Concrete and its Properties 1

This week we continue with the second part of our four part series on Concrete and its properties by considering the concrete work-ability.

Work-ability of Concrete

What is concrete work-ability? You may ask.

This is the ease with which concrete can be transported, placed and consolidated without excessive bleeding or segregation. For the concrete to be workable it should be consistent.  Consistence is sometimes taken to mean the degree of wetness. Within limits, wet concretes are more workable than dry concrete, but concrete of same consistence may vary in work-ability.

Because the strength of concrete is adversely and significantly affected by the presence of voids in the compacted mass, it is vital to achieve a maximum possible density. This requires sufficient work-ability for virtually full compaction to be possible using a reasonable amount of work under the given conditions. Presence of voids in concrete reduces the density and greatly reduces the strength of the concrete.

 Factors affecting concrete work-ability include;

1. Water content or Water Cement Ratio

More the water cement ratio more will be work-ability of concrete. Since by simply adding water the inter particle lubrication is increased. High water content results in a higher fluidity and greater workability. Increased water content also results in bleeding. another effect of increased water content can also be that cement slurry will escape through joints of form work.

High water content results in a higher fluidity and greater work-ability. Increased water content also results in bleeding.  Another effect of increased water content can also be that cement slurry will escape through joints of form work.

 2. Amount and type of Aggregate

  The greater the size of the aggregate the less water will be to lubricate it thus making available extra water for work-ability. Using smooth and round aggregate helps in increases the work-ability. Work-ability of the concrete may reduce in a case where angular and rough aggregate is used. Angular aggregates increase flakiness or elongation thus reducing work-ability. On the other hand round smooth aggregates require less water and less lubrication and it allows greater work-ability in a given water/cement ratio mix.

 3.  Aggregate Cement ratio

When the ratio of aggregate to cement is higher than recommended  the less the work-ability on the concrete will be since less cement means less water and this will result into a paste that is stiff thus reducing work-ability.

 4.  Weather Conditions

Weather conditions such as high temperature and strong winds affect both the setting and the work-ability of the concrete. Strong winds and high temperatures cause increase in the rate of evaporation leading to reduction in the amount of water in the concrete mix thus affecting work-ability.

 5. Chemical Admixtures

The use of certain chemical admixtures has been known to increase work-ability of the concrete. For instance the use of air en-training agent is known to produce air bubbles which act as a sort of ball bearing between particles and increases mobility and work-ability. This also helps to decrease bleeding and segregation. Use of fine Pozzolanic material produces better lubricating effect that helps in providing more work-ability.

 6.  Sand to Aggregate ratio

The higher the amount of sand in the concrete mix the more the work-ability will reduce. This is because sand has more surface area and more contact area and this may end up causing more resistance.

When preparing a concrete mix proportion the ingredients using the appropriate weight or volume. This will help to provide the desired strength and work-ability at minimum expense.

A low water-cement ratio may be applied to achieve a stronger concrete. It would therefore appear that by making the cement content high one could use enough for good work-ability and still have a low water/cement ratio. As true as it may be the problem is that cement is the most costly of the basic mix ingredients.