, Savar, Bangladesh). We will discuss technical issues of this structure; not political, administrative or legal factors.

This building was used as garments factories of several owners with markets and office spaces for institutions like bank etc. Being commercial building, to have uninterrupted electricity supply, several generator were used there. BGMEA confirmed that during collapse 3122 workers were working and a total 5000 workers were employed in different floors of garments factories. The rest workers do not joined work due to panic of cracks observed 23 April, the day before collapse.

Now come to the point earthquake. The construction industry of Bangladesh is not quite good. Here workmanship of worker is low and also many owner and construction contractors are looking for cheap, low quality work for more savings. Many building of the major cities like Dhaka and Chittagong (not considering suburban or rural areas) were constructed and still constructing disobeying rules of local and government authority.

Dear audience the image above was taken from the collapse in Christchurch earthquake, which resembles somewhat to rana plaza. The difference is that there was a release of huge strain energy due to deformation of plate boundary below South Islands (Australian plate and Pacific plate). The energy released by this earthquake was 6.3 (in magnitude scale). In savar not such agitation was felt. The structure was collapsed due to service loads, unexpected vibrations and its own weight.

The next image below was taken from Erics, Turkey; this failure seems more close to savar collapse. But this collapse was also associated with an earthquake of magnitude 7.1.

The last figure was taken from Mexico City. Here we can notice that bottom five floors were sandwiched. But this was due to one the great earthquake of the world; Magnitude 8.1 Mexico earthquake. The bottom floors had mass irregularity and sandwiched.

The structural irregularity and system limitation are published in this blog; you can visit this post for more information. We can conclude about rana plaza that there was instability in foundation, mass irregularity, inadequate confinement in joint of members (lack of ductility) poor construction materials and more prominently the lust of owner to be richer.

Now we are sharing some images of rana plaza collapse below. The structural weakness of this death trap will be discussed in our next post. Till then good bye.

What are the Naturally Occurred Pozzolanic Materials for Concrete?

We have already learned about pozzolana reaction in concrete. These materials may be natural and artificial. The artificial pozzolana that is well known is fly ash. We have published many posts about fly ash and will publish several posts regarding fly ash in this blog. Here our concern is natural pozzolana.

At first pozzolana meant to volcanic ash. This volcanic ash was used by Romans in Pozzuli. Pozzuli was a city of Naples province in Italy region. The original volcanic ash is pumicite.

Other than pumicite

-opanline

-shales

-santorin earth

-cherts

-burnt clay

-diatomaceous earth in calcined form

ASTM C618 defined above materials as class N. With these, rice husk can also be used as pozzolana for concrete. We have discussed about the application, properties and limitation of these in the previous post. There have pozzolanic cements which contains certain content of pozzolana which needs not to add separate pozzolanic materials. We will discuss this later.

These natural pozzolanas always have more or less some problems and these encourage to find and deriving artificial pozzolanas. We have mentioned that calcined diatomaceous earth as natural pozzolana. As an example we can include this pozzolana-

The physical properties of such pozzolanas results problems like these materials are angular and are also porous which requires more water to have optimum workability.

To avoid these problems some improvement techniques we applied. The most common is calcinations of them between (500-1100)0C; the temperature depends on the matter to be improved. Dear reader we are finishing this topic here. In the next post we will discuss calcined kaolinite clay to be treated an pozzolanic materials for concrete.

Application of Rice Husk for Pozzolanic Concrete

Dear reader we have discussed about pozzolanic reaction in the last post. Now we know that pozzolanas are siliceous or siliceous materials with aluminous content having some values to alter concrete properties. Here we will discuss about a siliceous material that is usually ignored in concrete production.

Dear reader we all know that rice husk is a waste product. This waste product is enriched with silica in very high percentage which can be used in concrete with some limitations. We will learn about properties and limitations in the next paragraph.

The rice husk is fired slowly and reached to (500-700)0C to produce an amorphous form of it which have porous structure. This ash derived from rice husk has very complex shape like origin of the plant. The specific surface of this ash is very high which can be reached even 50000 m2/kg; but has large particle size of between 10µm and 75µm.

These properties discussed above seek a greater water requirement when used in concrete. To have necessary workability and high strength, superplasticizers are used. But application of superplasticizers, reduce economy derived from using rice husk where processed rice husk is not easily available in any territory.

This ash is found effective in producing concrete strength at only (1-3) days. But there use in concrete may lead to increment in shrinkage.

Dear reader we are finishing this topic here, in the next post we will discuss about another amorphous natural materials having pozzolanic values, can be used in concrete advantageously.

Most Common Artificial Pozzolana for Concrete

In the last few post we have discussed about natural pozzolana that can be used in concrete to enhance it properties. We have discussed about volacanic ash, different siliceous clay, rice husk etc as natural pozzolana for concrete. Here, we are introducing an artificial pozzolanic material that is very common in concrete production. It is sometimes used in production of high performance concrete. We have published many posts regarding high performance concrete in this blog and other blogs of same authors. Follow the link to learn more.

Dear reader this is fly ash, also termed as pulverized fuel ash. The fly ash is derived from power station which uses coal as fuel. This is an ash that is precipitated mechanically or electrostatically form exhausted gases of such power station.

 The particle shape and size of flyash is very suitable for pozzolanic purposes. The particles are of spherical shape which is very important in regards of water requirement. The size is also very fine; diameter of majority particles lies between 100 µm to even less than 1 µm.

The specific surface of flyash particles are between 250 m2/kg and 600m2/kg when bline method is used in deriving them. The calcium hydroxides formed by the hydration of cement paste can react with flyash more effectively due to specific surface of such higher degree.

Dear reader we have discussed about the classification of flyash in our previous post. In the next post we will discuss about each classification elaborately.

What is Pozzolanic Reaction in Concrete?

Dear reader, we have used the term pozzolanic reaction in concrete in our many posts in our blog and many sites are using this term in discussing pozzolana related concrete properties like strength, durability etc. In this post we will learn about the pozzolanic reaction. Pozzolans are used in concrete to derive special properties, enhancing normal properties of concrete.

The pozzolana may be natural or artificial materials that are treated as cementitious materials in concrete remaining in latent form. These materials contain silica in the form of high reactive state. We are proving information here from ASTM 618.

The pozzolna is a siliceous or sometimes aluminous content may exist in it. The materials often have even no cementitious properties; if have, is in very little possession. Now why should we classified it as cementitious materials; why not filler materials in concrete? The answer is-these materials are always in finely divided state and when found moisture it reacts chemically with the calcium hydroxide at normal temperature and form compounds that have cementitious properties. Dear reader we will know the source of calcium hydroxide; we are escaping this formation. For detail understanding reaction that happened in concrete please follow the link below:

Pozzolanic Chemistry in Cement

We have emphasized about the condition “finely divided” as only in this state silica can react with calcium hydroxide formed by the hydration of cement. Another condition is moisture, that is, water. When these two condition satisfy calcium silicate, a stable form, is formed which have efficient cementitious properties, expected for pozzolanic concrete.

Regarding reactivity, the silica must be in amorphous form, as it is observed that crystalline silica shows lower reactivity, more precisely very low reactivity. Dear reader we are finishing this post here; in the next post we will discuss about natural pozzolana and next few posts will be about many aspects of pozzolana.

Specifications for Non-Agitating Equipment for Ready Mixed Concrete

We will discuss this topic in two parts. Let’s start with central mixed concrete. In this mixing arrangement, concrete is completely mixed in mixer that is stationary in a suitable location and then transported to required site to deliver concrete either by non-agitating equipment, will be discussed in this post, or by truck agitator which is operated at a designed agitating speed. 

The non-agitating equipment have to use in transporting, should be approved by the purchaser. The properties of the concrete also have to be approved by the purchaser with some limitations as follows:

Here we are going to provide some requirements for non-agitating equipment and the final product (concrete) transported by the equipment.

The equipment bodies should be water tight, smooth and metal containers should be furnished with a well equipped gates that will provide sufficient control over the discharging of concrete to the pouring site. For the construction work in extreme weather condition, the equipment should be equipped with suitable covers to have protection against weather as per requirements of purchaser.

Our aim is to have a thoroughly mixed concrete having mass uniformity and also requirements are focused on these facts. We can add that not only uniform mass of concrete in equipment is important but also this should be discharged satisfactorily having sufficient uniformity in delivery end without segregation. The degree of satisfaction is described in uniformity requirements of concrete. Follow the link below to learn about these requirements.

Non-Agitating Unit for Delivery of Ready mixed Concrete

In checking quick assessment of uniformity, slumps are checked for individual samples. The requirements for taking samples are:

The first samples should be taken at approximately after 15% of discharging concrete and second one after 85% of discharging concrete. Sample before 10% of discharging and after 90% of discharging from batch should be avoided. We know there have difficulties in determining actual discharge quantity of concrete from non-agitating equipments. But the samples must not be taken from just from beginning and end portion of a load.

Furthermore other requirements for taking samples is-time interval of taking two samples must not elapse more than 15 minutes. Now question is what should be expected difference between these slumps. The answer also refers to table for concrete uniformity. There we have provided limitations of difference between two samples in respect of slumps. This table also contains many differences of parameters to check uniformity of concrete delivered to site.

Now what should we do, if this equipment provides concrete not meeting concrete uniformity requirements. This equipment should not used until the conditions provided below are not corrected.

We are mentioning that the equipment should not be used when the equipment is in operation for maximum hauling time containing a concrete mix that are mixed for minimum time. But, when the concrete is hauled for shorter periods or mixed for relatively longer time, or combination of both, the non-agitating equipment can be used, if the conditions provide results which meet requirements of concrete uniformity.

Requirements for Batching Plant of Ready-Mixed Concrete ASTM C94/C 94 M

Dear reader we have discussed about measuring specification for constituents of ready mixed concrete earlier and also specification for checking accuracy of scales for correct measuring. Now we will discuss about some requirements of batching of ready mixed concrete. This specification also includes requirements for scales that are discussed in previous post.

The batching plant should be furnished with bins having sufficient separate compartments. These compartments are required to accommodate fine aggregates and coarse aggregates of required sizes. The compartments in each bin should have facility to discharge contained materials easily and efficiently into weighing hopper to have possible minimum segregation. In this regards, it is very important to control discharge into weighing hopper to have precise amount of constituents. So this discharging facility is controlled such that when desired quantity of particular constituent material reached, the discharge is shut with precision.

Weighing hoppers should be designed and maintained such that materials are not accumulated and discharged fully leaving no material in the hopper.

While charging hopper, the indicating devices, for convenient of operator, should be accommodated in such a way that he can read accurately and have a full view of indicator and should have easy accessibility to controls.

For measuring water and other ingredients some scales are used which should be maintained and checked for accuracy time to time. As informed above, the requirements for scales and their calibration specifications are discussed in “Requirements for scales in ready mixed concrete batching plant”. For measuring accuracy of water, we have published “How water is measured in ready mixed concrete batching”. Please follow the link to have sufficient and important information about measuring accuracy and calibration specifications as well.

Dear reader we are finishing here. In the next post we will discuss about central-mixed concrete as specified in ASTM C94/C 94 M.

Expected Strength Properties from High Performance Concrete

The early name of high performance concrete was high strength concrete. Now development of concrete technology expects high durable properties from high performance concrete too. We have learned earlier that constituents materials of this concrete are not revolutionary, rather they are normal materials usually used in construction of normal concrete. This is a controlled development of concrete technology that offer high strength and durability properties for mega structures and corrosion susceptible structures, expected to provide extended life to structures.

We will provide a classification of high performance concrete according to strength. In this post we will discuss about expectations with respect to strength perspectives. The requirements for strength may be very early or after 28 days or regular period of gaining.

Now how much strength makes a concrete to be treat as high performance. This is very difficult thing to make such demarcation about strength range. Once 6000 psi concrete was defined as high performance concrete (HPC) and later the strength requirements have grown up say 9000 psi and we expect that this requirements will grow up and up as construction industry are growing rapidly expecting small member size and always greater durability for a several hundred storied building or for a ultra-long span of bridge with slim members. Dear reader we will discuss few posts regarding high performance concrete having 12000 psi strength.

Strategic Highway Research Program (SHRP) implements different projects and developing concrete for different aspects of highway. SHRP categorize the HPC as follows:

a. Very Early strength (VES)-having minimum strength of 2,000psi at 6 hours

b. High Early strength(HES)-having minimum strength of 5,000psi at 24 hours

c. Very High strength (VHS)-having minimum strength of 10,000psi at 28 days

d. Fiber reinforced concrete-having minimum strength as that of High Early strength but have polymer reinforced criteria.

But in some publications HPC are sometimes classified according to strength like very high performance concrete; our opinion is that a concrete which is upgrading continuously should not be classified in such way.

Now question is which types of samples are taken to evaluate the strength of HPC that is either cubes or cylinders. Dear reader we will discuss about the test procedure and requirements in the next post. The strength criteria include not only compressive strength, but they also include modulus of elasticity, shrinkage and creep. Dear readers we will discuss every term in our blog, please stay with us. We have discussed about durability properties of HPC in our sister blog-“structural, concrete and foundation engineering”. Please follow this link to read this.