Bridge infrastructure deteriorates gradually, yet its root causes---salt intrusion, freezing, fatigue cracking, and others---remain invisible to the naked eye. Expert diagnosis relies on tacit knowledge built over years of practice. We address the challenge of automating this latent causal reasoning by proposing a Damage Cause Encoder that classifies 10-class damage causes from visible damage descriptions $S_i$ for use in autonomous bridge diagnostic agents. Our approach chains three components: (i)Knowledge Triple Extraction---a large language model extracts causal triples of the form (damage $\xrightarrow{\mathtt{caused\_by}}$ cause) from 15--35 diagnostic PDF manuals and indexes them in a FAISS vector store; (ii)Retrieval-Augmented Context---at training and inference time, relevant causal triples $\mathcal{C}_i$ are retrieved and concatenated with $S_i$, converting implicit domain knowledge into explicit Encoder context; (iii)Systematic Fine-tuning Comparison---we conduct a rigorous comparison of LoRA, QLoRA, and QA-LoRA on a fixed Golden Testset (116 stratified samples), demonstrating that QLoRA achieves the optimal trade-off: identical test accuracy (87.07%) to full-precision LoRA, 11% faster inference, 72% lower GPU memory, and superior generalization across diverse unseen inputs. A controlled Golden Testset---stratified, deduplicated, and difficulty-tagged---is introduced as a reusable benchmark contribution. QLoRA further outperforms LoRA by 13 percentage points on a 100-sample diverse evaluation spanning all 10 damage cause classes.These findings enable memory-efficient, high-accuracy diagnostic agents on consumer-grade hardware for edge deployment.
The Federal Highway Administration (FHWA) mandates that over 600,000 bridges in the United States be evaluated against the Recording and Coding Guide for the National Bridge Inventory (NBI). Manual compliance verification is labor-intensive, error-prone, and impractical in connectivity-limited field environments. This paper introduces BridgeGuard, a fully air-gapped agentic Retrieval-Augmented Generation (RAG) system for autonomous bridge inspection compliance. BridgeGuard integrates vector search over the FHWA Recording and Coding Guide with structured SQL queries against NBI tabular data, orchestrated by a stateful multi-step ReAct planning loop executing locally on commodity edge hardware. A section-aware chunking algorithm preserves hierarchical regulatory item boundaries, achieving 94.2% chunk integrity compared with 28.4% for naive fixed-size splitting. Evaluated on the full Delaware 2023 NBI inventory (874 bridges) and a Texas sample (200 bridges), the system achieves 99.77% and 100.0% classification accuracy, respectively, for Structurally Deficient bridge identification, with 100.0% citation accuracy, at 197.0 bridges per hour with out external network access. Ablation experiments confirm that both vector search and the multi-step agentic loop are necessary for correct compliance reasoning.