Learning Objectives Relevance of fresh concrete properties Tests on freshly mixed concrete Describe how fresh concrete properties are influenced by concrete constituents PECP06 2005

Relevance of fresh concrete properties Why are we interested in the properties of fresh concrete? Influence on the final quality of hardened concrete Helps to detect variations in material and process Uniform fresh concrete = Uniform hardened Last chance to identify unsatisfactory quality PECP06 2005

Relevance of fresh concrete properties Time Distance 50 40 30 20 10 Temperature º C Job site PECP06 2005

Relevance of fresh concrete properties Compaction ratio PECP06 2005

Tests on Freshly Mixed Concrete Procedures used for testing freshly mixed concrete are standardized Workability & consistency Bleeding and settlement Setting time Air content Temperature Density Used not only in the laboratory but also in the field Designed to eliminate random variations that may occur in the test results and lead to unnecessary disputes on the quality of concrete PECP06 2005

Workability & Consistency The ease of placing, consolidating and finishing freshly mixed concrete and the degree to which it resists segregation is called “workability” Properties related to workability include: Consistency Segregation/cohesiveness Mobility Pumpability Bleeding Finishability Degree of workability required is controlled by: Placement method Type of consolidation Type of concrete PECP06 2005

Workability & Consistency Factors influencing concrete workability are: Method and duration of transport Type and content of cement Aggregate characteristics (grading, shape…) Entrained air Water content Temperature (concrete & ambient) Admixtures PECP06 2005

Measurement of Consistency Although it is a different characteristic from workability, in practice the two terms are often confused and merged into one - the “slump” of concrete Slump ASTM - 143 PECP06 2005

Measurement of Consistency Workability Terminology Very stiff Stiff Plastic Soft (wet) Flowing PECP06 2005

Measurement of Consistency Flow table - DIN 1048 PECP06 2005

Influence of Water on Workability Only very minor adjustments to workability should be made by the addition of water alone Reduce strength and durability Potential for drying shrinkage cracking Therefore, adjustments to workability made by the addition of water should always be accompanied by an addition of cement to maintain a fixed W/C PECP06 2005

Influence of Water on Workability Relation between water requirement of paste and concrete PECP06 2005

Influence of aggregates on Workability Grading Affects workability, because the water demand necessary to wet all the solids in the mix will depend on the surface area of the aggregates The larger the particle size the greater the workability for a given cement content and W/C Particle shape Rounded or roughly cubically shaped aggregates will produce more workable concrete than that produced using flaky or elongated particles PECP06 2005

Sand PECP06 2005

Influence of temperature on Workability ASTM C 1064 Glass or digital readouts thermometers are available PECP06 2005

Influence of temperature on Workability Time Vs Slump 25 20 15 21 °C Slump (mm) 10 32 °C 5 -20 10 40 70 100 Time(min) PECP06 2005

Workability & Consistency Cohesiveness of concrete Is a measure of its ability to resist the segregation of its constituents during handling, placing and compaction Cohesive No Cohesive Segregation PECP06 2005

Cohesiveness PECP06 2005

Cohesiveness of concrete Factors affecting the cohesiveness of concrete: Water The higher the water content the greater is the risk of segregation and bleeding High water content retards the stiffening of concrete allowing sedimentation of heavier particles Cold weather can also retard setting and promote bleeding Dry mix can be friable and also prone to segregation Grading Mixes that are deficient in fine particles tend to segregate more readily during handling and bleed after finishing (entrained air can help) Excessive fines make concrete “sticky” and very cohesive PECP06 2005

Bleeding and settlement Bleeding is the development of a layer of water at the top or surface of freshly placed concrete Bleeding is normal and should not diminish the quality of properly placed, finished and cured concrete Some bleeding is helpful to control plastic cracking PECP06 2005

Excessive bleeding? Cracks PECP06 2005

Measurement of Bleeding Can be determined by the test method described in ASTM C 232 – rarely used in the field Typical values range from 0.1 to 2.5 % of mix water Or 0.01 to 0.08 mL/cm2 PECP06 2005

Setting time Can be determined by the test method described in ASTM C 403 by means of penetration resistance measurements Made on mortar sieved from the concrete mixture Initial and final time of setting corresponds to a penetration resistance equal to 500 psi (3.4 MPa) and 4000 psi (27.6 MPa) Typical values range Initial set: 2 to 6 hours Final set: 4 to 12 hours Factors affecting setting time of concrete: W/C ratio Cement type and content Temperature Admixtures PECP06 2005

Measurement Setting time Final set Initial set PECP06 2005

Setting time Methods Vicat ASTM C 403 Limestone cement Paste – w/c ~0.27 Concrete – w/c ~ 0.6 MSC Competitor Setting time Initial 110 min + 30 min 280 min - 50 min Final 140 min 400 min - 40 min PECP06 2005

Air content ASTM C231: pressure method ASTM C173: volumetric method Applied pressure compresses the air within the concrete including in aggregates pores Not suitable for lightweight aggregate concrete ASTM C173: volumetric method Removal of air from a known volume of concrete by agitating the concrete in an excess of water Suitable for lightweight aggregate concrete PECP06 2005

Density ASTM C138 Determine the volumetric quantity of concrete produced per batch Can also give indications of air content provided the relative densities of the ingredients are known PECP06 2005