# Presentation: 937849

## Structural Concrete Innovations:

***Structural Concrete Innovations:***
***A Focus on Blast Resistance***

**Hershey Lodge**
**Preconference Symposium**
**17 March 2008**

## Blast Overview

**Blast can effect structure in multiple way**
**Air blast**
**Drag**
**Ground shock**
**Primary and secondary fragmentation**
**Fire**

## Blast Loading

**Air blast design can be governed by max pressure, impulse, or combination**
**Function of size of explosive, standoff distance, and structure**

## Air Blast Loads

**Properties of the air blast load a function of the:**
**Size and shape of explosive**
**Distance to explosive**
**Orientation of specimen**
**Type of blast**
*Free air burst*
*Ground burst*
*Contained burst *

## Scaled Distance

**Convert explosive to equivalent weight of TNT**
**Determine scaled distance using ****	****	****Z = D / W^(1/3)****	****where ****	****Z = scaled distance****	****	****W= equivalent TNT weight****	****	****D = distance between specimen ****	****	****	****and explosive**
**Use figures in references (TM5-1300): “Structures to Resist the Effects of Accidental Explosions”**
**determine the expected peak pressure and impulse for determined scaled distance**

## Scaled DistanceFigure 2-7 TM5-1300

## Types of Cross Sections

**TM5-1300: 3 types of cross sections**
**Type I:  **
*Concrete is sufficient to resist compressive component of moment*
*Cover remains undamaged*
**Type II:  **
*Concrete is no longer effective at resisting moment*
*Equal top and bottom reinforcement*
*Cover remains in tact*
*Single leg stirrups used to resist shear*
**Type III:**
*Equal top and bottom reinforcement*
*Cover disengages*
*Lacing used to resist shear*

## Example Type II Cross-Section

**Example Type II Cross-Section**

## Motivation for Innovation in Blast Resistant Concrete

**Increased demand for impact and blast-resistant building materials**
**Need for practical, constructible options**
**Need for reduction in secondary fragmentation**

## Innovation

**Long (3”) fibers **
**Increased bond with concrete matrix**
**Length provides crack bridging, spalling resistance, increased ductility, energy absorption (through long-fiber pull-out)**
**Coated “tape” **
**Mix retains workability (no balling, etc)**
**Can be used with aggregate**
**Potentially economical**
**Carbon fiber yarn is waste product from the aerospace industry**
**No special mixers required**
**Lightweight “additive” reinforcement**
**Precast or cast-in-place**
**Molds to any shape**

## Experimental Program

**Mix design development**
**Workability**
**Static flexural strength**
**Small and large scale**
**Ductility**
**Impact testing**
**Small beams**
**Panels**
**Blast Testing**
**Finite Element Modeling**

## Experimental Program

**Mix design development**
**1.5% to 2.5% fiber content (by volume)**
**Various admixture combinations **
**Pozzolans (interground SF + GGBFS)**

## Preliminary Testing

**Mixture Design**
**Avoid balling**
**Increase workability**
**Increase fines and cement in mixture**
**Preliminary Static Tests**
**6” X 6” X 18” beams loaded at third points**
**Flexural Strength = 2112 psi**

## Slab Strips

**4” X 12” X 10’ slab strips loaded at midspan**
**Specimens:**
**2 control specimens with reinforcing mesh**
**2 fiber reinforced concrete specimens**
**2 fiber reinforced concrete specimens with mesh**
**Used to obtain load vs. deflection plot**
**Useful for obtaining toughness**

## Slab Strip Results

|  | Compressive Strength (psi) | Tensile Stress (psi) | Toughness (lbs-in) |
| --- | --- | --- | --- |
| Average Plane + mesh | 6151 | 750 | 186 |
| Average Fiber | 6652 | 1904 | 1834 |
| Average Fiber + mesh | 6619 | 2116 | 2619 |

## Impact Test Setup

**15 ft maximum drop height**
**50# weight**
**Panels 2’x2’x2”**

## Impact Testing: Panels

***Drop Height at failure***

## Impact Testing: Panels

***Drop Height at first cracking (top side)***

## Impact Testing: Panels(No Steel Reinforcement)

**Fiber addition controlled spalling**
**Failure in fiber specimens along weak plane due to fiber orientation**

**Plain panel**

**Fiber panel**

## Impact Testing: Panels(Steel Reinforcement)

**Fiber panel with steel reinforcement did not fail after repeated blows at top drop height**

**Plain panel**

**Fiber panel**

## Blast Testing

**6’ x 6’ x 6.5”**
**Heavily reinforced (as per TM5-1300)**
**resist shear failure at supports **
**evaluate comparison of materials under full blast design**

**Identical reinforcement in all specimens**
**Clear cover 3⁄4” to ties**

## Test Setup

**Slabs were simply supported on all four sides**
**Restraint provided along two sides to prevent rebound**

## Test Setup

**TNT suspended at desired height**
**Pressure gages record reflected pressure and incident pressure**

## Slide 24

## Hit 1: 75# at 6’ (scaled range 1.4)

- Extensive cracking, some spalling

- A few hairline cracks

**Standard Concrete**

**SafeTcrete**

## Hit 2: 75# at 3.2’ (scaled range 0.76)

**Standard Concrete**

**SafeTcrete**

## Hit 2: 75# at 3.2’ (scaled range 0.76)

**Standard Concrete**

**SafeTcrete**

- Some concrete loss due to pop out where reinforcement buckled (3/4” cover)

- Concrete rubble within steel cage

## Hit 2: 75# at 3.2’ (scaled range 0.76)

**Standard Concrete**

**SafeTcrete**

## Summary of Impact & Blast  Testing

**Much improved workability and dispersion of coated tape fibers**
**Increased ductility over plain concrete and further improved combined with standard reinforcement  **
**Significantly increased flexural strength under both static and impact loads**
**Complete control of spalling in panels under impact load**** **
**Excellent performance in blast testing**

## Potential

**Low cost fiber alternative**
**Applications  requiring impact and blast resistance**
**Protective cladding panels**
**Structural components: columns, walls**
**Barriers**
**Bridge piers**
**May be used as a replacement for, or in combination with standard reinforcement depending on application**

## Material Properties

**Stress-strain curves for material in both compression and tension needed for modeling**
**Compression: standard 6” diameter cylinders **
**Tension: dogbone specimens will be utilized**
**Varied load rates and fiber orientation**

## Tensile Properties

**New test method for tension in fiber concrete**
**Difficulties with direct tension**
**Size-effect with long-fibers**
**Dogbone specimens  32” high, 8” neck width, 16” top width **

## Concrete Dogbone

**Mechanical anchorages were used to load specimen**
**Anchorage consisted of 5/8”, 125 ksi threaded prestressing rod**
**LVDTs for displacement **
**Failure occurred in desired region**

## Tensile Properties

**Increase in energy dissipation**
**Testing will determine if cracking stress is affected by the addition of fibers**

## Finite Element Modeling

**Material model developed from testing**
**Comparison to field blast test and instrumented impact testing**
**Loading **
**CONWEP (built into LS Dyna)**
**Gas dynamics model (Lyle Long, AE)**
**Field data**

## Current Work

**Continued model refinement **
**Material model**
**Incorporation of fracture mechanics**
**Contact charges**
**Application specific testing**
**Durability**
**Reinforcement and fiber content variations**
**Specification development**

## Barrier Application Testing

**Use of fibers & polyurea for barriers**
**Large volume of concrete with small reinforcement percentage**
**Reduction in secondary fragmentation needed**

## Wall Testing: Spec Development

## Questions?

***Questions?***

**Hershey Lodge**
**Preconference Symposium**
**17 March 2008**