What are the Effects of Using Dry Aggregate in Concrete?

In case of concrete mix design, after all considerations for strength and durability, a water-cement ratio is fixed which is followed by necessary adjustment due to moisture deficiency or excess in aggregate mass. The normally adjustment is deficiency or excess amount of water is either added or cut off from water-cement ratio.

But this calculation is not so easy; as dry aggregate mass cannot absorb all moisture that is deficient in aggregate. Now several questions may be asked:

Is all moisture deficiency absorbed?

If not how much moisture is absorbed?

What are the effect of this on reach and lean concrete mix?

In case of coarse aggregate there have also movement of moisture form surface to interior mass of aggregate, which is not usual in fine aggregate. We have provided idea about pore size and porosity of aggregate in our previous posts.

Now why should we discuss about dry aggregate?

We can change in moisture condition before mixing in concrete. In case of dry and hot climate, excessive moisture may be loosed from aggregate before mixing and in some batching plant where conveyor belt is used; the dry air can absorb moisture from aggregate. Dear reader we have provided ACI code provision for hot weather concreting and also requirements for using conveyor belt in concrete production.

It is usually assumed that required water will be absorbed by aggregate to reach saturated condition. In dry weather, where there is no way to allow aggregate to lack moisture, actually aggregate cannot absorb sufficient water required for saturation.

This is due to rapid coating of aggregate with by cement paste. The coated aggregate losses its water supply from the concrete mix, required to reach saturated and our assumption becomes invalid.

This condition is prominent in coarse aggregate as it has option for water to travel from its surface to interior pores remain in aggregate. So the water-cement ratio that we assumed becomes higher than that of actual case of partial absorption of moisture into the aggregate.

The term ‘rapid coating’ is not always valid as it depends on cement or cementitious content in concrete mix i.e. coating rate becomes slower where cement content is low. So in case lean mix this partial saturation situation is not valid. So water-cement ratio is carefully set in case of rich mix.

The water absorption of desired aggregate for duration identical to the tentative coating time of aggregate is examined. Depending on the result (degree of saturation) water-cement ratio is designed. It is interesting to know that the behavior of mix is actually affected to some extent depend on sequence of feeding, in to the mixer, different ingredient of concrete.

What are the Influences of Porosity on Workability of Concrete?

Dear reader we have learnt some aspects of porosity aggregate of concrete in last posts. Where we come to learn that water absorption is related to porosity of aggregate. We have already learnt about different state of moisture in aggregate.

Normally, we consider that while setting of cement paste around aggregate, it is in the state of saturated and surface dry . But if it remains in dry condition several phenomena can be happened; but won’t discuss these here, we will learn this in next post.

What do you think about absorption of gravel and crushed rock? The water absorption of crushed rock is much higher than gravel due to inherent porosity in it as it is subjected to weathering and outer layer have a porous texture.

So if aggregate do not remain in ideal moisture condition, it will absorb water from concrete mix and due to their porous texture mobility of concrete is also reduced.

This result reduction in workability of concrete up to some duration. Are you surprised? Yes, reader, this reduction becomes small beyond 15 (around) minutes.

So it is not significant to determine total water absorption of an aggregate sample; more significant is a duration of (10 -30) minutes. Now question is why water absorption reduced or nullified or nullified after some duration? The answer is the aggregate particles are coated and water absorption is reduced.

State of Moisture Content of Aggregate for Concrete

We have already learnt that aggregate have pores which can absorb water form environment and also from fresh concrete. The absorption depends on pore size parameter as discussed in previous post. Here we will learn about state of aggregate depends on moisture content. These are

a. Oven dry

b. Air dry

c. Saturated and surface dry

d. Moist

We will learn each state of moisture and significance of them.

Oven dry

This is an artificial condition where prolonged drying of aggregates in oven reduces all moisture from them. This condition is sometimes called bone-dry. Thus the aggregate is completely dry on the other hand means fully-absorbent. This is usually done in laboratory for inspecting different properties of aggregate.

Air dry

The aggregate have some moisture but dry at surface. The moisture content is less than saturation. That means there have some degree of absorption affinity.

Saturated and surface dry

Usually known as SSD. This state of moisture defines all pores of aggregate are filled but have a dry surface. Thus the aggregates are saturation-surface dry.

Moist:

These have moisture in surface looks darker and which leave water to concrete mix.


These four states are very important in proportioning concrete mix. In mix design there has a step known as moisture adjustment where the moisture content of aggregates is considered to adjust W/C ratio. That is; if aggregates are in unsaturated water should be increased, in case of moist aggregate, the change is opposite to unsaturated condition.


As SSD is an ideal condition and difficult to maintain in aggregate, moisture adjustment is required.

Pore Size of Aggregate for Concrete

We know that concrete aggregates inherently have pores and their sizes can vary over a wider range. Of these, the largest may be only visible with microscope but sometimes they can be seen by naked eyes. In previous post we have learnt about the importance of pore in assessing concrete strength and durability.

So we have idea about size of pores in aggregates (maximum limit). But get pore of cement paste is smaller than the smallest pore size of aggregates. The common aggregates we used in concrete construction have porosity of (0~5) percent. Here we are providing porosity of some rocks and unconsolidated deposits below:

 

Pore size less than 4 µm in aggregates is considered especially as such aggregates are susceptible to be badly affected by alternating freezing and thawing.

Some pores don’t comes out to surface remain within solid volume but some pores reach to surface of aggregate particles. Cement paste have high viscosity than water so it cannot penetrate through the pores to a considerable depth.

Unlike cement paste, water can penetrate pores, thus size and extent of pores affect water demand of concrete. The degree of penetration depends on pore properties such as

-size

-continuity

-total volume

Aggregates, since, occupies three-fourth volume of concrete, it is obvious that a concrete produced from porous aggregate materially becomes porous concrete.

Why does Unsoundness Arise in Concrete Aggregates?

Dear reader we have some idea about soundness in our previous posts. Now we can differentiate volume changes depending on physical and chemical causes. Here we will provide reasons and particles that produce volume changes, that is, leads to unsound aggregate in concrete.

Impurities and aggregates itself responsible for unsoundness which can deteriorate concrete as discussed in last post. The causes are:

a. Porous flints

b. Cherts

c. Some type of shales

d. Limestones having thin layer of clay that are expansive.

e. Montmorillonite clay

f. Illite clay

Of these cherts and flints that are of lightweight having pore structure of fine textured cause expansion. Two clay minerals of montmorillonite and illite mean not only specific minerals but also the groups they hold.

Regarding flint we can include that it is a form of mineral quartz and fall in category of chert. We have discussed many posts about montmorillonite and illite in this blog.

For example, consider diabase which is used in construction industry as stone (crushed) is subjected to volume changes of the order of 600x10-6 in dimensions when set to wetting and dry cycles Which indicates that concrete made with such aggregates will also suffer such volume expansion and we can imagine it also performs same function in freezing and thawing.

Same disruption also observed in case of porous chert. Regarding diabase, we can include, it is also known as dolerite. It is volcanic basaltic rock used in concrete as crushed form.

Sampling and Recovery of Residual Soil

Subsurface investigation is essential for design with reliable safety. As discussed in last post, residual soils are competent but there have also exceptions which require careful investigation of the soil.

We have discussed about different sampling methods of soil and also discussed about difficulties in sampling of different types of soils. From the definition of residual soil, we know these soils hold their position even after weathering and weathered soil remains on the top of parent rock. 

 

Thus finer particles remains on surface and the particle size increases gradually with depth and larger particles remain at the interface between soil and parent rock.

While drilling for collecting sample, difficulties arise as rock fragments are encountered after drilling up to few depth. These depths depend on weathering condition and types of rocks. Ordinary drilling is often cannot serve the purposes.

In case of ordinary drilling, a hole becomes abandoned, in many cases, due to rock fragments and it is required to drill another hole, few feet apart from previous one.

When residual soil is saprolites, very often rock coring techniques have to apply to collect samples. The samples are tested for ascertaining probable settlements and bearing capacities to design foundation or to determine number of piles. Sample recovery, for such soils, and laboratory testing procedures are almost similar as that of sedimentary deposits.

Residual Soil: Laterites and Saprolites

We have discussed many posts about residual soil; here we will discuss about some residual soil. Our concern here is laterites and saprolites and there extent in the world.

Both soils are residual but formed by different weathering process. At this post we will learn about laterites. These soils are weather from some rocks having metallic oxides. The metal mainly involved in iron. Oxidation process produces reddish color. As oxidation is involved, they are weather in channel.

Their extent over the worldwide are:

a. Southeastern portion of united states

b. South America

c. Some regions of Africa

d. In some tropical regions

Saprolites are also residual soil, their main characteristics is that they retain parental fabric and structure of mother rock. But weathering process produces a soft deposit, unlike parent rock, and they are considered as soil (obviously residual) instead of rock.

They may be enriched with mica content to such amount that they cannot be used as foundation bed; require some improvement, like compaction.

In United States saprolites are usually found in regions at the foot of mountain range. But they also found in different geologic regions other than that of United States, over the world.

Dear reader we will learn about sampling of residual soil in next post.

Effect of Rainfall on Residual Soil

It should keep in mind that water/ moisture is the main enemy to soil and geotechnical engineering. While try to simulate a soil sample to represent actual filed/foundation condition, put at least to two conditions; they are:

a. overburden pressure

b. moisture content

Though in some apparatus we can control moisture content, usually, we test sample under saturated condition. It is assumed that fully saturated soil is the worst condition it can face in field.

In our upcoming post we will learn how shear strength of a partially saturated soil is greater than fully saturated soil. However, come to the point rainfall:

Two factor controls extent of internal poor material

-extent and degree of weathering

-amount of regional rainfall

This extent may be

-layers

-lenses

-zones

The interface where residual soil and its parent rock meets, water can accumulate to saturate (or almost saturate) soil as rocks are usually impermeable and water cannot penetrate and trapped into there.

Such soils are usually extended up to a thickness from 0 to around 20m. Thickness depends on types of rocks and climatic condition throughout a geologic period.

The upper or top layers of such deposits are found seldom saturated but as discussed above soil-rock interface may have 100% saturation. Though it is believed that residual soils are competent but many experiences were also observed.

How to Control Water Table Fluctuation beneath Foundation?

Dear reader we have discussed about water table fluctuation and its detrimental effects on foundation performance. Here we like to discuss about control of water table fluctuation. We have discussed a post relevant to this topic regarding controlling expansive potential of expansive soil like black cotton soil.

Here we will discuss about sump pump. The fluctuation problems can generally be solved providing some types of barrier to control accumulation around water building boundary. We know that rise in water table generates hydrostatic pressure which produce uplift below basement or foundation.

Providing a barrier against water accumulation is difficult or costly, as we discuss in “cutoff wall for mitigating foundation problem for expansive soil”; and we generally provide any form of drainage system that collect water to remove them from building site up to a safe distance.


In residential building and in some large building, drains are applied covering the perimeter of basement in regular pattern. In some cases, sloping basements are provided covered with granular materials which are provided such that a horizontal surface I achieved to provide a serviceable basement.


In such basement, a well is provided at the lowest point and have pumping facility. The well is called sump pit and the system is called sump-pump system. Now-a-days there have many pumps that can operate automatically when water level rise up to a critical level, obviously, in sump pit.


Now question is where to discharge this water? Usually municipalities in many regions, do not permit such water to discharge to sanitary sewers. Water table can be lowered permanently, but when concern to environment, it is not permitted.

Rock Residue and Contamination in Residual Soil

We know soil is of two types depending on origin, namely residual soil and transported soil; we have discussed many posts about both types of soil. Residual soil may be formed due to weathering of both hard rock and chemically reactive rocks.

Thus residual soil may be found by chemical and physical weathering of different types of rocks. The rocks may be of 

a. Sedimentary rock

b. Metamorphic rock

c. Igneous rock

The content of residual soil and contamination too depends on the rock types from which they formed and the process under which they weathered.

Soils formed in such manner usually contain silty sands or sandy silts. As contamination there may contain

-some mica particles

-some clay particles

Particle shapes of residual soil are found more angular than particles of sedimentary deposits. In many cases, soil contains pebbles of angular to subangular shape. Large pieces of rocks may remain in residual soil which are not weathered or remain under weathering.

Dear reader in our next post we will discuss about effect of rainfall on residual soil.