Chapter 5 : Concrete basic relations

 

 

 

↑ water → ↑ w/c (constant cement) → ↑workability

Increasing the water content will increase the workability as it will increase the paste volume for a constant cement content making the paste more diluted and available to the inter particle lubrication and coating of the aggregates.

 

↑ Water → ↑ w/c (constant cement) → ↓ strength

When water is add, the paste volume increases causing the paste to become more diluted and the density lowered because of the increase in the total volume of the capillary voids which will decrease the strength when crack in the mortar start to open when at  70% and more of the ultimate stress cause when the load is applied.

                                       

 

 

 

 

↑ water → ↓ Durability

Increasing the water content will decrease the durability, as the concrete becomes more porous and permeable resulting to a lowered resistance to external and internal attacks.

 

 

 

In general:

Coarse aggregates        →  affect strength

Fine aggregates             →  affect workability

 

Grading:

 

↑ the better the grading of aggregates → ↑ workability

The grading of the aggregates determines the paste requirements of the concrete and by that it controls workability. The grading of the  aggregates reflects on the volume of the voids between those aggretes that needs to be filled with paste.

 

Uniform grading : as only a few aggregates sizes dominates the grading. The aggregates won’t be effectively packed and more paste is required to fill the voids which will decrease the workability.

 

Gad grading: same as uniform grading more voids to be filled with paste.

 

Well grading: the spaces between the large particles are effectively filled with the medium ones and the voids between the medium aggregates are filled with the finer aggregates. This will produce maximum packing with minimum voids to be filled with paste thus increasing the workability.

 

↑ the better the grading of aggregates → ↑ strength

The better the aggregates pack together the less voids between the aggregate with less paste needed to fill these voids. As the aggregate is taking the place of paste and the aggregates is far stronger than the paste the strength increases. And as the paste and as the paste needed is lower the workability increases which give way for decreasing the water content thus lowering w/c ratio  which will increase the workability.

 

Fineness of sand:

↑ Fineness of sand → ↑ workability

↑Crushed sand → ↓ workability

A low fineness modulus of sand means that the sand is finer (smaller in size not smoother) with a larger surface area than coarser sand. This larger surface area needs more paste to coat it and lubricate it. This decreases the workability and increases the paste requirement. Crushed sand is coarser and more angular than natural sand.

 

↑ Fineness of sand → ↓percent of fine in aggregates

as the sand decrease in size, the volume of the voids increases and the weigh of a fixed volume of the fine aggregates is lowered. in the total weigh of aggregates the weight of the fines will decrease as they decrease in size forcing the percent of the coarse to increase to maintain the total weight of aggregates

 

 

Size of coarse aggregates:

 

↑ Max size of aggregate → ↑ compressive strength ↓ tensile strength

Increasing the maximum size of aggregates increases the compressive strength of the concrete as aggregates are usually stronger than paste. The cross section of the concrete now consists of more aggregates than paste leading to a higher strength. The effect is up to 1 ½” then the strength will decrease due to the placing requirements and spacing between steel bars in the concrete formworks which restrict the maximum size up to 1 ½”.

 

At the same time increasing the maximum size decrease the tensile strength because larger particles reduce the specific surface area of the aggregate which leads to a reduction in bond strength; also, larger particles are better in resisting shrinkage than the paste and tend to restrain volume changes in the cement paste and therefore induce some internal stress which will weaken the concrete.

 

↑ Max size of aggregate → ↑ Workability

Increasing the maximum size of the aggregates increases the workability as the total surface area is less and the extra paste can be also used elsewhere increasing the workability.

Increasing the maximum size of the coarse aggregates will lower the voids volume between those aggregates decreasing the volume of the sand needed to fill those voids which means decreasing the volume of the voids between the sand particle that needs less paste to fill them.

 

 

↑ Max size of aggregate → ↑ compressive strength ↓ tensile strength

Increasing the maximum size of aggregates increases the compressive strength of the concrete as aggregates are usually stronger than paste. The cross section of the concrete now consists of more aggregates than paste leading to a higher strength. The effect is up to 1 ½” then the strength will decrease due to the placing requirements and spacing between steel bars in the concrete formworks which restrict the maximum size up to 1 ½”.

 

At the same time increasing the maximum size decrease the tensile strength because larger particles reduce the specific surface area of the aggregate which leads to a reduction in bond strength; also, larger particles are better in resisting shrinkage than the paste and tend to restrain volume changes in the cement paste and therefore induce some internal stress which will weaken the concrete.

 

↑ Max. Size → ↑ Durability of concrete

Not because larger aggregates got a better durability (the smaller ones are more durable as there are less in total pores volume and lower in permeability) but because the larger the maximum size the larger the absolute volume of aggregate and the lower the paste volume which means lower paste permeability and higher durability.

 

Shape and texture:

 

↑ Smoother the aggregate → ↑  workability → ↓ strength

As the shape of the aggregates gets rounder the (surface area/volume) ratio decreases, the paste requirement to lubricate the surface is lowered which will increase the workability.

 

In addition for having a larger surface area, The rough texture aggregates interlock with each other needing more paste to lubricate them which will decrease the workability.

 

using a rough texture aggregates will increase the strength as it got a better bond with the cement paste than the smooth aggregates.

 

↑ Flaky and elongated aggregates → ↓ strength and durability

Elongated and flaky aggregates will lower the workability as they tend to orient themselves in one plane trapping water and air voids under them preventing the entrapped water on be mixed within the paste which will lower the water content of the paste reducing the workability.

 

↑ size and flatness→ ↓ Durability

Elongated and flaky aggregates will lower the durability as they tend to orient themselves in one plane trapping water and air voids under them preventing the entrapped water on be mixed within the paste this reduces the durability as the entrapped air pockets are more likely to be attacked by the freezing and thawing, in addition to reducing the air content of the paste by entrapping the air pocket beneath them not allowing them to be distributed evenly.

 

↑ Angular aggregates → ↑ sand content

Angular aggregates got more voids in between them so more sand is needed to fill the gaps other wise more paste has to be used.

 

Materials finer than 0.075 mm (#200 ASTM sieve):

 

↑ Percent passing #200 → ↓ workability & ↓ strength

The materials finer then #200 will decrease the strength of concrete (like clay coatings) as they prevent a bond between the aggregates surface and the cement paste and cause shrinkage cracks in the paste. Superfine materials will also decrease the workability because of their very large surface area (like loose silt) that will absorbed water from the paste.

Some fine materials (like organic materials) will cause retardation of setting and growth in strength.

Porosity of the aggregates:

 

↑ Porosity of aggregates → ↓ strength

The more porous the aggregates the lower the strength as the increased porosity will decrease the surface bond with the paste and increase the abrasion.

 

↑ Porosity of aggregates → ↑ freezing and thawing

The high porosity of the aggregates will decrease the freeze-thaw resistance as absorbed aggregates may freeze in inside the pores.

 

Absorption of the aggregates:

 

↑absorption od aggregates → ↓ workability

Increasing the absorption of the aggregates in an already proportioned mix will decrease the workability as it will absorb more water from the paste decreasing the paste volume resulting in a more concentrated paste.

 

Aggregates proportioning:

 

↑ % fine aggregate → ↑ workability

Increasing the fine/coarse aggregates ratio will increase the workability as the fine aggregates fill the voids in the coarse aggregates, minimizing the paste required to fill those voids.

But after 40-50% the workability will decrease as the volume of the fine aggregates will exceed the volume of the voids and the finer aggregates start to replace the volume of the coarse which will increase the surface area and thus increase the paste requirements.

 

 

 

↑ % fine aggregate → ↨strength

As the percent of sand increase the voids in the aggretes decreases which increases the workability and it is possible to reduce the cement content, but as the fines increase they will start to take the place of the larger aggregates which means more voids. Also the paste requirements increases and it’s important to increase the cement content.

 

 

Specific gravity:

 

↑ specific gravity of aggregates → ↑ density of the concrete

as the specific gravity of aggregates increase the volume of the aggretes remains constant but the density of the concrete is increase because the weight of the same volume of aggregates is larger.

 

 

 

↑Air voids → ↑workability

Increasing the air content will increase the workability as it is friction less and needs no paste to lubricate it as well as it take the place of the paste pushing it away  to where it could increase the lubrication of aggregates thus increasing the workability.

 

 

 

↑Air voids → ↓ strength→↓ durability

Increasing the air content well decrease the strength and durability as it will increase the porosity of the paste making it less dense. But after around 8% the durability will decrease because of the increases porosity and abrasion.

 

 

↑Air voids → ↓ freezing and thawing

Air voids acts as a release champers for the internal pressure generated by the frozen water

 

 

↑temperature→ ↓ workability

Increasing temperature will cause an increase in the rate of hydration and evaporation. Both of these effects lead to a loss of workability.

 

↑temperature→ ↑early strength ↓ ultimate strength

Increased temperature results in improved early strength and lower ultimate strength. The early strength gain is explained by the increase of the hydration process. The lower ultimate strength is more difficult to explain, but seems to be related to non-uniform development of the microstructure.

 

↑ age → ↓ workability

Workability will decrease with time due to several factors; loss of water through evaporation and absorption, increased particle interaction due to the formation of hydration products on the particle surface.

 

↑ age → ↑strength

as the concrete gets older more hydration products are formed increasing the bonds between the paste itself and the bond with the paste and the aggretes.

 

 

↑ Fineness of cement → ↑ workability

The increase in the fineness of cement lowers the friction by forming a rich paste but the increase in the total surface area of cement grains means more water needed for the hydration

 

↑ Fineness of cement → ↑ Hydration rate

As the fineness of cement increase there will be more surface area for the hydration to take place therefore the hydration rate increases.

 

↑ Rate of hydration → ↑ early strength  & ↓ Ultimate strength

If the hydration rate is increases, the hydrates tend to be more coarser and less favorable for strength development and because of the accelerated hydration, the there is larger quantities of hydrates that develop in a fast rate surrounding the unhydrated cement grains prohibiting their hydration.

 

↓ Rate of hydration → ↓ early strength & ↑ Ultimate strength

The decreased rate of hydration allows the slow formation of cement gel-like structure with a larger surface area which give more space for the hydration to take place and thus more cement grain get hydrated on the long run.

 

↑ Rate of hydration → ↓ permeability

As the rate of hydration increases, the hydrated paste is formed faster, blocking the inter connecting system between capillary voids.

 

↑ fineness → ↑ early strength ↓ultimate strength

The rate of hydration increases with the increase of fineness. Cement that hydrates faster will have lower initial strength but higher ultimate strength.

 

↑ cement content → ↑ early strength ↑ ultimate strength

As the cement content increases while the mixing water stay constant, the concentration of the pate increase allowing for more hydration product and more bonding.

 

 

 

↑ Compaction → ↑ strength

When the concrete is compacted more entrapped air get out lowering the porosity and the total volume of the voids. Making the concrete more dense so when crakes start in the pate when stresses exceed 70% of the ultimate stress it will take more time to connect the micro crack on the interface between aggregates and paste with each other which leads to failure.