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.