The Bridges.IN

Guidelines for Concrete Admixture Storage and Testing at Site

01) INTRODUCTION

Ever since the introduction of cement for concrete there have been various attempts to use chemical admixtures to modify some of the properties of concrete. In the early period of usage admixtures were generally regarded with suspicion; probably due to certain unpleasant experiences. Now the advantages of properly controlled usage of admixtures are successfully established and there use has greatly increased. The admixture manufacturers have also realized the market potential as well as the need for quality assurance in the manufacture and supply of products.

Admixtures influence the physical, chemical and mechanical properties of concrete and its durability. However, an admixture should be used only for the intended purpose and calls for knowledge of concrete technology and certain minimum level of quality assurance at site. Since large types of various brands of admixtures are available in the market, reliable technical information is essential to be provided by the manufacturer. It is obligatory on the part of the customers to use only those products which are backed by good technical service.

02) ADMIXTURE

A material other than water, aggregates, and hydraulic cement and additives like pozzolana or slag and fiber reinforcement used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state.

03) REFFERENCE

IS 9103:1999, Indian Standard, ASTM C 494

04) TYPES OF ADMIXTURES

Admixtures are classified according to type of material constituent or the characteristic effects of their use. Internationally the most widely used classifications are those of American Society for Testing Materials (ASTM), German and French Standards.

ASTM C 494 classifies the admixtures as:
  • Water reducing admixtures
  • Retarding admixtures
  • Accelerating admixtures
  • Water reducing / Retarding admixtures
  • Water reducing / Accelerating admixtures
  • Water reducing / High range admixtures
  • Water reducing / High range and Retarding admixtures
    •

IS 9103:1999 classifies concrete admixtures as:

  • Accelerating Admixture or Accelerator

An admixture when added to concrete, mortar or grout, increases the rate of hydration of a hydraulic cement shortens the time of set or increases the rate of hardening or strength development.

  • Retarding Admixture or Retarder
    •

An admixture which delays the setting of cement paste, and hence of mixtures, such as mortar or concrete containing cement.

  • Water Reducing Admixture or Workability Aid
    •

An admixture which either increases workability of freshly mixed mortar or concrete without increasing water content or maintains workability with a reduced amount of water.

  • Air-Entraining Admixtures
    •

An admixture for concrete or mortar which causes air to be incorporated in the form of minute bubbles in the concrete or mortar during mixing normally it’s used to increase workability and resistance to freezing and thawing.

  • Super Plasticizing Admixtures
    •

An admixture for mortar or concrete which imparts very high workability or allows a large decrease in water content for a given workability.

  • Retarding Super plasticizing Admixture
    •
Super plasticizing admixture that imparts prolonged workability retention and retards setting.

The German classification

Admixtures

Symbol

Container color Code

Plasticizer

BV

Yellow

Superplasticiser

FM

Grey

Air entraining Agent

LP

Blue

Waterproofing Agent

DM

Brown

Retarder

VZ

Red

Accelerator

BE

Green

Grouting Aid

EH

White

Anti-bleeding Agent

ST

Violet

The French classifications

  • NFP18

    • 103 – Definition, Classification
      •
      • 331–Setting accelerator, chloride – free
        •
        • 332–Hardening accelerator, chloride – free
          •
          • 333 – Superplasticiser
            •
            • 334 – Wateproofing agent
              •
              • 335 – Plasticiser
                •
                • 336 – Plasticiser, water reducing
                  •
                  • 337 - Retarder
                    •
                    • 338 – Air entraining agent
                      •
                      • 339 – Grouting Aid
                        •

.

05) SAMPLING OF ADMIXTURE FOR TEST

Whenever the concrete admixtures are supplied at site it is necessary that admixtures are stored properly and also it is essential that site carries out initial tests on these admixtures to ensure that admixtures supplied are same as earlier approved sample. In this connection certain guidelines are furnished herein below:

.

Liquid Admixture

Grab samples taken for testing shall represent not more than 9000 litres of admixture and shall have a volume of at least 1 litre. A minimum of four grab samples shall be taken Composite samples shall be prepared by thoroughly mixing the grab samples selected and the resultant mixture sampled to provide at least 4 liters for quality tests.

Admixture from bulk storage tanks shall be sampled equally from the upper, intermediate, and lower levels by means of drain cocks in the sides of tanks or a weighed sampling bottle fitted with a stopper that can be removed after the bottle is lowered to the desired depth.

.

Non-liquid Admixture

Grab samples taken for tests shall represent not more than 2 tonnes of admixture and shall weigh at least 1 kg. A minimum of 4 grab samples shall be taken and composite sample shall be prepared by thoroughly mixing the four grab samples. The resultant mixture sampled shall provide at least 2.5 kg from the composite sample. Grab samples shall be taken from different locations well distributed throughout the quantity to be represented.

Samples of packed admixtures shall be obtained in the same manner as described in IS 3 53 5.Samples shall be packed in moisture-proof, airtight containers.

Retain a sample of 4 liter of the approved admixture at site along with manufacturer’s chemical analysis certificate. The certificate should indicate at least the information contains below.

When the new supply of admixture is received, check the new supply with respect to the approved sample retained with us as mentioned below:

a) Colour }

b) Density } they should be same as approved sample.

c) pH value }

d) Compare the performance of the product with regard to slump, retention, etc. with respect to the approved sample. If the product does not meet the approved sample’s characteristics, the same should be intimated immediately to the supplier and P&S at H.O.

Further, check the other data such as solid content, chloride content, sulphate, impurity and ash content with respect to the chemical analysis certificate. If the chemical analysis certificate is not supplied by the manufacturer, please insist for immediate action to furnish. Physical properties of admixture to be tested for every lot of admixture.

06) Storage

Liquid admixtures can be stored in barrels or bulk tankers. Powdered admixtures can be placed in special storage bins and some are available in premeasured plastic bags.

All admixtures including approved sample should be stored in shade. If this is not possible for short duration of few days or so, then it is recommended to cover the sample with tarpaulin sheets. Further, storing of admixtures in cement go down should not be permitted as there is possibility of cement dust getting mixed with admixtures whenever container is opened for usage. Admixture container should always be closed unless it is required to be opened for usage.

Ligno blended admixtures particularly when stored in open and at high ambient temperatures may foam or generate gas due to aerobic bacteria. In such situation even the drums may get swollen under foam pressure. In such cases, allow the release of gas formation by opening the lid. The efficacy of the admixture will not get affected because of this generation of gas.

Dispenser tanks at concrete plants should be properly labeled for specific admixtures to avoid contamination and avoid dosing the wrong admixture. Most liquid chemical admixtures should not be allowed to freeze; therefore, they should be stored in heated environments.

Consult the admixture manufacturer for proper storage temperatures

Whenever admixture drums stored for long, these drums should be rolled or stirred well before withdrawing the admixture as there is possibility of sedimentation.MSDS (Material safety data sheet) for different types of admixture should be available at storage location.

In addition to the above, the following guidelines are also given which will help in proper dosage. If admixture dispenser is used, calibrate the system and weighing systems regularly at least once in 6 months. Clean the dispenser motor and hose regularly (once a week).