concrete retempering presentation

CONCRETE RETEMPERINGUSING WATER & SUPERPLASTICIZER

By : Mohamed SaberNourhan Ahmed

What Is Retempering? Retempering , the practice of adding

water or admixtures to concrete or mortar to restore workability, is considered good practice because it keeps Concrete or mortar in the right condition for Casting or laying brick, block, and stone.

What Makes Concrete (Mortar) needs retempering?

Causes Slump loss 1 – Hot weather Concrete Casting

2 -Delay In delivering ready mixed concrete causes Slump loss

What are the Conditions we are allowed to reptemper ready mixed concrete at the site?

1-only in accordance with procedures outlined in ASTM C 94, “Standard Specification for Ready Mixed Concrete.” You are not allowed to add so much water that the concrete exceeds the specified slump or the specified water-cement ratio, because that can affect concrete properties such as strength and cracking. When in doubt, consider adding a water-reducing admixture or superplasticizer to increase the slump

Continue Conditions to allow retempering at the site: Estimate or measure the slump from the

first portion of concrete discharged from the truck (no more than ¼ cubic yard). If both the slump and water-cement ratio are below the maximums allowed by the specifications, adding water is permissible. Measure and record the amount of water added, then mix the concrete for 30 revolutions of the mixer drum at mixing speed.

Water only or Water and Plasticizers?

In order to answer that question an experiment was made to prove which is better

Continue Water or Water and Plasticizers?

Procedure: Two types of concrete mixtures designated as

c25 and c35 according to European standard with 150 mm slump after initial mixing were planned to stationary mixer in the laboratory.

To take concrete samples to be tested, the mixer was briefly stopped after 5min and after the first , second ,third and fourth hours.


First, the temp. and the slump were determined. Then , the samples showing a slump of less than 15 cm were placed into smaller mixer and four different retempering processes were applied to bring the slump value up to the original value , 15 cm:

1-Retempering with plain water.2-Retempering with water + 1.5% superplasticizer by mass of

water.3-Retempering with water+ 3% superplasticizer by mass of water.

4-Retempering with water + 4.5% superplasticizer by mass of water.

In all cases the amount of water was adjusted to obtain the desired slump of 15

cm.


Results: The amounts of retempering water needed to

raise the slump of the concrete samples obtained from concretes c25 and c35 are given in the following fingures, depending on the solution concentration (from 0% to 4.5 % solid superplastizers by mass of retempering water.

Fig.1. Amount of retempering water added to concrete c25 for readjusting the slump to 15cm.

Fig.1. Amount of retempering water added to concrete c25 for readjusting the slump to 15cm .

Fig.2. Amount of retempering water added to concrete c35 for readjusting the slump to 15 cm


As it is seen from fig 1 and 2 less amount of water was needed when superplasticizer was used for retempering process.

As the dosage of superlasticizer used in retempering water increased the amount of water for slump readjustment decreased significantly.

Since c35 showed higher slump losses compared

to c25 the amount of water added to c35 for readjustment was higher than that added to c25.


Effects of retempering on Compressive Strenght:


Conclusion: 1-High slump losses are experienced as the

mixing time increases.

2-The inc. of concrete temp. , of the degree of hydration , and of the possible grinding action of the mixer is a main cause of slump losses.

3-High slump losses , of course , make the placing

, compaction and surface finishing of the concrete very difficult , if not impossible.

Therefore, retempering of the concrete at the job site is a common process to render the concrete more plastic. On the other hand , inclusion of extra water causes strength losses.


4-Less amount of water is needed when superplasticizer is used for the retempering process, as compared to the use of plain water alone.

5-The final w/c ratios of mixes retempered with

superlasticizer are lower that of mixes retempered with plain water alone.

Thus the dec. in the compressive strengths of thoso concretes retempered with superplasticizer is less than the dec. in the strengths of those concretes retempered with water.

Questions

Thanks for your Attention

Submitted By : Mohamed Saber

Nourhan Ahmed

Submitted to : Dr\Walid Fouad tawhed

concrete retempering presentation

Engineering