Guidelines for Cement Concrete Pavements

Questions and Answers

What is the characteristic flexural strength for rural road pavements?

3.8 MPa

A good compacted foundation layer below a concrete pavement is termed as subbase.

True

The minimum CBR of the subgrade shall be ___

Which of the following is an advantage of providing a sub-base below concrete pavement?

It supports construction traffic even if the subgrade is wet Signup and view all the answers

What is the recommended effective k value for granular subbases?

20% more than the k value of the subgrade Signup and view all the answers

What are the components in the relationship for flexural strength based on compressive strength?

f_f = 0.7f_c Signup and view all the answers

The coefficient of thermal expansion of concrete is ___ per °C.

10 x 10^-6 Signup and view all the answers

Which factor can induce the highest stress in the pavement during design?

Traffic loads Signup and view all the answers

What type of stresses for interior loading are generally lower than edge and corner loading?

Load stresses Signup and view all the answers

The maximum tensile stresses in the edge region of the slab are caused by simultaneous occurrence of wheel loads and temperature differentials.

True Signup and view all the answers

What is the recommended joint spacing for the design of a 180 mm slab?

3.75 m Signup and view all the answers

What is the design life of the pavement?

20 years Signup and view all the answers

How can temperature curling stresses be reduced?

By adopting a lower joint spacing Signup and view all the answers

The cumulative fatigue damage is considered safe if it is less than __________.

1 Signup and view all the answers

What does SCC stand for?

Self-Compacting Concrete Signup and view all the answers

The material cost of SCC is generally lower than conventional concrete.

False Signup and view all the answers

What is the effect of higher dosages of fly ash in SCC?

Enhances flowability Signup and view all the answers

Cementitious material is a combination of cement and __________.

mineral admixture Signup and view all the answers

Who developed Westergaard's equation for edge loading?

Westergaard Signup and view all the answers

What is the recommended upper limit for 20 mm aggregate in M30 to M40 grade concrete?

100 Signup and view all the answers

Which ingredient has the highest quantity in Trial 1 mix design for M30 grade concrete?

Crushed sand (0 to 4.75 mm) Signup and view all the answers

The slump-flow diameter test is used to assess the flowability and the flow rate of self-compacting concrete in the absence of ________.

obstructions Signup and view all the answers

Curing is only important for surface elements made with conventional concrete.

False Signup and view all the answers

What is the purpose of the V-funnel test in self-compacting concrete?

To assess the viscosity and filling ability of self-compacting concrete. Signup and view all the answers

What does w/c ratio stand for in concrete mix design?

Water to cement ratio Signup and view all the answers

What is the V-funnel flow time for self-compacting concrete measured in?

Seconds Signup and view all the answers

Match the following sizes of aggregates with their respective recommended lower limits:

20 mm = 95 10 mm = 50 4.75 mm = 35 2.36 mm = 25 Signup and view all the answers

What is filling ability in fresh concrete?

The ability of fresh concrete to flow into and fill all spaces within the formwork, under its own weight. Signup and view all the answers

Define flow ability in the context of fresh concrete.

The ease of flow of fresh concrete when unconfined by formwork and/or reinforcement. Signup and view all the answers

What is the meaning of fluidity in fresh concrete?

The ease of flow of fresh concrete. Signup and view all the answers

What does paste refer to in concrete?

The fraction of the concrete comprising powder, water, and air, plus admixture, if applicable. Signup and view all the answers

What is passing ability in fresh concrete?

The ability of fresh concrete to flow through tight openings such as spaces between steel reinforcing bars without segregation or blocking. Signup and view all the answers

What are powders (fines) in concrete?

Material of particle size smaller than 0.125 mm. Signup and view all the answers

Describe robustness in the context of concrete.

The capacity of concrete to retain its fresh properties when small variations in the properties or quantities of the constituent materials occur. Signup and view all the answers

What is self-compacting concrete (SCC)?

Concrete that is able to flow and consolidate under its own weight, completely fill the formwork even in the presence of dense reinforcement, maintaining homogeneity. Signup and view all the answers

Define segregation resistance in concrete.

The ability of concrete to remain homogeneous in composition while in its fresh state. Signup and view all the answers

What does slump-flow refer to?

The mean diameter of the spread of fresh concrete using a conventional slump cone. Signup and view all the answers

Describe thixotropy in materials like SCC.

The tendency of a material to progressively lose fluidity when allowed to rest undisturbed but to regain its fluidity when energy is applied. Signup and view all the answers

What is viscosity in relation to SCC?

The resistance to flow of a material once flow has started. Signup and view all the answers

What are Viscosity Modifying Admixtures (VMA)?

Admixtures added to fresh concrete to increase cohesion and segregation resistance. Signup and view all the answers

What is the role of mineral admixtures in SCC?

They are commonly used to improve and maintain the cohesion and segregation resistance of the fresh concrete. Signup and view all the answers

What is the significance of fly ash in SCC?

Fly ash provides increased cohesion and reduced sensitivity to changes in water content. Signup and view all the answers

What is the importance of fine aggregates in SCC?

The influence on the fresh properties of SCC is significantly greater than that of coarse aggregates. Signup and view all the answers

Define superplasticizers in concrete.

Admixtures that bring about the required water reduction and fluidity while maintaining their dispersing effect. Signup and view all the answers

What is the general requirement for mixing water in SCC?

Water conforming to IS:456 should be used in SCC mixes. Signup and view all the answers

Study Notes

Guidelines for Cement Concrete Pavements

Designed for low volume roads, published by the Indian Roads Congress in January 2014.
Uses a default value of 10% from Equation 3.1 for cumulative fatigue analysis; heavy truck loads require a spectrum of axle loads.

Subgrade Characteristics

Strength determined by modulus of subgrade reaction (k), assessed via plate bearing test (750 mm diameter).
Values are influenced by moisture; tests should be conducted post-monsoon.
Soaked California Bearing Ratio (CBR) values correlate to k values, establishing a minimum CBR of 4.

Importance of Sub-Base

A quality sub-base prevents settlement under wheel loads, promotes uniform support, and acts as a drainage layer.
Types of sub-base vary depending on traffic volume:
- Up to 50 CVPD: 75 mm thick Water Bound Macadam over 100 mm granular sub-base.
- 50 to 150 CVPD: Similar thickness of WBM/WMM over granular material or cementitious layer.
- 150 to 450 CVPD: Increased WBM/WMM thickness or alternate cementitious layers suggested.

Concrete Strength Specifications

Design based on flexural strength; compressive strength relationship established (Equation 3.3).
For low volume roads, 90-day strength utilized for design; minimum flexural strength set at 3.8 MPa for 28 days.
Recommended that characteristic 28-day compressive strength be at least 30 MPa.

Modulus of Elasticity and Other Properties

Modulus of Elasticity for concrete is taken as 30,000 MPa, and Poisson's Ratio as 0.15.
Coefficient of thermal expansion for concrete estimated at 10 x 10^-6 per °C.

Fatigue Behavior

Generally not a concern for rural roads; however, significant traffic may necessitate fatigue analysis.
Specific fatigue equations apply for low traffic scenarios with lighter commercial vehicles having negligible impact.

Slab Thickness Design

Stress analysis crucial due to traffic loads and temperature gradients.
Critical stress locations are identified at slab edges and corners, where curling stresses are highest during temperature fluctuations.
Joint spacing impacts safety; closer joints enhance load sharing and reduce stress on pavements.

Stress Calculation Methods

Uses Westergaard’s equation for edge loading to compute stresses due to wheel loads.
Examples demonstrate safe design based on computed stress levels and joint spacing adjustments.### Pavement Design Considerations
Adopt 180 mm thickness with 3.75 m joint spacing for granular subbase.
A 170 mm slab is safe for 2.50 m joint spacing; 180 mm is needed for 3.75 m spacing.
Cemented subbase reduces temperature curling stresses.
Joint spacing impacts wheel load; lower spacing reduces stresses.
Required joint spacing is influenced by multiple factors including cost and convenience.
Provide a non-erodible subbase to ensure subgrade support.
Design life for pavements is typically set to 20 years.

Stress Analysis and FATIGUE Considerations

Fatigue fracture of concrete needs consideration; includes total wheel load and temperature stresses.
Concrete gains strength over time, leading to residual strength even past 90 days.
In design scenarios, cumulative fatigue damage must be maintained below 1 for safety.
Examples of different designs include:
- 200 mm thickness with 4 m joint spacing led to cumulative fatigue damage of 193.31 (unsafe).
- 2.5 m joint spacing with 220 mm thickness leads to cumulative fatigue damage of 9.62 (unsafe).
- 2.50 m joint spacing and result of 0.01 cumulative fatigue damage indicates safe design.

Temperature Distribution Analysis

Non-linear temperature distribution is observed across slab thickness.
Thermal stresses from linear and non-linear temperature components need algebraic addition for total stress.
Daily temperature differences are significant; day time sees higher disparities than night.
Temperature differences during the day can lead to significant internal bending stresses due to different curvatures in the slab.

Curling Stresses during Day and Night

Daytime curling causes upper slab bending from linear temperature effects while the lower slab remains horizontal.
Nighttime trends reverse with compressive stresses developing at different slab depths.
Predicts compressive and tensile stresses resulting from internal bending moments during both day and night.

Self-Compacting Concrete (SCC) Overview

SCC developed to enhance performance and productivity without external compaction methods.
Compaction in SCC is achieved through gravity, ensuring filling and mixing uniformity.
Requires rheological properties to maintain desired flowability, workability, and homogeneity.
High deformability achieved through superplasticizers and low water-cement ratios.
SCC enhances productivity, reduces labor costs, and improves construction conditions.

Key Terms in Self-Compacting Concrete

Filling Ability: Ability to fill formwork under self-weight.
Passing Ability: Flow through reinforcement without segregation.
Robustness: Concrete's capacity to maintain properties despite material variations.
Thixotropy: Loss of fluidity when stationary, regained with movement.
Viscosity: Resistance to flow during casting and compaction.

Applications and Cost Considerations of SCC

SCC can replace traditionally placed concrete (TPC) in various applications.
Initial costs may be higher than TPC, but reduced labor and increased durability may offset expenses.
No standard guidelines for SCC use in India but references from developed countries are available.### Rheology of Concrete
Understanding rheology is essential for optimizing processes in concrete, mortar, and paste.

Workability

Self-compactability allows concrete to flow solely by gravity, filling formwork spaces without defects.
Characterized by three functional requirements: filling ability, resistance to segregation, and passing ability.

Filling Ability

SCC must deform under its own weight to spread effectively.
Evaluated using slump flow measurement to assess flow diameter (deformation capacity) and flow speed (velocity of deformation).
A balance between deformation capacity and velocity is crucial for effective filling.

Resistance to Segregation

SCC should not segregate during movement; indicators of segregation include:
- Bleeding of water
- Paste and aggregate segregation
- Coarse aggregate segregation, which can cause blocking
- Non-uniform air-pore distribution
Reduced movable water and lower water-to-powder (W/P) ratios enhance segregation resistance.
Viscosity Modifying Admixtures (VMA) can improve water and solid segregation resistance by increasing viscosity.

Passing Ability

SCC with excellent filling ability may still face blocking under certain conditions:
- Large maximum aggregate sizes
- High content of large-sized aggregates
Blocking can occur despite segregation tendencies of coarser aggregates.

Specification of SCC

SCC workability and stability defined by four key characteristics with preferred testing methods:
- Flowability: Slump-flow test
- Viscosity: Slump-flow test or V-funnel test
- Passing ability: L-box test
- Segregation resistance: Sieve test
Acceptable values for SCC vary based on field trials, with benchmark slump flow of 400 mm and V-funnel time of 8 seconds for village roads.

Constituent Materials

SCC materials conform to IS:456 standards, similar to traditional vibrated concrete.

Minimum Cement Content

Required minimum cement content must meet durability criteria based on exposure conditions.

Mineral Admixtures

Inert and pozzolanic/hydraulic additions enhance cohesion and segregation resistance.
Pozzolanic options include fly ash, silica fume, rice husk ash, and metakaoline.
Hydraulic materials such as GGBS are used for properties enhancement.

Fly Ash

Fly ash improves cohesion and reduces water sensitivity.
Must adhere to specific limits, including total sulfur (max 5%), total chloride (max 0.05%), and required fineness (min 320 m²/kg).

Aggregates

Normal-weight aggregates must conform to IS:383, ensuring durability.
Fines (particles <0.125 mm) are crucial for SCC rheology; insufficient fines may lead to segregation.
Coarse aggregates must meet IS:383, while fine aggregates significantly affect fresh properties.

Admixtures

High-range water-reducing admixtures are essential in SCC for increased fluidity and reduced water content.
Viscosity Modifying Admixtures (VMA) mitigate sensitivity to variations in moisture and material proportions.

Mixing Water

Use water conforming to IS:456 to ensure quality in SCC mixes.

Basic Mix Design

No standard SCC mix design method exists; different institutions use various guidelines based on constituent characteristics.
Strategies include optimizing the grading of the aggregate and assessing the flow/stability of paste and mortar before adding coarse aggregate.

Typical SCC Mix Composition

Water: 155-175 L/m³
Powder: 375-600 kg/m³
Fine aggregates: 40-60% of total aggregate weight
Coarse aggregates: 750-1000 kg/m³
Water to powder ratio: 0.76 to 1.0

Curing

Curing is critical for SCC to prevent drying, especially due to higher paste content and reduced bleed water.
Initial curing should begin immediately after placement to minimize surface crusting and shrinkage cracking.

Testing Fresh Concrete

Slump-flow test: Measures flowability and filling ability by assessing the diameter of spread concrete.
Test conducted with a cone, comparing maximum and perpendicular diameters of flow.
Maximum aggregate size for testing should not exceed 40 mm.

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Description

This quiz covers the guidelines for the design and construction of cement concrete pavements specifically for low volume roads as revised in January 2014. Test your knowledge on the standards published by the Indian Roads Congress. Understand key concepts related to pavement design and low volume road construction.